Clasps

18Removable Partial Denture Design

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• Multiple incisal rests should be connectedlingually by a plate of metal (Fig. 18.142b)

remember that when questioned about a parti-cular rest, they should also explain about thetechnique for preparing the corresponding restseat.

Direct Retainers

Definitions

Direct retainer “A clasp or attachment applied toan abutment tooth for the purpose of holding aremovable denture in position”.

“It is that component part of a removable partialdenture that is used to retain and prevent dislodge-ment, consisting of a clasp assembly or precisionattachment”—GPT

Retention “Retention is that quality inherent inthe prosthesis which resists the force of gravity, theadhesiveness of foods, and the forces associated withthe opening of the jaws” – GPT.

Direct retention “Retention obtained in a removablepartial denture by the use of clasps or attachmentswhich resist removal from the abutment teeth” -GPT.

Indirect retention “Retention obtained in a removablepartial denture through the use of indirect retainers”- GPT.

Classification

Direct retainers are broadly classified as:• Extracoronal direct retainers (Clasps):

• Manufactured retainers (Dalbo)• Custom-made retainers:

• Occlusally approaching (Circumferen-tial or Aker’s clasp)

• Gingivally approaching (Bar orRoach’s clasp)

• Intracoronal direct retainers (Attachments):• Internal attachment• External attachment• Stud attachment• Bar attachment• Special attachments

Extracoronal Direct Retainers (Clasps)

An extracoronal direct retainer is defined as, “Apart of a removable partial denture which acts as adirect retainer and/or stabilizer for the denture by

Fig. 18.141: Facial extension of the incisal rest to providea lock for the rest

Fig. 18.142a: A shallow channel like preparation should beprepared on the lingual surface of the abutment for theplacement of the minor connector

Fig. 18.142b: When multiple incisal rests are placed, each.One is not individually connected to the major connector.Instead they are interconnected with a metal plate which isconnected to the major connector

Indications: Full incisal rests may be given in thefollowing clinical conditions:• Tooth morphology does not permit other

designs.• When the incisal edge is completely lost, the

incisal rest can restore the lost contour.• When more stability is required.• Guidance is required for placement of the

restoration.After designing the location and position of

the rests, the rest seats should be prepared. Therest seats are prepared during prosthetic mouthpreparation phase (phase IV) prior to makingthe secondary impression. The outcome of therest is totally dependent on the rest seatpreparation. The rest is fabricated along withthe framework.

The technique for the preparation of each restseat is described in detail in the next chapter‘Prosthetic mouth preparation’. Students should

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18 Textbook of Prosthodontics

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Fig. 18.144: According to Prothero’s Cone theory, the toothis considered as a pair of cones sharing one base. Hencethe height of contour of the tooth is considered as the baseof the cone and all retentive components of the dentureshould cross the height of contour

partially encircling or contacting an abutmenttooth”- GPT.

“It is that component of the partial denture thatrests against the vertical enamel surfaces of anabutment tooth and aids in bracing and retention”.

General considerations A clasp should belocated at the undercut area in relation to thedetermined path of insertion and removal of theprosthesis (Fig. 18.143).

Component parts of a clasp (Fig. 18.145a)The component parts of a clasp have beendescribed in detail here. These components maybe rigid or flexible. The flexible components aredesigned below the height of contour so that theyprovide retention when they engage theundercut at the same time they can flex and passthrough the height of contour without requiringmuch effort during insertion or removal.

Fig. 18.143: The clasp should cross the height of contourand engage an undercut to provide retention

In a conventional clasp design, the tip of theretentive arm is the only flexible component. Allthe other parts are rigid and hence, placed abovethe height of contour (widest circumference ofthe tooth).

Retentive arm “A flexible segment of a removablepartial denture which engages an undercut on anabutment and which is designed to retain thedenture” – GPT.

It is that part of the clasp comprising of theretentive clasp arm and retentive terminal. Theretentive clasp arm is not flexible and is locatedabove the height of contour. The retentive termi-nal is flexible and lies below the height of contour(Fig. 18.145b).

Height of contour is defined as “A line encir-cling a tooth designating its greatest circumferenceat a selected position”—GPT. Kennedy named thegreatest convexity of the tooth as the height ofcontour. Cummer called it as the guideline thathelps in the placement of the components of theclasp. DeVan named the surfaces slopingsuperiorly as the suprabulge area and thesurfaces sloping inferiorly as the infrabulge area.

In 1916 Prothero proposed the Cone theoryto explain the basis for clasp retention. Hedescribed the shape of the crown of premolarsand molars to be equivalent to two cones (upperand lower) sharing a common base (Fig. 18.144).The upper cone resembles the occlusal half ofthe tooth and the lower cone resembles thecervical half of the tooth. A clasp tip that endsbelow the junction of the two cones will resistmovement in the upward direction, because ithas to deform (straighten out) to be releasedacross the junction of the bases of the two cones.The degree of resistance to deformationdetermines the amount of clasp retention.

Fig. 18.145a: (1) Retentive terminal (2) Retentive clasp arm(3) Reciprocal arm (4) Occlusal rest (5) Shoulder (6) Body(7) Minor connector



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Reciprocal arm “A clasp arm or other extensionused on a removable partial denture to oppose theaction of some other part or parts of the prosthesis”.- GPT.

It is located on the side of the tooth oppositeto the retentive arm. It resists the lateral forcesexerted by the retentive arm when it passesthrough the height of contour during the place-ment and removal of the RPD (Fig. 18.146).

General functions of the reciprocal arm canbe enlisted as follows:

• Provides stability and reciprocationagainst the retentive arm.

• The denture is stabilized against horizontalmovements.

• Acts as an indirect retainer (preventsrocking) to a minor degree.

Shoulder It is the part of the clasp that connectsthe body to the clasp terminals. It lies above theheight of contour and provides stabilizationagainst horizontal displacement (Fig. 18.148).

Fig. 18.146: The reciprocal arm (RC)serves to counter actthe forces of the retentive arm (RT) and stabilize the abutment

Fig. 18.145b: The retentive terminal (the only flexiblecomponent of the clasp) is the only component that crossesthe height of contour during insertion and removal

It is always placed in the supra-bulge area(above the height of contour). It may act as anindirect retainer when placed on an abutmentlocated anterior to the fulcrum line (axis of rota-tion) of the partial denture. Thus, the rigidreciprocal arm can resist the rocking of the den-ture base (Fig. 18.147).

Fig. 18.147: In a secondary abutment anterior to the fulcrumline (line of rotation of the denture during function) thereciprocal arm functions as a indirect retainer by preventingthe lifting of a denture

Fig. 18.148: Shoulder of a clasp

Body Part of the clasp that connects the restsand shoulders of the clasp to the minorconnectors. It is rigid and lies above the heightof contour (Fig. 18.149). The body of the clasp isdesigned such that it contacts the guide planeof the abutment during insertion and removal.The tissue surface of the body of the clasp, whichis closely related to the guide planes, is knownas a proximal plate of the direct retainer.

Figs18.149a and b: (a) Proximal view of the body of theclasp (b) Buccal view of the body of the clasp

Rest “A rigid (stabilizing) extension of a removablepartial denture which contacts a remaining tooth orteeth to dissipate vertical or horizontal forces”- GPT.

It is the part of the clasp that lies on theocclusal or lingual or incisal edge or surfaces ofthe tooth. It resists tissueward movement of theclasp by acting like a vertical stop (Fig. 18.150).The preparation and structure was described in



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detail in the previous section. The rest also helpsto maintain the retentive arm of the clasp inposition.

Minor connector “The connecting link between themajor connector or base of a removable partial dentureand other units of the prosthesis, such as clasps,indirect retainers and occlusal rests” - GPT.

Here, it joins the clasp with the remainingpart of the metal framework. In a gingivallyapproaching clasp it is also known as theapproach arm (Fig. 18.151).

6. Primary abutment clasp of a distalextension denture base should never exerttipping forces on the abutment.

7. It is preferable to place the reciprocal ele-ments at the height of contour and theretentive element below the height ofcontour.

Functional Requirements of a Clasp

The functional requirements of a clasp include• Retention• Stability• Support• Reciprocation• Encirclement• PassivityEach of these functional requirements are

provided by various components in a clasp. Theclasp and its parts should be designed appro-priately to achieve the functional requirements.

Retention “Retention is that quality inherent inthe prosthesis which resists the force of gravity, theadhesiveness of foods, and the forces associated withthe opening of the jaws” - GPT.

Retention is the most important function ofthe clasp; hence, it is the most important func-tional requirement. The purpose of a clasp is lostif the retention is lost. Retention is provided bythe retentive arm of the clasp. The tip of theretentive arm (retentive terminal) should lie inan undercut to the selected path of insertion.The undercut used for retention is known as aretentive undercut or preferred undercut.

This undercut should be identified during sur-vey. If an undercut is absent it should be createdusing any one of the four methods describedunder surveying. The retentive arm should befabricated according to the following designconsiderations.

Technical design considerations The followingfactors should be remembered while designinga retentive arm for a clasp:• The retentive arm of the clasp provides reten-

tion. The terminal third of the retentive armis flexible and it engages the undercut. Middlethird of the retentive arm has minimum

Principles of Clasp Design

The basic principle of clasp design is encirclementi.e. to obtain more than 180° of continous contactfor Aker’s clasp and a minimum of 3-pointcontact for Roach clasps. Other principles ofdesign include:

1. Occlusal rest should be designed to preventtissue-ward displacement of the denture.

2. Each retentive terminal should be opposedby a reciprocal component.

3. Balanced retention should be present (i.e. ifa buccal retentive clasp is present on oneside, the same should be present in theopposite side and vice-versa).

4. Path of escapement should never coincidewith the path of removal.

5. Only the minimum necessary amount ofretention should be used.

Fig. 18.150: An occlusal rest attached to the directretainer

Fig. 18.151: Approach arm of a Roach clasps



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The retentive terminal is forced to deformwhen a vertical dislodging force is applied.The retentive terminal exhibits a certainamount of resistance to deformation. Thisresistance is proportional to the flexibility ofthe clasp arm. It is this resistance to deformationthat generates retention (Fig. 18.155). Theflexibility of the clasp varies with the type ofalloy being used (Discussed below).

Fig. 18.153: Location and extent of the undercuts may varyaccording to the view angle. Similarly the location and extentof the undercut will vary according to the path of insertion

Fig. 18.154: The retentive terminal should always belocated at the undercut

Fig. 18.155: During removal notice that the retentive terminalat the undercut is forced to deform and cross the height ofcontour. The resistance to deformation offered by theretentive arm generates retention for the denture

Fig. 18.156: Path of displacement is any path of movementof the clasp without resistance. Hence, there will be no heightof contour obstructing the movement of the retentive armalong any path of displacement. If the path of insertion anddisplacement of the denture are parallel to one another itsimply means that there is no retentive undercut presentalong the path of insertion

Fig. 18.152: The proximal third of the retentive arm shouldbe placed above the height of contour. The middle third ofthe retentive arm should be at the height of contour. Onlythe terminal third is flexible and should be placed below theheight of contour to engage a undercut

flexibility. Proximal third is rigid and islocated above the height of contour. (Fig.18.152).

• The location and degree of a tooth undercutavailable for retention is relative to the pathof insertion of the partial denture. Path ofInsertion is defined as “The direction in whicha prosthesis is placed upon and removed fromthe abutment teeth “-GPT (Fig. 18.153).

• A clasp has rigid and flexible components.The rigid components of the clasp should beplaced in the non-retentive areas of the toothfor a given path of insertion. This is becausethey cannot deform to cross the height ofcontour.

• The clasp design for each abutment must beseparately considered. For a clasp to be reten-tive, the retentive terminal must be placed inthe undercut area of the tooth (Fig. 18.154).

• The retentive undercut will be present onlyin relation to a given path of insertion. Theretentive undercut is absent in conditionswhere the direction of dislodgement of theclasp arm is similar to the direction alongwhich the clasp arm was inserted (Fig.18.156). Hence it is important to maintain asingle path of insertion that does not coincidewith the path of displacement.

• Retentive undercuts should be located withthe help of a surveyor. The cast should betilted in a surveyor to achieve a unique path



356Fig. 18.158: The distance between the height of contourand the retentive terminal affects the length and flexibility ofthe clasp

of insertion. The following factors should beconsidered while determining the path ofinsertion.• Tissue undercuts• Location of vertical minor connectors• Origin of bar clasp arm• The denture bases.For a detailed discussion about path ofinsertion refer surveying.

• A good path of placement and removal isobtained by the contact of the rigid parts ofthe framework with the parallel surfaces ofthe abutment. These parallel tooth surfacesguide the denture during insertion andremoval and are called guiding planes. Theseguiding planes that are prepared on the toothact as an additional retentive unit.

Guiding planes are defined as “Two ormore vertically parallel surfaces of abutment teethso oriented as to direct the path of placement andremoval of partial dentures” -GPT. Guidingplanes should be as parallel as possible to thepath of insertion of the denture. If they arenot parallel, trauma to the teeth andsupporting structures will occur duringinsertion and removal of the denture. Thisleads to periodontal breakdown of theabutment teeth and strain to the parts of thedenture. In the absence of guiding planes, theretention from the clasp will be meagre orpractically non-existent.

• When the dislodging forces are not acting onthe denture, the retentive terminal should bein a passive relationship with the tooth. If theretentive arm is not passive, orthodonticmovement of the abutment will occur. Thisis due to the continuous pressure exerted bythe clasps on the abutment teeth.

Factors Affecting Retention

Depth of the undercutThe deeper the undercut, the greater is the reten-tion. A retentive undercut has three dimensions,namely:a. Buccolingual width of the undercutb. Distance between the survey line and the tip

of the clasp arm (occlusogingival height)

Fig. 18.157: Based on the depth of the undercut, the type ofmaterial for making the clasp is selected. Note: Whilemeasuring the undercut, the periphery of the disc and theshank of the undercut gauge should contact the tooth

c. Mesiodistal depth (gives the length of theclasp arm below the height of contour).

a. Buccolingual width of the undercut: This dimen-sion is the most important. It is measuredusing an undercut gauge in thousands of aninch. Clasp alloys are selected based on thismeasurement (Fig. 18.157). Alloys withgreater flexibility (low modulus of elasticity)are used against deeper undercuts.• For a 0.010-inch undercut - cast chrome

alloy is used.• For a 0.015-inch undercut - gold alloy is

used.• For a 0.020-inch undercut - wrought wire

is used.b. The distance between the survey line and the tip

of the retentive clasp: This affects the length ofthe clasp arm, which in turn affects theflexibility of the clasp (Fig. 18.158).

c. The Mesiodistal length of the clasp arm belowthe height of contour: Longer clasp arms offermore flexibility. The flexibility of the clasp isdirectly proportional to the cube of the length.Increased flexibility decreases the magnitudeof the horizontal stresses acting on theabutment (Fig. 18.159).



357Fig. 18.161: Half round clasp (flexes in a single direction)

Fig. 18.162: A bar clasp showing push type retention

Fig. 18.160: The retentive arm should taper such that itreduces to half its width from the proximal to the terminalend. To obtain a good functional taper, cobalt chromiumclasps should be atleast 15 mm long

d. Taper of the clasp arm: The clasp arm shouldtaper uniformly from its origin to the tip. Thedimension at the tip should be half of that inthe origin (Fig. 18.160).

Approach of the Clasp Arm

The gingivally approaching clasp arm has abetter retention as it pushes against the heightof contour during dislodgement (Fig. 18.162)(Push type retention).Fig. 18.159: For the same tooth, looping the retentive arm

in order to increase its length improves the flexibility

e. Cross-sectional form: A half round clasp armis flexible only in one plane (inward oroutward) whereas a round clasp, is flexiblein all planes. Round clasps are superior butthey are avoided due to difficulty in fabri-cation. Half round clasps flex in one direction(away from the tooth surface) (Fig. 18.161).A cast retentive clasp arm is used mainly intooth supported partial dentures because theyneed to flex only during placement andremoval of the denture.In a Kennedy’s class I situation, the clasp has

to flex during functional movements also. Itshould flex universally or disengage the toothwhen vertical forces are applied. A round claspis preferred in these conditions. Only a roundclasp can engage an undercut away from thedenture base.

Stability It is defined as, “The quality of a dentureto be firm, steady, or constant, to resist displacementby functional stresses, and not to be subject to changeof position when forces are applied” - GPT.• All components of the clasp except the

retentive arm provide stability.• Cast circumferential clasps offer greater

stability because it has a rigid shoulder.• Wrought clasps have a flexible shoulder and

bar clasps do not have a shoulder; hence, theyoffer lesser stability.

Support It is defined as, “To hold up or serve as afoundation or prop for” - GPT.• It is the resistance to the movement of the

denture in a gingival direction (along thepath of insertion). (Whereas retention is theresistance to the movement of the dentureagainst the path of insertion) (Fig. 18.163).

• It is provided by occlusal, lingual and incisalrests.

Reciprocation It is defined as, “The means bywhich one part of a prosthesis is made to counterthe effect created by another part”- GPT• It is provided by a rigid reciprocal arm.• It resists the stresses generated by the retentive

arm. It also stabilizes the denture againsthorizontal movement. In other words, it helpsto hold the tooth when the retentive arm isactive. If the reciprocal arm is absent therewill no resistance available for the action ofthe retentive arm (Fig. 18.164).



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Fig. 18.165: 180° encirclement provided by the retentivearm (RT) rest (RS) and reciprocal arm (RC)Fig. 18.163d: Support is provided by rests which act as

vertical stops

• It should be placed preferably at the junctionof the gingival and middle thirds of theabutment tooth (always above or at the heightof contour).

• It should contact the abutment tooth alongwith or before the retentive arm during inser-tion and removal.

• Other parts which offer reciprocation are:• Lingual plate major connector.• An additional occlusal rest placed on the

opposite side of the tooth alongwith theminor connector.

Encirclement “It is the property of the claspassembly to encompass more than 180° of theabutment tooth either by continuous or brokencontact to prevent dislodgement during function”

Each clasp must encircle more the 180° of theabutment tooth. Encirclement can be either acontinuous contact as in circumfrential clasp ora broken contact as in bar clasp with at least 3different areas of contact (Fig. 18.165). The threepoints of contact are:

• Retentive terminal• Occlusal rest• Reciprocal armThis embracement prevents the clasp assem-

bly from moving out of the confines of the toothduring function.

Fig. 18.164: Reciprocation is essential to stabilize thetooth against the deleterious effects of the retentive arm

Figs 18.163a and b: Retention is the resistance tomovement of the denture against the path of insertion

Fig. 18.163c: Support is the resistance to movement ofthe denture towards the path of insertion

Passivity It is defined as “The quality or conditionof inactivity or rest assumed by the teeth, tissues,and denture when a removable partial denture is inplace but not under masticatory pressure” – GPT.

The retentive function should act only whendislodging forces are present. If the clasp is notseated properly, the retentive forces act conti-nuously on the tooth leading to pain and tender-ness.



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Types of claspsThe types of clasps are:• Circumferential or Aker’s clasps• Vertical projection or Bar or Roach clasps• Continuous clasp

Cast circumferential clasp: “A clasp thatencircles a tooth by more than 180 degrees, includingopposite angles, and which usually has total contactwith the tooth (throughout the extent of the clasp),with atleast one terminal being in the infrabulge(gingival convergence) area” - GPT.

Vertical projection clasp / Bar clasp / Roach clasp:“A clasp having arms which are bar type extensionsfrom major connectors or from within the denturebase; the arms pass adjacent to the soft tissues andapproach the point or area of contact on the tooth ina gingivo-occlusal direction” - GPT.

Continuous clasp: “A metal bar usually restingon the lingual surface of teeth to aid in theirstabilization and to act as an indirect retainer”-GPT.

Cast Circumferential Clasp

They are popularly known as Aker’s clasps.These clasps embrace more than half of theabutment tooth. They may show a continuousor a limited three-point contact with the tooth.This architecture helps the clasp to hold theabutment firmly enough to prevent the rotationof the denture. They approach the undercutfrom an occlusal direction.Advantages:• Easiest clasp to make and repair.• Less food retention• Best when applied in a tooth supported

partial denture.• Derives excellent support, bracing and

retention.Disadvantages:• It covers a large tooth surface area. It also

alters the Buccolingual width of the crown(Fig. 18.166).This affects the normal food flowpattern leading to food accumulation. Thiscauses decalcification of the tooth structure.Damage to soft tissue will occur due to lackof physiological stimulation.

• Difficult to adjust with pliers because of it’shalf-round configuration.

Fig. 18.166: Notice that a cast circumferential clasp altersthe width of the tooth and hence the occlusal table

Fig. 18.167: An occlusally approaching Aker’s clasp shouldnever be used to engage the mesiobuccal undercut of aprimary abutment in a distal extension denture base

Fig. 18.168: Simple circlet clasp

• If these clasps are placed high (moreocclusally) on the tooth, the width of the foodtable increases leading to generation ofgreater occlusal forces.

• All cast circumferential clasps should neverbe used to engage the mesiobuccal undercutof an abutment adjacent to the distaledentulous space (Fig. 18.167). Hence, theycannot be used for cases with an undercutaway from the edentulous space.

Types of Cast Circumferential Clasps

Cast circumferential clasps can be of 11 differenttypes. They are:

1. Simple circlet clasp (Fig. 18.168)• Most versatile and widely used.• Best for tooth supported Partial dentures.• It approaches the undercut from the eden-

tulous space.



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Fig. 18.170: Multiple circlet clasp

Fig. 18.171b: Pontic clasp

• It engages the undercut, located away fromthe edentulous space.

• Clasp can be adjusted only in one direction(i.e. buccolingually but not occlusogingivally).

• They cannot be used for distal extension casesas they engage a mesio buccal undercut.

2. Reverse, circlet or reverse approach clasp(Fig. 18.169)

• This clasp is used when the retentive undercuton the abutment tooth is located adjacent tothe edentulous space.

• Consider a distal edentulous condition.Usually the clasp will arise from the distalsurface of the abutment to reach the mesialundercut. But this clasp is designed in such away that the clasp arises from the mesial sideand ends on the distal undercut.

• Usually Bar clasps are preferred for distalextension cases. These clasps are used whena bar clasp is contraindicated. E.g.• If there is an undercut area in the ridge• Presence of a soft tissue undercut caused

by buccoversion of the abutment tooth.• These clasps are used in distal extension den-

ture base to control the stresses acting on theterminal abutment teeth on the edentulousside.

Disadvantages:• If sufficient occlusal clearance is not present,

the thickness of the clasp has to be reduced.This will affect the strength of the clasp.

• The occlusal rest away from the edentulousspace does not protect the marginal ridge ofthe abutment tooth adjacent to theedentulous space. Hence, an additional restmust be placed to provide the necessaryprotection.

• Poor aesthetics as the clasp runs from themesial to the distal end of the facial surface.

• Wedging may occur between the abutmentand its adjacent tooth if the occlusal rest isnot well prepared.

Fig. 18.169: Reverse circlet clasp

3. Multiple circlet clasp (Fig. 18.170)• It is a combination of two simple circlet clasps

joined at the terminal end of the reciprocal arms.• It is used for sharing the retention with addi-

tional teeth on the same side of the arch whenthe principal abutment tooth has poor perio-dontal support.

• It is a mode of splinting weakened teeth.• It’s disadvantages are similar to that of simple

and reverse circlet clasps.

4. Embrasure clasp or modified crib clasp(Fig. 18.171a)

• It is a combination of two simple circlet claspsjoined at the body.

• It is used on the side of the arch where thereis no edentulous space.

• The clasp crosses the marginal ridges of twoteeth to form the double occlusal rest. Theclasp emerges on the facial surface and splitsinto two retentive arms. Each retentive armengages the undercut located on the oppositeside of the tooth.

• Interproximal tooth structure should be remo-ved to provide sufficient thickness of themetal. The clasp may break if the metal is toothin.

Fig. 18.171a: Embrasure clasp



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Fig. 18.172: Ring clasp

Fig. 18.173: Auxiliary bracing arm to reinforcethe ring clasp

Fig. 18.174: Hairpin clasp

Fig. 18.175: Onlay clasp

• Indications: It is used in Kennedy’s class IIand class III cases without any modifications.

• Occasionally, a very small edentulous spacecan be closed by a modified embrasure claspcalled pontic clasp (Fig. 18.171b).

5. Ring clasp (Fig. 18.172)• Consider a distal edentulous condition with

a distolingual undercut where a reverse circletclasp cannot be placed (no buccal undercut).In such cases, the retentive arm is extendedall around the tooth from the distobuccal endto termiante in the distolingual undercutacross the mesial side of the tooth.

• It is used in cases with lingually tipped molarabutments. Mandibular molars usually tipmesiolingually and the maxillary molars tipmesiobuccally. Hence, the retentive undercutwill be on the mesiolingual side for the lowermolar and mesiobuccal side for the uppermolar.

• As the clasp is long, additional supportshould be provided by adding an auxiliarybracing arm from the denture base minorconnector to the center of the ring clasp onthe buccal surface. (Fig. 18.173).

Disadvantages:• Alteration in the food flow pattern.• It cannot retain its physical qualities.

• Difficult to adjust or repair.• Increased tooth surface coverage.Contraindications:• If the buccinator’s attachment lies close to the

lower molar.• If the bracing arm will have to cross a soft

tissue undercut.

6. Fishhook or hairpin clasp or reverse actionclasp (Fig. 18.174)

• It is a type of simple circlet clasp, which aftercrossing the facial surface of the tooth loopsback to engage the proximal undercutbeneath its point of origin. It is used inconditions where the undercut is near theedentulous space.

• Upper arm is rigid and the lower arm isflexible. The upper arm should be positionedabove the height of contour in such a waythat it does not interfere with occlusion.

Indications:• The undercut is adjacent to edentulous area.• Presence of a soft tissue undercut.Disadvantages:• It has poor aesthetics.• It tends to trap and accumulate food debris.

7. Onlay clasp (Fig. 18.175)• It is an extension of a metal crown or onlay

with buccal and lingual clasp arms.• It is used in the occlusal surfaces of submerged

abutment teeth (that are below the occlusalplane) so that the normal occlusal plane canbe restored with an onlay.



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Figs 18.177a and b: Half and half clasp: (a) occlusal view,(b) proximal view

Fig. 18.178a: Back action clasp

• If the onlay clasp is made of chrome alloy,the opposing tooth should be protected witha gold crown. Because the chrome alloy canproduce massive attrition of enamel.

• As this clasp covers large amount of toothstructure, it may lead to breakdown of enamelsurfaces. Hence, it should be used only in acaries resistant mouth.

8. Combination clasp (Fig. 18.176)• A cast circumferential clasp cannot be used

when an undercut is adjacent to the edentu-lous space, as it will produce destructiverotatory forces on the distal abutment.

• In such cases, a flexible wrought wireretentive arm is used to replace the rigid castalloy retentive arm. These clasps are calledcombination clasps as they combine the two.

• As it has a greater flexibility it can be placedin a deeper undercut without any hazard tothe abutment.

Fig. 18.176: Combination clasp

• It is used in maxillary canines and premolarsdue to its superior aesthetics.

Advantages:The round configuration of the wrought wiregives two advantages• It has a thin line contact, which collects less

debris and is easy to maintain.• It can flex in all planes.Disadvantages:• Tedious lab procedures.• Easily breaks or distorts.• Poor stability.

9. Half and half clasp (Fig. 18.177)• It has a retentive arm arising from one direc-

tion and a reciprocal arm arising fromanother.

• Two minor connectors are needed for thisdesign. The first minor connector attaches theocclusal rest and the retentive arm to the

major connector. The second minor connectorconnects the reciprocal arm, which is similarto the bar clasp with or without an auxiliaryrest.

• This design produces large tooth coverage,which can be reduced by converting the reci-procal arm into a short bar with an auxiliaryocclusal rest.

• This design is intended to provide dual reten-tion.

10. Back-action clasp (Fig. 18.178a)• It is a modification of the ring clasp.• Here the minor connector is connected to the

end of the clasp arm and the occlusal rest isleft unsupported.

Disadvantages:• Lack of support to the occlusal rest reduces

its function.• It has both biological and mechanical

unsound principles.



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Approach Arm (Fig. 18.179)

It is a minor connector that connects the retentivetip to the denture base minor connector. It is semicircular in cross section and should cross thegingival margin at a right angle. The approachshould closely adapt over the soft tissues andcannot be fabricated over soft tissue undercuts.This is the only flexible minor connector designedin a RPD.

Retentive terminal (Fig. 18.180)It varies for each type of bar clasp. It should endon the surface of the tooth below the undercut.It can be paired or singular. The terminal, whichenters the undercut, is called retentive finger andthe other terminal is called the non-retentive finger.It helps to give a push type retentive force.

Fig. 18.178b: Grasso’s clasp or vertical reciprocalhorizontal retentive (VRHR) clasp

Fig. 18.179: Approach arm

Fig. 18.180: Retentive finger that engages an undercut

Advantages of bar clasps• It is easy to insert and difficult to remove.• It is more aesthetic, as it covers less tooth

structure.Disadvantages of bar clasps• It tends to collect food debris.• It has increased flexibility but reduced bracing

and stabilization.• Additional stabilizing units are needed.

11. Grasso’s clasp or VRHR clasp(Fig. 18.178b)

Developed by Grasso, this clasp consists of avertical reciprocal arm, an occlusal rest and ahorizontal retentive arm each arising separatelyfrom the major connector. It is more of aproposed concept.Advantages:• Minimizes tooth contact without compromise

in efficacy.• Does not require the preparation of guide

planes.• Suitable for posterior teeth with high survey

lines.• The placement of the retentive arm is more

aesthetic.• The balance between the retentive and reci-

procal components prevents the whiplasheffect of the retentive arm.

Disadvantages:• Difficult to maintain as the block out zone

between the base of the reciprocal arm andthe tooth tends to collect food debris.

Vertical Projection or Roach or Bar Clasp

These clasps approach the undercut gingivally.It has a push type of retention, which is moreeffective than the pull type retention providedby the circumferential clasp.

Parts of a Bar Clasp

Only the retentive arm of a bar clasp varies fromthat of a cast circumferential clasp. All othercomponents like the rest, shoulder, body, proxi-mal plate and the reciprocal arm are similar tothe ones present in a cast circumferential clasp.

The retentive arm in a bar clasp comprisesfor two parts namely the gingival approach armand the retentive tip.



364 Fig. 18.182: T clasp

Fig. 18.183: Modified T clasp

• Can also be used for a tooth supported partialdenture with natural undercuts. Since theclasp is designed to use the existing undercutswithout creating new ones, it is referred toas clasping for convenience.

• It should not be used on a terminal abutmenttooth if undercut is located away from eden-tulous space.

• Should not be used over a soft tissue undercut• The clasp has good aesthetics but due to the

flexibility of approach arm it lacks the bracingquality.

Modified T clasp (Fig. 18.183)• It is similar to T clasp but the non-retentive

finger (usually the mesial terminal) of the ‘T’terminal is removed.

• It is used in canines and premolar for betteraesthetics.

Design considerations• Approach arm should not impinge as it

crosses the soft tissue.• No relief is given so the tissue surface of the

approach arm should be smooth andpolished.

• Approach arm should cross the gingivalmargin at a 90° angle (Fig. 18.181).

• It is used if the favourable undercut is presentnear the edentulous area.

• The approach arm is a minor connectorarising from the denture base. It arises fromthe edentulous area near the undercut. It runsvertically upwards to the height of contourof the abutment where it splits into itsterminal ends.

• The tip of the retentive terminal shouldalways point to the occlusal surface.

• The bar clasp should be placed as low aspossible on the tooth.

Types of Bar Clasps

Bar clasps have been classified based on theshape of the retentive terminal. Each type isdescribed in detail below.

T clasp (Fig. 18.182)• Used in a distal extension denture base with

a distobuccal undercut

Fig. 18.181: The approach arm should cross the freegingival margin at a right angle

Disadvantages• It does not have 180° encirclement.

Y clasp (Fig. 18.184)Y clasp is basically a T clasp modified to suitcertain abutments where height of contour ishigh at faciomesial and faciodistal line anglesbut low at the center of the facial surface.

I clasp (Fig. 18.185)• Used on distobuccal surface of canines for

aesthetics.• Only the tip of the clasp (2-3 mm) contacts

the tooth. Hence, stability and encirclementis decreased.

Infrabulge clasp or mirror view clasp:by MM DeVan (Fig. 18.186)• The approach arm for the retentive terminal

arises from the border of the denture base,



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Advantages:• More aesthetic as it is placed more inter-

proximally.• Increased retention without any tipping

action on the abutment.• Resists distortion during handling.

I-barIt is a modified I type roach clasp introduced byKratochvil. It has a mesial rest arising from amajor connector, an I-bar retentive arm and along proximal plate.

It is designed to reduce tooth contact. Adetailed explanation about the design of an I-bar partial denture is described in chapter 22.Krol in 1973, modified kratochvil’s I-bar systemand introduced the RPI and RPA systems, bothof which have been described in chapter 22.

Intracoronal Direct Retainers (Attachments)

Intracoronal direct retainers are called so becausea part or the whole of the retentive componentsare located within the anatomical contour of theabutment teeth. They are of the following types:

• Internal attachment• External attachment• Stud attachment• Bar attachment• Special attachmentsGenerally all attachments have male and

female components that are manufactured usingstandard measurements. They are not customfabricated like a direct retainer. Hence, the onlydesign consideration for an attachment is pre-paring the tooth to place the female componentof the retainer.

Internal AttachmentIt is also known as precision attachment orfrictional attachment or key and keywayattachment or parallel attachment or slottedattachment.

Fig. 18.185: I clasp

Fig. 18.186: Mirror view clasp. Note: the lingual surface ofthe abutment is plated

Fig. 18.187: A half cut made to increase the flexibility ofthe approach arm

either as an extension of a cast base orattached to the border of a resin base.

• There are two occlusal rests on each abutmenttooth.

• The lingual aspect of the abutment may beplated (supported) or left open.

• It is more flexible because the portion of themetal base that gives rise to the approach armhas an incomplete cut. The cut is preparedeither by machining or placing a thin matrixband during casting (Fig. 18.187).

• The retentive arm can also be made ofwrought wire, which has higher flexibility.The wire may be soldered to the metal baseor embedded in the resin base.

Fig. 18.184: Y clasp



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Definition: “A retainer, used in removable partialdenture construction, consisting of a metal receptacleand a closely fitting part: the former is usually con-tained within the normal or expanded contours ofthe crown of the abutment tooth and the latter isattached to a pontic or the denture frame work” -GPT.

“A retainer consisting of a metal receptacle(matrix) and a closely fitting part (patrix). Thematrix is usually contained within the normal orexpanded contours of the crown on the abutmenttooth and the patrix is attached to a pontic or aremovable partial denture”.

The patrix engages the vertical walls built intothe crown of the abutment tooth to create fric-tional resistance during removal (Fig. 18.188).Dr Herman ES Chayes first formulated thisprinciple in 1906.

Some of the commonly used internal attach-ments are:• Ney-Chayes attachment.• Stern Goldsmith attachment.• Baker attachment.Advantages:• Elimination of visible retentive components.

• Elimination of visible vertical support elementthrough a rest seat.

• Provides some horizontal stabilization.• Stimulation of underlying tissues due to

intermittent vertical massage.Disadvantages:• Preparation of abutments and castings.• Complicated clinical and lab procedures.• Wear resulting in loss of frictional resistance.• Difficult to repair and replace.• Least effective in teeth with small crowns.• Difficulty to place it completely within the

circumference of the abutment tooth.Contraindications:• Large pulp (this limits the depth of the

receptacle.)

Fig. 18.188: Matrix (M) and Patrix(P) of a intracoronal retainer

Table 18.1: Differences between circumferential and bar clasps

Circumferential clasp Bar clasp

• It approaches the undercut from the occlusal aspect • It approaches the undercut from the gingival aspectof the abutment. of the abutment.

• It arises above the height of contour of the abutment. • It arises below the height of contour of the abutment.

• It has a rigid minor connector. • It has a flexible minor connector. The minor connector forthe bar clasp is called approach arm.

• It is easier to remove. This is because only the retentive • It is easier to seat but difficult to remove because the minorterminal should flex to be relieved from the undercut. connector should flex alongwith the retentive arm to be

relieved from the undercut.• It has a pull type retention. That is the retentive tip • It has a push type retention. That is the retentive tip

should pull occlusally to engage the undercut. should push occlusally to engage the undercut.• Due to continuous tooth contact, it has a good • Due to limited 3-point tooth contact, it has less bracing effect.

bracing effect.• It is less aesthetic, due to more metal exposure. • More aesthetic as it is present below the height of contour.• It has reduced food debris accumulation as it adapts • Increased food debris accumulation, because a space exists

more closely to the tooth. between the minor connector and the abutment surfaceand the length of the clasp assembly is more.

• Easy to repair due to it’s simple design. • Difficult to repair as the design is more complex.• It increases the width of the occlusal table because the • No such problem as it is placed in a lower position.

retentive arm arises near the occlusal surface of theabutment. It increases the occlusal load on the abutment.

• Due to increased tooth coverage it may cause • No decalcification due to limited 3-point contact.decalcification.

• It can be used in tilted abutments and in cases • It cannot be used in cases with tilted abutment andwith soft tissue undercuts. soft tissue undercuts.


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