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BIOMECHANICS OF REMOVABLE PARTIAL DENTURE

INTRODUCTION:

Biomechanics is defined as the application of the principles of

mechanical engineering in the living organism.An understanding of the

biological response to mechanical stimuli is of paramount importance

for promoting long term success of removable partial

dentures.Mechanical forces exerted on removable partial dentures

during functional mandibular movements should be properly directed to

the supporting tissues to elicit the most favourable response. It may

also be considered as the study of the problem of distributing the

energy generated by the muscles of mastication so that it will be

expressed at the occlusal surface with the maximum efficiency

consistent with the minimum damage to the supporting structures.

The primary consideration in partial denture construction is to

distribute the forces on the occlusal surfaces with the minimum

damage to the supporting tissues.Partial dentures are subjected to

many forces,such as chewing(vertical and lateral), lifting( sticky foods),

and actions of the tongue,lips and cheeks.

The manner in which alveolar bone surrounding the natural teeth

responds to force differs markedly from that of the residual bone

remaining after the extraction of the teeth. Fundamental to

understanding partial denture design is a solid grasp to simple

mechanical principles.It is necessary to understand the essential physics

involved in the working of the prosthesis.

Designing a removable partial denture which optimally satisfies the

prosthodontics requirement of support, function and esthetics is a

daunting challenge.When poorly designed without taking into

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consideration of the biomechanical principles involved would make the

removable partial denture as a tooth extractor especially in a distal

extension removable partial denture. All removable partial dentures

direct mechanical forces to bone which is the ultimate supporting

tissue.The mucosa of the residual ridge transmits compressive forces

through the submucosa to the underlying bone without changing the

nature of the forces frequently resulting in pressure induced resorption.

The natural teeth are attached to the bone by means of a periodontal

ligament which converts much of the masticatory compressive forces to

tensional forces favourably stimulating alveolar bone. In the oral cavity

one would find a number of sources of stress generation, the human

body is built in such a manner that it learns to adapt to any stressful

situation. However when we try to create an artificial replacement of

that natural component which is lost, we are at loss in making it fully

functional and adaptable.

Designing of partial denture necessitates a proper planning for the

form and extent of dental prosthesis and studying of all the factors

involved. The prosthesis must be designed following the most

favourable biomechanical principles, as the proper design helps in

reducing the harmful effects on the supporting structures. The optimal

goal is to provide useful, functional removable partial denture

prostheses by striving to understand how to maximize every opportunity

for providing and maintaining a stable prosthesis.

Because removable partial dentures are not rigidly attached to teeth,

the control of potential movement under functional load is critical to

providing the best chance for stability and patient accommodation. The

consequence of prosthesis movement under load is an application of

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stress to the teeth and tissue that are contacting the prosthesis.It is

important that the stress not exceed the level of physiologic tolerance,

which is a range of mechanical stimulus that a system can resist without

disruption or traumatic consequences.

In the terminology of engineering mechanics, the prosthesis induces

stress in the tissue equal to the force applied across the area of contact

with the teeth and/or tissue. This same stress acts to produce strain in

the supporting tissue, which results in load displacement in the teeth and

tissue. The understanding of how these mechanical phenomena act

within a biological environment that is unique to each patient can be

discussed in terms of biomechanics.

It is important for clinicians providing removable partial denture service

to understand the possible movements in response to function and to be

able to logically design the component parts of the removable partial

denture to help control these movements. The following biomechanical

considerations provide a background regarding principles of the move-

ment potential associated with removable partial dentures, and the

subsequent chapters explain various factors associated with removable

partial denture and how they are used to control the resultant move-

ments of the prostheses.

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REVIEW OF THE LITERATURE

Goodmann.J et al21(1963) stated that the design of a free-end partial

denture restoration required a careful balance between the requirements

of retention and the stresses that the retainers would exert on abutment

teeth He founded a simple solution to reduce stress to abutment teeth

through the use of Balance of force principle.

Augsburger.R26 (1963)postulated that mathematical equations could be

used to outline quantitative values in the design of removable partial

dentures and numerical values were used to simulate forces imposed

upon abutment teeth by retention and support components of the

denture. He concluded that this system of analysis could be applied to

designs of removable partial dentures but the factors of the patient’s

attitude toward cosmetics and functional comfort must be considered.

Maxfield et al37(1979)measured the forces applied to abutment teeth by

removable partial dentures computed by applying an extension of the

Pythagorean theorem,they found that the transmitted forces vary when

different removable partial denture designs were used.They also

suggested that improving adaptation of the extension bases to the

residual ridge was an excellent means for providing maximum support,

increasing patient comfort, and decreasing forces to abutment teeth.

Cecconi.T.B et al32 (1975) had performed an invitro study using several

types of rests to determine which type of rests transmits forces to

abutment teeth in the most favourable manner. He concluded that the

rests with gingival seats at maximum depth in abutment teeth

significantly decreased abutment tooth movement and bilateral loading

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of a removable partial denture caused significantly less abutment tooth

movement than unilateral loading.

Knowles E.L16(1958) reviewed the engineering principles associated

with removable partial denture and he proposed that the primary

biomechanical principles to be considered were support, bracing, and

retention.

Seong-kyum kin et al74 (2007) had conducted an invitro study to

investigate the biomechanical effects of mandibular Removable partial

denture with various prosthetic designs under unilateral loading using

strain gauge analysis.They concluded that splinting of two isolated

abutments by bridge reduced the peri-abutment strain in comparison

with unsplinted abutments under unilateral loading.

Asher L.M52(1992) had proposed biomechanical consideration for the

use of the rotational path removable partial denture for a patient with a

tooth-bounded ridge on one side and a distal extension ridge on the

opposite side.He concluded that by including a rotational path rigid

retentive element in a design that accommodated rotational movement

in function,exceptional stability was achieved with minimal stress to the

abutments.

Rachman Ardan76(2008) conducted an in vitro study about the

masticatory force on the fulcrum point of first class lever on the lower

jaw distal free end denture using two dimensional static model in

sagital direction.He concluded that the main problem of distal free end

Removable Partial Denture is lateral displacement. He also stated that

the first class lever retainer design on distal free end RPD generated

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leverage to the abutment tooth and created cumulative pressure in main

fulcrum point.

Devan M M4 (1952) had stated that mouth needing a bilateral partial

denture was in a state of mutilation.He suggested for the preservation

of the partial denture foundation the horizontal forces falling on the

saddles and transverse forces falling on the abutments should be

reduced.He also suggested the all-out use of every available tooth and

tissue bearing for preservation of partial denture foundation.

Arthur.R.C1(1951)proposed that the amount of force imposed upon the

denture may be reduced by maintaining the sharpness of tooth cusps

and by decreasing the size of the food table.

Kwin Chi Luk59(1979) had demonstrated the design of a unilateral

rotational path removable partial denture to restore a single

edentulous space with a tilted mandibular molar.He suggested that

the stability and retention of the denture were controlled anteriorly

by the buccal retentive clasp and lingual guide plate of the

conventional direct retainer, and posteriorly by the rigid retainer

with its buccally and lingually extended proximal plates.

Theodore Berg51(1992) had compared photo elastically the stress

distribution characteristics of maxillary bilateral distal-extension

removable partial dentures retained by light and heavy ERA

extracoronal attachments.He also compared the pattern of stress

distribution in photoelastic model with one prosthesis included

supporting rests and the other had no rests.He concluded that there

was significant difference in stress distribution.

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MC Cracken et al15(1958) discussed the design, location, and the

purpose of the various components of the partial denture and pointed

out that some of the violations of sound biologic and mechanical

principles were committed.

Lawrence W.A11(1956) had analysed the lateral force transmitted to

denture base location and clasp design.He constructed an experimental

model to simulate the type of tooth movement found in the

mouth. He also constructed partial dentures of various designs and

evaluated the torques and rotational patterns of removable partial

denture.

MC Cleod.S N3(1982) had shown that with a rotating retainer an

axis of rotation exists about the fulcrum line on either side of the

dental arch. He also concluded that lack of alignment of the

rotational axis on either side of the arch produces torque on the

abutments when the prosthesis was in function.

Ceconi T.B29(1971) conducted an in vitro study and determined the

effect of two types of partial dentures movement, stress breakers on

abutment tooth movement and ridge displacement. He measured

these movements when the stress breakers were both active and not

active.

MC Cleod34(1977) had shown that with a rotating retainer an axis

of rotation existed about the fulcrum line on either side of the

dental arch. He concluded that lack of alignment of the rotational

axis on either side of the arch produced torque on the abutments

when the prosthesis was in function. He also altered the retainers and

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accomodated the lateral movements associated with the rotational axis

and compensated the torque.

Lammie8(1954) stated forces acting on RPD always be resolved into

three components, a purely vertical force, uncomplicated by a

simultaneously acting horizontal component on a lower free-end

saddle.He explained the forces acting on bilateral free end saddle and

the treatment options for bilateral free end saddles.

McCracken7(1953) stated that two distinctly different types of

partial dentures exist,tooth borne and tooth tissue borne.He further

stated that the advantage of this method of classification was there

exists a definite relationship between each other.

Arthur R.C1(1953) had planned partial denture with special reference to

stress distribution based on the physiologic rest position of the

mandible. He stated that there were two major factors involved in

controlling the forces of mastication.They were the reduction of

the amount of force imparted to the denture during mastication

and, the wide distribution of the forces to the tissues.

Arthur J.Kroll20 (1963) had demonstrated clasp design on an extension

based removable partial denture.He considered that the factors of stress

controlled when there was minimal tooth coverage and gingival

coverage. He introduced the RPI clasp that minimised tooth coverage

and reduced stress on the abutment tooth.

Ceconi T.B29(1971) had studied the effects of the sagittal inclination

of the residual ridges.He compared bilateral vs unilateral loading on

abutment tooth movement and also load vs non load side movements

of the abutment teeth.He found out that the angulation of the

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residual ridge in a sagittal plane altered the direction and

magnitude of abutment tooth movement.

Chester Perry10(1956) stated few basic rules and few basic requirements

such as support,retention,stability and esthetics must be met if the

restoration had to function adequately with comfort.

Kratochovil20(1963) demonstrated the effects of rest placement using a

training aid. He found that as the denture base was followed

posteriorly, the arc of movement became nearly perpendicular to the

surface of the mucosa

ZBen Ur61 (1999) had discussed the factors affecting denture design

related to the position of the abutment teeth, the symmetry design,the

cross sectional shape of the residual ridges and the treatment of

complications of the edentulous distal extensions.

Aviv.I48(1989) had proposed that an axis of rotation was created through

most distally placed occlusal rests when distal-extension removable

partial denture was loaded.He further stated that if the residual ridges

were of unequal lengths, the axis of rotation may not be

perpendicular to the residual ridges.

Beckley W.R27(1969) had considered the correct distribution of stress

between the abutment teeth and the denture base had been a point

of contention among the three schools of thought advocating

broken stress, functional bases, and wide distribution of stress. He

proposed a technique that took advantage of beneficial aspects of each

method.

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Weinberg28(1971) had analysed the lateral force in relation to denture

base location and clasp design. He constructed an experimental model

to simulate the type of tooth movement found in the mouth.He

constructed partial dentures of various designs on the model and

tested torque and rotation patterns.

Ben-Ur et al61(1999) performed rigidity tests on maxillary major

connector with different designs and mandibular major connectors of

the lingual bar type with different cross sectional shape and

thickness.They concluded that the most rigid maxillary major

connector was anterior-posterior palatal bar and the most flexible was

the U-shaped palatal bar.

Nathan K.C. Luk49(1990) had analysed mathematically occlusal rest

design for cast partial dentures.He concluded that a decrease in occlusal

width had increased the bending stress and required thicker rest for

compensation.He concluded by his mathematical analysis that the

traditional spoon-shaped occlusal rest seat dimensions had complied

with the mechanical requirements for non-precious cast metal occlusal

rests in RPD.

Akaltan et al72(2005) had evaluated the effects of two distal extension

removable partial denture designs on tooth stabilisation and periodontal

health. They concluded that RPD treatment did not damage remaining

teeth and periodontal tissue if the dentures were carefully planned, the

prostheses and oral hygiene were checked at regular recall

appointments.

Miura et al46(1992) had examined the effect of direct retainer and major

connector designs on RPD dynamics under simulated loading.They

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found out that rigid direct retainers and rigid major connectors

decreased both RPD movement,abutment tooth displacement under

loading. Their results provided the basic data that supported the

importance of rigidity of the RPD components to control denture

dynamics.

RoyMacgregor41(1983) reviewed the literature on planning the

support of bounded saddles of removable partial dentures or fixed

bridges. He also used three-dimensional photoelastic analysis to

examine stresses on removable partial dentures of different designs.

Neil D J17(1958) discussed the problems associated with lower free end

removable partial dentures,the problems associated with not restoring

the dentition, the tissue damage associated with wearing of removable

partial denture,the problems associated with denture design and

technical consideration in fabricating lower free end removable partial

denture.

Beckerd L.S44(1988) had analysed the influence of saddle classification

on removable partial denture.He classified distal extension saddle

situation based on support it derives into class-1(tooth borne),Class-2

(mucosa borne),class -3(problem type have inadequate abutments to

support the saddle and probably also inadequate mucosa support).

Kelly K.E et al9(1953) explained physiologic approach to partial denture

design.He advocated several methods to reduce lateral stresses by

means of rigid lingual and palatal bars so these stresses would be

distributed over as many abutments as practicable and by using stress

breakers.

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Potter B.R24(1967) stated that lateral stresses applied to a natural

tooth are so destructive to the supporting structures, whereas

stresses in the direction of the long axis of the tooth were well

tolerated. He proposed that the design of removable partial dentures

must minimize lateral stresses by distributing these stresses over

as many teeth and as much supporting tissue as possible, and

occlusion that provide damaging stresses should be avoided.

Avant W.E28(1971) discussed retention and fulcrum lines in planning

for removable partial dentures.He described a primary retention line

and compared with a primary fulcrum line.

Bickley W.R27 (1969) discussed a method for constructing removable

partial denture, incorporating broad distribution of stress,the

principles of broken stress and functional bases.His technique

minimised lateral stresses by keeping all the forces in a vertical

direction and by allowing rotation without torquing of the teeth.

Mensor C.M25(1967) suggested the rationale behind resilient hinge

action stress breaker. He started with the known motion differences

between anchor tooth and the free-end denture base.He also made

an attempt to differentiate the entire movement complex of

mastication into individual components.

Davis M.M et al3(1952) had explained the design and force distribution

in removable partial denture.They suggested that movement of a

removable partial denture in function was rotary in that the

movement takes place in three planes.They also added that the

“instantaneous center of rotation” theory could be meaningfully

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applied to partially edentulous situations even though the theory

was based on movement in one plane.

Hindels W.G3(1952) discussed the load distribution in extension saddle

removable partial denture.He insisted that the method used to make

impressions of the supporting and retaining anatomic structures of

the mouth was of basic importance for obtaining optimum

distribution of the masticatory load in the construction of

removable partial dentures.

Blatterfein et al44(1988) evaluated loading forces on mandibular distal

extension prosthesis.They classified the concepts of design into 4 basic

categories the flexible denture base design,the floating denture base

design,the mucofunctional concept,the enodosseous implant concept.

Hirschritt et al14(1957) differentiated tooth and tissue bearing areas of

the mouth and simplified the design of each individual partial denture. He

said that a tooth borne unit, with its splinting and supporting ability,

protected the teeth against overstressing.

Deboer.J43(1988) reviewed the position of rests on occlusal surfaces of

abutment teeth for distal extension removable partial denture and

proposed rational alternatives to improve denture stability and

prognosis by the selection of sites for occlusal rests in distal

extension removable partial dentures.

McCartney38(1980) analysed motion vector of an abutment for a distal

extension removable partial denture.He determined intraorally the

effect of various rest placements and clasp designs of a

mandibular bilateral distal extension removable partial denture. He

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also found that the magnitude and direction of abutment tooth

movement under vertical loading of the denture base.

Boero.E et al30(1972) explained considerations in the design of

removable prosthetic devices with no posterior abutments. He explained

that the basis of good restorative dentistry consists of establishing

an equilibrium of forces so stresses were conducive to develop a

physiologic continuum rather than pathosis.

Marie.K.M21(1963) compared the average measurements of forces

required to dislodge two kinds of circumferential clasps in

different amounts of undercuts,one with a half-round retentive arm

and the other with a round retentive arm under tensile load.His

findings indicated the use of cast round clasps were advantageous

in clinical fit and reduction of transmitted forces to the abutment.

Steefel V18(1962) explained the importance of diagnosis and functions of

removable partial dentures.He proposed the objectives of removable

partial denture design as bilateral distribution of stresses, the various

types of retainers (direct and indirect), cosmetic effects, and

function. He explained certain methods to achieve these objectives.

Kaires K.A12(1956) studied the effect of partial denture design on

force distribution. He fabricated a mandibular model and tested partial

denture to determine the effect of various denture designs on the

distribution of stress.

Frechette R.A2(1951) analysed lower distal extension removable partial

denture. He determined the magnitude of forces imparted to

abutment teeth when known loads were applied to a denture.

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Wills D J et al36(1979) performed experiments on macaque monkeys.

He compared the support provided for base plates resting on

groups of teeth, palatal mucosa and a combination of both.

Ben Ur et al61(1999) proposed that retentive clasp components could be

created to minimize torquing forces on abutment teeth incorporated in

the support and retention of bilateral distal extension removable partial

dentures.

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EVOLUTION OF REMOVABLE PARTIAL DENTURE

Early Concepts- The band, the clasp, and sectional construction(before 1950)

The early concepts of RPD design were primarily developed by

dentists who recorded the techniques that were successful in their

practices.The first recorded description of an RPD was by Heister in

1711 when he reported carving a block of bone to fit the mouth(Fig-

1).Fauchard,' who is considered by many to be the father of modern

dentistry, described the construction of a lower RPD in 1728 using two

carved blocks of ivory joined together by metal labial and lingual

connectors. (Fig-1).

The first mention of a maxillary RPD using a palatal connector was

by Balkwell in 1880.Retentive clasps were first discussed by Mouton in

1746. In 1810,Gardette described the use of the wrought band clasp.(Fig-

2)The bands completely encircled the tooth and often extended into the

gingival sulcus. The destruction of the marginal gingiva and the tooth due

to constant vertical movement of the prosthesis led tothe first description

of an occlusal rest in 1817.

In 1817, Delabarre" referred to "hooks" (clasps) and the use of

"little spurs" (occlusal rests) to prevent irritation around the abutment

teeth.In 1899, Bonwill recorded his techniques for clasping abutments

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with individually contoured gold circumferential clasps that were then

soldered to "the plate" (major connector). Bonwill also advocated the use

of "lugs" (rest seats) so that the prosthesis would be supported by the

abutments.(Fig-3)

In 1913, Roach presented a wrought wire circumferential clasp as

an improvement over the wide wrought band clasp. The first mention of a

bar clasp or "infra bulge" clasp was by Henrichsen in 1914,, but the bar

clasp did not gain popularity until Roach"' promoted this concept in 1930.

'I'he concept of rotational factors, which the early writers called

"balance," was first described by Balkwell" in 1880. Prothero" is credited

with coining the term "fulcrum line." William Taggart(Fig-4) proposed

the lost-wax casting technique for dentistry in 1907(Fig-5). This principle

was applied to RPDs by Norman B. Nesbet.(Fig-6)

In 1916,he described the technique for casting clasp assemblies for

RPDs. His refinement of the alloy and prosthetic tooth attachment

allowed the successful creation of short spanned unilateral RPDs. Nesbett

described the “inlay fit” of the clasp assemblies attained afterassembling

the separately cast components on a plaster cast.

Chayes had developed a parallelometer(Fig-7) in 1920 to help

guarantee parallel alignment both clinically and in the laboratory.The first

commercially available instrument developed specifically for use in

surveying models of teeth was designed by Weinstein and Roach in 1921.

The leap into full-arch, one-piece RPD castings was officially

made by Akers(Fig-8) when he published this technique in 1925.

Although descriptions of line tracings on the teeth occur prior to this

time, the term “height of contour”is credited to Edward

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Kennedy.Willis(1935)was among the first to describe in writing the

technique for dental model surveying and blocking out undesirable

undercuts. He was the first to use the term “path of insertion” for RPDs in

relation to a chosen plane and described tripoding. Roach, who was the

first to describe reciprocation, was aware that most retentive clasps were

actively exerting force on the abutment teeth.

During the 1930s and 1940s, there was persistent disagreement as

to how to approach the two dissimilar tissues encountered with the distal

extension RPD-teeth and the mucosa covering the residual ridgeThe

discussion centered around how to equalize forces placed on the hard,

relatively immovable, abutment teeth and the soft, relatively movable,

edentulous tissue areas

According to Steiffel, the prominent clinicians of the time could be

placed into the following three groups:

(1) those advocating some sort of stress-breakers between the

abutments and the major connector

(2) those advocating broad stress distribution to multiple abutments

and the edentulous area and

(3) those advocating physiological or functional basing

Steffel placed himself into “the broad stress distribution” group but

conceded that all three methods could be successful if properly executed.

He rejected the common practice of constructing a distal extension RPDs

from a single impression.

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Before 1950, RPD concepts were mostly developed by a small

group of authors who presented their theories and techniques based

primarily on empirical observation.

INVESTIGATIVE YEARS 1950 TO 1970

It was in 1950s that some of these clinical debates were resolved in

an evidence based approach.During these years several longitudinal

studies have performed that showed extensive pathological changes in

the periodontium and increased caries activity for patients who wore

RPDs. These studies gave credence to the then prevailing attitude of the

profession, as well as of the public, that RPDs were detrimental to the

existing dentition and were considered an interim appliance on the

pathway to complete dentures those days.

It was pointed out that in the 1950s, the partial denture concepts in

Europe were vastly different from the accepted concepts in North

America. In Europe, the RPDs tended to provide a flimsy design with

wrought wire clasping and, usually, no rest seats. In North America, the

partial denture design tended to include rigid major connectors, cast

clasps, and rest seats.

In 1956,Kaires showed that the lingual bar of a lower RPD should

be rigid to distribute forces across the arch. Also, an increase in residual

ridge coverage reduced forces to abutment teeth. In 1956, Frechette

showed that multiple occlusal rests helped to distribute forces to more

abutments and, thus, reduced forces to the terminal abutments.

Holmes and Leupold both showed that distal extension partial

dentures constructed on one-piece casts exhibit more movement of bases

than those constructed using an altered cast procedure. The original

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altered cast technique was first presented to the profession by Applegate.

Advantages of the altered cast procedure have more recently been

confirmed by Vahidi and by Leupold et al.

During the 1960s and early 1970s, two influential clinicians

increased the popularity of the bar clasp concept started by Henrichsen

and Roach many years before. Kratochvil promoted the use of the I-bar

clasp with a mesial occlusal rest as a means of reducing the force on a

clasped abutment when dealing with distal extension RPDs. Krol

modified Kratochvil’s concept with his mesial rest proximal platc-I bar

(RPI) design.(F ig-9)

Some of the problems encountered included insufficient

vestibular depth, soft tissue undercut below the abutment tooth, and lack

of “I-bar usable” undercuts. As a result of these limitations for the I-bar

system, there evolved a modification that combined the I-bar and

circumferential clasp designs. This clasp design is called the mesial rest-

proximal plate-Akers clasp (RPA) and was developed by Krol and

Eliason.

The mesial rest and proximal plate are identical to the RPI system,

but the buccal retentive arm becomes a circumferential or Akers clasp

engaging a mesial undercut. The superior border of the rigid portion of

the Akers clasp should contact the tooth on the survey line.Nelson et al

suggested using a cast round clasp rather than the conventional half round

design to form the retentive Akers clasp.

RESEARCH IN EARNEST- 1970 TO PRESENT

During the 1970s, there began to appear a large number of studies

beginning with in vitro research. Cecconi et al showed that force to the

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abutment teeth was transmitted via the rest seats, and that this force was

the same with or without retentive clasps.

Robinson showed that forces to abutment teeth with distal

extension RPDs are minimized with a mesial rest (as opposed to a distal

rest) and that a wrought wire retentive clasp has the same force on the

abutment as an I-bar design when used with a mesial rest. He also

demonstrated that no clasp is passive, as had been deemed essential by

nearly all theoretical concepts proposed in the past.

Nally showed that a mesial rest created the least amount of

abutment movement and that abutment movement increased with the

removal of indirect retainers. Browning et al confirmed the value of the

mesial rest with either the I-bar or the wrought wire clasp design. Frank

and Nicholl showed that indirect retainers have little to do with retention

of a distal extension RPD; rather, it is the guide planes that create

retention in conjunction with clasping. They showed that indirect

retainers do help with force distribution and, thus, are a beneficial

component in RPD design. An earlier study by Fisher and Jaslows

supports the findings of Frank and Nicholls.

Photoelastic studies provided a new laboratory research tool for

evaluating RPD design. Kratochvil and Caputo showed that an RPD

framework that had been properly adjusted to fit the abutments created

less force to the abutments than a framework that had not been adjusted.

Thompson et al reported the most favorable force to abutments came

with a mesial rest and either a wrought wire or an I-bar retentive clasp.

Pezzoli et al confirmed the value of mesial rests, indirect retainers, and

multiple rest seats on force distribution.

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Clayton and Jaslow measured the movement of the clasps on the

corresponding abutments. Browning et al showed that the clasp moves

more than the corresponding abutment. The major reasons for using

wrought wire clasps are that the wire is more flexible than a cast clasp

and that wire can flex in three dimensions. The fallacy in Clayton and

Jaslous study is that movement of the clasp does not necessarily translate

into movement of the abutment, and, thus, comparisons of the force

placed on the corresponding abutment by measuring the movement of the

clasp is invalid.

This study has been widely misquoted as justification for using an

I-bar instead of the more flexible wrought wire clasp. Clayton and

Jaslow's study does confirm that there is no such thing as a passive clasp.

From the increased interest in scientifically evaluating the design

concepts of the past, there began to emerge the following sound basic

principles for RPD design:

1. Major connectors should be rigid.

2. Multiple rest seats appear to distribute forces favorably.

3. Mesial rests appear to provide some advantage when used

with distal extension RPDs.

4. Parallel guide planes are beneficial for retention and stability

of a prosthesis.

5. The I-bar or the wrought wire retentive clasp, in combination

with a mesial rest, may be a superior design for the distal

extension RPD

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6. The altered cast procedure reducrs movement of the distal

extension RPD at least initially.

PERIODONTAL AWARENESS

Clinical research began to gain momentum as periodontal

awareness increased. More valid and reliable concepts for RPD design

evolved that relied less on empirical observation. . In 1966, Rudd and

O'Leary did a brief longitudinal study in which they reported that, when

proper guide planes were established on periodontally treated patients,

mobility to abutment teeth remained the same or improved.

In 1977, Schwalm et al reported the results of a 2-year

investigation in which acceptable RPD design principles were used and

initial plaque control instructions and basic periodontal therapy were

instituted, but there was no periodic recall. Bergman and Ericson reported

that in a 3-year cross-sectional study, they found no adverse periodontal

results associated with the wearing of RPDs.

UNCONVENTIONAL DESIGNS

Swing lock design

The swinglock design was first introduced to the dental profession

by Simmons in 1963(Fig-10). Simmons took advantage of the casting

properties of the chrome cobalt metals to devise a hinge and lock system

that allowed for a retentive labial bar that can he opcncd and closed by

the patient. This radical technology alloys for successful use of

periodontally compromised abutment teeth, as well as situations in which

critical abutments are missing.

23


Bolender and Becker have suggested certain specific indications

for the swinglock design including: periodontal compromised abutments,

missing key abutments,abutment mobility, limited economics, and

maxillofacial prosthesis.They recommend the use of multiple rest seats

and suggest placing the hinge and clasp of the labial retentive bar atleat

one tooth distal to the terminal abutments.

Antos, Renner, and Foerth prefer no rest seats and place the hinge and

clasp of the labial retentive arm next to the terminal abutments.

The dual path or rotational path design

The dual path (or rotational path) RPD(Fig-11)concept is relatively

new, having been introduced by king and Graver in 1978. Initially, the

dual path design arose out of the need for an RPD that would be esthetic

when anterior pontics are present primarily, the desire to eliminate

anterior clasping. This technique uses proximal undercuts adjacent to the

edentulous spaces for retention without clasps. The “first path” of

insertion of the framework is into these proximal undercuts.

As soon as the framework has gained access to the desired

undercuts, it is rotated into the “second path” of insertion to complete

seating the prosthesis.Initially, the dual path design was limited to tooth

borne situations in which anterior teeth were missing.

The swinglock and dual path concepts are good examples of design

modifications that have evolved because of a need to solve special

problems.

24


Return to Precision:Sectional construction revisited

Following the introduction of Akers’ one-piece casting technique,

several authors maintained that sectional construction was preferred due

to the superior fit obtained. Several modern methods of sectional

construction have been discussed. More recently, improvements in laser-

welding technology have allowed predictable unification of metal

components.

Cecconi et al described a component approach in which individual

parts are fabricated and joined on the definitive cast by means of

autopolymerized acrylic resin or laser welding.Brudvik et al showed that

this technique reduced distortion of large castings, the cumulative effect

of which is optimum control of the framework fit.

Cecconi advocated the advantages of sectional casting as being

Eliminating the need for time-consuming trial

placement of the framework.

fabrication of tooth- and tissue-supported elements

can be done separately.

dissimilar materials may be used. In component

RPDs, cobalt–chromium or nickel–chromium alloys

may be used for rigid major connectors, and gold

alloysmay be used for clasp assemblies where

improved accuracy and flexibility may be required

Similarly, acrylic resin denture base and acrylic resin

teeth may be combined with metal or porcelain where

necessary.

25


Key turning points in RPD philosophy included Bonwill’s band-

free RPD design, Akers’ one-piece casting technique, and the

ramifications of the one-piece technique’s application

BIOMECHANICAL CLASSIFICATION OF REMOVABLE PARTIAL DENTURE(Based on the nature of the supporting tissues- Occlusal forces are

transmitted to the teeth used as RPD abutments)

A. TOOTH BORNE (Tooth supported /Dentoalveolar supported)(Fig-12)

1. Abutment teeth border all edentulous areas where tooth

replacement is planned.

2. Functional forces are transmitted through abutment teeth to bone.

B. TOOTH - MUCOSA BORNE (Tooth and Mucosa supported, Dento-

alveolar and muco-osseous supported or extension base)(Fig-13)

1. Exhibits one or more edentulous areas which are not bordered by

abutment teeth (extension base RPDs).

2. Forces are transmitted through abutment and mucosa to bone.

3. The majority of these are distal extension RPDs.

4. This category may apply to tooth bordered situations when

excessive abutment tooth mobility is present or when long span

tooth bordered edentulous areas are present precluding primarily

26


tooth support.

C. MUCOSA BORNE. (Muco-osseous supported)(Fig-14)1. Regardless of the natural teeth present, support is derived entirely

from the mucoosseous segment.

2. This category includes prostheses fabricated from hard or

combinations of resilient and hard denture base materials such as

stayplates which function as interim or transitional prostheses.

3. These prostheses usually do not contain a metal framework and

usually should not be considered definitive treatment.

MECHANICAL PRINCIPLES OF REMOVABLE

PARTIAL DENTURE

Definition:Dental Biomechanics is defined as the relationship

between the biologic behavior of oral structures and the physical influence of a dental restoration.

Bio------pertaining to living systems(eg:inflammation, Caries,resorption.. ..etc)

Mechanical----related to forces and its application to objects(Eg:looseness of teeth,bone resorpt ion . .e tc)

Mechanics may be classified into two general categories: Simple & complex.

Complex machines are combination of many simple machines.There are six simple machines(Fig-15)(Fig-16)(Fig-17)

1. Lever

2. Inclined plane

3. Wedge

27


4. Screw

5. Wheel

6. Axle and pulley

A removable partial denture in the mouth can perform actions of two

simple machines,LEVER & INCLINED PLANE

LEVER :

The lever is a simple rigid bar supported at some point along it is

length.It can be used to move objects by application of force(weight),

much less than weight of object being moved.

Types of lever:

Classification is based on location of fulcrum (support), load(resistance), and direction of effort (force).

Note:

The load is the weight or force to be acted upon.

The effort is the weight or force required to cause the action.

The fulcrum is the pivot about which these forces act.

In a perfect system which is static:

The effort × the distance from the fulcrum = the load ×the distance from the fulcrum.

There are three fundamental levers around which the whole removable

partial denture revolve.But, the first fundamental facts are

1. A lever system works at mechanical advantage when the effort is

less than the load.

28


Mechanical advantage =Effort arm /Resistance arm

The length of fulcrum to resistance is called resistance arm,

while the length of lever from fulcrum to the point of application of

force is called effort arm.

2. A lever system works at a mechanical disadvantage when the

effort is greater than the load.

3. To be in balance(equilibrium) the forces on either side of the

fulcrum should be equal. That is the effort multiplied by its

distance from the fulcrum is equal to the load multiplied by its

distance from the fulcrum.

4. Whenever the effort arm is longer than the resistance arm the

mechanical advantage favors the effort arm,proportionately to the

difference in length of the two arms.In other words when the effort

arm is twice the length of the resistance arm a 25lb weight on the

effort arm will balance a 50 lb weight at the end of the resistance

arm.The opposite is also true and helps in cross arch stabilization.

The fundamental levers are

1. The first class lever

2. The second class lever

3. The third class lever

THE FIRST CLASS LEVER:(Fig-18)

The fulcrum (F) is in center of the bar, resistance (R) is at one end

and the force (E) is at opposite end (called cantilever).

Cantilever: It is a beam supported only at one end, when force is

29


directed against unsupported end of beam cantilever can act as first class

lever.

Archimides said that “Give me a lever long enough, I can lift the whole

world”

CLINICAL APPLICATION OF CLASS-I LEVER

A cast circumferential direct retainer engages the

mesiobuccal undercut and is supported by the disto-

occlusal rest.If it is rigidly attached to the abutment

tooth, this could be considered a cantilever design, and

detrimental first class lever force may be imparted to the abutment if

tissue support under the extension base allow excessive vertical

movement toward the residual ridge.Every effort should be made to avoid

lever of Ist class as it causes more damage to the supporting structures.

THE SECOND-CLASS LEVER:(Fig-19)

The fulcrum at one end, the force at opposite end & the resistance

in center. This type is seen as indirect retention in removable partial

denture.Works at a mechanical advantage cannot work at a mechanical

disadvantage as the load is always near the load.

CLINICAL APPLICATION OF CLASS-II LEVER

Typical examples for clinical application of class-II lever in

removable partial denture is seen in indirect retention in removable

partial denture and Equipose removable partial denture.In equipoise

restoration the occlusal rest (F) located mesially, while the retentive tip

(R) positioned distally, and the saddle(E) located distal to the retentive

tip i.e.the (Resistance) located in between the (Fulcrum) & (Effort).

THE THIRD CLASS LEVER:(Fig-20)

30


The fulcrum at one end, the resistance at opposite end and the

force in the center.This type is not encountered in removable partial

denture.Eg: tweezers.

INCLINED PLANEInclined plane is nothing but two inclined surfaces in close

alignment to one another. The direct retainers and the minor connectors

slide along the guide plane of the teeth and can act as inclined planes if

not prepared correctly.

When a force is applied against an inclined plane it may

produce two actions:

Deflection of the object, which is applying the force

(Denture).

Movement of the inclined plane itself (tooth).These results

should be prevented to avoid damage to the abutment teeth.

31


POSSIBLE MOVEMENTS OF REMOVABLE PARTIAL

DENTURE(Fig-21)

Three fundamental planes and three axis as related to the human head.

I.SAGITAL PLANE

The first plane is a sagittal plane. Movement in this plane occurs

relative to a medio-lateral axis that is perpendicular to the sagittal plane.

One movement is rotation about an axis through the most posterior

abutments. This axis may pass through occlusal rests or any other rigid

portion of a direct retainer assembly located occlusally or incisally to the

height of contour of the primary abutments This axis, known as the

fulcrum line, is the center of rotation as the distal extension base moves

toward the supporting tissue when an occlusal load is applied.

The axis of rotation may shift toward more anteriorly placed

components, occlusal or incisal to the height of contour of the abutment,

as the base moves away from the supporting tissue when vertical

dislodging forces act on the partial denture. These dislodging forces result

from the vertical pull of food between opposing tooth surfaces, the effects

of moving border tissue, and the forces of gravity against a maxillary

32


partial denture. If it is presumed that the direct retainers are functional

and that the supportive anterior components remain seated, rotation rather

than total displacement should occur.

Vertical tissue ward movement of the denture base is resisted by

the tissue of the residual ridge in proportion to the supporting quality of

that tissue, the accuracy of the fit of the denture base, and the total

amount of occlusal load applied. Movement of the base in the opposite

direction is resisted by the action of the retentive clasp arms on terminal

abutments and the action of stabilizing minor connectors in conjunction

with seated, vertical support elements of the framework anterior to the

terminal abutments acting as indirect retainers. Indirect retainers should

be placed as far as possible from the distal extension base, affording the

best possible leverage against lifting of the distal extension base

II.HORIZONTAL PLANE

The second is the horizontal plane. Movement in this plane occurs

around a vertical axis that is perpendicular to the horizontal plane. The

movement is rotation about a longitudinal axis as the distal extension

base moves in a rotary direction about the residual ridge.This movement

is resisted primarily by the rigidity of the major and minor connectors and

their ability to resist torque. If the connectors are not rigid, or if a stress-

breaker exists between the distal extension base and the major connector,

this rotation about a longitudinal axis applies undue stress to the sides of

the supporting ridge or causes horizontal shifting of the denture base.

III.FRONTAL PLANE

The final plane is a frontal plane. Movement in this plane occurs

relative to an antero-posterior axis running perpendicular to the frontal

33


plane. The movement is rotation about an imaginary vertical axis located

near the center of the dental arch.This movement occurs under function

because diagonal and horizontal occlusal forces are brought to bear on the

partial denture.

It is resisted by stabilizing components, such as reciprocal clasp

arms and minor connectors that are in contact with vertical tooth surfaces.

Such stabilizing components are essential to any partial denture design,

regardless of the manner of support and the type of direct retention

employed. Stabilizing components on one side of the arch act to stabilize

the partial denture against horizontal forces applied from the opposite

side. It is obvious that rigid connectors must be used to make this effect

possible.

34


FORCES ACTING ON REMOVABLE PARTIAL DENTURE

Removable partial dentures (RPD) have to be in a state of

equilibrium, i.e., a state in which opposing forces or influences are

balanced. Keeping in mind Devan's statement ‘to preserve what

remains,’’ forces should be given major consideration while designing a

partial denture, to ensure the dynamics of these appliances without

deleterious effects to the supporting structure.

The Supporting structures for removable partial are structurally

adapted to receive and absorb forces within their physiological tolerance.

The ability of these structures to tolerate forces is largely dependent upon

the magnitude, the duration and the direction of these forces in addition to

the frequency of force application.

The magnitude of forces acting on partial dentures depends on age

and sex of the patient, the power of the muscles of mastication and the

type of opposing occlusion.Natural teeth are better able to tolerate

vertical directing forces acting on them. This is because more periodontal

35


fibers are activated to resist the application of vertical forces. On the

other hand, lateral forces are potentially destructive to both teeth and

bone. Lateral forces should be minimized in order to be within the

physiologic tolerance of the supporting structures.

Removable partial dentures are subjected to a composite of forces

arising from three principal fulcrums. One fulcrum is on the horizontal

plane that extends through two principal abutments, one on each side of

the dental arch, and generally is termed the principal fulcrum line.This

fulcrum controls the rotational movement of the denture in the sagittal

plane ( i e, denture movement toward or away from the supporting

ridge). Rotational movement around this fulcrum line is the greatest in

magnitude,but is not necessarily the most damaging.

The resultant force on the abutment teeth is usually mesioapical or

distoapical, with the greatest vector in the apical direction the fibers of

the periodontal ligaments are arranged so that axially aligned forces are

resisted 17 times greater than the non-axial loads. Therefore, horizontal or

lateral forces are of much less magnitude and can be more destructive to

the hard and soft tissues of the periodontium.

A second fulcrum line lies in the sagittal plane and extends through

the occlusal rest on the terminal abutment and along the crest of the

residual ridge on one side of the arch.In a Class I situation, there would

be two such lines, one on each side of the arch.

This fulcrum line controls the rotational movements of the denture

in the frontal plane (ie, a rocking movement over the crest of the ridge).

This movement is easier to control than the first and usually not as great

in magnitude. The resultant forces are more nearly horizontal and are not

36


well resisted by the oral structures. Therefore, these forces can be

moderately damaging and should be given thorough consideration in the

design process.

The third fulcrum is located in the vicinity of the midline, just

lingual to the anterior teeth.This fulcrum line is oriented vertically and

controls rotational movement in the horizontal plane (ie,the flat, arcuate

movements of the prosthesis). Due to its orientation, the force resulting

from this movement is almost entirely horizontal. Consequently,these

forces can be extremely damaging and should receive significant

attention during the design process.

Every effort must be made to control or minimize the rotational

movements related to these three principles

37

TYPES OF FORCES ACTING ON RPD

I.Vertical forces(Fig-22)

a.) Tissue-ward movements b.) Tissue-away movements

II.Horizontal forces:(Fig-23)

a.) Lateral movements b.) Antero-posterior movements.

III.Rotational forces:(Fig-24)

They are due to the variation in compressibility of supporting structures,

absence of distal abutment at one end or more ends of denture bases, and /or

absence of occlusal rests or clasps at any end of the bases.

a.)Rotation of the anterior and posterior extension denture base around coronal (transverse) fulcrum axis:

i.)Rotation of the denture base towards the ridge around the fulcrum axis

(joining the two main occlusal rests)

ii.) Rotation of the denture base away from the ridge around the fulcrum axis

(joining the retentive tips of the clasps.)

b.)Rotation of all bases around a longitudinal axis parallel to the crest of the residual ridge (Buccolingual or labiolingual).

c.)Rotation about an imaginary perpendicular axis, this axis either near the center of the dental arch in class I, or is the long axis of abutment tooth in class II partial denture.


I.Vertical forces

a) Tissue-ward movements.

Tissue-ward forces are,“Vertical forces acting in gingival direction

tending to move the denture towards the tissues”.They occur during

mastication, swallowing and aimless tooth contact. Biting forces falling

on artificial teeth are transmitted to the soft tissues and bone underlying

the denture base.

The partial denture should be designed to resist this movement by

providing adequate supporting components. This function of the partial

denture is called “Support”.

b.)Tissue-away movements

Tissue-away dislodging forces are, "Vertical forces acting in an

occlusal direction tending to displace and lift the denture from its

position”.Tissue-away forces occur due to the action of muscles acting

along the periphery of the denture, gravity acting on upper dentures or by

sticky foodadhering to the artificial teeth or to the denture base.

The partial denture should be designed to resist this movement by

providing adequate “Retention”.

II.Horizontal forces

a.)Lateral movements

Lateral forces are “Horizontal forces developed when the mandible

moves from side to side during function while the teeth are in

contact”.Lateral movements have a destructive effect on teeth leading to

tilting, breakdown of the periodontal ligament and looseness of abutment

teeth. The application of lateral forces causes areas of compression of the

38


periodontal membrane, which leads to bone resorption. Hence lateral

forces play a major role in bone resorption.

Partial dentures should be designed to prevent the deleterious

effects of lateral forces by using stabilizing or bracing components.

The magnitude of lateral forces could also be minimized by:

1.Reducing cusp angles of artificial teeth.

2. Providing balanced occlusal contacts free of lateral interference

The removable partial denture being anchored to both sides of one

arch and joined by a rigid major connector can provide cross arch

stabilization to forces acting in bucco-lingual direction.

b.)Antero-posterior movements

Antero-posterior forces are "Horizontal forces which occur during

forward and backward movement of the mandible while the teeth are in

contact". This may result in movement of the denture.There is natural

tendency for the upper denture to move forward and for the lower denture

to move backward.

Forward movement of the upper denture could be resisted by:

Anterior natural teeth.

Palatal slope.

Maxillary tuberosity.

The natural teeth bounding the edentulous space.

The backward movement of the lower denture could be resisted by:

The slope of the retromolar pad.

39


The natural teeth bounding the saddle area.

Proximal plates.

III.Rotational forces

Rotational forces are “Forces acting on the partial denture either in

vertical or horizontal direction causing rotation (torque) of the denture

base around an axis.In tooth supported removable partial dentures, the

abutment teeth on both sides of the edentulous area provide adequate

support and resistance to rotational forces through supporting rests and

clasps placed on them.In distal extension partial denture when vertical

forces are applied the difference in displaceability of the supporting

structures often results in rotation of the partial denture around a fulcrum

axis and application of torque on abutment teeth.

Rotational movements must be counteracted in the partial denture

design to minimize their destructive effect on both,teeth and the residual

ridge.Rotational forces acting on distal extension partial denture may

result in three possible rotational movements these are

i.)Rotation of the denture base around the fulcrum axis (Torque).

ii.)Rotation about a longitudinal axis formed by the crest of the

residual ridge (Tipping movement).

iii.)Rotation about an imaginary perpendicular axis near the center

of the dental arch (Fish tail movement).

a.)Rotation of the anterior and posterior extension denture base

around coronal (transverse) fulcrum axis:

Movement of the component parts of the denture lying on the

opposite side of the fulcrum axis occur in a direction opposite to that of

40


the applied force. This leads to rotation of the denture:The fulcrum axis is

an “imaginary line passing through teeth and component parts of the

partial denture around which the distal extension partial denture rotates

when a vertical force is applied”.More than one fulcrum lines may be

identified for the same removable partial denture depending on the

direction and location for force application.

i.)Rotation of the denture base towards the ridge around the fulcrum axis

This movement results from occlusal stresses occurring during

mastication and occlusion of teeth. The free extension denture base

moves tissue-ward while other components on the opposite side of the

fulcrum line moves away from the tissues.This result in rotation of the

denture about a diagonal supportive fulcrum line joining two occlusal

rests on the most posterior abutments on either side of the dental arch.

Tissue ward movement of the base could be limited by supporting

structures, which are:

Supportive form of the residual ridge,

Accurate and properly extended bases.

Artificial teeth set on the anterior two third of the base

Flexible clasps are preferred over rigid clasping to reduce stresses

and torque applied on abutments. If the clasps are rigid, the abutments

tend to rotate distally during tissue ward movement of the denture base

resulting in periodontal breakdown and looseness of teeth.

ii.) Rotation of the denture base away from the ridge.

41


This movement occurs due to the pulling effect of forces applied

by sticky food, gravity on upper dentures and the elastic rebound of soft

tissues covering the edentulous areas.

Tissue-away rotation of denture base is counteracted by:

Indirect Retainers: which are the components of partial denture

located on the side of the fulcrum axis opposite to the distal

extension base.

The retentive tip of the clasp arm.

Adequate coverage and extension of the base (direct indirect

retention)

Effect of gravity on mandibular bases.

b.)Rotation of all bases around a longitudinal axis parallel to the

crest of the residual ridge

This rotation occurs due to application of vertical forces on one

side of the arch only. It causes twisting of the denture base.

This movement is counteracted by:

Cross arch stabilization (The action of clasps on the opposite side

of the arch).

Broad base coverage.

Proper placement of artificial teeth (teeth on the ridge or

lingualized occlusion).

Narrow teeth bucco-lingually

The effect of rigid major connectors

c.)Rotation about an imaginary perpendicular axis, this axis either

near the center of the dental arch

42


Application of horizontal or off-vertical force results in rotation

around an imaginary vertical axis located either about the axis of

abutment in class II or near the center of the dental arch, lingual to

anterior teeth in class I.

It results due to the application of masticatory forces falling on distal

extension bases causing buccolingual movementof the base. This rotation

is called fishtail movement.

This movement is counteracted by :

Providing adequate bracing components in the partial

denture.

A rigid major connector.

Broad base coverage.

Balanced contact between upper and lower teeth.

Forces occuring through a removable restoration can be widely distributed,

directed, and minimized by the selection, the design, and the location of components of

removable partial dentures and by developing a harmonious occlusion.

FORCE CAUSE OF THE FORCE

COUNTERAC--TION OF FORCE

FUNCTION

I.Vertical

Forces

a.)Tissue ward

movements

Functional

movements during

mastication,

swallowing and

occlusion of upper

and lower teeth

-Rests placed on

abutments in

bound saddles

-Rests & proper

coverage in free

end saddles

-Maxillary

Support

43


connectors

b.)Tissue

away

movements

Pulling effect of

sticky food provides

gravity on upper

dentures and excess

muscle forces acting

on the periphery of

the denture

-Retainers

-Adhesion &

cohesion

between denture

base & tissues

Retention

II.Horizontal

forces

a.)Lateral

Forces

Side to side

movements of the

mandible while teeth

are in contact.

-Rigid bracing

clasp arms.

-Major

connectors.

-Balanced

occlusion.

-Maximum

extension of

the flanges

Bracing

b.)Antero-

posterior

forces

Forward and

backward

movement of

mandible

while teeth are in

contact

-Abutments

adjacent to

the denture.

-Guiding planes.

Stabilization

III.Rotational

Forces

a.Rotation of

the denture

Functional

movements

.

Supporting rests

and properly

Supporting

rests.

44


base towards

the ridge

around the

fulcrum axis

while teeth are in

occlusion

adapted bases -Properly

adapted bases

b.Rotation of

the denture

base aways

from the ridge

around the

fulcrum axis

-Sticky foods gravity

on upper

dentures,elastic

rebound of tissues

under the base

-Indirect

retainers

-Direct retainers

Indirect

retention

45


PHILOSOPHYOF PARTIAL DENTURE DESIGN

There are four design concepts, which can be used to distribute the

force evenly along the tissues and supporting tooth structure. They are :

Conventional rigid design.

Stress equalization.

Physiologic basing.

Broad stress distribution.

Physiologic basing(Fig-26)

The philosophy of design agrees in part with the first school about

the relative lack of movement of the abutment teeth in an apical direction

but denies the necessity of using stress directors to equalize the disparity

of vertical movement between the tooth and the mucosa. The belief is that

the equalization can best and most simply be accomplished by some form

of physiologic basing, or lining, of the denture base.(Fig-26)

The physiologic basing is produced either by displacing or

depressing the ridge mucosa during the impression making procedure or

by relining the denture base after it has been fabricated. The reason for

displacing the mucosa during the impression procedure is to record the

soft tissue in its functioning, not anatomic, form. If the tissues are

recorded in their functional state, the denture base, formed over the

displaced tissue, will be better able to withstand the force that is

generated.

46


It is obvious that in such situation, the artificial teeth will be

positioned above the plane of occlusion when the denture is in mouth and

not in function. To permit vertical movement of partial denture from

the rest position to the functioning position, the direct retainers or

retentive clasps must be designed with minimal retention and the number

of direct retainer must be limited. The occlusal rest and direct retainers

will also be slightly unseated at rest. They will be completely seated only

when the mucosa beneath the denture base is displaced to its functional

form.

ADVANTAGES

a) The intermittent base movement has a physiologically stimulating

effect on the underlying bone and soft tissue , which reduces the

frequency of relining or rebasing the prosthesis (there will be less

bone loss )

b) Simplicity of design and constructive because of the minimal

retention requirements results in a light weight prosthesis needing

minimal maintenance and repair

c) An additional advantage is gained by the minimal direct retention

used. The looseness of the clasp (combination clasp with wrought

wire retentive arms) on the abutment tooth reduces the functional

forces transmitted the abutment tooth. Hence the abutment teeth

are preserved for longer time duration.

DISADVANTAGES

1. The denture is not stabilized against lateral forces

47


2. The residual ridge receives the greater proportion of forces that

are transmitted by the denture, hence more chances of bone

resorption

3. The load of stabilizing and supporting the denture is limited to a

few teeth instead of being shared by a number of teeth as in other

design philosophies

4. There will always be slight premature contacts between the

opposing teeth and the denture teeth when the mouth is closed.

This is an uncomfortable situation to many patients and may

result a sense of insecurity

5. It is a difficult to produce effective indirect retention because of

the vertical movement of the denture and the minimum retention

of the direct retainer.

Broad Stress Distribution(Fig-25)

According to this philosophy of design, the occlusal load acting on

the denture should be distributed over a wider soft tissue area and

maximum number of teeth. This is achieved by increasing the number of

direct retainers, indirect retainers, and rests and by increasing the area of

the denture base.(Fig-25)

Advantages

This design with multiple clasps acts as a form of removable

splinting.

It increases the health of the abutment teeth (due to splinting

action).

Easier to construct and economical.

Disadvantages

Less comfortable.

48


Difficult to maintain adequate oral hygiene

Conventional Rigid Design

The denture is designed with rigid component which act like a raft

foundation to evenly distribute the forces on the supporting tissues. This

design is used in all general cases. The flexible component of these

dentures is their retentive terminal.

Advantages

Easy to construct and economical.

Equal distribution of stress between the abutment and the residual

ridge.

Reduced need for relining as the ridge and abutment share the load.

Indirect retainers prevent rotational movement and also stabilize

the denture

during horizontal movements.

Less susceptible to distortion.

Disadvantages

Increased torquing forces on the abutment teeth.

Rigid continuous clasping may damage the abutment teeth.

Dovetail intracoronal retainers cannot be used in these cases as

tipping forces from the denture base will be directly transmitted to

the abutment teeth.

49


Tapered wrought wire retentive arm (combination clasp) cannot be

used, as it is difficult to construct.

Relining is difficult and inappropriate relining leads to damage of

the abutment teeth.

Stress Equalization or Stress Breaker or Stress Directing Concept(Fig-27)

A stress breaker is defined as, “A device which relieves the abutment teeth of all or part of the occlusal forces" - GPT.

A stress director is a device that allows movement between the

denture base and the direct retainer which may be intracoronal or

extracoronal. Dentures with a stress breaker are also called as Broken

stress partial dentures or Articulated prostheses.(Fig-27).

We know that the soft tissues are more compressible than the

abutment teeth. In a tooth tissue supported partial denture, when an

occlusal load is applied, the denture tends to rock due to the difference in

the compressibility of the abutment teeth and the soft tissue As the tissues

are more compressible, the amount of stress acting on the abutments is

increased. This can produce harmful effects on the abutment teeth.In

order to protect the abutment from such conditions, stress breakers are

incorporated into a denture.

There are two types of stress breakers:

Type I

Here a movable joint is placed between the direct retainer and

denture base. This joint may either be a hinge or a ball and socket or a

50


sleeve and cylinder. Adding these stress breakers to the junction of the

direct retainer and the denture base, allows the denture base to move

independently.This decreases the amount of force acting on the abutment.

The combined resiliency of the periodontal ligament and the stress

director will be equal to the resiliency of the oral mucosa overlying the

ridge.

Examples for hinges include DALBO, CRISMANI, ASC 52

attachments.

Type II

It has a flexible connection between the direct retainer and the

denture base. It can be a wrought wire connector, divided or split major

connector or a movable joint between two major connectors.In a split

major connector, the major connector is split by an incomplete cut

parallel to the occlusal surface of the teeth into two units namely the

upper unit (more near to the tooth) and the lower unit. The denture base is

connected to the lower unit and the rests and direct retainers are

connected to the upper unit.

Advantages

The alveolar support of the abutment teeth is preserved as the

stresses acting on the abutment teeth are reduced.

The stress on the residual ridge and the abutment teeth are

balanced.

Weak abutment teeth are well splinted even during the

movement of the denture base.

Abutment teeth are not damaged even if relining is not done

appropriately (after the denture wears out).

51


Minimal requirement of direct retention.

Movement of the denture base produces a massaging effect on

the soft tissues.

This avoids the frequent need for relining and rebasing

Disadvantages of stress breakers

1. The broken stress denture is usually more difficult to fabricate and

therefore more expensive

2. Many stress breakers designs are not well stabilized against

horizontal forces.

3. The effectiveness of indirect retainers and cross arch stabilization

is reduced or eliminated altogether.

4. The more complicated the prosthesis, the less the patient may

tolerate it.

5. Spaces between components are sometimes opened up in function,

thus trapping of food and occasionally the tissue of the mouth

leading to injury and periodontal problems.

6. Flexible connectors may be bent and distorted by careless

handling.

7. Repair and maintenance of any stress breaker is difficult, costly

and frequently required.

8. If relining is not done whenever needed, it may result into

excessive resorption of residual ridge.

52


SUPPORT MECHANISM IN REMOVABLE PARTIAL DENTURE

Support is derived from bone, for it is to the bone that all forces are

ultimately transmitted, either via the mucosa and periosteum or via

the teeth and periodontal ligament. The mucosa is an inappropriate

tissue to resist occlusal forces, as any complete denture wearer will

attest to. In a partially edentulous situation, using mucosa only invites

iatrogenic damage.Therefore,vertical support must always be provided by

some of the remaining teeth for all removable partial dentures.

CHARACTERISTICS OF SUPPORT BEARING AREAS

The forces directed to the supporting tissues will be partially

absorbed and partially transmitted to adjacent tissues. The percentage of

force absorbed or transmitted will vary depending upon which tissue is

involved. Bone is the tissue which ultimately absorbs the greatest amount

of the force applied to both the muco-osseous and dento-alveolar

segments.

DENTO-ALVEOLAR SUPPORT

A. TEETH.(Fig-28)

Teeth should be

1.Structurally sound and

2.Anatomically favourable.

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1. Structurally sound.

Functional forces are transmitted by a partial denture to the tissues

with which it is in contact.If a denture is supported primarily by the

natural teeth most of the forces will be transmitted to the alveolar

bone through the fibres of the periodontal ligament.

Tooth structure:Structurally sound vital teeth are capable of

withstanding normal functional forces.Excessive forces applied to the

tooth may result in adverse effects such as

i. Tooth fracture.

ii.Tooth movement.

iii. Pulpal irritation.( Pulpal hyperemia or irreversible pulpitis.

Structurally compromised teeth may fail in response to normal

functional forces.Few examples are

i.Teeth with large intracoronal restorations.

ii.Endodontically treated teeth.

Endodontically treated teeth are structurally weak due to dessication of

dentin leading to loss of its organic content which ultimately makes the

dentin brittle.This brittle dentin when subjected to occlusal forces may

fracture and loss of teeth structure.

2.Anatomically favorable.

a. Root surface area.

b. Root morphology.

c. Presence of multiple roots.

54


d. Presence of divergent roots

e. Crown to root ratio.

f. Axial Inclination

B.PERIODONTIUM(Fig-29)

These includes gingiva, crevicular epithelium, junctional

epithelium, connective tissue attachment, cementum, periodontal

ligament and alveolar bone. Healthy periodontium permits force

absorption without damaging effects.Excessive forces may increase the

width of the periodontal ligament and result in increased tooth mobility.

Health periodontium should be

1.Anatomically favorable.

a. Normal epithelial and connective tissue attachment.

b.Adequate zone of attached gingiva.

2.Absence of periodontal disease

Plaque induced inflammation may compromise the

periodontium. It can lead to apical migration of the crevicular

epithelial attachment (functional epithelium) and destruction of the

fibroblasts and connective tissue of the connective tissue attachment.

In the presence of inflammation normal functional forces may

accelerate the rate of periodontal attachment loss.

The presence of plaque induced periodontal disease is

associated with a loss of bone height. Moderate forces may

accelerate the

55


disease process resulting in further bone loss, less bone support, and

increasedmobility of the teeth.

A healthy periodontium should have

a.Gingival indices within normal limits.

b.Absence of mobility or hyper mobility.

C.ALVEOLAR BONE.

Residual ridge support(Fig-30)

As has been said, the entire root surface area ot an arch ot teeth is

about 45 cm2 . It is as well to consider this in terms of the area

remaining when the teeth arc lost. It has been calculated that the

entire denture bearing area when all teeth are lost is about 20 cm for

the maxilla, and about 12 cm2 for the mandible. Hence in the

partially edentulous situation, ii will always be preferable, just from this

consideration alone, to use the teeth for support. It residual ridge support

is to be used as well, then it follows that full use should be

made of a fully extended denture base.

56


The residual alveolar ridge, though, has forces transmitted to it by

the overlying mucoperiostium, and this too will resist forces in a

manner which will depend on its morphology. There is a wide

variety of thickness and type of ridge mucosa, with some areas being

almost seven times thicker than others.There are three main

histological types of mucosa.Buccal mucosa is partially keratinised and

has underlying elastic tissue; mucosa of the floor of the mouth is similar

but non-keratinised. Both these types are not firmly attached to the

underlying bone, in contrast to the third type, the attached ridge mucosa,

which is usually keratinised and much more able to withstand loads.

FORM OF RESIDUAL RIDGE

The residua ridge itself is also uneven in shape, and this will affect

not only resistance to loading forces, but also resistance to laterally

directed forces,inother words, the stability of the denture base overlying

the ridge.In A, a flat ridge will provide good support but poor

stability. The varying thickness of the mucosa and the sharp and

often spongy ridge in B provides poor support. In C, neither good

support nor stability are present, because of the flabby and

displaceable tissue over the ridge.

57


The ridge often becomes sharp and uneven because of the uneven

resorption of bone following tooth extraction.This depends on many

factors, such as the nature and health of the alveolar bone prior to

extraction of the teeth, and the manner of resorption of the smooth

conical bone, which varies from individual to individual. Also

varying, is the type and position of the muscle attachments, which

may form sharp and pointed ridges.

With age. some of the tendinous attachments may become calcified,

and with increasing resorption, all attachment sites can become

relatively prominent (this is common at the genial and mental

tubercles and along the mylohyoid ridge).

PRESSURE-TENSION THEORY:

Bone tends to resorb in response to compressive force and to be

stimulated by tensional force. In order to preserve remaining alveolar

bone,it is important that functional forces be transmitted to bone

primarily as tension rather than pressure whenever possible.

In tooth borne situations the majority of functional forces are

transmitted as tension to bone through proper rest design and rest seat

preparation.In tooth mucosa borne situations some of the vertical seating

forces are transmitted as tension to the bone through the rests.

Horizontal forces are transmitted as a combination of compressive and

tensional forces to the alveolar bone(e.g.those forces directed through

bracing clasps,proximal plates and minor connectors contacting proximal

tooth surfaces and guiding planes).Vertical displacing forces are

transmitted to the bone as both compressive and tensional forces

(e.g.sticky foods or retentive clasps engaging undercuts).

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BONE INDEX

The response of bone to pressure varies in terms of the rate of

resorption depending on genetic,nutritional, hormonal and biochemical

and other intrinsic factors. The bone index is determined by analyzing the

previous response of bone to force. The bone index of the alveolar bone

surrounding natural teeth may differ from that of the bone comprising the

residual ridges.

CORTICAL VS. CANCELLOUS BONE

The residual ridge crest is comprised mainly of cancellous bone

and is less resistant to resorption. The facial and lingual inclines of the

residual ridges are comprised of cortical bone and are more resistant to

remodelling. The rate of cancellous bone resorption has been described as

being approximately three times that of cortical bone. Excessive forces

may increase the rate of bone resorption.

Moderate forces may result in accelerated bone resorption when

intrinsic factors, local abnormalities or systemic disorders compromise

the bone index of the individual.

MUCO-OSSEOUS SUPPORTA.MUCOSA.

Keratinized and Firmly bound.

B.SUBMUCOSA.

1. Normal sub mucosa serves as an "hydraulic cushion".

2. Firmly bound and dense.

C.BONE.

1. Cortical bone.

2. Favorable bone index.

3. Presence of muscle attachments which direct tension to bone (or the

59


equivalent in terms of resistance to pressure induced resorption).

DISPLACEMENT CHARACTERISTICS OF PERIODONTIUM AND RESIDUAL RIDGE MUCOSA

The previous discussion of the types and sites of support available

for a partial prosthesis leads to a consideration of the different

characteristics that the support mechanisms may have. A denture that

uses both tooth and residual ridge support is dento-mucosally supported,

and because the nature of the tissues varies, one must be aware of the

loading characteristics of each type When load is applied to a material

there are basically three ways in which the material can react, depending

on its nature,as shown in figure where D is the displacement, and T-time.

A purely elastic material will be displaced immediately, and then

immediately recover to its original form or position on removal of the

force applied it obeys Hooke's law). When a viscous substance, such as

oil, is subjected to load,it will gradually be displaced to reach a resting

state, and will not recover on removal of the force (it behaves as a

Newtonian element).

60


A combination of these two effects occur in viscoelastic materials

which behave as a voigt model.Application of the load will cause a

relatively free phase of displacement or distortion, the rate of which will

lessen until equilibrium is reached. Removal of the load will reveal a

relatively fast recovery phase followed by a prolonged and gradual return

to the original state. Hence the response depends on the rate of loading as

well as the magnitude and duration of the load.

Teeth and mucosa behave as viscoelastic materials, but with quite

different characteristics. When a tooth is loaded,there is an initial rapid

displacement as a result of movement of tissue fluids and cell distortion,

followed by a stiffer more gradual response as the periodontal fibres are

loaded.When the applied force is removed, the tooth recovers to its

original position rapidly,within a minute or two.

The response of oral mucosa,however,is much more akin to a

classical visco-elastic response, and depends far more on the magnitude

and the duration of the loads applied. This has been tested by applying

different loads to an acrylic plate placed on palatal mucosa.For static

loading, when the load was applied suddenly there was an instantaneous

elastic displacement, and as the load was maintained constantly for 10

minutes a further gradual displacement (creep) occurred.

On sudden removal of the load there was an instantaneous elastic

recovery, followed by a viscoelastic recovery that can last upto four

61


hours( the heavier the load, the longer the recovery).For dyanamic

loading an increase in the loading rate reduces the amount of mucosa

displacement at 4 Newtons per second the plate was displaced 500µ and

at 100N/sec it was displaced only 375µm.

More importantly, under functional conditions in the mouth,

loading varies with each chew, and the effects of simulating this have

also been studied. With successive chews, there is a progressive

displacement, but also a progressive failure to recover, so that an

equilibrium at a displaced position relative to the starting position is

reached.

These displacement characteristics of mucosa can be explained by

considering the structure of mucosa itself its thickness and fluid flow

characteristics when depressed will cause the variety of responses,

together with the general physiological tissue characteristics of the host.

This latter becomes an important consideration when age is taken into

account.

It has been shown that there is a decrease in mucosa thickness with

age, and a significant difference in recovery characteristics ,the tissues in

elderly people take many hours to recover from the effect of moderate

mechanical force, whereas the tissues of a 25 year old,for example,

require much less lime to recover from the same force. In figure the

62


mucosa of'young patients (age range 15-25 years recovered faster and

further than that of older patients (age range 72-86 year).

The choice of support

It should be apparent from the above discussion that oral mucosa

presents a much more varied and greater response to loading than the

periodontium.

The diagram illustrates the displacement of tooth-borne and

mucosa-borne plates when a load was applied and maintained for 30

seconds.The tooth-borne plate displaced the least, with a load of 1N as

shown by the line A. The mucosa-borne plate at the same load displaced

further, as shown by the line. B. When this plate was given a load of 4N

the greatest displacement was measured, as the lineC.

Under certain conditions, mucosa can be displaced up to twenty

times that of the periodontium, and this can create many clinical

63


complications especially with dento-mucosally supported dentures.

The practical application of this problem will be dealt with under a

number of different sections following, but it is essential to understand

the biomechanical nature of the problem. It should be obvious now that it

would be preferable to use the teeth for support at all times, and to avoid

any loads on the mucosa at all but there are occasions when the mucosa

must also be used for support, and when it is, there must be some

compensation made for the difference in displacement characteristics of

the mucosa and the teeth.Otherwise, at every bite, the denture will move

in a manner such that not only will the patient will be unable to control

the movement,but the movement may cause iatrogenic damage to the

teeth or tissue.

The most obvious situation where the residual ridge must be used

is when there are no teeth distal to the gap the dento-mucosally (usually

called dento-gingivally.) supported denture. The residual ridge must be

used to support that part of the denture carrying the missing teeth.Less

obvious, are the situations where the state of the teeth and periodontium

are such that they could not carry the extra loads of a prosthesis without

exceeding their physiological tolerance to do so.In this case, once again

iatrogenic damage to the teeth and their supporting tissues may occur.

Over the years, a variety of clinicians have offered suggestions for

classifications of dentures based on support. For any classification to be

useful, it should be:

Consistent

Unambiguous

Generally accepted

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No one classification is ideal, and perhaps the most useful is that

outlined, in one form or another, by Beckett (1953), Craddock (1956) and

Osborne and Lammie (1974)

Class I: Denture supported by mucosa and underlying bone

Class II: Denture supported by teeth.

Class III: Denture supported by a combination of mucosa and tooth-borne means.

We consider that this classification should now be extended to include a

further type, namely:

Class IV: Denture supported by implants.

It must be realized that this classification is not ranked in order of

precedence but could perhaps be considered in order of complexity of

planning. For this reason, the support options will be discussed in the

above order.

Class I Dentures (deriving their support from mucosa and underlying

bone)

Wills (1978) clarified some misconceptions on the displacement

and deformation properties of oral mucosa with their research on

primates. They determined that the effects of loading mucosa over a long

period were to compress it by up to 45% of its original thickness and,

further, that its recovery was visco-elastic in nature. The time required for

recovery from the displacing forces has also been found to increase with

age. What this clearly means, however, is that prostheses which derive

their support from mucosa and the underlying bone will inevitably do two

things:

65


Displace mucosa

Result in further loss of alveolar bone (this is perhaps of greater importance).

From the above, it is clear that in mandibular dentures especially,

mucosa-borne partial prostheses ought to be considered as a last resort, or

possibly as a transitional phase to complete dentures.More latitude exists

in the maxilla, however, where the hard palate affords additional support,

but this is often abused.

Class II Dentures (deriving their support from teeth)

Tooth-supported prostheses gain their support from the teeth via

the supreme qualitative and quantitative support agent, namely the

periodontal membrane. Pressure down the long axis of the tooth imparts

tension in the periodontal membrane, which in turn helps to maintain

alveolar bone. Clearly this is the most desirable form of support and

should be used whenever practical. It has traditionally been taught that

dentures may gain tooth support from incisal rests, occlusal rests or

cingulum rests.

The statements in the foregoing paragraph indicate that,

theoretically, support derived from teeth is more desirable than any other

single form of support, and this is a scientifically established fact.

However, on occasion the clinician has a need to be empirical and to

prescribe what is most appropriate for the patient. For example, a patient

who has been treated for chronic periodontal disease may have lost

considerable bony support,and a cast metal framework utilizing occlusal

rests and cast cobalt chromium clasp assemblies may impart

inappropriate forces on a tooth.

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Class III Dentures (deriving their support from a combination of mucosa

and tooth borne means)

In tooth tissue supported RPD attention must be given to both

abutment and edentulous ridge. For the abutment teeth these

consideration are periodontal health, crown and root morphology, C/R

ratio, bone index area, location of abutment in the ridge, and opposing

dentition. For edematous ridge these consideration are the quality of the

ridge, the extent of the ridge covered by denture base, the type and

accuracy of impression technique, and the partial denture design.

The greatest difficulty occur in transition area where tooth support

ends and mucosa support begins ,when functional occlusal load is applied

to denture base,an axis of rotation is created ,the denture tend to rotate

about its most distal abutment inducing heavy torisonal stresses on the

abutment teeth and possible traumatization of the ridge.

The degree and direction of the denture base movement are greatly

influenced by the quality of the supporting residual ridge, the design of

RPD and the extent of the forces exerted on the denture during

function.When RPD with both anterior and posterior denture bases

present a stress problems, since the length of the ridge area extends

anterior and posterior to the fulcrum clasping areas produces a double

acting lever problem for the abutment teeth.

It is perhaps no coincidence that clinicians and patients alike have

embraced the shortened dental arch philosophy. The option to do nothing

or to use a fixed prosthesis to replace one dental unit(e.g. by cantilevering

one unit from the terminal abutment) is seen as being less problematic

than providing a removable prosthesis to replace several missing teeth.

67


From the clinician’s viewpoint this is because of the very real and

problematic differences between the two supporting elements,and from

the patient’s perspective because of intensive tissue

coverage.Extrapolating the results of Wills and Manderson (1977) and

Wills et al. (1980), the clear fact emerges that, long after abutment teeth

have returned to their resting positions (after mastication, for example),

the mucosa will remain displaced; this displacement is of the order of 20

times that of the teeth even on the basis of a maximally covered saddle. It

will be self-evident to state that mucosa under minimally covered saddles

will be displaced even more than under maximally covered saddles.This

support differential is thus problematic,and the inherent tendency for a

prosthesis to demonstrate rocking (instability) has resulted in

philosophies of clasping which were based on homeostatic principles of

stress-breaking, whereas others were based on more biological

principles(Kratochvil, 1963; Krol, 1973).

Class-IV Dentures(Tissue Implant supported removable partial

denture)

The design and maintenance of bilateral and unilateral distal

extension partial dentures (Kennedy Class I and II) present challenges for

clinicians,as these dentures require support from the teeth, the mucosa

and the underlying residual alveolar ridges. In particular, the distal

extension removable partial denture is subjected to vertical, horizontal

and torsional forces due to the different resiliencies of alveolar mucosa

and periodontal ligament of abutment teeth that may have adverse effects

during functional and para-functional activities.

To prevent displacement of the denture, precision attachments or

conventional clasps have been widely used. However, the rotational

68


tendency of the RPD after long-term use cannot be eliminated

completely, regardless of design and fit of the denture. To overcome this

clinical challenge, single implants may be placed bilaterally at the distal

extension of the denture base to minimize the potential for dislodgement

of the denture.

The chief goal of placing an implant under the posterior-most

molar of the distal extension denture base is to stabilize the RPD in a

vertical direction. Distal implants effectively convert a Kennedy Class I

or II denture to a Kennedy Class III denture. Therefore, a tooth- and

implant-supported RPD is cheaper (because fewer implants are needed)

and more stable, and may therefore be a better option for patients with

limited financial resources than an implant-supported fixed partial

denture.

IMPLANT CORRECTED REMOVABLE PARTIAL DENTURES

The classification will always begin with the phrase "Implant-

Corrected Kennedy (class)," followed by the description of the

classification. It can be abbreviated as follows:

(i) ICK I, for Kennedy class I situations,

(ii) ICK II, for Kennedy class II situations,

(iii) ICK III, for Kennedy class III situations, and

(iv) ICK IV, for Kennedy class IV situations.

ICK I, for Kennedy class I situations

The Kennedy Class I partially edentulous arch has bilateral distal

extensions. The functional load is transmitted to the teeth and the soft

tissue. Implant location depends primarily on the dimensions of the

69


residual ridge and the biomechanical considerations of the RPD design.

Two distally positioned implants in the area of the second molars would

effectively trans-form the Kennedy Class I configuration to a more

favorable Kennedy Class III.

Theoretically, the implants should be located as distally as possible

to provide maximal support and stability. This is of special importance in

the mandible because of the significant displacement of the denture base

that is not supported by the major connector. The implants might be used

for support only using healing caps or for retention with resilient

attachments connected to the implants. A low-profile attachment is

preferred to decrease the off-load forces to the implants.

Note:Endodontically treated abutments would be specifically

beneficial when used for support only without direct retainers applying

unfavorable lateral displacing forces.

Drawbacks:

An inadequate posterior ridge dimension could restrict implant

placement to a more anterior location.

The Implant therapy is versatile

In the future, the patient might select to restore the edentulous

ridges with fixed implant-supported restorations(in such case the

implants should be located more medially, adjacent to the existing

abutments, to allow future prosthodontic use)

ICK II, for Kennedy class II situations

The Kennedy Class II partially edentulous arch has a unilateral

distal extension. An ISRPD should be usedwhen the tooth loss is

70


extensive.Otherwise,just as when only the molars are missing, the patient

might not use the prosthesis. When the patient has no functional problem,

a shortened dental arch concept,with no prosthesis, should be considered

Placing a single implant in the posterior regionwould modify the

Kennedy Class II configuration to a Kennedy Class III and increase the

stability and reten-tion of the prosthesis. The same considerations dis-

cussed for the Kennedy Class I tissue ISRPD also apply.

ICK III, for Kennedy class III situations

The Kennedy Class III partially edentulous arch has edentulous

space bounded by teeth. Therefore, implants should be used when the

edentulous space is long, the abutments are compromised, and when

thepatient objects to the appearance of the clasps.The implants should be

placed adjacent to the abutments.

ICK IV, for Kennedy class IV situations

The Kennedy Class IV partially edentulous arch hasa single,

anterior edentulous space that crosses the midline and is bounded by the

remaining teeth. The implants should be placed as medially as possible

tothe abutments to provide optimal support. The labial flange of the

prosthesis might serve to restore the lip support in these ISRPDs. The use

of implants inKennedy Class IV partially edentulous patients renders the

use of retentive clasps and elaborated dual-path RPD designs

unnecessary.

Clinical guidelines for Implant Supported removable partial denture

1. Place implants in area of second molars in distalextension patients.

71


2. Place implants adjacent to distal abutment in case future fixed

restoration is an option, distal abutments are poor, or patient is

concerned about unesthetic clasp showing.

3. Place implants medially in Kennedy Class IV arch.

4. Use short or narrow-body implants if necessary.

5. Use resilient attachments on the implants.

6. Design a simple RPD; use rest seats and guiding plates similar to

conventional RPD.

7. Use rigid major connector design for maxillary arch.

8. Minimize mandibular lingual flange if difficult for patient to adjust

9. Incorporate retentive elements to denture base under functional

load.

10. Schedule patient for checkups and maintenance appointments

Advantages

Improved esthetics by the elimination of visible clasp assemblies.

Ability to change fulcrums in the arch providing more favorable

biomechanics.

Minimizing rotational and lateral forces on direct and indirect

abutment teeth.

Controlled additional vertical support especially significant in

partially edentulous patients with distal extensions.

72


Provide additional retention and stability to the prosthesis by

incorporating an attachment mechanism.

Simplify prosthesis design and base extension.

Highly predictable treatment.

Easy to maintain depending on prosthesis design and attachment

system.

Minimize excessive pressure and trauma to soft tissues and

supporting ridge with alteration of the biomechanical forces.

Disadvantages of using implants in removable partial prosthodontics.

Additional costs for treatment.

Additional surgical procedures.

Extended treatment time.

Involve careful treatment planning and interdisciplinary approach

More technique sensitive than a conventional RPD.

Additional maintenance over time depending on prosthesis design

and attachment systems used.

Manual dexterity can be challenged in certain patient populations,

eg., rheumatoid arthritis, limited mobility.

Increased costs to overall treatment.

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FACTORS INFLUENCING MAGNITUDE OF STRESSES

1) Length of the edentulous span

The longer the edentulous span is, the longer the denture

base will be, and the greater the leverage force transmitted to the

abutment teeth will be. For each distal extension base, the fulcrum

is located at or near the occlusal rest on the most posterior

abutment tooth. During function, a load is applied to the artificial

teeth, and the length of the lever arm (i.e,denture base) determines

how much force the associated abutments must withstand.

Therefore, the practitioner must always be aware of the

forces that are generated as a result of removable partial denture

design. Although other factors such as the thickness of the mucosa

and the total area of the residual ridge may affect clinical

outcomes, the length of the edentulous span remains a factor that

warrants particular attention.

When treatment is being planned, every effort should be

made to retain an abutment posterior to the edentulous

space.Preserving a posterior tooth to serve as vertical support, even

as an overdenture abutment, results in improved patient service.

Similarly, the placement of an endosseous dental implant can result

in an equally valuable service.

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2) Quality of support of Ridge

The form of the residual ridge can play a large part in

distributing forces generated by the function of the partial denture.

Large, well-formed ridges are capable of withstanding greater

loads than are small, thin, or knife-edged ridges. Broad ridges with

parallel sides permit the use of denture bases with longer vertical

surfaces. These surfaces help stabilize the removable partial

denture against lateral forces.

The thickness and health of the mucoperiosteuma so

influence the loads transferred to abutment teeth.A healthy

mucoperiosteum approximately 1mm in thickness is capable of

bearing a greater functional load than is thin, atrophic mucosa.

Soft,flabby, displaceable tissue contributes little to the vertical

support of the denture base. This type of tissue allows excessive

movement of the denture base and permits forces transmitted to the

associated structures.

3) Occlusal relationship of the remaining teeth and orientation

of the occlusal plane

Many patients exhibit deflective occlusal contacts that

generate horizontal force vectors. These vectors can be magnified

by removable partial dentures and can be transmitted to the

abutments and residual ridges. To prevent the transmission of

destructive forces, the practitioner must be fully aware of occlusal

conditions and of the mechanics of partial denture movement.The

opposing occlusion can play an important role in determining the

load generated during closure.Some individuals with natural teeth

can exert closing forces of 300 pounds per square inch.

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In contrast, many denture wearers may not be able to exceed

30 pounds per square inch. Therefore, a removable partial denture

that opposes an intact dentition may be subjected to much greater

loading than a removable partial denture opposed by a complete

denture. The area of the denture base against which the occlusal

load is applied also influences the amount of load that is transferred

to the abutment teeth and the residual ridge. If an extension base is

loaded adjacent to the neighboring abutment, there will be minimal

movement of the denture base. As loading moves far away from

the abutment, movement of the denture base will be greater.

Ideally, the occlusal load should be applied in the center of

the denture-bearing area, both anteroposteriorly and faciolingually.

In most mouths, the second premolar and first molar regions

represent the best areas for the application of the masticatory loads.

Artificial teeth should be arranged so that the bulk of the

masticatory forces are applied in these areas.

4) Qualities of clasp

A flexible retentive clasp arm decreases the stress that

will be transmitted to the abutment tooth.

A wrought wire clasp is more flexible than a vertically

projection clasp, hence, it decreases the forces acting on the

abutment tooth and increases the forces transferred to the

edentulous ridge. It provides less resistance to more

destructive horizontal stresses.

5) Clasp design

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A clasp that is designed to be passive when it is completely

seated on the abutment tooth will exert less load on the tooth than

will one that is not passive. As a result, the fit of a removable

partial denture framework must be carefully refined to ensure that

the prosthesis is completely seated. Only when the framework is

completely seated will the retentive clasp arms be passive. If a

clasp's retentive tip is designed and constructed to lie in a 0.010-

inch undercut, but the framework is not completely seated, the

retentive tip will not be passive. Instead, it will exert a continuous

load on the abutment.

Refinement of the framework's fit is best accomplished by

uniformly coating the tooth-contacting surfaces of the framework

with a disclosing wax.As the framework is seated, wax is

displaced. A tooth to metal binding will show through the wax.

These areas are adjusted until the framework is completely seated

and the clasp arms become passive.

A clasp should be designed so that during insertion or

removal of the prosthesis, the reciprocal arm con-tacts the tooth

before the retentive tip passes over the greatest bulge of the

abutment. This will stabilize or neutralize the load to which the

abutment is subjected as the retentive tip passes over the greatest

bulge of the tooth.

6) Length of clasp

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The flexibility of clasp depends on its length. Doubling the

length increases the flexibility five times. This decreases the

stress on the abutment tooth using a curved rather than a

straight clasp on an abutment tooth will aid to increase clasp

length.

7) Material used in clasp construction

A clasp constructed of chrome alloy will normally exert

greater stress on the abutment teeth, than a gold clasp because

of its greater rigidity. To compensate for this property, clasp

arms of chrome alloys are constructed with a smaller diameter

than a gold clasp.

8) Surface characteristics of abutment

The surface of a gold crown or restoration offers more frictional

resistance to clasp arm movement than does the enamel surface of a

tooth. Therefore, greater stress is exerted on a tooth restored with gold

than on a tooth with intact enamel.

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STRESSES INDUCED BY THE REMOVABLE PARTIAL

DENTURE

The service expectancy of a removable partial denture will be

proportional to the degree of control which is exercised over the stresses

induced by it. This is such an important factor especially in the success of

the extension-base type of prosthesis that it should be emphasized by

analyzing each stress and suggesting clinical and constructional

procedures for bringing about its most effective control. Functional stress

stimuli, within certain limits, are necessary for maintenance of the

supporting structures. Beyond an optimal amount, which may vary to a

considerable degree, stress may become an irritant, however, and may

actually cause retrogressive changes to begin.

In the case of the partial denture, one sure method of avoiding

overload is by the reduction of functional stress loads to a minimum

which is consistent with a conservative restoration of function. In fact, the

total stress load should be well below the estimated tolerance of each

patient. This provides a safety factor to accommodate a variation in the

amount of stress which the structures may tolerate at different periods.

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Such variations are seen between one individual and another, where

extreme differences may be noted, but there are also variations from one

period to another in the life of the same individual.

PRESERVING THE ORAL STRUCTURES

Certain basic precautionary measures are indicated to assure that an

oral rehabilitation program will be kept within the tolerance limits of the

prosthodontic patient. The restoration of masticatory function is desired,

but the degree of restoration must be adjusted to the individual's ability to

sustain such increased workloads on the supportive structures.

In addition to limiting the beginning functional load given to the

individual, one must also provide a margin of safety to accommodate for

the depression periods of reduced tolerance limits. Even for the young

patient an occasional subnormal period may be expected. For older

patients there is the added certainty of a slowing up in physical processes

as they approach senescence.

These low tolerance periods and slumps in metabolic function may

come on very gradually and without the patient's recognition. One of the

important reasons for scheduled, periodic rechecks, as a part of partial

denture maintenance service, is to detect evidence of any stress overload

and to correct for this if possible. It is strongly urged that prosthodontists

concentrate less on the idea of restoring full masticatory function for the

partially edentulous patient, and that they exhibit more concern about

maintaining the oral structures which still remain.

INDUCED STRESSES TO BE RECOGNIZED

This is very important to determine how each component part may

assist in the reduction or elimination of induced stresses. For further

80


emphasis of this important matter of stress control, each of the following

potential overloads should be analyzed,its effects upon the supportive

structures noted, and measures for its control outlined. To have a better

overall picture of this problem, however, the induced stresses first should

be enumerated.

The principal ones are:

1. Stresses resulting from an inaccurate appliance;

2. Stresses caused by an interference to appliance insertion and

removal;

3. Stresses which may cause impingement of the gingival

structures adjacent to the remaining teeth;

4. Stresses which develop as a result of the use of a sloping tooth

surface for the support of an abutment occlusal rest;

5. Stresses resulting in impingement by a major connector;

6. Stresses which torque or twist the abutment of an extension-

base prosthesis;

7. Stresses which cause the proximal or lateral tilting of an

abutment.

1.STRESS RESULTING FROM APPLIANCE INACCURACY

When a removable partial denture (of any type) is either oversize

or too small, there will be a continuous pressure on all teeth and other

structures with which it makes contact. The direction of the pressure will

be variable and dependent upon which unit of the prosthesis is

transmitting the contact effect.

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The first result of this stress will be orthodontic in nature. If severe,

it may induce hyperemia and discomfort. Usually, the tooth so effected

will respond to the pressure, as in intentional orthodontic therapy, and

will alter its position enough to release the pressure.

As a result of the induced movement, a relation of malocclusion

will usually be produced as a second effect of the inaccuracy of appliance

fit. This has quite serious potentialities unless it is soon rectified. Unre-

lieved occlusal prematurities of this type can result in periodontal

disturbances, not only about the tooth moved but also about those in

adjacent and/or occlusal contact. Such pressures are capable of causing

compression areas in the periodontal membranes of the affected teeth and

may easily lead to destruction of the enveloping alveolar bone.

Ramfjord has said, "Traumatic occlusion may result when

pressure contacts force a tooth into a position having an occlusal relation

which in turn rocks the tooth into another position when functional or

bruximatic stress is applied."

A third effect of appliance distortion may be noted in the cast bases

of inaccurate extension-base partial dentures. Impingement of sub-basal

structures sometimes occurs in the mucosal pad over the mandibular

ridge. It may occur bilaterally and apparently results from a slight

"rebound" of the horseshoe-shaped casting when the sprues are cut.

Mills refers to distortion of this nature in a very valuable study of

volume change as a result of various factors concerned with the making

of large bilateral removable partial denture castings. An inaccuracy as a

result of volumetric change during the congealing of cast metal would be

most noticeable near the free ends of the long castings, such as in a Class

I mandibular appliance.

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This type of mucosal pad impingement would not be encountered

in a partial denture of resin base construction. The resin base having been

related to the cast metal framework after the casting process had been

finished, there would be compensation for the former distortion as far as

the ridge relationship of the base is concerned. This is another advantage

of the resin base.

SOME COMMON CAUSES OF APPLIANCE INACCURACIES

In order to prevent the damage which may arise from a distorted

appliance, the various contributing factors should be appraised. Most of

these can be completely avoided, and all can be reduced to discrepancies

which are quite within the average range of tissue tolerance.

a.FAULTY IMPRESSION

A faulty impression is the first cause of inaccuracy to be

eliminated. The discrepancy may be in either the impression of the

prepared dental arch or the hydrocolloid duplication impression of the

master cast. Partial displacement of the impression from the tray is more

frequent than is suspected. When this accident is apparent (usually at one

heel of the lower tray), there is the temptation to rely upon what seems to

be an accurate replacement by the repositioning of an otherwise perfect

impression back into contact with the tray.

Since the slightest failure to reseat it perfectly will result in a gross

error in a casting the size of an average dental arch, the risk in making

such an attempt is evident. The value of properly locking the impression

material in the tray is apparent when the eventual cost of this error is

taken into account. This distortion could have been avoided by proper

placement of the material in the tray. It could be corrected in a very short

time by retaking the impression.

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A second source of damage to the impression of the dental arch is

from overstraining the material by forcing the impression from a deep

undercut area in a direction which will result in the greatest strain. First

release the surface tension seal at the periphery in the area of least

undercut (let air under the impression). Then remove the impression as

quickly as possible, allowing the direction of the snap removal to take the

line of least resistance. This method will allow less chance of

permanently deforming the elastic impression material.

Improper care of the hydrocolloid impression is a common reason

for the inaccuracy of the resulting appliance. A volumetric change (either

shrinkage or expansion) is only one discrepancy to be guarded against.

Injury to the surface of the resulting master cast by the contact of the

setting stone against hydrocolloid is another danger which also must be

avoided In this connection, an advantage of rubber-base material

(mercaptan) is superior surface of the stone cast which can be obtained.

b.ERRORS IN DUPLICATION

In duplication, the hydrocolloid will have been diluted and, hence,

is more easily abraded. All undercuts which are not to be used should

have been blocked out (filled) to lessen the strain of removing the master

cast. A further safety feature in this connection is to use a duplicating

flask or ring which permits complete displacement of the impression from

it before removal of the master cast is attempted .Mills found that one of

the several factors affecting the degree of accuracy of prosthetic castings

was the use of a non conducting ring of the duplicating flask. When such

a flask is used,gelation of the hydrocolloid will take place from the

bottom upward. This avoids the risk of internal shrinkage voids.

Greater accuracy of the casting resulted, according to the Mills report.

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c.CASTING INACCURACIES.

Incorrect proportions of water to investment can be the source of

casting inaccuracies at two points in the development of a partial denture.

First, the master cast may be affected in this way. The master cast must

be an exact replica of the dental arch. The proper portion of water to in-

vestment should be used to produce a stone which will have minimal

expansion.

The amount of water to investment for the refractory cast also may be

incorrect so that there will be an insufficiency of the necessary setting

expansion of this cast. It takes the combined expansion which is obtained

in three ways to negate the contraction of molten metal as it congeals to

the desired form of the casting:

1. The setting expansion of the refractory material of the proportions

found to give the maximum expansion while still having a

consistency which permits proper handling;

2. The hygroscopic expansion achieved by having water contact the

refractory material as soon as the impression has been filled, and

before this investment begins to set;

3. The thermal expansion of the refractory investment when it is

heated to eliminate the wax pattern (1300° F. for three hours).

Improper W-P ratio is one of the most critical of the several possible

causes of appliance undersize or oversize. Even when every precaution is

taken, it is barely possible to control this factor. It must be admitted that

very often the large removable partial denture casting is not perfect in this

respect the minimal goal that is accepable is one that keeps this error

within the range of tissue tolerance.

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Larger, heavier removable partial denture castings, especially those

for maxillary cases, present a need for maximum control of metal

shrinkage. An effective way to accomplish this, is by altering the water-

powder ratio. For instance, from the recommended ratio of 28 cc. of

water to 100 grms of powder, for the average bulk of Ticonium casting,

this may be varied to amounts of water ranging as low as 25/100. A lower

ratio of "Vestic," the more recent refractory investment recommended for

Ticonium castings, has given excellent clinical fits.

Surface abrasion of the casts used in making a partial denture may

easily be responsible for a larger error in appliance.The effect of

hydrocolloid on the surface of casts made of gypsum products is that of a

retarder. If a soft, chalky surface is present on the master cast it will

certainly be reduced by abrasion and the casting will be that much

undersized. To offset this retarding action of the hydrocolloids it is

possible to employ a "hardening" solution (2 per cent potassium sulfate)

as a wash into which the impression is immersed for a few minutes before

it is to be filled.

The surface of the refractory cast may be more easily abraded because

it is much softer than the improved stone of which master casts are most

often made. A potassium sulfate solution may be used again to give a

harder surface to the refractory cast. To further lessen the chances of

abrading the refractory cast, at the time of removing the hydrocolloid, it is

suggested that the removal of the cast be delayed for about 30 minutes

after the allotted setting time. During this additional period the hydro-

colloid is intentionally dehydrated by removing the duplicating

impression from the flask and exposing it to air (a stream of compressed

86


air also may be directed on the cast and exposed hydrocolloid). As the

latter loses water it dries the surface of the cast by absorption.

Removal of the duplicating material can be done with less abrasion by

breaking it away do not attempt to withdraw the refractory cast. If a V-

shaped piece is removed from the hydrocolloid impression in the palatal

and lingual areas, those portions of the impression can be removed with

less rubbing of the lingual surfaces of the abutment teeth. It is also an aid

to use compressed air as a means of loosening these pieces. The re-

fractory cast should not be rubbed or brushed and it should be handled

with great care while it is drying and during the placement of the wax

pattern. If the refractory cast has been reduced, the casting will fit neither

the master cast nor the patient's dental arch.

Proper thermal expansion of the casting mold will be listed here for

emphasis,although it was mentioned in connection with the W-P ratio. It

is usual to maintain a temperature of 1300° F. for three hours to insure an

adequate temperature in the mold center. One important precaution is that

the oven Pyrometer be tested frequently enough to insure its complete ac-

curacy. It occasionally happens that the temperature may be lower than

the pyrometer shows. This condition will give an undersized casting.

Distortion of the cast units during heat treatment or soldering

operations has been reported in only a small percentage of cases

tested.This should be regarded as a possible cause of discrepancy of

appliance fit, particularly if the casting had shown a correct relationship

when tested on the master cast, and then, after the reheating of the

casting, it no longer seemed to fit as well as before.

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Even only two or three out of 100 appliances would seem too great a

percentage of failures from this cause. These failures can be avoided by

the use of alloys whose physical properties are acceptably near the ideal

and do not require heat treatment, and by use of a method of construction

which does not entail soldering.

Excessive polishing is responsible for many misfits among partial

denture castings. The reduction of the tooth surface of an occlusal rest

can result in the lowering of the appliance. When some other unit (such as

the suprabulge sector of a clasp) is resting on an occlusally inclined

surface, it then will exert a lateral or proximal pressure on the abutment,

if the appliance is allowed to settle as it would by the reduction of the

undersurface of an occlusal rest. Actually, the appliance no longer fits. It

should be a rule, therefore, that the tooth surfaces of a casting should be

burnished and buffed lightly never ground.

2.STRESS FROM INTERFERENCE TO APPLIANCE

INSERTION

This stress, unlike that from an appliance inaccuracy, is an

intermittent disturbance of tooth alignment. It occurs when a contacting

rigid area of a removable prosthesis passes over a surface bulge of the

abutment tooth. This interference to appliance insertion or removal results

from the failure in mouth preparation to establish parallelism between the

tooth surfaces with which the prosthesis is to make contact. Sometimes it

is not possible or desirable to achieve a parallel relationship of the total

area on which an interference is found, in which case some of the

undercut will remain and must be eliminated at the stage of final survey.

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If the degree of interference is slight, the type of tooth disarrangement

which it causes is of brief duration only.

A second source of this kind of stress arises from the movement of

a retentive clasp out of the infrabulge area and over the abutment height

of contour. In this action, pressure develops from the temporary distortion

of the clasp arm. Ideal clasp design provides a reciprocal support to

counteract the force generated by the retentive clasp.

The advantage of having this reciprocal terminal placed on a

surface which had been made parallel to the path of appliance movement

was emphasized. When this is done, the retentive stress is neutralized

throughout the total period of its generation. Even when unreciprocated,

this stress (retention which is generated by the retentive terminal) is also

very briefly only the time that would elapse in the movement of the

retentive clasp terminal on the infrabulge incline to and over the crest of

abutment contour.

While these two stresses are not to be desired, and can, with proper

preparation of the abutments, be entirely eliminated, the potential damage

which they may cause is undoubtedly much less than the stress generated

by an appliance of inaccurate adaptation.

In the first place, the stress from interference to appliance

placement is brief and not continuous for the entire time that the

prosthesis is in position. As compared to the above mentioned constant

pressure (from a distorted or an inaccurate appliance), it certainly would

be unlikely to cause any increase of trauma, and usually it would be much

less.

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Of greater significance, however, is the fact that during the period

of this stress application, the teeth cannot be in occlusal

contact.Therefore, the added trauma arising from occlusal imbalance,

which accompanied the stress caused by the inaccurate appliance, is com-

pletely avoided in the case of stress arising from interference.

After the mouth preparation changes have given parallelism, there

still will be need for the elimination of slight interferences in most

instances. When the master cast is completed, the degree of improvement

may be accurately measured by another study of the cast on the surveyor.

Almost always there will be need to block out remaining undercuts of

minor extent. When this is done,it is possible to make a refractory cast

which will be almost entirely free of interference.

Following the above precautions, any remaining interference

should be very slight. If care is taken in studying the relationship of the

casting to the master cast, these points of interference can be detected

before damage to the cast surface has occurred. Relief of the appliance

can be made to remove the final degree of interference. This method

should be used only as a last resort.

If relief by grinding the appliance is used excessively, the metal

structure may be weakened to an objectionable degree. A more serious

objection, however, is the development of space for the retention of

debris and stagnant saliva between the appliance and the tooth surface.

This is especially hazardous when the prosthesis is resting on an enamel

surface of a caries-susceptible patient.

3.GINGIVAL IMPINGEMENT BY THE REMOVABLE

APPLIANCE

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The gingivae are most susceptible to injury by any pressure

induced by a removable prosthesis. Even minor contacts seem to promote

an unfavorable reaction in these areas. Inflammation in the areas of

contacts made by the units which must cross the gingivae is soon

followed by edema. As these structures become distended, the pressure

increases and a vicious circle of retrogressive change is established.

The end result is a resorptive loss of the adjacent alveolar process

with a pocket formation. Loosening of the abutment follows, and as the

bone level is lowered, the tilting and twisting stresses on the abutment

become more and more an overload. If the abutment tilts, the

impingement of the periodontium in areas of compression will closely

follow.

It frequently is easier to prevent this unfortunate sequeala than to

reduce the condition after it has become well established. There may be

need to give the structures a rest period with the appliance removed from

the mouth for all except the periods of meals. At the time of first seeing

the patient who has this situation, a careful examination subgingivally

should be made to check on the possible presence of subgingival calculus.

Such deposits are at times the cause of this irritation because, as the

gingivae are pressed away "from the cervical area by the accumulating

mass, they are pressed against the overpassing unit of the prosthesis.

After the root surface is freed of deposits and has been polished, a

short rest period then follows. It is best to defer any reduction of the

prosthesis until the patient is again seen, when the amount of adjustment

(if any is needed) can be determined more exactly. Not infrequently, the

edges of the metal base or the connectors are found to be too sharp, or

91


angular. This angularity alone is quite sufficient to initiate the process of

inflammation.

On the side of prevention there are certain definite precautionary

measures to observe. Most important is to be sure of proper occlusal rest

preparations. Without adequate occlusal rest stops, it is useless to expect

the gingiva to escape impingement in these crossing areas. Some have

suggested the use of clasp retainers without occlusal rests. If such

unsupported clasps are under even the slightest tension (as when distor-

tion might have occurred), there will be a cervical pressure generated

enough to produce gingival impingement of increasing severity.

For added emphasis, it seems well to urge again that soft alloys not

be used for a restoration in which to prepare an occlusal rest seat. Silicate

and resin should not be used in this way, and an amalgam filling which is

in situ should not be used if it seems soft or poorly condensed. Getting

the proper rest support still is not enough,it must be protected. The tooth

surface of an occlusal rest must not be reduced in the process of polishing

the prosthesis. Any reduction of the rests will allow the appliance to

move toward the subbasal structures to impinge, first of all, the gingival

crossings.

At the time of construction a slight relief should be made at each

gingival crossing. Particular care should be given to the matter of

rounding the edges of the prosthesis which are adjacent to or which cross

the gingivae. Each time the partial denture patient is seen for maintenance

inspections, the gingival crossings should be checked again for evidence

of over-contact.

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It is quite possible that such a condition may develop even after

years without such trouble. This might come from occlusal rest wear,

from intrusion of the abutment teeth apically, or as a result of subgingival

calculus deposits. The consequence of gingival irritation warrants that

every safeguard be utilized to avoid its beginning. Its cure is not always

easy.

4.STRESS FROM OCCLUSAL RESTS PLACED ON INCLINES

The frequent necessity of using a cuspid tooth for abutment service

makes the problem of effecting a safe transfer of partial denture occlusal

loads to one that is constantly with the prosthodontist. The lingual anat-

omy of the valuable cuspid abutment is frequently steeply inclined. In

fact, some mandibular cuspids present almost a vertical lingual surface.

To apply rests on such surfaces would produce very unfavorable leverage

on the abutments, resulting in areas of impaction in the periodontal mem-

brane. An abutment support cannot accept this destructive overload, even

when the host is capable of normal bone maintenance under increased

stress loads.

A second unfortunate sequela of applying a partial denture loading

on an inclined surface is the possibility that the appliance will slip as the

occlusal load is applied. Appliance movement of this kind can easily

induce the gingival irritation which was discussed in the preceding

paragraphs.

While the most serious situation pertaining to the problem of the

inclined support relates to the use of a cuspid abutment, bicuspids and

molars (especially those with single or fused roots) are also subject to

similar damage unless the rest recess is favorably formed.

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In some mandibular extension base partial dentures, the placement

of a distal occlusal rest on a surface which slopes cervically toward the

edentulous area may result in repeated impingement of the subbasal pad

at the retromolar periphery of the base. This is produced as the prosthesis

slips posteriorly on the inclined surface of the abutment.

Preventing stress which would be caused by locating an occlusal

rest on cervically sloping abutment surfaces can be attained only by

considerable clinical effort. The operator must come to evaluate this extra

expenditure of time and exertion as being an excellent investment in

longevity for his service, and the patient must be sufficiently aware of its

potential value to accede to the considerable additional cost. There is al-

ways the temptation on the part of both to take an easier shortcut. After

seeing the tragic loss of fine abutment teeth from this type of stress, the

prosthodontist of long experience can attest to the merit of proper mouth

preparation.

Specific measures to be taken in the direction of avoiding damage

from this source can be accomplished at the time of preparing the mouth

for partial denture service. The first, and by far the most frequent, is the

making of an adequate occlusal rest recess in bicuspids or molar

abutments. Of primary significance in stress control is that the floor of the

prepared recess must slope from the abutment margin toward its center.

This form creates an angle which is less than 90 degrees between the rest

floor and the vertical minor connector. Then, under stress loads, an

abutment is held firmly against the vertical guiding plane of the minor

connector, thus preventing lateral pressures which would cause

periodontal impingement.

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In the situation of the cuspid abutment,the form of this tooth will

seldom be such as to permit the placing of an adequate occlusal rest. The

reshaping of a cuspid tooth can be done best by the placement of a three-

quarter veneer crown restoration, in which a groove is placed on the

lingual surface just above a raised cingulum.

Occasionally, for some good reason, reconstruction of the cuspid

may be impossible. Than the labio-incisal (embrasure-hook) unit has been

used instead of the raised-cingulum restoration. Another substitute

measure may be suggested for the posterior tooth where an ideal rest

recess cannot be executed for some reason. This is the use of a secondary

(auxiliary) occlusal rest to compensate for any pressure in the mesial

direction which would be generated by the use of the rest on a distal

incline.

As noted, however, this reciprocal action of the auxiliary occlusal

rest is operative only as long as the mesial rest remains perfectly seated.

Should there be a resorptive loss in the sub-basal structures which would

permit rotation of an extension base prosthesis at its cross-arch fulcrum

line, the compensatory action of the mesio-occlusal rest would be

nullified. Thus, again, the best procedure, as in the case of the rebuilding

of the cuspid, is to place a restoration which would permit the proper

occlusal rest recess on the distal portion of the occlusal surface.

5.STRESSES THAT A MAJOR CONNECTOR MAY CAUSE

There are three different ways in which a major connector may

produce impingement of the structures over which it passes. If it is not

rigid, workloads may cause it to flex. When these loads are such as to

cause an extension base to move lingually, the non rigid connectors

95


(particularly the lingual bar type) may be forced to flex toward the

subbasal structures. At the weakest point in its anterior arc between the

right and left abutments, the flexible bar will, because of these flexures,

repeatedly press against the mucosal covering.

Localized inflammation, followed by edema, increases this

pressure and soon the underlying bone is involved. The lesion is not

usually very painful and may escape the notice of both patient and dentist

unless the area is carefully examined. If allowed to continue, this type of

impingement may eventually produce a perforation of the mucosal pad.

The small hole is quite smooth and well defined. Through this aperture

one may probe the bone, which may be denuded with the periosteum

detached in an area much larger than the tiny opening. Not infrequently a

sequestrum may be exfoliated, and occasionally the lingual cortical plate

is entirely lost in this area.

A second type of major connector impingement may follow a

lateral shifting of the appliance. This, too is more commonly seen in the

mandibular prosthesis. It usually accompanies the property of flexibility

of the lingual bar, but in this condition, the bar has a tendency to become

straightened (its arc reduces). The result is that the connector moves

laterally to impinge the area lingual to one or the other, but not usually

both sides of the arch. This movement is quite frequently associated with

an occlusal imbalance in which the prosthesis tends to move toward the

side being impinged.

Here, again pressure contacts lead to an inflammatory process, and

this trauma may produce edema to increase the pressure and thus

establish a vicious circle. In less time than one may realize, bone

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destruction may ensue, with definite pocket formation on that side of the

abutment. Mobility of the abutment increases and its loss may be the end

result of the unfavorable sequelae of this impingement.

A third major-connector traumatization may be seen, but with less

frequency than either of those referred to above. This condition is a

generalized contact pressure which results from a change in the relation-

ship of the connector to the underlying structures, when the tooth-borne

portion of the partial denture settles or depresses. While this does not

happen often, it is a situation that can be the result of several conditions,

most of which fortunately can be prevented. This is another problem

which is much easier to prevent than to correct.

Actually, when impingement is found throughout a major

connector, the disturbance is so painful that it will be necessary to remove

the partial denture at once. This is usually seen more frequently in

connection with the mandibular partial denture. There is no reason why

the causes of appliance settlement cannot occur in the maxillary arch.

A probable reason that it is not so frequently associated with the

upper partial denture is that the anterior major connector (palatal bar) is

usually much broader than a lingual bar. As a result, any impingement

would be more widely spread and therefore less likely to exceed tissue

tolerance. It is a clinically observed fact, that the structures of the anterior

palatal area are much less likely to be irritated than those of the lower

arch.

a.Trauma from flexing

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This type of impingement most often occurs with the single lingual

bar,and it is always seen in the case of a bar which is too flexible. During

heavy occlusal impacts, the arc between the right and left abutments

alters in such a manner that the bar springs against the mucosal pad. This

will occur in the area of its greatest flexibility, or at its most acute

curvature. Since length is associated with flexibility, the longer connector

will be most prone to show this defect. One problem that is always

encountered in the design and construction of a lower Class II prosthesis

is that of overcoming the tendency for the long connector to flex.

This was a constant difficulty when the use of the wrought lingual

bar was common practice. It also is usual to find occlusal imbalance

accompanying this situation. The type of prematurity or cuspal

interference for which one should be most watchful is that which would

carry the lower extension base appliance in a horizontal direction.

Prevention of flexure impingements:

1. Use a cast connector employ a less flexible alloy.

2. Increase the bulk, when the connector is long .

3. Alter the form (use a half-pear form instead of half-round or flat).

4. Some alloys of gold that are rigid or may be made rigid by heat

treatment.

5. Add a secondary lingual bar across the cingula of the lower

anterior teeth.

6. Use a linguoplate connector, which will be more rigid because of

being in two planes and somewhat corrugated in form.

7. Widen the anterior palatal bar to include two planes of the palatal

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surface add bulk between the rugal crests.

8. When the palatal arch is high and the rugae are prominently

developed to provide a corrugated undersurface then it is

unnecessary to also use a posterior palatal bar.

When the palate is low and flat (with a less well developed system of

rugae), it is necessary to use both posterior and anterior palatal

connectors. A principal reason that the assembled partial denture,

utilizing a wrought connector, has proven less than satisfactory is because

it is too flexible; this is especially true of the lingual bar. By casting this

bar, it was possible to change the form and vary the bulk to remove this

objection. The selection of a less flexible alloy is possible, when it is to

be cast. Such choice is much more limited in the ready-formed bars.

Since it is not easy to draw the less ductile alloys, they are avoided in the

manufacture of the ready-made wrought bar.

b.Trauma from lateral appliance movement

A lateral shift of the partial denture may tend to occur in certain

conditions, with the result that there is a pinching of the tissue beneath

the major connector. Such movement would be encouraged by the use of

weak tooth support, especially when this condition is accompanied by the

use of a flat ridge from which a base could not gain much resistance to

lateral stresses.

If considerable occlusal disharmony is added to these conditions,

there is a probable chance that an area of thin, unyielding tissue might be

pinched between the base and the surface of the bone. If this trauma

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continues, the chronic irritation may result in bone necrosis. Fortunately it

is possible to utilize certain preventive measures.

Prevention of impingements from lateral shifting:

1. Provide a slight space beneath the lingual bar by placing a thin block-

out material before duplicating the master cast.

2. Employ more rigid stabilizing units (reciprocal clasp arms, auxiliary

occlusal rests, indirect retaining units, etc.

3. Reduce the cuspal inclines of the opposing occlusal surfaces. When

unused teeth present cuspal inclines that are steeper because the teeth

have not been in function and have had no abrasive wear, such teeth

should be adjusted.

4. The height of their cusps and the steepness of their cuspal inclines

should be made to correspond to that of the remaining natural teeth.

5. Restore the best possible occlusal level of extruded teeth by

grinding,or by restoration when they must be shortened so much that

the dentin would be exposed. Occasionally, such teeth may have to be

extracted because their malposition is so extreme.

6. Relieve the major connector in an area of anticipated impingement

after the casting has been made (or when irritation has occurred);

reduction by grinding may make the major connector flexible, in

which case one trauma would be likely to replace another.

7. Since this type of lesion is associated with lateral appliance

movement, it is doubly urgent that the mandibular base be extended to

maximum flange length, especially on the lingual. If the ridge height

is subnormal and there is a sharp lingual edge, surgery should be

utilized to make possible a longer lingual flange by recontouring the

area.

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8. Splinting to provide multiple abutment support will effect more

adequate stabilization and reduce the possibility of appliance

movement.

It is interesting to note that of the above measures suggested for the

control of a very annoying and too frequent partial denture difficulty, all

but one may be said to be preventive. Six of the seven are planned and

executed before delivery of the prosthesis. Four of the seven are measures

to be completed at the mouth preparation stage of the proposed service.

Only one measure (relief of the casting) can be classed as remedial, and it

is suggested only in a limited way. With careful attention directed toward

the six preventive aids, the rehabilitation program will have improved

chances of success. If it fails, it will probably do so in spite of the lone

remedial measure.

c.Trauma from connector settlement

Removable partial denture without adequate occlusal rests is

seldom encountered in modern prosthodontics. A strong plea has been

made for utilizing tooth support (gained by the use of proper rest units) to

prevent gingival irritations. No less forceful is the claim that inadequacy

of occlusal rests can be cited as a cause of major connector impingement.

Particularly in the instance of a mandibular partial denture can it be

stated that even maximum extension of the bases will not alone be able to

gain sufficient support to avoid an occasional appliance settlement. Even

with the aid of tooth support, there still will be some situations where

such settlement will occur.

One such occasion is that in which one or both abutments have had

no recent occlusal work loads. An abutment of this category is certain to

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take a position (after assuming abutment service) which will alter its

relation to the alveolar walls that are to give it support. It may be

expected that this change will result in an increasing contact between the

lingual bar and the surface of structures subjacent to it.

When too little free-way space has been provided for a patient with

unusually heavy occlusal force loads, there may be intrusion of the

supporting teeth. Hypercontact of the connector is sure to follow.A

similar result is encountered when the bone of the alveolar process is

subnormal. If the patient is incapable of normally maintaining his bone

structure, it is certain that the bone tolerance limit will be more quickly

reached. Under this condition and with too heavy occlusal loading,

abutment intrusion is possible. In this connection it would be profitable to

review the section on determining the probable stability of the alveolar

bone.

The use of unsuitable materials to support an occlusal rest is about

the same as not using one. Obviously, a soft filling (such as silicate) in

the area of an occlusal rest site will reduce. As the support for an occlusal

rest is lowered, the appliance settles to closer contact with the mucosal

surface beneath it. The same effect can be the result of grinding the

supporting surface of an occlusal rest during the finishing and polishing

of a removable partial denture casting.

Avoiding major connector settlement:

1. The primary preventive measure to be taken is an attempt to adjust

for any metabolic imbalance, when there is evident failure to

maintain the alveolar bone.

2. If there has been previous loss of supportive bone, splinting of the

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adjacent teeth will be a major factor in avoiding overload after the

abutment service is added.

3. Adequate occlusal rest units should be provided. In polishing the

undersurface of these, no reduction is to be made burnish and

polish lightly.

4. Restorations made of easily abraded materials must be avoided in

locating primary occlusal rests.

5. When abutment teeth have not had recent occlusal function, digital

exercise will help to reduce the amount of positional adjustment

after prosthetic loading occurs. The patient should be instructed to

place his finger so that an occlusal force may be simulated as to the

amount and direction. Such exercises should precede the final

impression by a few days, during which time the exercise should

be frequently repeated.

6. In designing a removable partial denture, where the possibility of

overload is suspected, auxiliary occlusal rests can profitably be

included, in order to spread the work load more widely.

7. A lingual bar wax pattern should be thickened when there is a

chance that later reduction may be needed.

8. A supporting base should be extended as widely in a buccal

direction and over the retromolar area as possible, when an un-

stable condition is suspected. Include all of the basal bone surface

which can be used without encroaching on moving structures.

9. Finally, reduce the occlusal table (both in width and length) to

lessen the force loads which may be received in any single contact

with a food bolus. In establishing the occlusal pattern, care also

should be taken (especially in these situations) to avoid the

overreduction of free-way space. Continuous occlusal pressure

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from muscle tension must be avoided.

10.If, in spite of the above preventive measures, major-connector

settlement does occur, the impingement certainly will be of less

degree and perhaps can be entirely relieved by a reduction of the

under (or tissue) surface. Any such reduction, how ever, is quite

definitely limited—the bar must not be made flexible.

With the exception of the first of the above corrective measures,

which will often require specialist management, all are either to be done

in mouth preparation procedures by the prosthodontist or are to be under

his direction and executed by a technical assistant.

6.STRESSES WHICH TORQUE OR TWIST THE ABUTMENT

The stresses resulting in the various impingements of the major

connector, which have been discussed in the preceding paragraphs, may

be caused by a tooth-borne removable prosthesis as well as by the ex-

tension-base type. However, the stresses which cause torque or twisting

action will be found to operate to an exaggerated degree in the partial

denture having an extension base and practically not at all in the tooth-

borne appliance. This is because the prosthesis with the free end produces

twisting and tilting forces because of its lever action. Since this base is

supported by structures having some yield, both its lateral and vertical

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movement will need maximum control, even when the base is the best

possible.

The term "torque" is used in the following analysis to designate

that stress which tends to twist or turn an abutment in its alveolus, as

distinguished from a force which leads to the tilting of the abutment

laterally or proximally. Conditions which encourage the lateral movement

of the extension-base prosthesis make this stress a constant problem, and

its control one of the reasons that this prosthesis has been called the most

difficult of prosthodontic assignments.

Lateral movement of the extension base becomes aggravated when

the sub-basal ridge is low and flat in form. This movement results

principally from inadequate flange length. It also may be increased by the

presence of a flabby, movable pad of mucosal structures over the ridge.

Another critical factor in the development of torque stresses is the

presence of high cuspal inclines, especially if these are surfaces which are

not in occlusal balance. This lack of occlusal harmony occurs frequently

in the partial denture on which substitutes have been placed in relation to

teeth which had migrated from normal alignment, and which had been out

of occlusal function for a long period.

On these unused teeth, the cuspal height and inclination are both

excessive as compared to the existing condition of the remaining teeth

that have been subjected to abrasive wear. When such teeth govern the

excursive movements of the jaw, then the supplied teeth (and those with

which they occlude) cannot possibly be in harmonious balance until their

surfaces also have been made to conform.

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In addition to the above conditions, torque stresses will be most

destructive

When the occlusal loads are heavy; when the abutment has a

round, tapered root

When the abutment root is single (or fused).

When there has been previous alveolar bone loss about the

abutment teeth

When the occlusal table is long, and the number of remaining teeth

are few and

When the patient has a well established habit of bruxism.

Preventive measures in torque control:

Surgical recontouring of flabby and hypertrophic tissue on the

alveolar ridge.

Splinting the adjacent teeth, if the root is short or tapered,

which gives counter leverage advantage of multirooted

abutment.

Maximum extension of denture base within the physiologic

limit.

Use a rigid connector which extends to a remote anchorage in

order to effect adequate counter leverage.

Utilize a combination clasp to provide its stress breaking

action.

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Since torque stresses originate with lateral appliance movement, it is

most important in the mandibular extension-base prosthesis to obtain a

stable base before the abutment has suffered damage from torque. Very

frequently the undersized lower ridge is also further handicapped by

having been out of function for many years. Until it has been

reconditioned, by having received work stimuli, it cannot assume the

support of functional loads at once without further resorptive loss.

There are two ways to handle this temporary instability. The

prosthesis may be completed and then "rebased",or a prosthesis without

teeth may be worn with only light digital exercise to stimulate the

alveolar process to become "re-organized". Further loss of basal

structures should be avoided. There is no more certain way to induce

torque stress loads, and there is no stress which is more destructive.

Since much of the control of torque stresses will be dependent upon

the amount of force received on the occlusal table, the matter of

achieving harmony in occlusion is a very vital factor in the control of this

type of stress. The need for adjusting the occlusal anatomy of opposing

unused teeth has been stressed.

Another matter, equally important, is to coordinate the occlusal

relations of the supplied teeth to those opposing so that there will be har-

mony throughout all ranges of excursive jaw movement. The method of

doing this by having the patient wear an occlusal wax record,during

which period jaw movements are exercised, is strongly recommended.

The measures for the reduction of stresses which place a twisting force

on the partial denture abutment diminish in effectiveness as the length of

the occlusal table increases. The most frequently occurring partial denture

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situation, the mandibular Class I, must make use of the bicuspid for

abutment support. This means that the appliance lever is long, while the

form of most bicuspid roots is least resistant to the turning action

stimulated by the torque stress.

At the same time it should be recognized that in this situation (the

most frequently occurring partial denture case), the measures for con-

trolling this induced stress are less than maximum. It would seem that this

unfortunate combination of circumstances attaches the greatest emphasis

to the need for reducing, at its origin, that force responsible for torque.

There is urgent need, then, for complete occlusal harmony, not only

during the voluntary effort of masticating but also throughout an

involuntary muscular contraction like bruxism.

7.STRESSES WHICH TILT AN ABUTMENT

It has been emphasized that stress loads can be transferred to the

supporting bone of the jaws most ideally, from a physiologic point of

view, through the periodontal membrane. But this is true only when the

force loads are received in a trajectory which is parallel to the long axis of

the abutment. When the tooth is tilted by forces that are not parallel to its

longitudinal axis, certain areas of the membrane fibers are compressed

instead of being tensed, and impingement trauma results.

Tooth-borne prosthodontic appliances often are made removable

and bilateral in design in order to avoid these lateral tilting stresses. In

bilateral design, the principle of cross-arch splinting can be applied to

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develop counter-leverage by which effective control of tilting stresses is

gained.

Control of forces which induce proximal (mesiodistal) tilting is not

so readily accomplished in the extension-base prosthesis, however. In

tooth-borne appliances there is little possibility of abutments being tilted

proximally; in the extension appliance this stress is a major problem. Any

slight yield of the mucosal pad structures, not to mention actual resorptive

change in the sub-basal supporting bone, tends to produce varying

degrees of vertical movement of the base, and proximal tilting follows.

The ultimate result of compressive trauma of the periodontium is

bone resorption in the area of the alveolar walls. As the tooth is tipped, it

assumes a position of increasing malocclusion, with the forces generated

by occlusal imbalance being added to the traumatic injury already

sustained.

As the abutment becomes mobile, lateral shifting of the prosthesis

may result to produce major connector impingement which further

accelerates the process of damage. Particularly in the maxillary extension

partial denture (because of gravity) there may also be a mesial proximal

tilting. Hence, in severely unfavorable situations, the supporting bone

may be overloaded from all directions because of lateral and proximal

tilting of an abutment.

It has been shown that extension partial denture rotates at its cross-

arch fulcrum line in two directions, toward and away from the sub-basal

structures.As a result, the periodontal pressure developed may be on

either the mesial or the distal surfaces of the abutment alveolus. The exact

109


location of the pressure areas will be determined by the direction of the

tilting force.

a.Limiting Stress Caused by Base Movement Toward Ridge

Any movement toward the sub-basal structures would indicate lack of

sufficient support to sustain the occlusal load. If the partial denture is of

the type that is distally extended, the abutment will be tilted in that

direction. The first control is that of most direct approach improve the

support. There are two ways of doing this:

1. Improve the ability of the supportive structures to carry a greater

load. This can be done, in many instances, by surgical procedure at

the time of mouth preparation. If the mucosal pad shows excessive

mobility, it frequently may be improved by excision of some of the

hypertrophic mass to provide a more stable foundation.

2. If there has been prolonged lack of functional activity, a second

way of improving the support is the program of exercise therapy.

This has been found to so recondition the supporting bone that the

base may not require the usual rebasing procedure later.

Another effective aid in the problem of appliance instability is to

increase basal coverage. The beneficial result of increasing the

supportive area of the base is two fold. Not only can a greater occlusal

load be borne safely, but the wider distribution of the applied load will

lessen the possibility of resorptive change. Hence, proximal tilting of the

abutment may be avoided more frequently and for longer periods.

There is a very definite limit to extending the size of the base,

especially in the mandibular edentulous ridge area. Surrounded by

110


functionally moving structures, the peripheral limits of the base are

reached much too soon before an adequate area of coverage has been

attained, in many instances.

One more important measure may be taken when this impasse is

reached. This is to relate the base to the supportive structures in such a

way that all units of the surface are giving support. Care should be taken

always to keep the applied load well within the limit of the physiologic

tolerance of sub-basal structures.

However, distributing the functional load as uniformly as the nature of

the various component structures will produces the least chance of

overloading the firmer areas so as to induce resorptive change in them.

As has been pointed out, these measures too frequently are not enough

to insure stability of the base. The available mandibular area is too

limited. There is, however, another approach which is as direct as the first

this is to reduce the load at its source. To accomplish this, it is better to

reduce the buccolingual width of the occlusal surfaces supplied on the

prosthesis rather than to shorten the mesiodistal length of the occlusal

table. If the most posterior of the opposing teeth is not given occlusal

contact, it would tend to extrude, a condition which must be avoided.

The reduction in occlusal width does not always solve the problem of

overload . While it greatly diminishes the force generated by any single

occlusal masticatory contact on a food bolus, it in no way eliminates the

overload which occurs during bruxism. In the latter stress, only one point

of contact on the occlusal table is needed to transmit the full load.

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Unfortunately, the bruximatic force is usually the more dangerous

because it may be continued for long periods without interruption. In

mastication, the occlusal forces are applied intermittently, usually with

less biting pressure and for shorter intervals. There is a very dependable

way to reduce the possibility of bruxism, however; this is to carefully

eliminate all occlusal prematurities. Occlusal imbalance is considered to

be a primary cause of the habit of bruxism.

When these methods have been utilized to the limit,some curtailing

induced stresses, others augmenting the quality and degree of the support

then the last defense is again called into play. It is best to assume that (at

least in periods of subnormal tolerance) the demand on the supportive

structures may approach or exceed their maximum capability.

Accordingly, every effort should be made to include some safety measure

such as stress breaking type of flexible clasp as a last resort.

b.Limiting Stress Caused by Base Movement Away From Ridge

This stress does not develop in a tooth-borne prosthesis when a

direct retainer is functioning at each terminal abutment. Any force

tending to cause the extension-base appliance to leave its contact with the

sub-basal structures does produce tilting stress on the abutment. It is

suggested that the section relating to the indirect retainer be reviewed at

this point, since it has a pertinent application with the problem relating to

the stress being considered at this time.

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The amount of the force tending to induce movement of an

appliance away from the supporting ridge will vary, but its magnification

always will be directly proportional to the extended length of the ap-

pliance. In general, the forces which tend to move the prosthesis away

from its supporting structures will be less than those of occlusal origin

which move it toward them.

These forces are: the pull of sticky substances (upon which one

may have bitten) when the jaws are again separated; the pressing of

circumjacent structures as they are in functional movement against the

border or side of the prosthesis; sudden expulsions of air from the the

lungs (such as coughing or sneezing); and the force of gravity in

maxillary extension base appliances. The effect of this leverage stress on

the abutment tooth is to cause it to be tilted proximally in its alveolus.

The direction of the stress application will, be such as to tip the

abutment away from the edentulous area. Again, this stress will cause

zones of compression in the periodontal membrane, as did the stress

developed by movement of the base toward the ridge surface. While this

impingement may be less in magnitude, it may be more prolonged. The

effect of gravity (in displacement of the maxillary appliance, for instance)

is continuous for most of the time whenever the teeth are not in occlusal

contact.

Certain factors in the control of this stress are favorable, however.

The extension-base partial denture is predominantly of the Class I or II

variety. This means that a stress resulting from movement of the base

away from ridge contact would tend to tilt the abutment in a

mesioproximal direction. Usually, the abutment will make contact

113


mesially with an adjacent tooth. The tilting force will, therefore, be

partially shared by this contacting neighbor.

There may be two or three such contacting and supporting teeth in

the arch. Also, when the abutment is multiple that is, adjacent teeth are

rigidly splinted not only is there wider distribution of this stress, but also

a more favorable leverage advantage is developed. Both of these

influences tend to reduce this type of stress now being considered.

This statement is not presented to minimize the importance of

seeking to reduce this type of stress, however.The end result of its

continuation can be very destructive, culminating in permanent injury to

the periodontium.

In order to limit the stress caused by the free end of an extension base

appliance tending to loose ridge contact:

1. Reduce the weight of the maxillary partial denture of the extension

type to lessen the effect of gravity.

2. Avoid peripheral encroachment on moving circumjacent structures

in the attempt to enlarge the area of the base.

3. Reduce the base peripheries, if there has been overextension.

4. Contour and finish the appliance so that there is less chance of a

sticky bolus adhering to its surface. Position the supplied teeth so

that the contact of tongue and cheeks will tend to displace the

appliance least.(Reduce the lower teeth, if necessary, on their

lingual surfaces.)

5. Employ complete palatal coverage to obtain surface tension

support (as for a complete denture) as an aid to the less effective

indirect retainer in extensive Class I cases

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6. Utilize the most efficient indirect retention which can be obtained

under the existing conditions.

7. Dissipate the remaining leverage stresses by the use of flexible

retentive clasp arms as stress-breaking units.

8. If a problem is anticipated in the adequacy of the control measures,

or if a weakened abutment tooth must be used, it is well to utilize a

multiple abutment if splinting is possible.

It would seem, then, that the most destructive stresses induced by

the partial denture are those which twist or tilt an abutment tooth. This

is because the functional forces produced on the occlusal table are

magnified by the appliance, acting as lever, and are then passed on to

the abutment.

Certain measures can be taken to prevent this and, in fact, to

accomplish a reduction of the stress load in many cases. If the various

measures for controlling these stresses, as outlined in the foregoing

pages, are applied with meticulous insistence at the time of mouth

preparation, during design and construction of the prosthesis, and at

each appointment for maintenance service, then the removable partial

denture so produced will be most likely to give a long period of

satisfactory service.

SPLINTING TO IMPROVE STRESS CONTROL

A very common predisposing cause of alveolar breakdown is a

previous loss of bone support. As the bone level at the alveolar crest is

reduced, so also is the surface of the alveolar wall remaining for root

support. The stress assumed by each square unit of bone surface becomes

greater as the depth of the alveolus decreases. If the abutment happens to

115


be one which has a tapered root form, this decrease in percentage of area

of remaining alveolar support is quite rapid. Add to this situation an

unfavorable root form and it need not surprise one to find the surrounding

tissue overloaded.

An abutment with a single root is always more vulnerable because

of its reduced area of surface support, but when this one root is round it

also becomes very susceptible to torque. Often a single root is round and

tapered and this form is accompanied by a previous loss of bone around

it, indicating a susceptibility to alveolar atrophy. To use such teeth for

abutment support is a matter of questionable wisdom to say the least.

Stress control actually starts,at the diagnosis stage of partial

denture service.The wise prosthodontist will prescribe complete denture

service for a case in which he knows that it will be impossible to control

the stress load which the contemplated partial denture is likely to induce.

Where there are especially urgent reasons for retaining the

remaining teeth under some or all of the above conditions, there is one

possible way of controlling the stress load with reasonable success. This

is by the use of multiple abutments. The most effective way of

accomplishing a division of abutment work is by the actual union of two

or more teeth. Restorations in the adjacent teeth may be soldered at their

contact points to make such union.An other application of this idea is to

use the fixed bridge, uniting a tooth which is standing alone to one which

is separated by only a one or two tooth space.

This splinting of weak teeth produces an abutment support which is

comparable to that of a multirooted tooth. A molar with two or more

widely separated roots is accepted as an ideal bridge abutment. By such

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union a great advantage is gained against torque and a favorable leverage

is developed to combat proximal tilting.

Damage which formerly was attributed to the rigid fixation of

teeth, in the light of present-day knowledge seems to have been caused by

lack of attention to some other phase of stress control. Until alveolar

atrophy can be controlled through other remedial measures, the splinting

of these weakened teeth offers hope for saving at least the majority of

them.

THE COMBINATION CLASP IN STRESS REDUCTION

A combination clasp is one in which the retentive arm is made of a

round, flexible, wrought structure. In spite of all efforts at stabilization,

the base of an appliance that depends upon the subjacent structures for its

major support will have a variable amount of lateral and vertical motion.

Some device, therefore, is necessary to eliminate, or at least reduce, the

resulting stress before it is transmitted to an abutment tooth and the

surrounding area of supporting tissue.(Fig-41)

Stress-breakers of varied types have be entried from time to time to

reduce the work of a partial denture abutment. Most of these have

incorporated an idea of a broken joint between the clasp and the

appliance. This device allows some movement laterally or toward the

ridge but does not let a prosthesis move away from the tissue.

There is, usually, too much movement allowed, stresses are not

uniformly distributed and the very valuable stabilizing leverage of the

bilateral design is then lost. This type of moveable attachment is

complicated to make and adds materially to the cost.

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A simple, inexpensive but very effective approach to the stress-

breaker control of forces which escape all other means of elimination is

the combination clasp.Because this clasp uses a retentive arm made of

wrought alloy, it is flexible and, being fibrous, has a toughness that

permits its use in very small gauges.

Also, a round form of retentive arm is given to this clasp to make it

equally flexible in any direction. For this reason it is as effective against a

twisting stress as one which tends to tilt out of vertical. Hence, no stress

which would shift the abutment can pass through this flexible arm. Any

stress which can reach this point is simply dissipated, because the

wrought arm will yield (flex) before the pressure generated against the

tooth is enough to cause periodontal injury.

The wrought arm of a combination clasp has the additional and

very practical advantage of being adjustable. An increase or decrease in

the amount of retention requires that the clasp arm be moved cervically

(into the undercut) or occlusally to a level nearer the height of contour. If

the arm is half-round, as the cast clasp is, the above adjustment would

require an edgewise bend. This is a most difficult change to make in a

cast structure without permanently injuring it.

METHODS OF STRESS ANALYSIS

BRITTLE LACQUER COATING TECHNIQUE

This technique was developed by DeForest et al in the 1940s.

It gives a qualitative and roughly quantitative analysis of the strain

patterns in a previously deformed body. The technique is particularly

useful in detecting and measuring strains at the surface of a

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structure as well as indicating the direction and sequence of the

tensile strains. The qualitative analysis involves spraying a lacquer

onto the surface of the body to be tested. This is allowed to dry

and loads are applied in the desired way.(Fig-31)

Cracks appear in the lacquer in areas of maximum tensile

stress. Increasing the load causes cracks to form at other points

where the tensile stress has exceeded that required to fracture the

lacquer. The lacquer selection is critical and depends on the

humidity,temperature and sensitivity required. The prime constituent

of the lacquer used is colophony resin and spraying is carried out

using the equipment specifically designed for this purpose. The

major drawback with this technique is that the cracks which are

relatively easily seen on load application disappears once the load is

removed.

Additionally as the cracks do not run out onto the surface,

the top surface has to be etched away before developing with a

dye. The technique is also sensitive to fluctuations in temperature

and humidity and only gives a qualitative assessment of the stresses.

Due to its simplicity this method has been widely used in dentistry

and was first applied to examining stresses in dentures by Matthews

and Wain.

Qualitatively. the technique gives a quick and easy test as a

guide to the need for primary modifications. It has been suggested

that for quantitative measurements in dentures the brittle lacquer

technique is used in conjunction with electrical strain gauges.

ELECTRICAL STRAIN GAUGES

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Electric strain gauges have been used to give a quantitative

analysis of the stresses encountered. For any material a well-defined

relationship between stress and strain exists. If the strain in a certain

area of the denture is measured,the value of the stress can be

calculated provided the elastic modulus of the material is known. It

is this principle that is utilized in employing the electrical strain

gauges for the measurement of stress.(Fig-32)

An instrument measures the strain and using the relationship

described above,the stress is calculated. This instrument belongs to a

class of strain gauges which depend on the alteration of some

parameter associated with the flow of an electric current for the

measurement of strain.

When the load is applied strains in the surface of the

specimen under examination are transmitted to the wire filament via

a paper backing cemented onto the surface. This results in a change

of resistance of the wire filament which is then measured by some

associated electrical current.

Three factors have to be considered before the gauges are

fixed to the surface of the specimen:

(1) location of the gauge.

(2) size of the gauge to be used

(3) orientation of the gauge with regard to the specimen.

Strain gauges have been widely used in clinical research on

dentures. Studies have been carried out which examine both

mandibular and maxillary denture rigidity and denture deformation.

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Kegli and Kyddx were the first to apply the strain gauges to study

mandibular base deformation.

Regli and Gaskill” studied the deformation of plastic denture

bases using strain gauges and concluded that dentures with high

ridges exhibited torsion deformation during mastication and those

with flat ridges exhibited compression. They also found that the

ability of the denture base to resist deformation was an important

factor in adequate stress distribution to the supporting structures.

ADVANTAGES

The main advantage of using this technique is the relatively

small size of the gauges which causes minimal interference during

use. However the disadvantages encountered are far greater than the

advantages.

DISADVANTAGES

The gauges have to be sealed effectively from the oral

tissues. if used intraorally, to prevent short circuits and they must

be adhered firmly to the surface of the appliance.They must also be

placed in relevant parts of the denture as well as aligned in the

correct direction. Additionally with these gauges only the surface

strain at selected points is measured and although stresses may be

calculated from the strain measurements,this requires time.

Strain gauges have been used in conjunction with the brittle

lacquer coating to overcome some of the problems discussed above.

The initial analysis with the lacquer indicates the areas of high

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stress concentration thereby enabling location of the best areas to

cement the gauges.

The coating also gives the direction of the tensile stresses

and hence aids in the alignment of the gauges. Wain”’ employed a

combination of the two methods. The lacquer was coated onto the

denture and loads were applied. The gauges were then applied

where the cracks were seen and strains measured.

PHOTOELASTIC ANALYSIS

The photoelastic method is a well-recognized engineering

method of stress analysis and was first applied to dentistry in 1949

by Noonan. His study employed this method to evaluate amalgam

restorations and cavity design. Since its initial application. the

method has been used widely in the field of dentistry. The technique

involves construction of a model of the structure to be investigated

from a photoelastic material.(Fig-33)

The direction and magnitude of the applied forces and the

way it is supported and its shape must simulate the conditions of

the actual structure to obtain a true analysis of the stresses. The

temporary double refraction under stress of photoelastic materials is

utilized for photoelastic analysis. The incident ray of light is

resolved into two rays which travel at different velocities along the

principal plane of the material and emerge retarded with respect to

each other.

The amount of retardation is directly proportional to the

difference between the principal stresses and is measured using a

polariscope. The coloured fringes obtained are used for the stress

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determinations. The application of the method to dentistry was

reviewed by Mahler and Peyton. They concluded that the technique

was particularly applicable to dental problems because of the

irregular shapes encountered.

Initial studies utilized two dimensional models but with

improvements in technology the three-dimensional model is being

used. Despite this three dimensional photoelastic studies have been

limited due to the complexity involved in the determination of the

complete state of stress in three dimensional irregularly shaped

structures.

Although the photoelastic method is widely used in dentistry

there is little documentation of its use in dentures. The majority of

the studies relate to partial dentures and few have been reported in

complete dentures.

ADVANTAGES

The advantage of the photoelastic method over the earlier

methods discussed is that it provides a visual display for the

observation and measurement of stress distribution throughout the

model under investigation. However the method requires special

equipment and expertise to perform adequately. Since the

introduction of the computation era,stress analysis of dental

structures has been made easier with the use of the finite element

method.

DRAWBACKS:

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The specimen preparation for this method is arduous since it

is critical that the model is of uniform thickness. Tanner” has

examined the factors that affect the design of photoelastic models

for two-dimensional analysis. He concluded that the mode of support

was the most important factor affecting the relationship between the

experimental model and the original structure.

The elastic modulus of the material used for specimen

preparation may not conform to the actual material used for the

prosthesis. Additionally no absolute value for the magnitude of the

stress is obtained and only the maximum shear stresses can be

analysed. Separation methods have to be used to obtain other

components of the stress tensor which can be lengthy and

demanding.

REFLECTION PHOTOELASTICITY

This is a new method of detecting stresses in prosthetic appliances.

Through the observations of fringe patterns created upon loading,

reflection photo elasticity gives immediate identification of stress fields

in parts of the studied object accessible to normally incident light. This

method has been widely used in testing industrial prototypes but

experiments in dentistry are limited.

FINITE ELEMENT ANALYSIS

The finite element analysis is a computerized numerical

method used to determine the distribution of stresses and

displacements in a structure subjected to mechanical load. Initially

developed for use in the aircraft industry.The method has seen

widespread use not only in aerospace engineering but also in civil

engineering. Prior to the advent of the computer the technique

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required considerable mathematical ability. However. with the

availability of a number of software programs the method has

become more versatile.(Fig-34)

The basic concept of the method is the idealization of the actual

continuum as an assemblage of a finite number of discrete structural

elements,interconnected at a finite number of points called the nodal

points. The finite elements are formed by figuratively cutting the

original continuum into a number of appropriately shaped sections

and retaining in the elements the properties of the original material

(such as the elastic modulus and poisson’ s ratio). In structures

having a regular simple geometry relatively small numbers of

elements will be adequate, however in more complex shapes a

higher number of elements would be required to improve the

accuracy of the analysis.

The analysis process consists of satisfying compatibility within

each element and equilibrium conditions at the nodal points. By

concentrating the equivalent forces at the nodes, equilibrium

conditions are satisfied in an overall sense.

The information required to calculate the stresses is:

The total number of nodal points.

The total number of elements.

The type of boundary conditions.

Evaluation of the forces at the external nodes.

Coordinates of each nodal point.

The elastic modulus and poisson’ s ratio.

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Once these are specified, the displacements. as well as the stresses.

can be immediately calculated with the help of the program. The

validity of the finite element results depends on the precision by

which the geometry, material properties and interface conditions,

support and loading are in accordance with the physical reality.

The finite element method, due to its simplicity and relative

ease of use is becoming more popular for the stress analysis of

dental structures. Additionally its other advantages are that the oral

conditions can be simulated reasonably easily and different

parameters can be altered relatively simply. Although initially used

in two dimensions the popularity and improved accuracy of the

three dimensional model is becoming more apparent. The three-

dimensional model has been used in the stress analysis of the

mandible and other structures.

This method has proved to be valuable in stress

analysis.However the limitations of the method lie in the validity

and accuracy of the model. The latter problem can be overcome by

the use of convergency tests where subsequent mesh refinements of

the model make the results concerge. The validity of the analysis

should be established by either comparing results with clinical

observations or laboratory tests.

The limitations of a two- dimensional design must be

appreciated where the analysis involves these models. Additionally in

the finite element analysis it is assumed that the interfaces between

different materials are in perfect adhesion, with the elements

comprising different materials being joined at common nodes.

Another drawback is that the computer package for the analysis can

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be quite costly. Despite these limitations the method seems to be

promising and appears to play a valuable role in the stress analysis

of all dental structures.

In the finite element method all the stress components can be

calculated and these can be calculated at each point in the model.

Changes of relevant parameters and loads can be easily incorporated

into the calculation and hence conducting the analysis and

assimilating the results is quicker than the photoelastic method.In as

much the accuracy of the calculation results can be easily increased

by increasing the number of elements and the three-dimensional

analysis is likewise easily within its range of possibilities.

HOLOGRAPHIC INTERFEROMETRY

While holography is often used to obtain recreations of 3-

dimensional objects, many industrial applications of holography make

use of its ability to record two slightly different scenes and display the

minute differences between them. This powerful technique, called

interferometry,is an invaluble aid in design, testing, quality control, and

stress analysis.(Fig-35)

Holographic techniques are non destructive, realtime,and

definitive in allowing the identification of vibrational modes,

displacements, and motion geometries.If the object under study is

changed or disturbed in some way during the hologram exposure or from

one exposure to the next,then a pattern of “fringes” will appear on the

image itself, making the object look striped.

These fringes really represent maps of the surface displacement

caused by the force or stress that disturbed the object.Such a displacement

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map represents an extremely sensitive picture of the actual motion the

object has experienced, with a single fringe contour representing lines of

equal displacement.

Holograms can record motions and displacements, deformations

and bends, and expansions and contractions on virtually any object. The

typical optical laser used in holographic interferometry gives an accuracy

better than a half wavelength (about 10 millionths of an inch), and both

qualitative and quantitative information can be derived from the fringe

patterns.

This allows us to look at the effects of vibration, temperature,

stress and strain,and other physical forces in an entirely nondestructive

way. A powerful feature of holographic interferometry is that information

is obtained over the entire illuminated surface of the object being

studiedas a full and continuous field, which is important in understanding

what is happening to the object as a whole.

Holographic interferometry is used in vibration and modal analysis,

structural analysis, composite-materials and adhesive testing, stress and

strain evaluation, and flow, volume/shape, and thermal analysis.All these

applications derive from one or more of the three basic methods of

applied Holographic interferometry

Real-time,

Multiexposure, and

Time average holography.

Interferometric nondestructive testing can be accomplished with either

continuous or pulsed lasers of almost all wavelengths.Continuous lasers

are ideal for real-time studies of displacement and motion. Pulsed lasers

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can be synchronized with motion and also can record holograms of

extremely fast transient phenomena.

REAL-TIME HOLOGRAPHY

Real-time holography allows one to observe instantaneously the

effects of minute changes in displacement on, or in,an object as some

stress affects it. This is done by superimposing a hologram of an object

over the object itself while it is being subjected to some small force or

stress.

MULTIEXPOSURE HOLOGRAPHY

Multiexposure holography creates a hologram by using two or

sometimes more exposures. The first exposure shows an object in an

undisturbed state. Subsequent exposures, recorded on the same image, are

made while the object is subjected to some stress. The resulting image

depicts the difference between the two states.

TIME AVERAGE INTERFEROMETRY

The third technique,time average holography, involves creating a

hologram while the object is subjected to some periodic forcing function.

This yields a dramatic visual image of the vibration pattern.All these

techniques reveal the shape,direction, and magnitude of the stress induced

displacements in the structure under study. An important key to

holographic interferometry’s success is that it allows the use of very low

level, non destructive stress to gather data that once required destruction

of the material.

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STRESS CONTROL BY DESIGN CONSIDERATIONS

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It is often argued that the theoretical aspects of partial denture

design are of primary importance. In reality,clinical observation and

experience must be used to balance what should happen with what will

happen. It has been stated that "No removable partial denture can be

designed or constructed that will not be destructive in the mouth." This

statement can be fully justified if all forces and movements are

considered.

There is no mechanism to counter all forces that may be applied to

a removable partial denture. Nevertheless, a design philosophy that

strives to control these forces within the physiologic tolerances of the

teeth and supporting structures can be successful. Therefore,the design

philosophy of this book is a combination of theoretical and clinical

knowledge that a practitioner can learn and then use to achieve

predictable results.

Past arguments about partial denture design philosophies have

resulted in noticeable confusion. As a result, many practitioners have

abandoned their design responsibilities. The stresses induced by a

removable cast partial denture can be managed by keeping design

considerations for the various components of the partial denture in

mind. They are as follows

I. Direct Retention

The retentive clasp arm is the element of a removable partial

denture that is responsible for transmitting most of the destructive forces

to the abutments. Consequently, a removable partial denture should be

designed to keep clasp retention at a minimum, and yet provide adequate

retention to prevent dislodgment of the denture by unseating forces.

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Other components of a removable partial denture may contribute to

the retention of the prosthesis,thereby allowing a reduction in the amount

of retention provided by clasps. Exploiting this retentive potential in

widely separated areas of the mouth can re-sult in reduced loads on the

abutment teeth. As a result, the support and stability of the prosthesis also

may be improved.

POTENTIAL SOURCES OF ADDITIONAL RETENTION

a) Forces of adhesion and cohesion

For prosthetic purposes, adhesion may be defined as the

attraction of saliva to the denture base and soft tissues, and

cohesion may be defined as the attraction of saliva molecules for

one another. Although it is impossible to develop a peripheral seal

around the borders of a removable partial denture, adhesion and

cohesion can still contribute to retention. To maximize this effect,

each denture base must cover the maxi-mum area of available

support, and it must be accurately adapted to the underlying

mucosa.

b) Frictional control

The partial denture should be designed so that “Guide planes”

are created on as many teeth as possible. Guide planes are areas can

the teeth that are created so that they are parallel to each other to

the path of insertion and withdrawal from the mouth. These planes

may be created on the enamel surfaces of the teeth or in restorations

placed on the teeth. The frictional contract of the prosthesis against

these parallel surfaces can contribute significantly to the retention

of the denture.

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d) Neuromuscular control

The innate ability of the patient to control the actions of the

lips, cheeks, and tongue can be a major factor in the retention of

a removable prosthesis. A patient who lacks the ability or

coordination to control the movement of these structures may

not be able to retain a prosthesis. The design and contour of the

denture base can greatly affect the patient's ability to retain a

removable partial denture.

Any overextension of the denture base can contribute to

displacement of the prosthesis. As a result, clasping mechanisms

will no longer be passive and will apply undesirable forces to the

abutments. These forces may produce noticeable tooth

movement and /ordiscomfort. Properly contoured denture bases

prevent such difficulties and can enhance retention and stability

of a removable partial denture.

d) Clasp position

The position or the relation of the retentive clasp to the height

of contour is more important in retention and in controlling

stresses.

The number of clasps used in the design will determine the

type of stress developed within a denture. Removable partial

dentures with four clasps are described to have a “Quadrilateral

configuration”. Similarly RPD with three and two clasps are

described to have “tripod” and “bilateral” configuration

respectively.

Quadrilateral configuration

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The quadrilateral configuration is indicated for Class-III arches,

particularly when there is a modification space on the opposite side of the

arch. A retentive clasp assembly should be positioned anterior and

posterior to each edentulous space. This creates a stable mechanical

situation in which leverage is effectively neutralized.(Fig-38)

For a Class III arch in which no modification space exists, the goal

should be to place two clasp assemblies adjacent to the edentulous space,

and two clasp assemblies on the opposite side of the arch. The clasp

assemblies on the intact side of the arch should be separated for

additional mechanical stability. Consequently, one clasp assembly should

be placed as far posteriorly as possible, and the other should be

positioned as far anteriorly as space and esthetics will permit. This

maintains the quadrilateral concept and represents an effective method of

controlling loading.

Tripod configuration

This design is used primarily for class II edentulous arches. If there

is a modification space on the dentulous side, the teeth anterior and

posterior to the space are clasped to bring about the Tripod configuration.

If the modification space is not present, one clasp on the dentulous side of

the arch should be positioned as far posterior as possible, and other as far

as anterior as factors. Such as interocclusal space, retentive undercut and

esthetic considerations will permit.(Fig-37)

The design is not effective as quadrilateral configuration but is

most effective in neutralizing leverage in class II situation.

Bilateral configuration

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In case of bilateral distal extension group, or class I ideally

the single retentive clasp on each side of the arch should be located

near the center of the dental arch or denture bearing area. In

bilateral configuration the clasp exerts little neutralizing effect on

the leverage induced stresses generated by the denture base.(Fig-

36)

e) Clasp design

1) Circumferential cast clasp

The conventional circumferential cast clasp originating from a

distal occlusal rest on the terminal abutment tooth and engaging a

mesiobuccal retentive undercut should not be used on distal extension

RPD. The terminal of this clasp reacts to movement of the denture base

towards the tissue by distal tipping, or torquing forces on the abutment

tooth.(Fig-39)

A cast circumferential clasp that approaches a distobuccal

undercut from the mesial surface of the terminal abutment tooth is

acceptable. As an occlusal load is applied to the denture base, the

retentive terminal is moves further gingivally into undercut area

and looses contact with the abutment tooth. In this manner the

torque is not transmitted to the abutment tooth.

2) Vertical projection clasp or bar clasp

This clasp is used on the terminal abutment tooth on a distal

extension partial denture when the retentive undercut is located on

the distobuccal surface. It is never indicated when the tooth has a

mesiobuccal undercut. The bar clasp functions in similar manner to

reverse circumferential clasp. As the denture base is located

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towards the tissue, the retentive tip of the bar clasp rotates

gingivally to release the stress being transmitted to the abutment

tooth.(Fig-40)

3)Combination clasp

When a mesiobuccal undercut exists on a abutment tooth

adjacent to a distal extension edentulous ridge, the combination

clasp can be used to reduce the stress transmitted to the abutment

tooth. Wrought wire clasp by virtue of its internal structure is more

flexible than a cast clasp. It can flex in any spatial plane, where as a

cast clasp flexor in horizontal plane only. The wrought wire

retentive arm has a stress breaking action that can absorb torsional

stress in both the vertical and horizontal planes. A cast

circumferential clasp under some situation would transmit most of

the leverage induced stress to the abutment teeth.(Fig-41)

f) Splinting of the abutment teeth

Weak abutment teeth should be splinted with the adjacent

teeth for strength and stability. Splinting helps to share the stresses

produced in a weak abutment tooth. It will stabilize the weak teeth

in mesiodistal direction. Usually splinting is done by fabricating

full veneer crowns over the teeth to be splinted or by clasping more

than one tooth on each side of arch with numerous rests for

additional support and stabilization.(Fig-42)

Guide planes helps to increase the horizontal stability of the

denture. Hence, additional clasps can be used to increase the guide

planes and also increase the cross arch stabilization.

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Indications

Abutments with tapered or short roots.

Terminal abutments located on the edentulous side of a distal

extension denture base.

Fixed splinting is given if there is some loss of periodontal

attachment, after a periodontal disease or therapy.

II. Indirect Retention

An indirect retainer is a part of removable partial denture that

helps direct retainer to prevent displacement of the distal extension

denture by resisting the rotational movement of the denture around

the fulcrum line established by the occlusal rests. The indirect

retainer is located on the opposite side of the fulcrum line from the

denture base.(Fig-43)

Indirect retention is based on lever principle. It is produced by

moving the axis of rotation of the denture away from the point of

application of force.

In class I situation, indirect retainers are necessary and they

should be positioned as far anteriorly to the fulcrum line as

possible.

In class II situation, the fulcrum line runs through the most

posterior abutment on the dentulous side and the terminal

abutment on the distal extension side. Adding another rest

perpendicular to this fulcrum line provides indirect retention.

In class III situation, indirect retention is usually not

required. In some case there is a buccolingual placing of the

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denture, which is prevented by placing rest on the dentulous

side, perpendicular to longitudinal axis of rotation of denture.

III. Denture Base

- The denture base should be designed to cover the maximum

amount of soft tissue available.

- The denture base should have long flanges, within the

physiological limits of the soft tissues in order to stabilize the

denture against horizontal movement.

- Distal extension denture base must always extend onto the

retromolar pad area in mandibular denture and cover the entire

tuberosity in the maxilla.

- The denture base will displace the soft tissues on the ridge

during functional occlusal load. A functional impression is

recorded to fabricate the denture in order to improve its

adaptation and avoid excessive tissue displacement.

IV. Major Connector

In the mandibular arch the lingual plate major connector that

is properly supported by rests can aid in the distribution of stresses

to the remaining teeth. It is particularly effective in supporting

periodontally weakened anterior teeth. It also contributes to the

effectiveness of cross arch stabilization.

In the maxillary arch the use of a broad palatal major

connector that contacts several of the remaining natural teeth

through lingual plating can distribute stresses over a large area.

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The major functions o the major connector includes rigidity,

retention and stability.

V. Minor Connector

The major connector joins the major connector to the clasp

assembly and the guiding planes located on the abutment tooth

surface. The minor connectors used for auxillary rests aid in

indirect retention.

- These provide horizontal stability to the partial

denture against lateral forces on the prosthesis.

- The abutment tooth receives stabilization

against lateral forces by the contact of the minor connector.

VI. Rests

Properly designed rests help in control the stresses, by

directing the forces, acting on the denture to the long axis of the

abutment tooth. The floor of the rest seat should be less than 90° to

a tangent line drawn parallel to the long axis of the tooth(Fig-46).

Adding rests on the additional teeth decreases the amount of

occlusal load on each tooth and helps to distribute the occlusal load

equally to all the abutment teeth.

STRESS BREAKERS

A stress breaker is defined as "A device which relieves the

abutment teeth of all or part of occlusal forces" GPT-6.

All vertical and horizontal forces, applied to the artificial

tooth are distributed throughout the supporting portions of the

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dental arch. Broad distribution of force is accomplished through the

rigidity of the major and minor connectors.

In distal extensions situations, the use of rigid connection

between the denture base and supporting teeth must account for the

base movement without, stress on the abutment teeth and residual

ridge is minimized through the use of functional basing, broad

coverage, harmonious occlusion and correct choice of direct

retainers.

The concept of stress-breaking exists that insists on

seperating the action of the retaining elements from the movement

of the denture base by allowing independent movement of the

denture base for its supporting framework and direct retainers

Aims:

1) To direct occlusal forces in the long axis, of the abutment

teeth.

2) To prevent harmful forces being applied to the remaining

natural teeth.

3) To share the forces as evenly as possible between the natural

teeth and distal extension area according to the ability of these

different tissue to accept the forces.

140


4) To ensure that the part of the load applied to the distal

extension area is distributed as evenly as possible over the

whole mucosal surface.

Dentures with a stress breaker are also called as a "broken

stress partial dentures".

In a tooth tissue supported partial denture, when an occlusal

load is applied, the denture tends to rock due to the difference in

the compressibility of the abutment and soft tissues. As the tissues

are more compressible, the amount of stress acting on the

abutments in increased, which can produce harmful effects on the

abutment teeth.

To protect the abutment from such conditions, stress breakers are

incorporated into the dentures. A stress breaker is a hinge like joint

placed with in the denture framework, which allows the two parts of the

framework on either side of the joint to move freely.

I. Movable joint between the direct retainer and denture

base. This group includes hinges, sleeves, and cylinders and

ball and socket devices. Being placed between the direct

retainer and denture base, they may permit both vertical

movement and hinge action of the distal extension base. This

prevents direct transmission of tipping forces to the abutment

teeth as the base moves tissue-wards under function. E.g.

Dalbo attachment, Crismani attachment, ASC 52 attachment.

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II. Flexible connection between the direct retainer and

denture base includes the use of wrought wire connector and

split major connector(Fig-47).

Advantages:

- Vertical forces acting on the abutment teeth are

minimized and alveolar support of abutment teeth is preserved.

- Intermittent pressure of denture bases massage the

mucosa thus providing physiologic stimulation, which prevents

the bone resorption and eliminates need for relining.

- Minimal requirement of direct retention.

- Weak abutment is well splinted even during the

movement of the denture base.

Disadvantages:

- Design is complicated and expensive.

- The assembly is very weak and tends to fracture very

easily.

- Difficult to repair.

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STRESS BREAKERS OR STRESS EQUALISERS

Stress Breaker

A stress breaker is a device which relieves the abutment teeth of all

part of the occlusal forces(GPT-2005).

Stress Director

A stress director is a device that allows movement between the

direct retainer which may be intracorornal or extra coronal.(GPT-2005)

Introduction:

The resiliency of the tooth secured by the periodontal ligament in

an apical direction is not comparable to the greater resiliency and

displaceability of the mucosa covering the edentulous ridge.Due to this

forces are transmitted to the abutment teeth as the denture bases are

displaced in function.

It is agreed that a rigid connection between the denture and the

direct retainer on the abutment tooth is damaging and that some types of

stress director or stress equalizer(a flexible or movable joint between

teeth and metal frame work so that the clasp) is essential to protect the

vulnerable abutment teeth.It allows independent movement of the denture

base and the direct retainers separates the action of the retaining elements

from the movement of the denture.

The need for stress breakers on free end RPDs has been recognized

on the basis that the resiliency or displaceability of the mucosal tissue

ranges between 0.4 mm to 2mm, while the vertical resiliency of a normal

healthy tooth in its socket is approx. 0.1mm. This tissue resiliency

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differential of 20 to 40 times the axial displaceability of a normal tooth in

its socket dictates the necessity for some form of stress direction in the

partial denture design.

TYPES OF DESIGNS:

RESILIENT JOINTS AND HINGE JOINTS

Joint attachments are used as retainers for unilateral and bilateral

distal extension partial dentures.They allow various degrees of movement

between the body of the prosthesis and the abutment teeth. The

movement

may be:

Rotation around a transverse axis

Vertical bodily movement

Based on the type of movement, joints are classified either as:

Resilient hinge joints that allow both vertical bodily movement and

rotation around a transverse axis

Pure hinge joints that only allow rotation around a transverse axis

RESILIENT HINGE JOINTS

Joints can be connected directly to an abutment crown through

either the female or the male part. In these cases the two parts are

separated in the mouth as the denture is removed.

There are, however, joints in which the entire joint construction

can be separated from the abutment. The male and female elements,

which comprise the resilient part,are connected to the abutment tooth by

means of a sliding attachment (sliding attachment resilient joint). Joints in

which one part can be removed from the other at the abutment crown are

called separable joints. Those that cannot be separated in the mouth are

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designated as linked joints and are connected to the abutment teeth by

means of additional retainers, such as cast clasps or double crowns.

Each individual form of joint is offered by the manufacturers in

many variations, allowing a wide range of applications. Thus, there are

special types for extracoronal installation, and for both unilateral and

bilateral use.

When using a jointed connection between the body of the

prosthesis and the abutment teeth, the compressibility and the resilience

of the mucosa must be taken into account The stress-breaker effect

prevents the transmission of excessive forces to the abutment teeth during

chewing. The springs built into the joints cushion the loading forces and

return the denture bases to their rest positions.

Dalbo attachment

This attachment is one of the oldest and most successful

extracoronal attachments and is classified as an adjustable, directed-hinge

distal extension attachment.This system features lateral stability, vertical

resiliency, and hinge movement.The advantages of the Dalbo system are

the intrinsic direct retainer and excellent stability owing to the vertical

beam. The attachment may be used in unilateral or bilateral applications.

The unilateral configuration provides a larger vertical bar for enhanced

lateral stability. The attachment is offered in two sizes, although the mini

version lacks vertical resiliency.(Fig-48)

The resilience hinge joint by Dalla Bona is available in separable

(Dalbo extracoronal attachment) and linked(Dalbo-Fix) forms.With the

separable variety,the ball-shaped male section is attached to the abutment

crown,either by soldering or by being luted to the wax pattern

145


beforecasting. The removable female housing with its enclosed coilspring

is embedded in the denture base.

The vertical resiliency is rendered through the presence of a spring

and found only in the standard unilateral and bilateral designs.The

difference between the standard and the mini is approximately 2 mm in

clinical crown height requirement, 1.7 to 2.0 mm in preparation depth,

and 1 mm in faciolingual width requirement. As in all extracoronal

attachments, the amount of space required in the denture base is

approximately 5.5 to 6.0 mm.

This often creates difficulty with tooth placement and inadequate

strength for the resin. The minimum amount of resin recommended

should be strictly adhered to so as not to compromise the strength of the

denture base in the region of the attachment. This extracoronal retainer

offers a mechanism to "lock" the attachment for reline procedures.

ASC-52 ATTACHMENTS

The functional properties of the ASC 52 resilient joint attachment

stressbreaker is based upon the original adaptation of the CARDAN

JOINT principle. The ball screw spring joint ASC-52 from Degussa is a

separable attachment, that is,it can be disconnected in the mouth. The

female part is attached to the abutment crown, and the male unit, to the

removableprosthesis.The male unit is made up of a ball tipped sliding

bolt, enclosure, spring,and screw(Fig-49).

It is most useful due to the following reasons:

The removable part of the prosthesis can accomplish a wide variety

of movements according to the specific case

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This removable part is well anchored to the abutment teeth, but it

does not overload them.

In this way a perfect retention of the partial prosthesis in the mouth

is assured,the prosthesis can be easily inserted and removed

without any risk for the abutment teeth.

The action of the inner part can be regulated according to the

specific needs.

An increase or decrease in prosthesis moveability is achieved by

adjusting the spring tension(screw or unscrew the small nut),

It is possible to replace any detail of the inner part at any time

(wear, damages, accident),dental technicians will find ther joint

attachment easy to handle.

DSE HINGE

The DSE Hinge is intended for use on bilateral clasp retained free

end removable partial dentures to reduce loading or torquing of

abutments. The small size is easy to work with and eliminates multiple

inventory requirements.The unique design provides for easy freeing after

casting and provides total lateral stability.For patients, it allows patient

comfort and abutment protection by allowing independent unilateral

function eliminating torquing leverage on the abutments on the

nonfunctioning side. The miniaturized size allows utilization in short

vertical spaces and provides for good esthetics(Fig-50).

The FR system

The FR system for removable partial dentures is a technological

revolution for what many call semi-precision or "mill-ins." A

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tapered,friction retained intracoronal precision attachment for removable

partial dentures with a lingual arm and for segmented bridgework or non-

parallel abutments in fixed bridgework.The female is actually cast against

the prefabricated male for total accuracy, improved proximal wall

contacts, and tremendous time and labor savings(Fig-51).

This simple and inexpensive attachment uses a single investment

technique, eliminates porcelain in the female due to the silicone male,

allows for easy and accurate duplication, no soldering is needed, easy

separation of male and female, and the miniature size allows for use in

‘close bite’situations. Easy insertion because of the tapered male,

improved esthetics (no metal on the occlusal),excellent retention and

reduced wear account for this being one of the most popular attachments

in dentistry.

The UNOR

The UNOR is a screw adjustable retention precision attachment for

intracoronal use. The beveled male is adjustable so retention may be

eitherincreased or decreased, allowingfor easy patient insertion and

removal, thus less wear. Vertical height may also be altered for short or

‘close bite’ situations. The female may be directly cast with precious or

semi-precious alloys for easy fabrication.(Fig-52)

A female ceramic former is available for creating a female in non-

precious castings. The excellent external wall contact allows for guide

plane stability. The male may be connected to the cast frame by acrylic

resin, composite resin, or solder.The system also allows for conversion of

a fixed bridge to a removable partial denture if distal abutments are lost.

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CEKA ATTACHMENT

Definition:

An attachment which has a patrix conical portion with a split head

for activation and a matrix cap portion.

Indications

The Ceka attachment was developed as an extracoronal attachment.

However, it can also be used for both root face abutments and bars. In the

latter case it allows increased retention of the superstructure where a clip

may not be provided. If the bar is short the placement of a clip may not be

possible and therefore the use of such an adjustable attachment can

provide the solution.

Advantages

The attachment can be used for many different clinical situations . The

matrix ring retainer can be placed in a variety of locations and the patrix

component comes in different forms allowing it to be cast, soldered or

bonded into place. The patrix has a cross split allowing for activation of

this attachment with wear.

The Traditional Ceka and Ceka Revax systems provide hinge,

vertical, and rotational movements to provide maximum abutment

protection. Each attachment consists of three angulations of plastic

female profiles with precision metal insert,male spring pin, and retention

component. The three angulations allow the user to design the case for

the patient’s needs(Fig-53)

149


Disadvantages

The attachment requires adequate space and the correct angulation

relative to the path of insertion of the denture.

VERTIX

The Preci Vertix and Vertix "P" provides for hinge and vertical

movements. It should besupported by a broad based ridge. TheVertix is a

very inexpensive and popular system that provides patient comfort and

abutment protection for both mandibular and maxillary bilateral

removable partial dentures. Unilateral free end removable partial dentures

should be cross arch stabilized(Fig-54).

The Vertix features time-saving and simple routine techniques,

requires no additional tools, may be cast in any alloy to eliminate

soldering and dissimilar alloys, and provides outstanding space-saving

aesthetics. The plastic female absorbs negative movements to protect

abutments and provide patient comfort. It requires only a routine full

coverage abutment preparation and provides easy patient insertion and

removal. The only servicing requirement is the occasional, fast, easy

female replacement. Three different female retention clips are available to

accommodate all your retention needs.

O-SO Distal extension

The OSO is a popular extracoronal attachment that provides free

movement in all planes for maximum protection. A proven retentive

system utilizing an easily replaceable rubber O ring. This is a resilient

attachment with vertical and hinge stress-breaking action for free-end and

bounded partials. Maybe used in conjunction with other attachment(Fig-

55).

150


Universal Ic attachment

The IC attachment is a popular spring loaded retaining attachment

that provides free movement for abutment protection without requiring an

abutment crown. The IC attachment requires a 180 degree reciprocal

lingual arm. The attachment consists of a male anchor and female inlay.It

is made of a stainless, chrome-alloy like those used for casting partials. It

will not tarnish or corrode, and when properly installed,will not

malfunction even after years of wear.Other benefits include no pulpal

involvement,no gingival retraction before impressions, easy to adjust at

the chair ,and this is a reversible procedure.

Mays unilateral attachment

Designed specifically for the unilateral distal extension, the Mays

is the first attachment with a lingual locking arm. It can not be

dislodged,but yet is easily removed for patient hygiene. Does not require

a lingual or palatal arm. No cast chrome framework required or soldering;

the male portion casts with the crowns. No parallelism is necessary ,even

on bilateral cases.(Fig-56)

HINGE JOINTS

Hinge joints exhibit only one type of freedom of movement,

namely, rotation around a transverse axis. Since there is no vertical bodily

movement, they have a somewhat more favorable topographic and

dynamic relationship to the distal abutment teeth than do joints with both

rotation and bodily movement. There is no direct mechanical irritation of

the gingival margin.Hinges that cannot be separated in the mouth (linked

hinges) can only be used on removable prostheses (partial dentures and

151


telescopic bridges). A bilateral application is also conceivable provided

the two denture bases are not rigidly connected to each other.

Hinges that cannot be separated in the mouth (linked hinges) can

only be used on removable prostheses (partial dentures and telescopic

bridges). A bilateral application is also conceivable provided the two

denture bases are not rigidly connected to each other.

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SPECIAL CLASP DESIGN TO CONTROL STRESS

RPI system

In 1963, Kratochvil introduced the “I-bar design philosophy”.This

philosophy was based upon use of an I-bar retentive element, a mesial

rest, and a distal proximal plate. Proponents of the I-bar philosophy

claimed that the resultant clasp design minimized torquing forces and

directed occlusal loads parallel to the long axes of abutments.The I-bar

rationale, especially the use of a mesial rest, emerged as a popular by-

product of Kratochvil's design principles(Fig-57).

The RPI clasp is a current concept for bar clasp design, as the full

“T” bar should not be used since it covers an unnecessary amount of

tooth structures compared with the RPI clasp.

Components of the I-bar System

Kratochvil's I-bar system includes a mesial rest, I-bar retainer, and

a long distal guiding plane that extends to the tooth-tissue junction. Each

component must function properly to ensure success of the I-bar system.

The RPI clasp fulfils the requirements of proper clasp design The

practitioner must understand that the I-bar retentive clasp is only one

element in the design equation.For this clasping system to function

effectively, all components must be properly designed, constructed, and

fitted.

The rest, located on the mesial occlusal surface of the abutment

tooth, acts as the point of rotation and exerts a mesial force on the tooth

rather than a distal displacing force. Pressure exerted on the extension

base moves the proximal plate tissueward without torquing the tooth. The

153


I bar also moves mesiogingivally away from the tooth under masticatory

load.

Rests

For distal extension base partial dentures where a bicuspid serves

as the abutment tooth, a mesial rest preparation is made.

For posterior teeth, where restorations are not placed,the rest seal

can be prepared in the appropriate triangular fossa.

Sufficient bulk of metal must be provided to permit the rest to

function without fracturing or bending.

Gold requires larger and deeper preparations than the non-precious

metals (chrome cobalt, nickel cobalt, etc.).

This preparation should be rounded and fully polished to permit

some rotation when depression of the extension base occurs.

If a cuspid is to serve as the abutment, a mesio lingual rest

preparation is made.

The rest seat must be deep enough to prevent the mesial rest from

slipping gingivally.

As a general rule, mandibular cuspids have a thin enamel covering

and when preparing an adequate rest seat, penetration into the

dentin is often inevitable.

If dentin is exposed, the preparation should be deepened and

modified to accept a gold foil, amalgam, or other restoration which

can be properly contoured.

154


Guide Planes

A guide plane is prepared on the distal surface of the abutment

tooth at the occlusal one third as proposed by Potter.

It should extend lingually just far enough so that the proximal

plate together with the mesial minor connector will prevent lingual

migration of the tooth.

The guide planes should be approximately 2 to 3 mm. in height

occlusogingivally.

This guide plane will often permit the proximal plate and the

mesial minor connector to contact the tooth simultaneously and

provide proper reciprocation against the force exerted by the

retentive buccal clasp arm during the seating and removal of the

denture.

If the mesial minor connector and proximal plate cannot contact

simultaneously, as may occur with cuspid abutments, then the

retentive I bar should engage the mesiobuccal undercut and receive

its reciprocation from the proximal plate alone.

Proximal Plate

It is placed on a distal guiding plane, extending from the marginal

ridge to the junction of the middle and gingival third of the

abutment tooth.

155


The proximal plate minor connector should contact approximately

1 mm of the gingival portion of the guiding plane in distal

extension cases.

The bucco-lingual width of the proximal plate is determined by the

proximal contour of the tooth.

The proximal plate extends lingually just far enough so that the

distance between the minor connector and proximal plate is less

than the mesiodistal width of the tooth.

It should be 1mm thick and join the framework at right angle.

The proximal plate together with the mesio-lingually placed minor

connector provides stabilization and reciprocation of the assembly.

I Bar

The approach arm of the I bar extends from the framework so as to

remain at least 3 mm from the gingival margin and then crosses

the gingival margin at right angles.

Approximately 2 mm of the I bar contacts the tooth surface,

usually at the gingival one third of the tooth.

The bottom ponion of the I bar contacting the tooth surface should

engage 0.01 inch undercut.

The I bar should taper slightly from the base to the tip. It is usually

placed at the greatest mesiodistal prominence on the buccal surface

or towards the mesial, but not toward the distal.

Slight relief is necessary when the arms crosses the gingival

margin.

156


Advantages

Vertical masticatory force on the distal extension base causes the I-

bar to move mesiogingivally away from the tooth and the proximal

plate to move further into the undercut of the tooth.

Thus, both the I bar and the proximal plate disengage the abutment

and thereby reduce torquing of the tooth.

The mesial minor connector together with the proximal plate

provide the necessary reciprocation and eliminate the need for a

lingual arm.

The mesial rest eliminates the potential "pump handle" effect

that a force. on the base often induces with a distal rest.

The RPI clasp contacts the tooth minimally and is advantageously

used on caries prone patients.

The I bar itself makes very little contact with the tooth, it is usually

more esthetic than most other clasp arms.

Indications

The RPI clasp is indicated

In distal extension cases, as it provides a stress releasing action.

When tissue undercuts are not severe.

Contraindications

157


The RPI clasp is contraindicated with

Shallow vestibule (the base of the I-bar should be at least 3mm from the gingival margin).

High floor of the mouth which necessitates the use of lingual plate.

When buccal undercut is absent or only distobuccal undercut exists.

In cases with severe tissue undercut to avoid food or tissue trap.

If the facial surfaces of teeth are facial to the tissue surface, the RPA Clasp may be used.

158


CONCLUSION

Many individuals have contributed to the progressive

advancement of partial denture service and have written extensively

to document their experiences and philosophies in this field. Their

objective are universal, to provide means of restoring function,

esthetics and comfort, which promotes the oral health.Partially

edentulous arches exist in a great variety of forms.

A thorough knowledge of the mechanical principles involved is

very important and it should be understood properly because it is an

integral factor in design of removable partial denture.Designing of the

appliance play an important role because it is through the structure

of the denture that the forces of mastication are transmitted from

the occlusal surfaces of artificial teeth, natural teeth and underlying

tissues.

Generally it is very important that, the design which provide

broad bases, rigid connectors, multiple rests and properly selected

retainers are most likely to effect favourable distribution of force

and maintain the integrity of remaining tissues. Hence while

designing removable partial denture biological as well as physical factors

should be considered.The biological factors includes the denture support,

avoiding the deleterious effect on the abutment teeth, proper stress

159


distribution and the physical factors includes the strength of the denture

base used, whether it accommodates the future relining etc.

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