Prosthodontics

Design principles for cantiievered resin-bondedfixed partial denturesMichael Botelho, BDS, MSc, MRD

Recent ciinicai studies show 2-unit cantiievered resin-bonded lixed partial dentures to be as retenuve otmore retentive than their lixed-fixed counterparts. The fact that the 2-unit prosthesis is successful addsvalue to the ciinicai use ol resin-bonded fixed partial dentures because the single-abutment prosthesisis even simpler and more cost effective than fixed-fixed designs. However, there is no evidence-basedinformation relating to design principies tor abutment preparation and framework design tor the single-abutment, singie-retainer prosthesis. The aim of this report is to suggest principies of design for the 2-unitcantiievered fixed partiai denture, based on information gained from studies on fixed-fixed designs.(Quintessence internationai 2000:31:613-619)

Key words: abutment, oantilevered, fixed partial dentures, fixed-fixed design, resin-bonded

Evidence-based information continues to support theclinical appropriateness of resin-bonded fixed par-

tial dentures (RBFPD) with recent clinical studiesshowing improved retention rates over earlier studies.'-'It is noteworthy that clinical studies show 2-unit can-tiievered RBFPDs to be as successful, if not more suc-cessful, than their fixed-fixed counterparts."*-̂ However,the use of cantiievered prostheses is contrary to estab-lished recommendations in prosthodontic textboolis,"^clinical papers,^'" and evidence-based information."

There are only a handful of reports relating to can-tiievered RBFPDs and fewer stili that mention designprinciples of tooth preparation and framework designfor these prostheses. The aim of this report is to sug-gest design principles for tooth preparation and frame-work designs for 2-unit cantiievered RBFPDs. Thesedesign principles are extrapolated from in vitro and invivo research on fixed-fixed RBFPDs.

ADVANTAGES OF SINGLE-ABUTMENT,SINGLE-RETAINER RBFPDs

If 2-unit cantiievered and 3-unit fixed-fixed RBFPDsshowed identical retention rates, the cantiieveredprosthesis would be the preferred restorative optionbased on fundamental clinical advantages. The 2-unit

Assistart Professor, Discipline of Orai Rehabilitation, Uniuersity ol HongKong. Prince PhJiip Denial Hospital, University of Hong Kong, Hong Kong.

Reprint requests: Dr Mioliaei Boielho, Discipline of Orai Rehabiiitation,University oí Hong Kong, Prince Phiiip Dentai Hospital, 34 Hospitai Road,Hong Kong. E-maii: boteltio@tii(usua,liku.hk

RBFPD is not only more conservative to tooth tissuethan its fixed-fixed counterparts, but is quicker to pre-pare, easier to record an impression for, and simplerto cement {Fig 1). These factors along with lower labo-ratory costs should make the treatment item a moreaffordable restorative treatment option for patientsseeking dental care. Also, 2-unit canfilevered designsdo not have the troublesome possibility of carlesunder a partially debonded retainer, as may occur withfixed-fixed RBFPDs; a debonded cantiievered RBFPDwill simply fall out. and the pafient will return for pos-sible recementation.

CLINICAL STUDIES ON CANTILEVERED RBFPDs

Three of the earliest reports describing cantiieveredRBFPD's longevity were from the United Kingdomand were part of larger studies investigating tbe ciini-cai performance of RBBs provided in dentalschools'^"''' (Table 1). Wben combined, tbese studiesinvestigated 919 RBBs, of wbich 160 were can-tiievered RBFPDs, and, in all 3, cantiievered RBFPDswere more retentive tban their fixed-fixed counter-parts {Table 1), In a later study, Brabant̂ also showedcantiievered designs to be more successful tban fixed-fixed designs; bowever, the mean service life andactual number of cantiievered RBFPDs placed wasnot stated {Table 1).

Hussey and Linden'' were the first to exclusivelyexamine the ciinicai performance of cantiieveredRBFPDs. They analyzed data from 142 RBFPDs andshowed a 78% retention rate witb a total mean life of36.8 montbs. The results showed that ali of the 17

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Fig 1 Caritiievered prosthesis stiowing an improved geometriccanfiguiation to the retainer to increase ngidity in comparison tottie iraditionai C-shaped retainer. An ocolusai bar is extendedbetween the retainer ends to form a more rigid D-shaped retainer.

Fig 2 This 3-unit fixed-fixed RBFPD inas partiaiiy debonded ontiie canine abutment. Note the occiusal contact marks seen on thewear facet of ttiis abutment in laterai excursion. The bebond ismost iiisely due to the dynamic occiusai contacts on the tooth,forcing it away from the fixed retainer and the poorer resislanceform ot this abutment, in comparison (o (he premoiar.

TABLE 1 Clinical results from reports investigating cantilevered resin-bonded fixed partial dentures

No. of RBFPDs

Hussey et al.1991'^

400

No, ot cantilevered HBFPDs 70Mean service iiteRetention rate ofcantilevered RBBs

32.4 mo

8 3 %

Dunne/Miliar1993'3

382

47

101 mo

79%

Gilmour/Ali.1995'-'

137

43

24.5 mo

72%

Brabant,1997«

838

12.1%1984-1994

97%

Hussey/Linaen,1996"

142142

36 mo

88%

Briggs et al.1996=

5454

27 mo

80%

3oteiho et ai.2000'5

3333

30 mo

97%

debonds occurred in the maxillary arch with 11 of thefailures occurring when the central incisors or canineswere replaced. The cause of the higher risk of failureof these prostheses was attributed to the smaller sur-face area available for bonding when a lateral incisorwas used as an abutment and to the higher occlusalforces applied to canine teeth.

Briggs et ai"' reported 11 debonds out of 54 can-tilevered RBFPDs with an average service life of 26.7months. Ten of the 11 cantilevered RBFPDs debondedin the maxilla. In this study, debonded fixed partialdentures appeared to remain successful atter rccemen-tation, and no RBB debonded more than once.

In a more recent review, Boteiho et al'^ reportedonly 1 debond out of 33 cantilevered RBFPDs, giving a96% success rate with a mean service life of 30 months.It was also reported that none of the prostheses wasobserved to have tipped or drifted, despite 7 can-tilevered prostheses judged as having greater than 50%bone loss. All of these reports, however, describe short-term results, and further investigations are required toassess their long-term use and clinical success.

The clinical success of cantilevered RBFPDs isclaimed to be based on the independent free-standing

nature of the prosthesis rather than on any modifica-tion in tooth preparation or framework design. Thisdesign is thought to eliminate adverse interabutmentstresses on the cement bond during function.''•^•'̂Occlusal contacts on tootb tissue rather than on theretainer framework have been reported fo contributeto debonding of fixed-fixed RBFPDs (Fig 2)."-ieBecause of the freestanding nature of cantileveredFPDs, such adverse occlusal contacts are not possible.

In ciinical studies on cantilevered RBFPDs, toothpreparation appeared to vary considerably, with only3 reports briefly describing tooth preparations,Hussey and Linden'' performed simple tooth prepara-tion with intraenamel preparation allowing wrap-around and cingulum rests. It was not known if the11 posterior cantilevered RBFPDs were placed withthe use oí occlusal rests. Briggs et al' described mini-mal preparations with guide planes, cingulum stops,and occlusal rests. A less conservative preparationwas described by Brabant^ for fixed-fixed and can-tilevered fixed partial dentures with a chamfer finishline, grooves on opposing surfaces wifh an apicalstop, and a "central pinhole" if a second groove wasnot possible.

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It appears that both minimal preparatíoti and reten-tive tooth preparations for RBFPDs are quite success-ful; however, the minimal preparation designs didshow a 12% to 20% debond rate over a relatively shorttime period.

Because of the potential improvement for the reten-fion rate for cantilevered RBFPDs, the use of resis-tance features is preferred to maximize clinical success.However, hecause the main advantage of RBFPDs isthat they conserve tooth tissue, preparation designsshould not be complex or extensive as this woulddefeat the purpose of conservation.

DESIGN PRINCIPLES FOR2-UNIT CANTILEVERED PROSTHESES

There is no evidertce-based information regarding can-tilevered design options for RBFPDs. A clinical casereport has described a cantilevered RBFPD design withan extension of the occlusal framework on abutmentswith medium-sized restorations.'* The mesio-occlusodis-tal (MOD) amalgam restorations were replaced with sil-ver-cement-glass-ionomer cement, which was later pre-pared with mesial and distal hoxes and an occlusalisthmus. In addition, the retainer covered the abutmentwifh a '.'i crownlike coverage, although the buccal andpalatal cusp tips were not covered. The mesial and distalends of the retainer were joined through the occlusalisthmus that was said to increase the retainer rigidityand surface area for bonding. However, this type oftooth preparation is not appropriate for teeth with mini-mal or no restoration. Also, the use of an abutment witha moderately sized MOD restoration and 'i coverageretainer may not be appropriate for a RBPPD becausethe tooth resistance form may be derived from a poten-tially weakened palatal cusp. Long-term loading maylead to fatigue fracture of the cusp.

As little or no evidence-based information exists forthe preparation design of the single-abutment, single-retainer RBFPD, it is necessary to transfer principlesof design based on fixed-fixed studies. The design ofcantilevered RBFPDs relates to tooth preparation andframework design. Tooth preparation should be con-servative while, at the same time, allowing optimalresistance form. Ideally, it should be confined withinenamel and should maximize the surface area of theabutment for bonding. The framework should be rigidand have optimal resistance form while allowing goodoral hygiene practices.

Surface area for bonding

Maximizing the surface area for bonding is one of themost important features for success of any resin-

bonded restoration. To achieve this, axial preparationis recommended to lower the height of contour,'' andwith a carefully chosen path of insertion, the surveyline can be lowered so as to limit fhe amount of toothpreparation. Most often, a lingual or palatal path ofinsertion will mean that the approximal surface andocclusal surface of the tooth will reqtiire preparationand that the iingual and palatal surfaces require littleor no reduction (Fig 3). To keep the preparation asconservative as possihie, a knife-edge margin is chosenbecause it can easily and accurately be achieved bycontouring the wax to an acute emergence angle onan investment model (Fig 4).

Abutment resistance form

Abutment resistance form is necessary to prevent Iheretainer from being displaced during function; this isachieved by wraparound, occlusal coverage, and theuse of resistance features such as grooves or pinholes.

Coverage of the occlusal surface of posterior fixed-fixed RBFPDs has been shown to improve retentionof retainers in vitro.-" In vivo, the use of multiple restseats,' occlusal channels (slots),̂ and occlusal struts'^have been shown to be clinically successful.

Proximal grooves have been recommended byinvestigators for anterior fixed-fixed RBFPDs to com-pensate for the lack of wraparound.''^' In vitro, the useof 2 grooves per abutment, in comparison to nogrooves, has been shown to significantly increaseresistance to debonding on both anterior^---' and pos-terior-"* prostheses. These in vitro findings are sup-ported by clinical data in which anterior teeth pre-pared with grooves show better clinical retention thanteeth with no grooves.''^"' From this evidence, thenatural progression is fo use grooves for anterior sin-gle abutments; however, the 3 clinical studies report-ing exclusively on cantilevered fixed partial denturesdo not mention the use of grooves as part of thepreparation design.-*''^ While this might imply thatgrooves might not be essential for success of can-tilevered RBFPDs, it should he remembered that thefailure rate is up fo 2O''/ii over short time periods.Therefore, on anterior teeth, the use of grooves on theproximai surface of the single-abutment retainer ispreferred to maximize clinical retention (Fig 5).

Retainer resistance form

Retainer resistance form is necessary to prevent func-tional and parafunctional stresses that flex the retainerand break the cement bond. Prevention of such flexingis possible through increasing retainer thickness orchanging the geometric configuration of the retainer(see Fig 1), While studies have shown thicker retainers

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Fig 3 A lingual path ot insertion will lower the height of contouron the palatal surlace negating the need tor signiticant tooihpreparation on this surface. Preparation is then confined lo themesial surtace where the height ol contour is usjally high. A smallamount ot preparation may aiso be required on the distopaiatalsurtace ot the abutment to lower the survey line The preparationof the ocolusal bar is most satisfactodly performed with a roundbur to create a U-shaped trough that will allow a range of pathsfor insertion for the retainer A midpalatai groove is prepared witha No. 169 or No t70 tungsten carbide bur to give resistance tcocclusal loading on the pontic.

Fig 5 Opposing axial grooves are prepareO a( light angies to thetooth surfaoe to give the abutment resistance form The groovesare prepared ¡usi palatal to the conlacf point and should not begrealer than the widlh of the bur To ensure there is no undercut,grooves are prepared as slightly tapering toward each other.

Fig 4 P'elormed wa« sheets ol 0.8-mm thickness are heated inwarm water, adapted to ttie investment die. and seaied down atthe margins with a heated instrument. The gingival margin is thensmoothed to create an acute emergence profile that is compatiblewith good oral hygiene practices.

Fig 6 The second premolars were nol seieoted as abutments(as is usually recommended] because the residual tooth tissuewas considered compromised Instead, a distai cantilever fromthe canine was chosen with eiiminatlon ot the paiatai cusps on thepontic to eliminate adverse occiusai contacts. Because of theciosed-bite between the maxiliary and mandibular canines, it wasdecided to cement the prostheses in supraocciusion to preventthe need (or any tooth reduction. Nole the paiatai cingulum stieitto direct supraocciuding forces axiaily

to decrease the stress on the cement bond" andincrease bonding strength,̂ ^ there is no evidence-basedrecommendation for fhe optitnal retainer thickness forRBFPDs. The range of thicknesses investigated hasbeen 0.3 to 0.7 mm,̂ '-̂ ^ and as thicker frameworks willbe more rigid, it is suggested that retainers should bemore than 0.7 mm thick.

Occlusion

It has heeti suggested that occlusal contacts on can-tilevered ponties should be kept light to minimize

forces that may debond the retainer.'" One clinical sit-uation requires special mention. The use of a canineabittmenf-premolar pontic distal cantilever shouldhave special design modification because of the poorresistance form of the palatal surface of the canine toocclusal loading on the premolar pontic. In this case,fhe palatal cusp of the premolar may be retnoved tominimize adverse forces on the abutment (Pig 6).

The restoration of anterior teeth with a "closed-hite" requires the creation and tnaintenance ofocclusal clearance for the retainer. While palatalpreparation of the maxillary incisors and incisai edges

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Fig 7a The small 2-surtace cavity still leaves this potential abul-meni with sufficient enamel tor tDonding and residual tooth tissuelor resistance torm to retain a prosthesis.

Fig 7b The cavity was restored with resin composite.

Fig 7c Axial giooves were prepared on the mesial surfaoe of therestoration to give additional resistance form.

Fig 7d The final prosLhesis with an occlusal bar cemented withresiri adhesive cement.

of the mandibular incisors is possible, predictable pro-visionalizaticn is not easily achieved, and loss of someof the prepared occlusal clearance may require furtherreduction of the mandibular incisors or retainer at thetime of cementation. An alternative technique hasbeen suggested whereby the finai restoration iscemented in supraocclusion, allowing the RBFPD toact as an orthodontic appliance until intercuspal toothcontacts are re-established^ (Fig 6),

Previously restored abutments

The presence of caries on a tooth does not precludeits use as an abutment for a cantilevered RBFPD. Infact, if the restoration is small and the location strate-gic, it can be incorporated into the framework designfor additional resistance form. Dentai materials thatare compatible wifh the final adhesive luting cementprior to preparation shouid be used for such restora-tions. As a guide, abutments with small 2-surfacerestorations are considered appropriate for RBFPDs(Figs 7a to 7d).

The use of endodontically treated teeth for abut-ments is usually avoided; however, if the remainingtooth tissue is virtually intact and the alternative abut-ment compromised, the root-filled tooth can be used.The restored access cavity can be prepared with intra-coronal pinholes, which will not weaken the tooth,instead of the extracoronal surface (Fig 8).

Proposed cantilevered designs

Anterior abutments. The first stage of cantileveredRBFFD preparation for anterior abutments begins withselecting a path of insertion that will create a low sur-vey line and therefore minimize the amount of toothpreparation. Axial tooth preparation to the height ofcontour of the tooth is performed when appropriate,and finally, the opposing grooves or pinholes are pre-pared (Fig 5).

It is considered most appropriate to preparegrooves with a tapered tungsten carbide bur such as aNo. 169 or No. 170, Grooves should be positionedjust palatal to the contact point and be oriented on

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Fig 8 To minimize further tootii tissue destruction, the restoredaccess cavity ol this root-fiiled tooLh was prepared wilh 2 intra-coronai pinhoies rattier than extraccronal axiai grooves This abut-ment was chosen because Ihe iaterai incisor has a smali surfacearea tor bonding.

Fig 9 For any abutment, the path of insertion of the tramework islirst planned (as shown by the surveying rod), and then the heigintof contour is reduced to maximize the surface area tor bonding.The grooves are prepared aiong the path of this insertion ¡ustpaiatal to the contact point.

Fig 10 This fixed-fixed RBFPD was remade with a D-shapedcantiievered retainer when it was discovered that the 0.6 mm C-Shape molar retainer couid be distorted with finger pressure.

the path of insertion of the framework {Fig 9), To bcconservative, grooves should bc no greater tban thewidth of the bur. To provide adequate resistance,grooves sbould have a definite apical stop and palatalaxial wall.

It is not always possible lo place grooves of suffi-cient length on tbe distoproximal surface of tbe abut-ment A pinhole or slot can be placed on the lingualsurface parallel to the opposing groove. However, theuse of grooves for resistance form is preferred to pin-hoies because they are easier to prepare, record animpression, cast, and fit the final framework.

The use of cingulum stops has been proposed toprovide resistance to gingival displacement;''^ how-ever, this use does not take into account tbat most, ifnot all. Class I or II incisor tootb contacts directforces obliquely to the long axis of the tooth and notin a gingival direcfion. Also, if grooves are used, thenthe need of cingulum stops is superseded.

Posterior abuttnents. The design and preparationof posterior abutments is the same in principle foranterior abutments with some minor modifications.Tbe use of opposing interproximal grooves is not usu-ally possible, because of adjacent teeth, and not appro-priate, because tbeir placement would not resistocclusal forces on the pontic. Clinical experience basshown, wben trying in castings, that tbe use of a mid-palatal groove offers some resistance form to occlusalloading on the ponfic {sec Fig 3),

The geometric shape of RBB retainers on posteriorabutments has classicaily been C-shaped. Personalclinical experience has sbown that C-shaped retainerscan be distorted between tbe fingers {Fig 10), and it isrecommended that the retainer ends should he joinedto "brace tbe arms" of the prosthesis.' This will con-vert a flexible C-shape retainer to a rigid D-shaperetainer {sec Fig 1). Tbe provision of an occlusal barbetween the retainer ends may not require toothpreparation if there is sufficient occlusal clearance. Ifocclusal clearance is sufficient, it is recommended thatcoverage of the palatal or lingual cusp be achieved;'this will not only increase retainer rigidity but also thesurface area for bonding.

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CONTRAINDICATIONS FORCANTILEVERED RBFPDs

Abtitments requiring cuspal proteetion or having arestoration larger than a Class II are not good candi-dates for cantilevered RBFPDs. Also, patients with ahistory of drifting teeth or poor bone support withuncontrolled periodontal disease may not be suitablebecause of the possibility of abuttnent drifting. Atpresent, it is not recommended to cantilever a molar-sized pontic from a molar abutment because thegreater leverage forces from the pontic may causeuncontrolled tooth movement.

CONCLUSION

The single-abutment, single-retainer RBFPD is thepreferred prosthesis design when replacing single pon-tics anterior to the premolar tooth. Such a design isnot only more cost effective to the operator and thepatient but also appears to be a more simple and pre-dictable prosthesis than its fixed-fixed counterpart.Further studies are required to detennine if the use ofgrooves, pinholes, and occlusal bars offer improvedsuccess over more conservative designs.

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