Planning fixed partial dentures for severely misaligned abutments

Prosthodontics

Planning fixed partial dentures for severely misaligned abutmentsGabriel R. Zuckerman*

Abstract fVhen the abtttment teeth of a fixed partial denture are severely mlmligned.insertion ofthe pmsthesis becotnes tnore difficult. Some ofthe tnethodstraditionally recomtnended to overcome the probletns associated with this situationare examitied. Otie design that simplifies treatment and improves the prosthesis issuggested. (Quintessence hit 1996:27:527-532.)

Clinical relevance

A segmented fixed partial denture design with anextracoronal interlocking attachment made fromprefabricated plastic patterns is tnore esthetic andrequires less tooth reduction than do traditionaldesigns recommended for ftxed partial dentures onseverely misaligned abutments.

Introduction

Placement of a fixed partial denture (FFD) can becomplicated by misaligned abutment teeth. Ideaily, thelong axes ofthe abutment teeth of an FPD shouid beparallel to one another. When the abutment teeth areseverely misaligned, the design for the proposedprosthesis must be modified. Careful platining prior totreatment is necessary if the prosthetic result is to bepredictable and successful. Misalignment is mostfrequently encountered when a mandibular first molaris to be replaced after the second and third molars havedrifted and tilted mesially and lingually (Fig I ).

Traditional procedures

Orthodontics

Use of orthodontic treatment to upright the mesiallytipped molar has been suggested.'- Aithough thisapproach is ideal, patients are often reluctant to

* Private Practice, East Nonhpon, New York.

Reprint requests: Dr Gabriel R. Zuekerman, 1199 Fifth Avenue, EastNorthporl, New York 11731.

commit the time and funds necessary for this extendedand extensive treatment, requiring the coordinatedservices of an orthodontist and a restorative dentist.

A Itered path of insertion

It is possible to prepare the molar abutment with a pathof insertion that is compatible with the premolar prep-aration, but this approach has several disadvantages.The retention and resistance of the molar preparedunder these conditions are severely compromised,-"'*and the risk of creating a mechanical exposure ofthemesial pulp horn ofthe molar is increased. Further-more, the mesial marginai ridge ofthe third molar, ifpresent, would interfere with insertion ofthe prosthe-sis (Fig 2).

Mesial half-crown retainer

Rosenstiel et a!̂ and Shillingburg et ai- have recom-mended the use ofthe mesial half crown as a retainerfor tilted molars (Fig 3). A mesial half-crown retainerhas several disadvantages:

1. The mesial half crown cannot be used if the molarhas previously been restored on the distal surface.

2. Preparation ofthe molar could resuit in exposure ofthe mesial pulp horn.

3. The patient may object to the use of a cast-metalrestoration.

4. Caries occurring on the unrestorcd distai surface ofthe second molar abutment after insertion oftheprosthesis jeopardizes the restoration.

Shillingburg et al- have suggested use of a completecoping with retentive grooves and a teiescopic mesialhalfcrown for the molar retainer {Fig 4). A retainer ofthis design requires more tooth reduction and could

Quintessence International Volume 27, Number 8/t996 527

Fig 1 The missing mandibular first molar has allowedsecond and third molars to tilt mesially. Note the severemisalignment between the long axes of the second pre-molar (A) and the second molar (B),

Fig 2 Three-unit FPD with rigid connectors. The prosttie-sis will not seat because the distal margin of the molarretainer engages the mesial marginal ridge of the thirdmolar during insertion. Note the proximityof the mesial pulphorn to the mesial axial wall of the molar preparation.

Fig 3 Three-unit FPD with rigid connectors. The secondmolar abutment has been prepared for a mesial half-crownretainer with facial and lingual grooves for added retention.Note the proximity of the mesial pulp horn to the mesial axialwall of the molar preparation.

Fig 4 fr/ghíí Three-unit FPD with full coping and telescopicmesial half crown on the molar abutment.

result in pulpal exposure or an overcontoured retainer.Preparation of the tooth for the complete-coveragecoping would be excessively tapered. Even if retentionbetween the mesial half crown and the coping wereideal, retention ofthe coping to the abutment toothmight fail because of the excessively tapered toothpreparation,^-^

Segmented FPD

The use of a segmented FPD, in which each section ofthe prosthesis has its own path of insertion and there isa mechanical interlock between segments, has been

advocated. The literature contains a variety of seg-mented prosthetic designs recommended to overcomethe problems associated with insertion of an FPDwhen the abutments are divergent,

Shillingburg et a|- have suggested the use of asegmented FPD with a nonrigid attachment betweenthe second premolar retainer and pontic. This designrequires tooth preparation ofthe premolar with a boxon the distal surface to accommodate the femalecomponent of a nonrigid connector (Fig 5), Toothpreparation of the premolar to accomrnodate thisconnector could jeopardize the pulp and the retentive

528 Quintessence International Volume 27, Number 8/1996

Fig 5 Segmented FPD witb nonrigid connector betweenpremolar and pontic. Excessive tooth reduction ¡s requiredto accommodate the female section of the interlock on thedistal surface of the premolar retainer.

Fig 6 Segmented FPD with an interlock between thesecond molar retainer and the ponfic. Excessive toothreduction is required on the mesial surface of fhe secondmolar abutment to accommodate the female portion of fheinterlock.

properties ofthe preparation. The ocelusal surface ofthe nonrigid attachment is visible on the occlusalsurface ofthe anterior retainer and can be cosmeticallyobjectionable to the patient. It is also possible todesign the prosthesis so that the female component ofthe nonrigid connector is on the mesial surface ofthemolar retainer (Fig 6)- However, the preparation ofthe molar abutment to accommodate this configura-tion increases the risk of pulpal exposure and com-promises the retention ofthe molar preparation,

O'Connor et a!̂ have proposed a segmented FPDwith a split pontic containing a nonrigid connectorThe nonrigid connector is oriented parallel to the pathof insertion ofthe molar retainer. The gingival half ofthe pontic is connected to the distal surface of thepremolar retainer and contains the male component ofthe nonrigid cormector. The occlusal half of the ponticis attached to the molar retainer and houses the femalecomponent ofthe nonrigid connector (Fig 7). Thiscomplex design is time consuming to produce and,when completed, results in an unesthetic display ofmetal at the base of the pontic.

Moulding et al̂ have advocated the use of asegmented FPD with a nonrigid cormector betweenthe premolar retainer and the pontic. The malecomponent ofthe connector is inverted and placed onthe distal surface ofthe premolar retainer, parallel tothe path of Insertion ofthe molar retainer (Fig 8),When the nonrigid connector is positioned in thislocation, there is an unsightly display of the metalguiding planes of the connector, and a large gingivalembrasure is created below the connector

Fig 7 Segmented FPD with a split pontic containing anonrigid connector.

Alternative procedure

A segmented FPD with a nonrigid connector betweenthe pontic and the molar retainer can be designedwithout the disadvantages of the previously describedprostheses. The male portion of tbe nonrigid connec-tor can be placed on the mesial surface of the molarretainer and oriented parallel to the path of insertion ofthe premolar retainer (Fig 9 ), The female section ofthe

Quintessence International Volume 27, Number 8/1996 529

Fig 8 Segmented FPD with a nonrigid connector. Thepremolar has the maie portion of the inleriock on its distalsurface. Note the iarge gingival embrasure between thepremolar and the pontic.

Size 1mm)

Smaili_arge

A

70

ao

B

2.83.5

C

2.02.5

D

6.77.0

E

2.43.0

F

2333

Fig 9 Segmented FPD with the maie portion of tiieinteriook on the mesial surface of the second molar retainerThe tooth preparation of the second molar is retentive hutwithout r̂ sii oi pulpal exposure.

Fig 10 (left) Design and dimensions of PDC components.

Fig 11 (below) ivtale portion of the PDC on tfie mesiai axiaiwail of the moiar retainer. (1) Guiding piane; (2| crest ofridge; (3] gingival end of male PDC contoured to ridge.

nonrigid connector is located under the pontic in theanterior segment ofthe prosthesis.

This design is ideal for metal-ceramic restorations.Both abutments are prepared for metal-ceramic com-plete yeneer crowns with a path of insertion parallel totheir long axes. This allows the dentist to prepare theteeth with maximum retention and minimal risk ofpulpa! exposure.

The male and female components of this nonrigidconnector are manufactured in two sizes and are listedin the catalogs of denial supply dealers as Plastic

Dovetail Connector (PDC} attachments (Fig 10). ThePDC attachments are designed so that the matingsurfaces of male and female components restrict themovements of the two segments of the FPD to oreplane. If the vertical height for the connector is limited,the PDC components may be reduced in height about40% and still remain effective.

Disadvantages

Use of this design for FPDs with long spans should beayoided, particularly when the connector has beeti

530 Quintessence internationai Voiurne 27, Number 8/1996

Zuckerman

Fig 12 Molar preparation with palh ot insertion along ¡tslong axis.

Fig 13 Molar retainer with male porlion of the PDC on Ihemesial surface. The nonrJgid conneotor is aligned parallel toIhe path ol insertion of the premolar preparation.

Fig 14 Anterior section ot FPD placed over the PDC andon the premolar abutment.

Fig 15 Completed prosthesis atter insertiorî.

reduced in height. Slight movement of the abutment towhich the pontics are rigidly connected can produceexcessive muvement between the components of thenonrigid connector.

Another disadvantage of this design is that theiaboratory costs for a segmented FPD are higher thanthe cost of a rigidly assembled FPD. Furthermore,most laboratory technicians are unfamiliar with thisunusuai application ofa nonrigid connector and willhave to he instructed accordingly.

Technical procedure

1. The dies and casts are fabricated and articulated asfor any FPD.

2. Wax patterns are made for both retainers.3. The male pattern of the PDC is recessed into the

mesial surface of the pattern for the molar retainer.The male attachment is positioned parallel to thepath of insertion of the premolar retainer.

4. The guiding plane of the connector is located in thecontact area between the molar retainer and thepontic. The gingiva! end of the connector iscontoured to establish even contact with the crest ofthe ridge (Fig 11).

5. The female pattern of the PDC is piaced over themale pattern. A wax pattern for the pontic is madeover ihc female pattern. The gingival facing end ofthe assembled connector should present a smooth, un-interrupted continuation of the ridge lap surface ofthe pontic when the prosthesis has been completed.

6. The patterns of the pontic and the premolarretainer are connected and cast as one piece.

7. The sections of the FPD framework are assembledand the gingival embrasure between the moiar andpontic is contoured with an abrasive separating disk.

Figures 12 to Í5 illustrate the clinical application ofthe PDC, which is completely concealed within theFPD,

Quintessence International Volume 27, Number 8/1996 531

Zuckerman

Summary

Segmented FPDs are a well-established method re-commended to overcotne the problems associated withthe insertion of FPDs when the abutments are severelymisaligned. Traditional designs used to create seg-mented FPDs were examined, and their disadvantageswere discussed.

A new design was suggested that does not have mostofthe disadvantages associated with segmented pros-theses. Manufactured plastic patterns that can be usedto create this segmented FPD design are available.These economical, prefabricated, burnout patternssimplify laboratory fabrication and produce uniform,consistent, and reliable nonrigid connectors. Theextracoronal application of an interiocking attachmentfor segmented FPDs provides a practical solution to adifficult probiem.

References

1. Lubow RM, Cootey RL, Kaiser D. Periodontat and restorativeaspects of molar uprighting. J Prosthet Dent I982;47:37.1.

2 Shillingburg HT, Hobo S. Whitsett LD. Fundamentals of FixedProsthudonties, ed 2. Chicago: Quintessence, i9Bi.

1 Zuckerman GR. Faetors that influence the meciianicai retention oíthe complete erown. Int J Proslhodont 19SB;i; Í36-20O.

4. Zuckerman GR. Resistance form for the complète veneer crown:Principles ofdesign and anaiiïis. Int JProsthodont t9äS;l:302-:!07.

5. Rosenstiel SF. Land ME Fujimoto J Contemporary Fked Prostho-dontics. ed I. St Louis: Mosby. 19S8

6. O'Connor RP. Caughman WF. Bemis C. Use of the split ponticnonrigid connector with the tilted molar abutment. J Prosthet Dent1986;56:249-2;i.

7. Moulding MB, Holiand GA. Sulik WD. An alternative orientation ofnondgid connectors in fixed partial dentures. J Prosthet Dent

8. D

532

Aesthetic Designfor Ceramic Restoratioiis

The mostnatrural,

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porary' techniques—and this hook descriheshow. Investigated are the characteristics ofnatural dentition, tissue tnanagement, impres-sions, occlusal records, waxing techniques,color, and lahorator)' techniques, includingmethods for an aesthetic ceramic margin,opalescence, and development ofthe dentinemamelon. Filled with practical tips, this hookis ideal for dentists and technicians.

159 pages; 292 color illus; ISBN 1 -85097-034-3

Contents1 Studies of Natural Dentition

2 Dentist-Technician-Paticnt Communication

3 Tissue Management for Aesdieticand Biological Harmony

4 Impressions artti Occlusai Records

5 Aesthetic Wax Diagnostic Control6 Accurate Registration and Communication

of Colour Characteristics

7 Advanced Laboratory Techniques8 Case Studies

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