David K. Burns, DM0Assistant Professor

A Review of Attachmentsfor Removable Partial

Denture Design: Part 2.Treatment Planning and

Attachment Selection

¡ohn E, Ward, ODS, MSDAssociate Professor

Department of Removable ProsthodonticsVirginia Commonwealth UniversitySchool of DentistryBox 566 MCV StationRichmond, Virginia 23298

Part 2 follows the initial review of attachment partial denturespresented in Part 1 and considers the analysis of specifictreatment requirements to provide a rationale for appropriateattachment selection. The tooth/tissue-supported removablepartial denture is carefully reviewed and several treatmentphilosophies for attachment use in this situation arediscussed. Int i Prostbodont 1990;3:169-174.

P art 1 addressed the definition, classification,functions, and indications of attachments for

removable partial dentures.' The development ofgood clinical judgment in using attachments forremovahle partiai dentures is not an easy task. Thecomplexity of attachments dictates that a thoroughunderstanding of their selection, treatment plan-ning, and use be a prerequisite to any clinical expe-rience. This paper analyzes a number of differentciinicai situations in which attachments may be usedand presents a method of determining an appro-priate removable partial denture design.

Application of Prosthodontic Principles

Although there are few scientific data to aid inattachment selection, there are some prosthodonticprinciples that should be used. One principle to befollowed, whether the prosthesis uses clasps or anattachment, is that forces should be widely distrib-uted to all available tissues. The denture base oftooth/tissue-supported removable partial denturesshould be extended to cover all of the residual ridgewithin the limitation of functional muscle move-ments. The teeth and denture-supporting areashould both be used to provide support, bracing,retention, direct-indirect retention, and stability. Ifone of these tissues is incapable of providing thesefunctions, other restorations (eg, complete denturesor a restoration using dental implants) should beconsidered.

It is important that the removable partial dentureframework can be properly related to the teeth andthe denture base to the framework. This principleis satisfied if the entire framework is rigid and the

framework contacis three or more teeth, preferablywidely separated and with rest seat preparations.Contact of the framework with only two abutmentteeth is inadequate if there is no other way to pos-itively relate the framework to the teeth. If a resilientattachment is used, there must be additional contactbetween the framework and the abutment teethother than the attachments themselves, or theremust be a way to deactivate the attachment, makingthe prosthesis rigid and thus allowing evaluation ofthe relationship between the base and the residualridge.

Treatment Analysis

It is most important to select the appropriateattachment for a specific clinical situation. There isan enormous variety of attachments available to thedental profession,- but the average dental practi-tioner does not need to become proficient in theuse of a large number of attachments. Many of theavailable attachments cannot be used universallyand are only suited for a specific clinical situation.Likewise, some involve complex mechanisms,which may result in early failure or an inability torepair them. Therefore, practitioners need to iden-tify a few attachments that can be used in the clinicalsituations they treat and should develop an exper-tise in their routine use.

Clinical success using attachments requires anawareness of the potential forces a prosthesis cantransfer to the teeth and residual ridges, as well asthe methods available to reduce or distribute theseforces. Analysis begins by classifying the edentulousspaces using a system such as the Kennedy Cias-

— 3, Number 2, 1990 1 6 9 The Inlernational ¡amr\a\ of Frostliodontii

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sification Systertn.' This will iidentify the type, loca-tion, anid number of eidentulous spaces (Fig 1).

If the partially edentulous arch is a Kenneidy ClassI or II situation ¡tooth/tissue-supported), then thestabilizing fulcrijm line is also identified. This imag-inary line connects the points of contact betweenthe framework and the most distal abutment toothon both sides of the arch around which the denturetheoretically rotates when occlusal forces areapplied on the denture base.''

Attachment Selection

Kennedy Class ¡II Partially Edentulous Arch

For a totally tooth-supported prosthesis restoringa posterior edentulous space, there is no tissuewardmovement of the denture and therefore no stabi-lizing fulcrum line. In such a situation there is littlecontroversy regarding the best treatment-a rigidinternal attachment. This attachment not only pro-

vides good retention, but also excellent support andbracing because of its rigid interlocking compo-nents. However, if the long-term prognosis for oneor both of the posterior abutment teeth is ques-tionable, then a stress-director type of attachmentcan be used with the anterior abutments in contin-gency planning for the future loss of ihcse teeth.

Kennedy Class I and II Partially Bdentuhus Arch

Philosophies of Attachment Use for the DistalExtension Removable Partial Denture. The most dif-ficult type of treatment plan is the one involving thedistal extension removable partial denture, whichrelies on support from both hard and soft tissues.The distal extension removable partial denture mustbe considered differently and in more detail thanthe totally tooth-supported situation when selectingattachments. Such a situation is controversial andthere are a number of treatment philosophiesdescribed in the literature.

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Fig 2 Advocates of tine stress-director philosophy believethat when the components of an attachment are rigidly con-nected, the loading of the distal extension can cause rotationand torquing of the abutment tooth around an axis within theroot and potentially cause periodontal damage.

Some practitioners believe that the distal exten-sion removable partial denture should use a non-rigid or stress-directing attachment. The theory isthat some basal movement is inevitable during thefunction of a distal extension base that rests on softtissue. Advocates of the stress-director philosophyhelieve that the loading of the distal extension willresult in rotation and torquing of the abutment toothwhen the components of an attachment are rigidlyconnected.^'* fhis may result in damage to the per-iodontium (Fig 2)."^ Opponents of this philosophybelieve that stress-director attachments allow theapplication of excessive force to the residual ridge,causing premature résorption of the denture-bear-ing area. Also, stress-director attachments arethought to be mechanically more complex thanrigid attachments and may be subject to increasedwear and breakage.

Another concept advocates the routine use of therigid attachment in constructing the distal extensionremovable partial denture.'" Advocates indicate thatthe edentulous ridge, with a precisely fitting denture

base, can provide as much support as the ahutmentteeth.'^ A removable partial denture with a castmetal hase developed from a mucostatic impressiontechnique is desired. Opponents of this philosophybelieve that the edentulous ridge cannot provide asmuch support for the prosthesis as the abutmentteeth, stating that when tissueward loading of theprosthesis occurs, forces are transferred to the rigidattachment and abutment teeth. Tfiis applies poten-tiaily damaging torquing forces to Ihe ahutmentteeth.

Another philosophy is known as tbe stable baseprecision attachment removable partial dentureconcept'^ or the floating denture base concept."Tbis concept incorporates rigid internal attachmentsand a cast metal base made from a mucostaticimpression of the residual ridge. The male portionof the attachment is connected to the denture base,allowing complete seating within the abutmenttooth component only when the prosthesis isloaded with tissueward force. Therefore, at rest, thetissues of the denture-bearing area are in their ana-

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Atlachmenls for Rer.ioval)ie Parli,.! Dentures, Pari 2 Burns/Ward

Fig 3 At rest, the tissues of the denture-bearing area are intheir anatomic torm and the attachment is not completelyseated.

Fig 4 In tunction, ttie tissues of the denture-Dearing areaare displaced and the vertical stops of the attachment comeinto contact, aliowing the attachment to resist turther occlusaiforces.

tomic form and the attachment is not completelyseated (Fig 3), The denture at this stage is totallytissue supported. In function, the tissues of the den-ture-bearing area are displaced into their functionalform and the vertical stops of the attachment con-tact, allowing the attachment to resist further occlu-sai forces. Only at this stage is the denturesupported by the teeth as well as the edentulousridge (Fig 4), Proponents of this concept claim thatthe stimulation of the tissues under such a denturebase prevents or retards residual ridge résorption,'»Opponents suggest that the rigid internal attach-ment allows only vertical movement of the denturebase. This does not adequately allow for any rota-tional movement of the base that might otherwiseoccur in function. Therefore, the attachment maybind, producing adverse forces on the abutmentteeth.

Force Distribution in the Distal Extension Remov-able Partial Denture. An important factor to con-sider in selecting a philosophy for treating patientsrequiring distal extension removable partial den-tures with attachments is balance between theforces applied to the residual ridge and abutmentteeth. Philosophies vary greatly regarding theamount of support provided to the prosthesis bythe individual structures. If the periodontium of theabutment teeth is healthy, providing good supportfor the teelh, and the residual ridge is composed ofadequate, well-rounded, dense bone, then the ridgeand teeth should be considered equally capable ofproviding support and the distribution of supportshould be equally balanced, 1 his is particularly truein the mandibular arch, where palatal support is nota factor. If the teeth are weak periodontally and the

residual ridge is composed of knife-edged bone ofpoor quality, then the ridge and teeth are both weakand the distribution of support should still beequally balanced. However, if the teeth are weakand the residual ridge is strong, the ridge shouldprovide more support than the teeth. Conversely,if the leeth are strong and the ridge weak, thengreater support should be supplied by the teeth.Other factors also influence attachment selection.These include interarch relationships, existing andproposed occlusal designs, interarch space, spaceavailable for attachment selection, treatment prog-nosis, and prosthesis design. Further considerationof these factors is necessary for a thorough under-standing, but they cannot be addressed in this over-view presentation.

Unfortunately, analysis of the action of attach-ments used for distal extension removable partialdentures under function is difficult. Regardless ofthe many theoretical considerations and philoso-phies that influence the designs, adequate researchanalyzing the forces developed in all of the differentsupporting structures is sparse, Kratochvil et al'^used photoelastic analysis to investigate the forcesdeveloped in the bone supporting the abutmentteeth by distal extension removable partial denturesusing attachment retainers of different designs. Theyfound that the use of stress-director-type attach-ments resulted in low-intensity forces on abutmentteeth, in contrast lo rigid attachments, which had apronounced tendency to torque the primary abut-ment tooth distally. They also reported that thesplinting together of abutment teeth resulted in bet-ter force distribution and reduced tortjuing of theteeth involved. In a similar study, Shohet^" found

Tlie internatiorial lournai of Prosliiodontii 172

Burní/Ward Attachments for Removable Paitial Dentures, Pan 2

that the greatest torquing force was produced witha rigid attachment and a nonsplinted, single abut-ment tooth. These studies do support the use oí thenonrigid or stress-director attachment with the distalextension removable partial denture, but they arebased on analysis of forces within the abutmentteeth and only infer indirectly the type of forcesbeing applied to the residual ridge.

When stress-director types of attachments areincorporated in removable partial dentures derivingpart of their support from the teeth and part fromthe residual ridge, analysis of the edentulous spaceand the axis of rotation will aid in determining thetype of stress-directing attachment to select. Thehinged or directionally oriented attachment, such asthe Dalbo attachment (APM-Sterngold, San Mateo,California), should be used, when possible, to pro-vide additional bracing or resistance to lateral move-ment. This becomes especially necessary when theresidual ridge is of poor quality and size, and it hasinsufficient vertical height to provide adequateresistance to lateral forces from the prosthesis.-^ Thehinged attachment is best suited when it can beplaced parallel to the edentulous ridge and per-pendicular to the stabilizing fulcrum line. If usedbilaterally, these attachments must be placed par-allel to one another to prevent hinding. Therefore,hinged attachments are best suited for symmetricKennedy Class I removable partial denture designs.However, they can be used successfully in othersituations, such as asymmetric Kennedy Class I andClass II, if parallel placement is maintained. Therotary or multidirectional attachment, such as theCeka attachment (Preat Corp, San Mateo, Califor-nia), does not restrict movement in a particularplane. The attachment can be successfully usedwhere ridges are not parallel to one another andwhere asymmetry exists.

The hinged and rotary type of attachments areusually extracoronal. They should be used in con-junction with a properly designed major connectorand a ledge or rest within a surveyed crown uponwhich the removable partial denture frameworkrests, providing support, bracing, and stability. Theattachment itself provides resiliency, retention, andsome additional stability.^^^^ This contact betweenthe prosthesis and the teeth in an area other thanthe attachment will also provide a way to deactivatethe attachment, accurately relate the framework tothe teeth, and provide an evaluation of the fit ofthe denture base to the edentulous ridge. Thehinged or rotary attachments are generally those ofchoice for most Kennedy Class I or Class II situa-tions. The use of the stress-director attachment inthis manner should result in a suitable distribution

of forces being shared by both hard and soft sup-porting tissues."•"

Some practitioners prefer to use the stable baseprecision attachment concept in the treatment ofKennedy Class I and II situations. Adequate researchcomparing the stress-director concept and the sta-ble base concept does not exist. This is a contro-versial area and practitioners must decide on tlieirpreference based on training and experience. As arule, bowever, tbe use of rigid attachments withdistal extension removable partial dentures is notrecommended.^"*

Kennedy Class IV Partially Edentulous Arch

Missing teeth in the anterior of the arch are thefinal consideration. This includes teeth missing onlyin the anterior (Kennedy Class IV), but may alsoinvolve patients with missing anterior teeth in con-junction with missing posterior teeth (KennedyClass I and II with an anterior modification space).These clinical situations are best treated with a fixedpartial denture in the anterior edentulous spacewhenever possible. However, on occasion, attach-ment-type removable partial denture designs shouldbe considered. This is particularly true when theedentulous ridge has a noncorrectable defect com-promising esthetics and contraindicating the use ofa fixed partial denture. The removable partial den-ture will incorporate a tissue-colored base that willsubstitute for the missing lissues and provideacceptable estbetic results. The ideal removablepartial denture design for such situations involvesthe use of a tissue bar placed close to the eden-tulous ridge and connected as a fixed unit to theabutment lecth on either side of the space, usingcrowns or resin retainers.

If the situation involves no missing posterior teethor a unilateral posterior edentulous space, then thebar is used in con]unction with posterior teeth toprovide support. Retention is provided by using aretentive clip or stud that is incorporated into thedenture base and either snaps over or into the bar.

Tbe retentive mechanism for the bar can beplaced so that some movement of the frameworkin function can occur before retentive resistance ismet. This avoids binding of the components in sit-uations such as the movement of a unilateral distalextension denture base during functional loading.This can be accomplished by placing a temporaryspacer between the har and the retentive mecha-nism when the mechanism (clip or stud) is beingconnected to the prosthesis. The spacer is subse-quently removed and the resulting space providesfreedom for movement of the prosthesis. The

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amount of freedom is determined by the thicknessof the spacer.

If the situation involves a biiateral distal extensiondesign, the tissue bar must be straight and flat, andit should be placed parallel to the stabilizing fulcrumline and perpendicular to the sagittal plane. Theremovable partial denture framework rests solely onthe bar to gain support and retention. Under func-tion, the prosthesis will rotate around the bar. Theretention is provided by a clip that snaps over thebar and allows rotational movement at the bar/clipinterface. The stud attachment will not allow thistype of movement and should not be used. Theguiding planes must be designed to provide bracingand fixation without interfering with movement.When this design is used, the proximal plate of theframework must be trimmed flush with the adjacentmarginal ridge on the most posterior abutment toserve as a third point of reference and allow amethod to relate the framework to the teeth forevaluation.

Conclusion

The decision lo use attachments in removablepartial denture design should be carefully consid-ered. Clasp-type removable partial dentures shouldbe used whenever practical because of their lowercost, ease of fabrication and maintenance, and thepredictability of results. However, if an attachmentremovable partial denture is the treatment of choicebecause of esthetics, abutment alignment, or theneed for greater cross-arch bracing, it must be usedwith a thorough knowledge and understanding ofprosthodontic principles and attachment use, aswell as an awareness of the intricacies and specialproblems associated with attachments. In treatmentusing the attachment-retained distal extensionremovable partial denture, the development of astress-directing attachment design as weil as theproper distribution of forces between the residualridge and abutment teeth should be goals for suc-cessful treatment.

References

1. Burns DR, Ward|£: A review of attachments for removablepartial denture design: Part 1. Classification and Selection,Intj Prosthodont 1990;3;9e-102.

. Zahler |M: Intracoronal precision attachcuont^. i.'L-nt CImNorth Am 1980;24:1 31-141.

. Henderson D, McCivney GP, Ca Hfiberry DJ:McCracken'i Removable Partial Prosthodontics, ed 7. SiL0UÍ5, CV Mosby Co, t985, p 17,

Academy of Denture Proslhetlcs: Glossaiy of Frostho-dontic Terms, ed 5. St Louis, CV Mosby Cu, 1987.Goodman J], Goodman HW. Balance of force in precisionfree-end restoration. / Prosthet Dent 1963;13.302-308.Henderson D, Blevins WR, Wesley RC, Seward T: Thecantilever type of posterior fi^ied partiai dentures: A lab-oratory study. / Prosthet Dent 1970;24:47-67.Cohn LA: The physiologic basis for looth fixation in pre-cision-attached partial dentures, / Prosthet Dentt9S6;6:220-244.

Mensor MC: The rationale of resilienl hinge-action stress-breakers. / Prosthet Dent 1968,20:204-215.Andersen JA, Lammie CA: A clinical survey of partiai den-tures. BrDent ! 1952;92:59-67,Carlsson CE, Hedegard B, Koivumaa KK: Studies in partialdenture prosthesis. Ill: A longitudinal study of mandibularpartial denlures with double extension saddles. ActaOdontol Scand 19&2;20.95-119,Fenner W, Gerber A, Muhlemann HR: Tooth mobilitychanges during treatment with partial denture prosthesis.; Prosthet DeiiM 956:6:520-525.Seeman S: A study of the relationship between periodor-tal disease and Ihe wearing of partial dentures. Aust Dent! 1963,8:206-208,Lammie CA, Osborne |: The bilateral free-end saddlelower denture. / Prosthet Dent 1954;4:640-652.Grosser D. The dynamics of internaf precision attacii-men!s. / Prosthet Dent 1953,3:393-401.Leff A: Precision attachment dentures. ; Prusthet Dent1952;2:84-91.

Clayton |A: A stable base precision attachment removablepartial denture (PARPD): Theories and principles. DentCiin North Am 1980;24:3-29.Monteith 6D: Management of loading forces on mandib-ular distal-extension prosthesis. Part I: Evaluation of con-cepts for design, / Prosthet Dent ¡984;52:673-681.Long |H: Unpublished dara.Kratochvil F|, Thompson WO, Caputo AA: Photoelasticanaiysis of stress patterns on teeth and bone wirh attach-ment retainers for removable partial dentures. / ProsthetDent 1981:46:21-28.Shohet H: Relative magnitudes of stress on abutmentteeth with different retainers. J Prosthet Dentt969;21:267-282.Weinberg LA: Lateral force in relation lo the denture baseand clasp design. / Prosthet Dent I956;6:785-800.Singer F: Improvements in precision-attached removablepartiai dentures. / Prosthet Dent 1967;I7:69-72.Frechette A: Infiuerrce of partial denture design on dis-tribution of force on abutment teeth. / Prosthet Dent1956;6,-I95-212.

Henderson D, McGivney GP, Caslleberry D|:McCracken's Removable Partial Prosthodontics ed 7 StLouis, CV Mosby Co, 1985 p 79.

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