The Construction of Crowns for Removable Partial Denture Abutment Teeth David R

The construction of crowns for removable partial denture abutment teeth David R. Burns* / John W. Unger*

The construction of crowns for teeth that will serve as abutment teeth for removable partial dentures is an important and technically demanding procedure. The preparation of the tooth itself must include provision for the components and path of placement of the removable partial denture and at the same time conform to all the parameters associated with proper tooth preparation. Some of the factors unique to surveyed crowns that must be considered include the position of the tooth in relation to the proposed oeelusal plane with the removable partial denture, the type of facial and lingual looth contours desired for the direct retainer, the size and suitability of existing restorations, and the proposed path of placement and removal of the removable partial denture relative to the orientation of the abutment tooth as well as to other adjacent teeth if the abutment also is to be used as a fixed partial denture retainer. The design requirements for the removable partial denture must be determined before treatment is initiated, to account for all of these factors. (Quintessence Int 1994:25:471^75.)

Introdnctiou and undercuts as required by the RPD design. Further- more, the tooth must be prepared so that it simultane- Dentists are frequently faced with the need to place a ously accounts for the path of placement and removal crown on a tooth that will serve as an abutment tooth of both the crown and the RPD. Without careful plan- for a removable partial denture (RPD). As greater em- ning and execution, a predictable result cannot be phasis is placed on retaining teeth and with the achieved. profession's increased ability to restore teeth, the hke- It is the purpose of this article to review some of the lihood increases that teeth that may have otherwise situations where abutment crowns may be indicated been extracted will now be maintained as useful abut- and to describe the preparation of these teeth as well as ments through the placement of crowns. the fabrication of the finai restoration. An abutment tooth must always be prepared in ac- cordance with sound prosthodontic principles for the Treatment planuing crown preparation itself, but additional considerations in preparation design that are unique to the abutment There is perhaps a no more critical aspect of prostho- crown for an RPD must also be incorporated. The com- dontics than the treatment planning process. This is es- pleted crown will reflect these additional preparation pecially true in situations involving removable partial requirements to provide correctly developed heights of dentures and abutment crowns. As part of a complete contour (survey lines), guiding plane areas, rest seats, oral examination, the teeth are examined clinically for caries, pocket depth, attachment loss, mobility, and oeelusal interferences. In addition, radiographs are used to confirm and expand on the clinical findings. Mounted diagnostic casts are an absolutely essential Associate Professor, Department of Prosthodontics, Virginia component of the treatment planning process. The Commonwealth University. School of Dentistry, PO Box 566, mounted casts allow for an accurate assessment of both MCV Station, Richmond, Virginia 2329S. Associate Professor and Chairman, Department of Prosthodon- the interarch distance and the plane of occlusion. Fur- tics, Virginia Commonwealth LIniversity. thermore, individual tooth position as well as the occlu-

Quintessence International Volume 25, Number 7/1994 471 Prosthodontics sal contacts between the teeth can be evaluated. The gument for placement of complete metal crown resto- casts can also be placed in the surveyor so that the path rations when esthetics is not a concern. When the oc- of insertion and removal for the removable partial den- clusai surface and the angle of cervical convergence are ture can be determined in relation to tlie teeth. An ac- appropriate, but the survey lines are slightly deficient, curate assessment of the height of contour and the requiring additional bulk of tooth structure, a crown amount and location of the available undercut is also continues to be a sound treatment choice. Composite determined with the surveyor (Fig 1 ). resin has proved successful in these situations as well, Specific attention is directed at the potential abut- when acid etched to the abutment tooth surface to alter ment teeth and the role they will play not only in rela- survey lines to the desired morphology. Dimpling of tionship to the RPD but as a part of the total prostho- the tooth to develop an undercut will often result in a dontic treatment plan. Teeth that have been weakened rough surface that can promote dental caries and so by extensive caries, wear, or fracture are generally un- should be done only with great care. suitable as abutment teeth without the placement of a It is often necessary to place crowns on RPD abut- crown,' In addition, teeth that are improperly posi- ment teeth for reasons other than the contour of the tioned in either a faciohngual or mesiodistal direction teeth or the size and extent of any existing restorations. often cannot be corrected relative to the planned path An RPD abutment tooth may also serve as a retainer of placement and removal of the RPD without place- for a fixed partial denture, or the need for splinting ment of a crown. Potential abutment teeth that arc lo- teeth will necessitate the placement of a crown on an cated above the desired plane of occlusion frequently otherwise appropriate abutment tooth. Additionally, require crowns for restoration to an acceptable level,- an attachment-retained RPD most often involves the Establishing the correct level and orientation of the oc- placement of crowns on the abutment teeth; the attach- clusai plane is a crucial process that is central to the de- ments are either attached to or incorporated into the velopment of a properly functioning occlusion (Fig 2). coronal contour of the crown (Fig 4). The type of restoration to be provided for the poten- When any of these circumstances occur, the abut- tial abutment tooth must reflect the functional require- ment tooth is subject to the same considerations as pre- ments of the tooth as well as the design requirements of viously stated: however, additional requirements may the RPD. Dentists are accustomed to selecting the type come into play as well. For the fixed partial denture re- of cast restoration based on the severity of dental caries tainer, the tooth must be prepared for the path of place- or the size of existing restorations. For abutments, se- ment and removal of the individual retainer, the fixed lection of the type of restoration must also be based on partial denture, and the RPD. When the proposed the location of existing survey lines on natural tooth RPD design includes components that will cross the oc- structure, the amount of exist ing undercut, the angle of clusai surface of the abutment crown, alterations in the cervical convergence, the location of the tooth relative normal tooth preparation are required. In situations to the desired plane of occlusion, and so on. like this the buccal, lingual, and occlusai embrasures of Class II amalgam and gold inlay restorations that re- abutment crowns must be modified to increase their store either marginal ridges or occlusai areas planned size and provide additional space for the clasp assem- for an RPD rest seat should generally be replaced with bly to be placed (Fig 5). an onlay or crown. When these restorations are re- Careful consideration of the proposed treatment of placed or when the plane of occlusion requires modifi- the abutment tooth relative to the RPD design as well cation, an onlay or three-quarters crown restoration as the tooth itself requires a thorough understanding can be suitable if the survey lines on the natural teeth not only of preexisting conditions discovered in the ex- are otherwise appropriate. amination process, but also of the final treatment plan Unfortunately, this is not always the case and a com- in its entirety. Problems and complications result when plete veneer crown must be planned. Tliis restoration fixed restorative treatment is carried out independent offers the greatest flexibility, because survey lines as of RPD design considerations. well as the occlusai surface can be incorporated into the restoration. If a metal-ceramic restoration is planned for an abut- Clinical treatment ment tooth, the desired survey line is developed with The preparation phase for an abutment tooth begins at the use of the surveyor and by adding or reducing por- treatment planning with the development of the RPD celain' (Fig 3). This method is cumbersome and an ar- design. The diagnostic cast is placed and examined on

472 Quintessence International Volume 25, Number 7/1994 Prosthodontios

(- Fig 1 Cast in plaoeon asurveyortable with the analyzing Fig 2a Cast of mandibular teeth demonstrating an irregu- rod used to determine the alignment of the tooth surface. lar occlusal plane.

Fig 2b Cast of mandibular teeth following restoration of Fig 3 Height of contour marked on a porcelain-fused-to- the ocolusal plane. metal crown.

Fig 4 Era attachment (APM-Sterngold) on the distal sur Fig 5 Clasp components crossing the occlusai suriaces of face of an abutment tooth. teeth. Additional space is required to avoid interference between the clasp assemblies.

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when the dentition includes supraerupted teeth, ta- cially positioned maxillary posterior teeth, or mesially and lingually tipped mandibular molars (Fig 6). To begin the intraoral procedure, guiding plane surfaces and proper faciolingual contours on those RPD abutment teeth that will not receive crowns are estah- lished. The abutment teeth that will receive crowns are prepared initially to reflect the desired plane of occlusion. With supraerupted teeth, this may necessitate root canal therapy. Whenever the need for root canal therapy can be reasonably anticipated, it should be made a part of the initial treatment plan. Any additional modifications in preparation design related to abutment tooth position are completed at this time by using the previously made index. Next the teeth are prepared following the tenets of conventional crown preparation, varying the design based on the type of material planned for the crown fabrication. For complete metal crowns, reduction of at least 1.0 mm of tooth structure is usually recommended, whereas, with metal-ceramic crowns, reduction of 1.5 to 2.0 mm is frequently required. Once this is completed, final alteration in the preparation of the tooth as dictated by the position of the component parts is accomplished. This most often involves providing the necessary reduction for rest seats and guiding planes. Fig 6 Index constructed on the diagnostic cast for use in leveling the occlusal plane. (A) Supraerupted first molar re- Occasionally, the required reduction of the axial quires modification. (B) Index is fabricated on diagnostic tooth surfaces can be quite extensive. The retention cast and provides reduction guide intraorally. Hash marks and resistance form of the completed crown can there- indicate area of tooth reduction necessary to correct plane fore be compromised. Where the tooth is tipped more of occlusion pnor to conventional tooth preparation. than 30 degrees from the path of insertion of the RPD, consideration should be given to other treatment op- the surveyor. It is at this time that the path of insertion tions, such as orthodontic repositioning of the tooth, re- of the RPD is determined and the component parts of duction of the tooth to serve as an overdentnre abut- the RPD are selected and located. The positions of the ment, or, in extreme cases, extraction of the tooth. Ii teeth that are to receive abutment crowns must be care- tnay be possible to improve the retention and resis- fully analyzed in relation to the proposed path of inser- tance form of a tipped tooth by altering the preparation tion for the RPD. Tt is a fundamental principle that Ihe design. When a tooth is tipped mesially. the mesial wall completed abutment crowns be contoured to be consis- of the tooth is prepared parallel to the path of insertion tent with this path. of the proposed RPD. Axial grooves are placed on the buccal and lingual surfaces of the preparation. This When the position of the abutment tooth differs sig- tooth preparation design can be adapted to fit other in- nificantly from the previously determined path of in- stances of abutment tooth malposition by altering the sertion, an alteration in the normal preparation of the location of the grooves. Pins may also be used to im- tooth is required to ensure the development of a cor- prove the retention and resistance form of some prep- rectly positioned height of contour without resorting to arations. improper contouring of the crown. Modification that is consistent with the desired tooth preparation is com- Once impressions are made and casts are formed and pleted on the diagnostic cast. An index of this prepara- articulated, a wax pattern can be developed for the tion may be fabricated on the diagnostic cast and later abutment crown. Before the formation of the crown in transferred to the mouth to aid in the clinical reduction wax, the path of insertion for the RPD is reestablished of the abutment tooth. This is particularly important on the surveyor with the master cast to ensure that the

474 Quintessence Intemational Volume 25, Number 7/1994 Prosthodontics crown is constructed in harmony with this path. Rest seats, guiding planes, and axial contours are developed in wax to fil the design of the RPD and lo parallel the path of insertion and removal."'-^ It is generally best to wax the crown to nomial contours consistent with the anatomic characteristics of the tooth being restored and in harmony with the desired plane of occlusion. Modification of the wax pattern to parallel the path of insertion and to provide for the component parts of the RPD is then accomplished (Fig 7). The pattern is cast in the appropriate metal, finished, polished, and made ready for the addition of any ve- neering material. The appropriate height of contour and amount and location of undercut must be correctly Fig 7 Blade attached to surveyor used to develop surfaces established on the veneered surface. The crown is then in Ihe wax pattem paraliel to the path of ¡nsertJon of the removable partial denture. completed and made ready for intraoral try-in. Following the necessary clinical procedures to fit the crown to the tooth and the adjacent and opposing teeth, a final verification of the crown contours is made, once again, on the cast surveyor. Tlie crown is then luted in place and the fabrication of the RPD can commence. References 1. Stewart KL. Rudd KD. Kuebker WA. Cürical Removable Par- tial Piosthod on tics. St Louis: Mosby, 1983:2yy. Summary 2. Unger JW, Btirns DR. The use ot the splint bar iti conjunction The task of fabricating a crown for an RPD abutment with removable partial denture Ireatment. Quintessence lnt tooth is a complex one. All too often, inadequate con- 3, Malotie WFP, Koth DL, Cavazos E. Kaiser DA, Morgaño SM. cern is given to the unique aspects of this restoration. Tylnan's Theory and Practice of Fixed Pruslhodonlics, ed 8. St The nature of the planning process, design of the prep- Louis: IshiyakuEuro America, 1989:374. aration, execution of this design, and the fabrication of 4, Seals RR. Schwartz IS. Successful integration of ii\t(l and re- the crown itself makes the RPD abutment crown one of movable piosthod on tics. J Prosthet Dent 1985 ;53:763-766. 5, Seals RR, Stratton RJ. Surveyed crowns: a key for integrating the most challenging and demanding restorations in all fixed and removable prosthodontics. Quintessence Dent Tech- of dentistry. noll987;n:43^9. O

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