Biomechanics of Implant

8/12/2019 Biomechanics of Implant

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JOMI on CD-ROM, 1993 Jan (19-31 ): The Biomechanics of Force Distribution in Implan Copyrights 1997 Quinte

The Biomechanics of Force Distribution inImplant-Supported Prostheses

Lawrence A. Weinberg, DDS, MS

Force distribution with natural teeth depends on micromovement induced bythe periodontal ligament. The location and cusp inclination of the toothqualitatively alter the force pattern. Osseointegrated implants do not havemicromovement associated with force distribution. Force distribution to theosseointegrated implant interface is completely different than with naturalteeth. Alterations in tooth location and cusp inclination are suggested to limitimplant overload. Force distribution in splinted natural teeth andosseointegrated prostheses are compared. The mechanism of interface force

distribution and the consequences of poor interface fit are interrelated. Thedifferential mobility of splinted natural teeth affects diagnosis and treatment.However, combining natural teeth with an osseointegrated prosthesis requiresnew design principles. (INT J ORAL MAXILLOFAC IMPLANTS1993;8:19-31.)

Key words: differential mobility, force distribution, impact area, interface forcedistribution, micromovement, micron movement, modulus of elasticity

The biomechanics of force distribution in implant-supported prostheses isqualitatively different than when natural teeth serve as abutments. The essentialdifference is caused by the periodontal ligament, which permits micromovements,compared to the osseointegrated implant, which has none. This article describes theprinciples of force distribution as applied to diagnosis and treatment ofimplant-supported prostheses.

Principles of Force Distribution

The character of force distribution between members of a system depends on therelative stiffness/deflection of each member1-3(Weinberg R, personalcommunication). However, there is a paradox concerning the role of rigidity and

deformation (flexibility) when comparing tooth-supported andmultiple-implant-supported prostheses. There are structural differences between thetwo entities and the supporting medium (ie, periodontal ligament versusosseointegration), which are diametrically opposed physiologically. The former hasthe maximum flexibility of any portion of the system whereas the latter, bydefinition, has none.

The prostheses of both systems are considered stiff. A fixed prosthesis is usuallypermanently cemented to the natural teeth, forming one stiff structural unit.


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However, the vertical elements of each system have opposing characteristics. Theimplant-abutment-prosthesis interfaces introduce minute degrees of flexibility as theresult of retaining screw deformation. These factors have a profound effect on theconcepts of force distribution when systems are compared and introduce the risk ofclinical failure when teeth and implants are combined in support of prosthesis

without an understanding of these fundamental differences.Character of Force Distribution. A scientific analysis of force distribution is

statistically indeterminate1because of variable factors that prevent quantifyingmeasurements. For instance, cortical and medullary bone have different elasticities.1The attaching screws have much more deflection (flexibility) than the prosthesisframework. The relative intimacy of interface fit of the prosthesis to the abutmentswill alter force distribution.1Cantilever force application and the geometric locationof the fixtures further alter force distribution patterns.

Force Distribution Analysis. Finite element analysis of fixture design (ie,

computer mathematical models) has shed light on the distribution of force withvarious implant configurations.4However, Brunski3has pointed out the enormity ofthe problem when considering all of the variables involved in the evaluation of the invivo total prosthesis-implant-bone system. In the absence of quantified forceanalysis, clinically pertinent estimates of force distribution in natural teeth andfixture-supported systems may be made1with simplified models2,5and/or simplifiedassumptions.6,7Simplified approximation of force distribution is an essential firststep in diagnosis and treatment planning; the following discussion is made within theparameter of these limitations.

Definitions

Macromovement. Movement of a tooth or prosthesis component more than 0.5 mmand easily observable.

Micromovement. Movement of a tooth, prosthesis, or implant systemcomponent 0.1 to 0.5 mm and not readily observable but subject to measurement.

Micron-movement. A term (coined here) to describe angstrom level(microscopic) movement below 100 m (less than 0.1 mm) that is not observable orsubject to measurement in vivo by ordinary means.

Force Distribution With Natural Teeth

Because of micromovement permitted by the periodontal ligament, as well as theshape of the root itself, vertical occlusal force (O) produces a resultant line of force(F) that has its center of rotation (CR) located in the apical third area6,7(Fig 1a).

Impact Area. The impact area is the cuspal contact of opposing teeth (arrow,Fig 1a). The resultant line of force is always perpendicular to the impact area.6,7Therefore, a cusp-to-fossa contact produces a vertical force, while a cusp-to-inclinecontact produces lateral forces. For instance, when a vertical force is applied to the


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buccal cusp incline, the resultant line of force, perpendicular to that inclination, fallsat a great distance (D) from the center or rotation of the tooth6,7(Fig 1a).

Torque. Lateral force is expressed as torque, which is the force multiplied bythe perpendicular distance from the center or rotation (Fig 1a). As shown in Fig 1b,lateral force can be effectively diminished by reducing the cusp inclination of the

impact area so the resultant line of force passes closer to the center of rotation of thetooth. Compressive and tensile forces are exerted on the periodontal ligament as thetooth exhibits micromovement about the center of rotation (Figs 1a and 1b). Thelength of the root significantly enhances the distribution of force to the alveolarbone.

Force Distribution With Implants

Bone Modulus of Elasticity. The modulus of elasticity of bone permits a degree ofdeflection measured in microns. (Titanium fixtures are more rigid than the investing

bone.1) However, osseointegrated implants have no micromovement (such as thatpermitted by a periodontal ligament) sufficient enough to cause distribution of force

equal to that of natural teeth.

Torque. Because the lack of micromovement of implants, most of the forcedistribution is concentrated at the crest of the ridge.4Vertical force on cylindricalimplants would be concentrated at the apex, while threaded implants would producecrestal and apical force on the bone.4Lateral forces in both designs would result increstal force distribution.4(A screw-type fixture is used in all illustrations forsimplicity; however, the discussion applies to cylindrical design as well.)

As shown in Fig 1c, vertical force (O) on a cuspal incline would produce aresultant line of force (F) perpendicular to the impact area. The perpendiculardistance (D) from the crest of the ridge, multiplied by the resultant force (F) is thetorque value, which is concentrated at the crest of the ridge4rather than distributedalong the surfaces of the implant as it is in natural teeth (Fig 1a). This concept isconsistent with bone loss found in implants, which is almost always initiated at thecrest of the ridge.

Reduction of Torque. The cusp inclines can be reduced, which will flatten theimpact area, thus producing a more vertical resultant line of force (Fig 1d). The

perpendicular distance (d) from the crest of the ridge to the resultant line of force isreduced, thus effectively reducing the torque (lateral force) on the crestal bone. Atrue cusp-to-fossa relationship should be created in centric occlusion, with nocontact in working- or balancing-side relationships where possible.

Alteration of Anterior Impact Area

The maxillary single tooth restoration is vulnerable to loosening of the retainingscrew (regardless of design, precise interface fit is essential). As shown in Fig 2a, theocclusal impact area usually produces an inclined resultant line of force (F) because


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of the vertical overlap of the incisors. High levels of torque are produced on theretaining screw (FxD). Optimal implant orientation, as well as a therapeuticalteration in occlusal impact area (discussed later), effectively reduces torque.

The resultant line of force can be redirected more vertically by altering theocclusal impact area to provide a horizontal stop (Fig 2b). The resultant line of force

is more in line with the implant orientation and its supporting bone, thus effectivelyreducing torque on the retaining screw and alveolar bone. In general, the locationand impact area (inclination) should be given serious consideration in the restorativephase of implant-supported prostheses. The favorable force distribution associatedwith splinted natural teeth6,7(resulting from the micromobility of natural teeth) doesnot apply to multiple-implant-supported prostheses. The micron-mobility ofosseointegration (less than 100 m) tends to concentrate force distribution in thearea of force application.

Location of the Impact Area

Buccolingual Location of the Impact Area. The buccolingual location of theimpact area (O, Fig 3a) is under the control of the clinician. In the posteriormaxillary areas, the implant is most often placed lingually and slightly inclinedbecause of bone topography. When a normal buccolingual occlusion is used, evenwith diminished cuspal inclination as discussed previously, the resultant line of force(F) falls at some distance (D) from the crestal bone. This produces unwanted torque.When the teeth are arranged in crossbite relationship, occlusal force exerted on thesame cuspal inclination produces a resultant line of force that falls much closer to thecrestal bone (d), thus reducing torque (Fig 3b).

Vertical Level of the Residual Ridge Crest. The vertical distance from theocclusal end of the implant to the occlusal impact area (O, Fig 4a) is significantbecause it represents a lever arm with the fulcrum at the crest of the alveolar ridge.Posterior maxillary implants are usually inclined palatally because of the dictates ofbone anatomy. Occlusal force applied to a relatively flat cuspal inclination producesa resultant line of force that is inclined in relation to the orientation of the fixture.Even with relatively flat cusp inclines and minimal palatal inclination of the implant,as the vertical level of the crest of the residual ridge moves superiorly, the distanceof the resultant line of force to the crest of the ridge increases (x, y, z, Fig 4a). This

increases the lateral torque on the crestal bone.

On most implants, the resultant line of force from occlusal contact is rarely inthe long axis of the implant itself. As in Fig 4a, the greater the distance fromocclusal contact to the crest of the bone, the larger the lateral torque exerted on thecrestal bone. As the palatal inclination of the implant increases, and/or the resultantline of force becomes more inclined, the more exaggerated are the increases intorque as the crest of the residual bone is located more apically (X, Y, Z, Fig 4b).

To summarize, micromovement of the periodontal ligament allows the


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distribution of force along the surfaces of natural tooth roots around the center ofrotation in the apical third. An osseointegrated implant has no equivalentmicromovement; therefore, the forces are concentrated at the crest of the ridge.Torque can be reduced on the implant by creating true cusp-to-fossa occlusalrelationships and/or decreasing the inclination of the occlusal impact area. Alteration

of the occlusal relationship (such as a cross bite) provides contact more in line withthe implant rather than lateral to it. Lateral eccentric contact on a posteriorimplant-supported prosthesis should be eliminated when possible. The vertical levelof implant location is dictated by anatomy. The greater the implant-occlusaldistance, the more torque is produced on the crestal bone.

Force Distribution With Splinted Natural Teeth

A detailed analysis of force distribution with splinted natural teeth has beenpresented previously.6,7Therefore, only comparisons will be made as they relate tosimilar configurations involving multiple-implant-supported prostheses.

Lateral Force Production. The chewing stroke has a lateral components that isexerted on teeth through the food bolus, whether the teeth actually contact or not.When the bolus can no longer escape by deformation, it exerts similar lateral force tothe teeth as if the teeth were together. However, during bruxism, a slight canine risewill eliminate lateral torque on posterior implants and is advisable. The exceptionwould be when poor alveolar bone supports a natural canine, or when a caninelocation is an implant site.

Splinted Natural Teeth. Occlusal force on the buccal slope and lingual cusp

incline of a mandibular tooth produces a resultant line of force inclined lingually (Fig 5a). (In the maxillary arch the resultant line of force is inclined buccally.6,7) In astraight arch, multiple-tooth splint, if an occlusal force is applied only on the firstpremolar (hard bolus), the lingually inclined resultant line of force would initiatemicromovement about the vertical center of rotation located in the middle abutment (Fig 5b). The periodontal fibers would distribute compression, tension, and rotationalforces on all the roots (arrows, Fig 5b). However, as shown in Fig 5c, if a hard bolusapplies force only on the middle abutment (O), all of the teeth would tend to rotatelingually (arrows) about the horizontal center of rotation, passing through the apicalthird of all teeth. The force would be distributed more simply, as a compression or

tension on the periodontal ligament, depending on the location in relation to thecenter of rotation.

Force Distribution on Multiple-Implant-Supported Prostheses

The force distribution with multiple-implant-supported prostheses is completelydifferent than with natural teeth. Occlusal force on the first premolar as previouslyshown in Fig 5bproduces a similar lingually inclined resultant line of force.However, the crestal bone on that tooth absorbs the brunt of the force (arrow, Fig 5d). The internal force on the retaining screws and its effect on force distribution will


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be discussed later. When occlusal impact force is exerted on the middle tooth of theimplant-supported prosthesis, similar to a natural tooth splint shown in Fig 5c, thelingually inclined resultant line of force distributes most of the force to the crest ofalveolar bone of the middle implant (arrow, Fig 5e) with little distributed to theadjacent implant sites.

Micromovement and Force Distribution. The same principle is applied tomultiple-implant-supported prostheses that is applied to a single implant; namely, inthe absence of micromovement provided by the periodontal ligament, there is noeffective force distribution to multiple implants in the same prosthesis. This isbecause the prosthesis is stiff (rigid) and the implants and bone have onlymicron-movement, which is not great enough to effectively distribute force to all ofthe implants. However, multiple-fixture force transmission can take place because ofthe deformation of retaining screws1,2and possible overload caused by poorinterface fit between prosthesis and abutments.1

Retaining Screw Deformation and Stress Distribution. Because of theirreduced size and metallurgical composition, the abutment and prosthesis retainingscrews permit more deflection (flexibility) than other members of the totalprosthesis-fixture-investing bone system. Whatever force transmission takes placebetween multiple implants finds its origin in the deformation (flexibility) of theretaining screws.1,2However, this is extremely difficult to quantify on multipleabutments.3

Rangert et al2found that retaining screw deformation permitted 100-m(0.1-mm) vertical depression of a natural tooth that was splinted to a fixture on an

experimental model. It is debatable whether 100 m of vertical movement is enoughto distribute clinically significant force to the periodontal ligament. Certainly 100m of lateral movement is not enough to distribute force to the periodontal ligament,because "normal" tooth movement is in the range of 0.5 mm (previously defined inthis text as micromovement, which distributes stress).

Modulus of Elasticity of the Gold Retaining Screws. Gold retaining screwsare not rigid. This can be demonstrated by screwing a rigid multiple-implant metalceramic-casting into place with different patterns of gold screw tightening. Changesoccur in the abutment/gold coping interface not because the rigid metal ceramic

material flexes, but because the gold screws can elongate. Gold screws are,therefore, the most "flexible" portion of the system and permit enoughmicromovement to distribute force (to the fixtures). However, as demonstrated bythe Rangert et al2experiment, the magnitude of the deflection of the retaining screws(abutment and gold screws) is in the extreme lower end of the range ofmicromovement, defined here as 0.1 to 0.5 mm. Pending three-dimensional finiteelement analysis of multiple-fixture-supported prostheses, it is unknown at whatmicron deflection range force transmission will be effectively transmitted to allfixtures.


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Resultant Force Distribution. If an occlusal force is applied to the buccal slopeof the middle tooth of a multiple-implant-supported prosthesis (Fig 6a), a lingualresultant line of force will be generated. The degree of force distribution to theadjacent implants depends on the relative of elasticity of the gold screws and theinterface fit of all the components of the system. Over a period of time, more force

will be distributed to adjacent implants (arrows) because of the micromovement ofthe gold screws. To prevent undetected gold screw fracture caused by metal fatigue,with possible overload of the remaining implants, it is advisable to replace the goldscrews during the lifetime of the restoration. Cantilevered prostheses or angulatedabutments place greater stress on gold screws and the more rigid titanium abutmentscrews.

Sheer Stress. As shown in Fig 6b, an occlusal force that results in a buccal orlingual line of force creates shear force on the gold retaining screw, titaniumabutment screw, and the superior portion of the implant itself (arrows). To prevent

loosening and/or breakage, a number of factors are significant: (1) inclination of theimpact area (cusp inclines); (2) vertical distance from the occlusal impact to theimplant and to the abutment; (3) location of the impact area lateral to the axis of theimplant; and (4) the inclination of the implant relative to the line of force generatedby the impact area (occlusion).

Mechanism of Interface Force Distribution

The gold retaining screw must be tightened sufficiently (10 Ncm) to preload theinterface between the prosthesis (gold cylinder) and the abutment2,3(Fig 7a). As

long as the force application does not exceed the abutment-prosthesis interfacepreload value, the interface bears the load (arrows). Thus, the preloading of theabutment-prosthesis interface effectively limits shearing force on the retainingscrew. If the preload value is exceeded, the interface will begin to open as the goldscrew deforms and bears an increasing load until fracture. In this case the load isthen shifted to the remaining fixtures with the possibility of overload.

Gold Screw Tightening. If the gold screw is insufficiently tightened, lessocclusal force is required to separate the interface, and this can directly load the goldscrew. The gold screw can be distorted or break because of insufficient preloadingand/or poor interface fit. Excessive tightening can strip the threads and deform thegold screw.

The Effect of Poor Abutment-Prosthesis Interface Fit

Poor abutment-prosthesis interface fit can place more shear stress on the goldretaining screw than it was designed to withstand. As shown in Fig 7b, when theinterface fit is defective, the resultant line of occlusal force is not optimally resistedby the abutment-prosthesis interface. Although some force is distributed to theabutment at the point of contact, excessive shearing force is exerted directly on thegold screw.


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Multiple-Implant-Supported Prosthesis. As stated previously, poor interfacefit leads to a high incidence of gold screw metal fatigue and eventual failure. Insingle tooth restorations, loosening, and gold screw failure become clinicallyobvious. However, in multiple-implant-supported prostheses, poor interface fit andsubsequent gold retaining screw failure shifts the occlusal load to those sites where

there is good preloaded interface fit. As a result, the remaining implants can besubject to occlusal overload. This is particularly critical if the gold screw failuretakes place on a distal abutment. A lever arm that puts greater load on the adjacentfixture-abutment-prosthesis configuration is created.

Titanium Abutment Screw. Titanium abutment screws are stronger than gold(cylinder) retaining screws. Therefore, metal fatigue will usually produce gold screwfailure before the titanium abutment screw is affected.

Single Tooth Abutment. Poor interface fit usually causes loosening or fracture

of the retaining screw and continual failure. When a UCLA abutment-typerestoration is required, premanufactured cylinders should be used rather than plasticwaxing sleeves, which are technique sensitive fit. Several factors can reduce theshear stress on the retaining screws: (1) reduction of the inclination of the impactarea (Fig 1d); (2) narrowing of the occlusal table, and/or moving the occlusal contactarea more in line with the implant location (crossbite, Fig 3b); (3) improved implantorientation with the use of computerized tomography9and surgical guides10; and (4)alteration of the impact area for anterior maxillary single tooth restorations (Fig 2b).

Vertical Component Force

Vertical component force (O) on the distal implant of a cantilevered prosthesisresults in most of the force distributed to that implant (Fig 8a). If the implant iscylindrical, the force is concentrated at the apex; if it has a screwlike configuration,the force is concentrated at the outside edges of the screw threads.4Verticalcomponent force on the cantilever, however, will tend to have apically directed forceon the distal implant (distributed as above) and occlusally directed force on theprosthesis anterior to it (Fig 8b). This force distribution is created by themicromovement between the prosthesis, abutments, and implants resulting fromretaining screw elongation rather than implant movement in bone.

Significance of Differential Mobility in Splinted Natural TeethWhen mobile natural teeth are splinted to firm teeth, the force transmission patterndepends on the degree of differential mobility between the teeth, their strategiclocation, and the direction and point of the application of force. If the macromobileteeth (Class II, macromobility) are surrounded by firm teeth (Class I,micromobility), the weak mobile teeth are protectively splinted (Fig 9a). The greaterthe differential in mobility between the strong and weak teeth, the more the force isdistributed to the firm teeth and relatively little to the more mobile teeth.


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For example, lateral force applied to the firm posterior abutment (arrow, Fig 9a)initiates a rotation about the center of rotation located approximately in the firmcanine on that side. However, the lateral force is distributed to the remaining firmteeth only, with very little to the more mobile premolar teeth included within thesplint because of their high differential mobility (Fig 9a).

Mobile Terminal Abutment. Mobile terminal abutments create specialdiagnostic and clinical problems because of the high differential mobility betweenthe abutments and their strategic location. As shown in Fig 9b, when the terminalabutments of splinted teeth have a high differential mobility, lateral force (arrow)initiates a rotation about the firm canine on that side, as shown in Fig 9a. However,the mobile terminal abutment creates a lever arm mechanical advantage that cancause a metallurgical failure of the prosthesis, loosening of the castings on theanterior teeth, and /or periodontal breakdown of the firm anterior teeth.

In summary, force transmission depends on micromovement. Differentialmobility between natural tooth abutments distributes forces disproportionately to thefirm teeth. Mobile posterior abutments within a multitooth splint can cause lever armoverload forces on the strong anterior abutments.

Combined Prosthesis Using Implants and Natural Teeth

Clinical Considerations. Most clinicians agree that whenever possible,implant-supported prostheses should be free standing. Assemblage procedures aresimplified and the vast difference in differential mobility complicates diagnosis.However, in combined prosthesis design, natural tooth intrusion tends to separate

internal attachments and/or telescopic copings vertically. Posterior working sideforces (Fig 1a) and anterior incisal guidance (Fig 2a) tend to produce buccallyinclined resultant forces in the maxillary arch,6,7which can produce horizontalseparation of the prostheses. Splinted natural maxillary teeth can move buccallyaway from adjacent free-standing implant-supported prostheses. Decreased incisalguidance and posterior cusp inclines, and an optimal buccolingual occlusalarrangement (Figs 1b, 1d, 2b, 3b) can effectively reduce this hazard.

In the mandible, the resultant lines of force are usually lingually directed (Fig 5a)6,7and have less tendency to separate the component prostheses horizontally.

However, combined implant- and tooth-supported prostheses may be clinicallyrequired, which necessitates diagnostic evaluation because of the vast difference indifferential mobility. Methods must also be used to prevent separation. Whencombining a fixed retrievable implant-supported prosthesis with natural teeth,internal attachments or telescopic copings can be used.

Internal Attachments. Because the implant-supported prosthesis is fixed andretrievable, the female portion of the attachment is located in the natural tooth crown(Fig 10a). However, the natural teeth can be intruded vertically into the sockets,leaving the male portion of the attached extruded occlusally. To prevent this, a


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U-shaped pin can be added to the internal attachment.

U-Shaped Pin Attachment. A U-shaped pin (Selle R, CDT, So-Mar DentalStudies, Jamaica, NY; personal communication, 19XX) can be placed through theinterface between the male and female internal attachment (from the lingual aspect).Special laboratory procedures are required for semiprecision or precision

attachments. Because the male and female relationship is so precise, it is suggestedthat the U-shaped pin be in place during cementation.

Telescopic Copings. An alternative technique for combining natural tooth andimplant-supported prostheses uses substructure copings that are permanentlycemented to the natural teeth (Fig 10b). The superstructure telescopic copings overthe natural teeth are attached to the fixed-retrievable implant-supported prosthesis.The telescopic superstructure coping is temporarily cemented to the natural toothcoping; the implant portion of the prosthesis is screw retained (Fig 10b). Someclinical problems can occur: (1)the telescopic copings can separate and the natural

tooth can be intruded vertically into the alveolar bone; (2)"temporary" cementbetween the telescopic copings can harden and prevent separation; and (3)secondarycaries can occur.

Force Transmission in Combined Natural Tooth and Implant-SupportedProstheses

Differential Mobility. The discussion of differential mobility of teeth and its effecton force distribution (Figs 9a and 9b) does not apply to the comparison of naturalteeth and implants involved in the same prosthesis. As pointed out previously,

micromovement is essential for force distribution. Osseointegration, by definition,permits no movement other than the modulus of elasticity of bone measured inmicrons.

Lever Arm Effect. Internal attachments or telescopic prosthesis fabricationinvolving many splinted natural teeth do not support implants. It is the reverse: theimplants support the natural teeth. Figure 11aillustrates two posterior implants witha long pontic attached. The splinted anterior teeth provide an internal attachmenterroneously intended to support the posterior implant-retained prosthesis. Mutualsupport would take place only if natural teeth were in the place of the implants. In

that case, micromovement of all the teeth would facilitate mutual support and forcedistribution. The most rigid framework provides the best force transmission.5-7

However, because implants provide no micromovement (onlymicron-movement below 100 m), the normal movement of the anterior splint exertsenormous force through the long lever arm of the pontic (Fig 11a). The configurationof the prosthesis should be redesigned to reduce the stress on the osseointegratedinterfaces.

New Design Principles for Combined Prostheses


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Reduced Implant Lever Arm. The following principles should be applied:

l. A cantilevered pontic extended from a free-standing implant-supportedprosthesis should never exceed that which can be supported by the implantsalone.

2. Attachment of splinted natural teeth to an implant-supported cantilever increasesthe load on the implants rather than supporting them.

3. Implants support the teeth and not visa versa.

4. When combining splinted natural teeth with an implant-supported prosthesis, thelever arm on the implant-supported portion should be reduced as much aspossible (ie, shorter than if it were a free-standing prosthesis).

5. A cantilevered pontic should be extended from the splinted natural teeth, as far asappropriate, as if it were free standing.

6. The method of attachment between the two prostheses should be relativelynonrigid (ie, semiprecision with or without a U-shaped pin).

7. Rigid attachments (eg, precision attachments, cantilevered screw attachments)cause possible lever arm overload on the implants.

Combined Prosthesis Design. The erroneous design in Fig 11ashould bemodified by applying the above principles. Cantilevered pontics are extended fromeach segment that can be supported by the respective natural teeth and implants (Fig11b). The internal attachment between the two should not be overly rigid and is

designed to prevent horizontal separation rather than force distribution. This designreduces the stress on the implants without overloading the natural teeth.

Summary

Force distribution between members of a system depends on a complex relationshipbetween the relative stiffness of the structural parts with its investment medium(periodontal ligament or osseointegration). A rigid prosthesis is necessary todistribute force in all types of multiple-unit-supported prostheses. When force isapplied to one portion of a multiple-tooth-supported prosthesis, the micromovementof the periodontal ligament (0.5 mm range) initiates movement of the whole rigid

structural entity (teeth and prosthesis). This micromovement distributes force to theremaining natural teeth.

With a multiple-implant-supported prosthesis, force application to one portion isdistributed to the nearest osseointegrated fixture interface. The force is concentratedat that interface. The amount of distribution to the remaining fixtures depends on thedegree of deformation (flexibility) of the investing bone, fixture, abutment, retainingscrews, and prosthesis. The range of deformation of the most flexible part of thesystem (the retaining screws) is at the lower end of micromotion (in the range of 100


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m). Therefore, the amount of force distribution to the remaining fixtures is muchless than that found with a periodontal ligament, which can permit 0.5 mm ofmovement (500 m).

Paradoxically, because of the relative "flexibility" of the periodontal ligament,force distribution is dependent on a rigid structural entity of teeth and prosthesis.

Conversely, because the osseointegrated interface permits no movement, forcedistribution depends on some deformation of the fixture-abutment-retaining screwcomplex.

Combined prostheses using implants and natural teeth should be approachedwith caution. Internal attachments and/or telescopic coping construction have beenused. However, force transmission is completely different in both segments.Implants always support the natural teeth, rather than visa versa, because of theoverwhelming differential in mobility between periodontal ligament micromovementand the osseointegrated implant interface. New design principles have been

recommended to avoid implant overload.


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1. Skalak R. Biomechanical considerations in osseointegrated prostheses. J ProsthetDent 1983;49:843-848.

2. Rangert B, Gunne J, Sullivan DY. Mechanical aspects of a Brnemark implant

connected to a natural tooth: an in vitro study. Int J Oral Maxillofac Implants1991;6:177-186.

3. Brunski J. Biomaterials and biomechanics in dental implant design. Int J OralMaxillofac Implants 1988;3:85-97.

4. Rieger MR, Mayberry MS, Brose MO. Finite element analysis of six endosseousimplants. J Prosthet Dent 1990;63:671-676.

5. Weinberg LA. Lateral force in relation to denture base and clasp design J ProsthetDent 1956;6:785-800.

6. Weinberg LA. Force distribution in splinted anterior teeth. Oral Surg Oral MedOral Pathol 1957;10:484-494.

7. Weinberg LA. Force distribution in splinted posterior teeth. Oral Surg Oral MedOral Pathol 1957;10:1268-1276.

8. Posselt U. Studies in the mobility of the human mandible. Acta Odontol Scand1952;10(suppl 10):3.

9. Weinberg LA. CT scan as a radiologic data base for optimum implant orientation. JProsthet Dent 1993 (in press).

10. Mecall RA, Rosenfeld AL. The influence of residual ridge resorption patterns onimplant fixture placement and tooth position. Part II. Presurgical determinationof prosthesis type and design. Int J Periodont Rest Dent 1992;12:32-51.


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Figs. 1a to 1d: Comparison of torque production in natural and implant-supported prostheses relativeto changes in cuspal inclination. O = vertical occlusal force; F = resultant force; CR =center of rotation; D, d = distance; T = torque.

Figs. 2a and2b : Modification of the anterior impact area can reduce torque. F = resultant force; D =distance; T = torque.


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Figs. 3a and3b : Location of the impact area more in line with the implant can reduce torque(posterior cross bite, right). O = vertical occlusal force; F = resultant force; D = distance;T = torque.

Figs. 4a and 4b: Vertical level of the implant can influence torque production (left). When the implant isseverely inclined (right), the increased vertical level of the residual bone can exaggeratetorque production. O = occlusal impact area; F = resultant line of force; x, y, z =increasing distance of line of force to crest of ridge; X, Y, Z = torque increases as thecrest of the ridge is located more apically.


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Figs. 5a to 5e :Comparison of force distribution with a natural tooth splint compared to animplant-supported prosthesis. F = resultant line of force; CR = center of rotation; O =occlusal force.

Figs. 6a and6b : Gold screw micromovement permits force distribution (left). Lateral force producesstructural shear Stress on all the components (right). O = occlusal force; F = resultantline of force.


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Figs. 7a and7b : Interface fit affects shear stress on gold retaining screws. F = force application.

Figs. 8a and8b : The location of vertical force changes the force distribution to the implants. O =vertical component force.


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Figs. 9a and9b : When the terminal abutment of a prosthesis is firm, the weak included teeth are

mutually supported (left). A mobile terminal abutment produces a lever arm that canovercome strong anterior teeth (right). R = center of rotation; II = Class II, macromobility;I = Class I, micromobility.

Figs. 10aand 10b : An internal attachment with a U-shaped pin can prevent vertical separation( left). A telescopic coping can be used to combine natural teeth with a fixed-retrievableimplant-supported prosthesis (right).


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Figs. 11aand 11b : A long lever arm is produced when an implant-supported prosthesis iscantilevered to a natural tooth prosthesis (left). Less torque is created if cantilevers areextended from both prostheses and joined with an internal attachment (right).

Biomechanics of Implant

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