Zeev Ben-Ur, DMD*

Designing Clasps for theAsymmetric Distal

Extension RemovablePartial Denture

Colin Corfil, DMO"

Ariel Shifman, DMD***

The Maurice and Gabriela Goldscbteger Schoolof Dental Medicine

Tel Aviv UniversityTel Aviv, Israel

Retentive clasp components can be created to minimize torquing forces onabutment teeth incorporated in the support and retention of bilateral distalextension removable partial dentures. Functional movement of the combinedtooth-mucosa-borne prosthesis about the axis of rotation necessitatesconsideration of retentive element design modifications, particularly as theyrelate to bilateral asymmetric distal extension situations, Intj Prosthodont1996:9:374-378.

The distal extension removable partial denture(RPD) derives its support from the relatively sta-

ble supporting abutment tooth or teeth and the re-silient soft tissues overlying the residual edentulousridge. These two tissues exhibit different degrees ofdisplaceability,' One source- states that "Forcesthat produce torque on abutment teeth and thealveolar residual ridge should be controlled andminimized in the design of direct retainers for dis-tal extension removable partial dentures,"Increased mobility of the tooth serving as a retainerfor the distal extension RPD might cause damageto the supporting tissues, A chain reaction may fol-low, with increased movement of the entire RPD,loss of occiusai contacts, and further continueddestruction of the supporting tissues.

Forces Acting on the ClaspedDistal Abutment Tooth

In the distal extension RPD, functional forces ap-plied to the denture base create an axis of rotation

'Clinical Lecturer, Section of Oral Rehabilitation.**Senior Clinical Lecturer, Section of Operative Dentistry.

***Senior Clinical Lecturer, Section of Oral Rehabilitation.

Reprint requests: Dr Zeev Ben-Ur, Section of OralRehabilitation, The Maurice and Gabriela Goldschleger Schoolof Dental Medicine, Jet Aviv University, Tel Aviv, Israel 69978.

around the most distal abutment teeth. When theclasp tip is placed mesial to the axis of rotation, thetorquing effect on the clasped teeth can be com-pared to the action of a Class I lever (Fig 1 ), Both thedegree and direction of this movement are greatlyinfluenced by the quality of the supporting residualridge, the design of the RPD, and the extent of theforces exerted on the denture during function. Thiseffect is most discernible in the mandibular distalextension RPD, where support is gained only fromthe residual edentulous ridges. Properly designedmaxillary RPDs with a major connector in the formof a broad strap or complete palatal coverage whenpalatal morphology allows, gain additional supportfrom the hard palate,' Use of such designs dependson the quality of the tissue covering this area, theneed to avoid areas of bony tori, a prominent me-dian suture line, sensitive incisive papillae, andpalatal form, A high-vaulted palate wil l providegreater stability but less support than a flatter palate,

Stress-Controlling Clasp Designfor the Distal Extension RPD

The literature describes four major stress-releasingclasp designs for the distal extension RPD,

1, The RPI clasp with a mesial rest, a proximalplate, and l-bar^'" (Fig 2, A)

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clasps for tile Asymmetric Distaf Extension Removable Partial Denlu

Fig 1 Class I iever, L = retentive clasp arm; F = fuicrurr

Fig 2 Four common ciasp de-signs for the distai extensionRPD, A = RPI; B = RPL: C =RPA; D = Equipoise; F = fuicrum;F, = distaiiy shifted tulcrum in theasymmetric biiaterai extensionbase on the shorter edentuiousside.

2, The RPL clasp with a mesial rest, a proximalplate, and L-bar̂ (Fig 2, B)

3, The RPA clasp with a mesial rest, a proximalplate, and a circumferential clasp'' (Fig 2, C)

4, The Equipoise back action-type clasp' (Fig 2, D)

All of these retentive clasp system designs forabutment teeth adjacent to a distal extension ridgeuse the Class II lever effect (Fig 3), allowing rota-tion of the RPD base towards the tissues withouttorquing the clasped tooth. The axis of rotation isdesigned around the mesial rest, far from the distalextension, and the retentive arm is designed to benear the distal extension. Occlusal forces cause the

Fig 3 Ciass ii iever, L = retentive ciasp arm; F - tuicrum.

5tv Volume 9, Number 4, 1996 375 The Internationai Journai of Prosthodontics

clasps tor the Asymmetric Distal Extpnsiiin Reniuvabif Partial Dentut

Fig 4 Symmetric bilateral distal extension bases. F = axis otrotation.

Fig 5 Asymmetric bilateral distal extension bases. F, - axisof rotation.

distal base and the retentive arm to move towardthe tissue around the mesially designed axis ofrotation, thus resulting in a stress-releasing effect(Class II lever effect).

Symmetrical Distal Extension

The most accepted stress-releasing clasp design de-scribed in the literature for the distal extension baseis the RPI system.^'''•'•' The point of application of

the i-bar is described as engaging 0.010 inch ofbuccal undercut at the greatest mesiodistal promi-nence or to the mesial of the abutment tooth. In thisclassic description, only the symmetrical KennedyClass I situation is considered.'•'''''•'' The fulcrum ofrotation as viewed in the occlusal plane is themesial rest, and the greatest mesiodistal promi-nence is the morphologic point of retention. This istrue for the symmetric Kennedy Class I distal exten-sion RPD in which the axis of rotation around themost distal abutment is nearly perpendicular to thelong axis of the denture bases (Fig 4).

Asymmetric Distal Extension

For the asymmetric distal extension RPD, the axisof rotation around the most distal abutments movesdistally on the shorter edentulous side and mesiallyon the longer edentulous side (Fig 5). The shiftingof the fulcrum must affect design of the clasp as-sembly, especially on the shorter edentulous side(Fig 2, F|). The shifting of the fulcrum distally rela-tive to the retentive tip of the retentive arm canpossibly create a torquing Class I lever effect onthe clasped abutment tooth.

Clasp Design for the Asymmetric Distal ExtensionBase

The functional fulcrum line or axis of rotationaround the most distal abutments must be estab-lished for the asymmetric bilateral distal extensionbase. The clasp must be designed so that the reten-tive arm will always be placed distally to this axisof rotation, thus creating the desired Class II levereffect. The true fulcrum line must be establishedcarefully for the I-bar retentive arm, as the verticalocclusal forces on the distal extension cause the I-bar to move mesiogingivally. Shifting the axis ofrotation distally on the shorter edentulous side maypotentially cause the I-bar to function as a Class Ilever. In the asymmetric distal extension base, thismovement will cause the I-bar to move laterallyand perpendicularly to the fulcrum line. Thesefacts must be considered when designing the I-barplacement (Fig 6). Placing the I-bar distal to the ful-crum line and in a position that will not interferewith any I-bar anterior movement minimizes thepotential torquing effect.

Because of their extreme distal placement, theRPL and Fquipoise retentive arms do not pose adesign problem (Fig 7). It logically follows that theRPA clasp design is not suited to asymmetric bilat-eral distal extension RPDs, especially on theshorter edentulous side.

The Internationai tournai of Prosthodontics 3 7 6 Volume 9, Number 4, 1995

Ben-Ur et al Clasps for the Asymmetric Distal Extension Removable Partial Denture

Fig 6 Desired position of the l-bar when the axis of rotationis shifted distally. F, = shifted axis of rotation fulcrum; R =l-bar; V = anterior direction of the l-bar movement (perpendic-ular to the fulcrum line).

Discussion

Stresses transmitted to the abutment teeth by theclasps of distal extension RPDs may result in in-creased tooth mobility.'"-!' A stress-releasing claspsystem design is advocated to protect these teethfrom torquing forces. A theoretical model of astress-releasing clasp assembly using the Class IIlever effect on clasped abutment teeth was sug-gested by Kratochvil in 1963' and later by Krol in1973.'' This was named the RPI clasp and used amesial rest which acted as a fulcrum, an l-bar directretainer positioned near the distal extension, and aproximal plate acting as as the element of recipro-cation. This theoretical design provided a clasp as-sembly with Class II stress-releasing effects onclasped abutment teeth and was widely acceptedfor the design of distal extension RPds.^-' Later re-search using photoelastic analysis of various claspdesigns arrived at the following conclusions'^:

1. The design of a retainer with a mesial rest in con-junction with a buccal l-bar exhibited the mostfavorable distribution of vertically applied forces.

2. Retainer designs with a distal rest tend to movethe clinical crown distally and the root mesiallyat the apex, resulting in horizontal forces in thebone.

3. Placing rests of distal extension RPDs more an-teriorly provides an axis of rotation that directsapplied forces in a more vertical direction.

Fig 7 Retentive clasp suggested for asymmetric distal ex-tension RPD. l-bar (top); Equipoise (center); and L-bar(bottom).

4. The distal rest in conjunction with circumferen-tial retainers developed greater horizontalforces within the supporting structures.

In addition to the RPI clasp assembly, RPL, RPA,and back action-type Equipoise clasp assemblieshave been suggested in the literature as stress-releasing components in the design of distal

Volume 9, Number 4, 1996 377 The International Journal of Prosthodontics

clasps for the Asymmetric Distal Extension Removable Partial Denture

extension RPDs.''"' In the authors' theoreticalmodel it is presumed that with the asymmetric dis-tal extension partial denture, the shifting of the axisof rotation that occurs with denture function cancause a leverage effect on clasped abutment teeth.The use of the suggested clasp design may helpovercome this problem, but experimental researchis necessary to further examine asymmetric designs.

Summary

The functional shifting of the axis of rotation in theasymmetric distal extension removable partial den-ture base has been considered. It is postulated thatthis movement may result in an undesirabletorquing effect on the clasped abutment tooth adja-cent to the distal extension base. This torquing canbe avoided by placing the clasp tip distal to thepredicted shift in the fulcrum line. Design sugges-tions for suitable clasp systems have been made.

References

1. Kratochvll FJ. Influence of occlusal rest position and clasp de-sign on movement of abutment teeth. J Prosthet Dent 1963;13:114-123.

2. Academy of Prosthodontics. Principles, concepts and prac-tices in prosthodontics, 1994.1 Prosthet Deni 1995;73:73-95.

3. Boucher LJ, Renner RP. Treatment of partially edentulous pa-tients. St Louis, MO: Mosby, 1982:46-49.

4. Krol AJ. Clasp design for extension-base removable partialdentures. J Prosthet Dent 1973;29;408-415.

5. Ben-Ur Z, Aviv I, Cardash HS. A modified direct retainer de-sign for distal-extension removable partial dentures. ) ProsthetDent 1988:60:342-344.

6. Eliason CM. RPA clasp design for distal-extension removablepartial dentures. I Prosthet Dent 1983;49:25-27.

7. Goodman JJ, Goodman HW. Balance of force in precisionfree-end restorations. J Prosthet Dent 1963;13:302-308.

8. Demer WJ. An analysis of mesial rest-l-bar clasp designs. JProsthet Dent 1976;36:243-253.

9. Berg T. l-bar: Myth and countermyth. Dent Clin North Am1984:28:371-381.

10. Rissin L, House SJ:, Conway C, Loftus ER, Chauncey HH.Effect of age and removable partial dentures on gingivitis andperiodontal disease. J Prosthet Dent 1979;42:217-223.

11. Browning JD, Jameson WE, Stewart CD, McGarrah HE, EickJD. Effect of positional loading of three removable dentureclasp assemblies on movement of the abutment teeth. JProsthet Dent 1986:55:347-351.

12. Ogata K. Longitudinal study of torque around the sagittal axisin lower distal extension removable partial dentures. J OralRehabii 1990:3:256-265.

13. Thompson WD, Kratochvil EJ, Caputo AA. Evaluation of pho-toelastic stress patterns produced by various designs of bilat-eral distal extention removable partial dentures. J ProsthetDent 1977:38:261-273.

Literature Abstract -

Comparison of the dimensional accuracy of one- and two-step techniqueswith the use of putty/wash addition silicone impression materiais

This stutdy compared the accuracy of tbe putty/vi/asb otie-step technique with the putty/wash

two-step technique using an addition-type siiicone impression material on undercut and

nonundercut abutments. A stainless steel base with three metal-tapered abutment prepara-

tions was used as the master model, and 15 impressions were made and poured in im-

proved die stone. The results demonstrated an increase in the interabutment distances (one

nonundercut, two with different undercut designs) for both techniques. The intra-abutment

distances with undercuts diminished in size for both techniques, while abutments without

undercuts increased. The authors stated that although statistioally significant differences in

accuracy could be found between the techniques, these differences were not considered to

be olinically significant.

Idris B, Houston F, Claffey N. J Prosthet Dent 1995:74:535-541. References; 26. Reprints: Frank

Houston, School ot Dental Science, Trinity College, Dublin 2, Ireland.—Seung-/'/ Eom, DDS. Advanced

Education Program in Prosthodontics. New Yori< University Coiiege of Dentistry, New Yori<. New Yorii

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