10 THE PARTIAL VENEER CROWN, INLAY, AND ONLAY PREPARATIONS
An extracoronal metal restoration that covers only part of the clinical crown is considered to be a partial veneer crown. It can also be referred to as a partial-coverage restoration. An intracoronal cast metal restoration is called an inlay or an onlay if one or more cusps are restored. Examples of these restorations are presented in Figure 10-1. Partial veneer crowns generally include all tooth surfaces except the buccal or labial wall in the preparation. Whenever feasible, a partial-coverage restoration should be selected, rather than a complete veneer, because it preserves more of the tooth’s coronal surface. However, the preparation is more demanding and is not routinely provided by practitioners. Buccolingual displacement of the restoration is prevented by internal features (e.g., proximal boxes and grooves). The partial veneer can be used as a single-tooth restoration, or it may serve as a retainer for a fixed dental prosthesis (FDP). It can be used on both anterior and posterior teeth. Because it does not cover the entire coronal surface, it tends to be less retentive than a complete crown and is less resistant to displacement. Unless the partial veneer is very carefully prepared, the reduced retention may contraindicate its use. Inlays and onlays are even less retentive than partial veneer crowns and are not recommended for FDP retainers. However, they provide the advantages of a casting, with less enamel removal than for a crown. When carefully prepared, they can produce an exceptionally long-lasting restoration.
Fig. 10-1 A, Partial veneer crowns serving as retainers on the premolars for a four-unit fixed dental prosthesis replacing the maxillary first molar. B, Maxillary premolars restored with gold inlays, and molar restored with gold onlay. These restorations have served for about 35 years.
The indications, contraindications, advantages, and disadvantages of partial veneer crowns are described first, and any specific deviations that pertain to a given preparation are identified with that type.
Partial veneer crowns often can be used to restore posterior teeth that have lost moderate amounts of tooth structure, if the buccal wall is intact and well supported by sound tooth structure. They are also commonly used as retainers for an FDP or where restoration or alteration of the occlusal surface is needed. Anterior partial veneers are rarely suitable for restoring damaged teeth, but they can be used as retainers, to reestablish anterior guidance, and to splint teeth. They are particularly suitable for teeth with sufficient bulk because they can accommodate the necessary retentive features.
Partial veneer restorations are contraindicated on teeth that have a short clinical crown because retention may not be adequate. They are also contraindicated as retainers for long-span FDPs. They are rarely suitable for endodontically treated teeth, especially anterior teeth, because insufficient supporting tooth structure remains for the retentive features. Likewise, they should not be used on endodontically treated posterior teeth if the buccal cusps are weakened by the access cavity or on teeth with an extensively damaged crown. As is true of all cast restorations, partial veneer restorations are contraindicated in dentitions with active caries or periodontal disease.
The shape and alignment of teeth are important determinants of the feasibility of partial veneer crowns. The alignment of axial surfaces should be evaluated, and partial veneer crowns should not be placed on teeth that are proximally bulbous. Making the necessary proximal grooves on these teeth is likely to leave unsupported enamel. It may be similarly impossible to prepare adequate grooves on thin teeth of restricted faciolingual dimension.
Partial veneer crowns are usually prepared parallel to the long axis of the tooth, and poorly aligned abutment teeth may not be suitable. When poorly aligned teeth are being prepared for a partial-coverage restoration, problems with unsupported enamel often result.
The primary advantage associated with partial veneer crowns is conservation of tooth structure. Another advantage is reduced pulpal and periodontal insult during tooth preparation. Access to supragingival margins is rather easy and allows the operator to perform selected finishing procedures that are more difficult or impossible with complete coverage restorations. Access is also better for oral hygiene. Because less of the margin approximates the soft tissues subgingivally, there is less gingival involvement than with complete coverage.
During cementation of a partial veneer, the luting agent can escape more easily, which produces relatively good seating of the restoration. Because of direct visibility, verification of seating and cement removal are simple. When the restoration is in service, the remaining intact facial or buccal tooth structure permits electric vitality testing.
Partial veneer restorations have less retention and resistance than do complete cast crowns. Preparing the tooth for this type of coverage is difficult, primarily because only limited adjustments can be made in the path of placement. The preparation of grooves, boxes, and pinholes requires dexterity of the operator. Some metal is displayed in the completed restoration, which may be unacceptable to patients with high cosmetic expectations.
The following discussions cover the teeth most commonly prepared for partial veneer restorations. The use of partial veneers on anterior teeth has declined because of the difficulty in achieving an esthetic result. The technique illustrated may be suitable for posterior teeth and, with minimal variation, for other teeth. On both posterior and anterior teeth, meticulous care and precision are required if partial veneer restorations are to be a successful (conservative) alternative to complete-coverage restorations.
This is the typical armamentarium for a partial veneer crown preparation. Depending on operator preference, additional instruments can be used. The regular- or coarse-grit diamonds are used for bulk reduction, and the fine-grit diamonds or carbides are used for finishing. Pinholes are prepared with the twist drill and finalized with a tapered carbide. The fissure burs are recommended for preparing boxes and ledges, and the inverted-cone carbide is recommended for preparing incisal offsets. Hand instruments can be used to finish proximal flares and bevels. A periodontal probe is invaluable for assessing the direction and dimension of the various steps.
The three-quarter crown preparation (Fig. 10-3) derives its name from the number of axial walls involved. Except for a slight bevel or chamfer placed along the bucco-oclusal line angle, the buccal tooth surface remains intact. The other surfaces (including the occlusal surface) are prepared to accommodate a casting in the same manner as a complete crown preparation (see Chapter 8), differing only in the need for axial retention grooves.
Fig. 10-3 The maxillary premolar three-quarter crown. A, Initial depth holes are placed in the mesial and distal fossae approximately 0.8 mm deep. B, They are connected by a guiding groove that extends through the central groove. Additional guiding grooves are placed on the lingual cusp similar to those for a complete cast crown (see Fig. 8-8). The depth cut placed on the triangular ridge of the buccal cusp becomes shallower as it approaches the cusp tip. C, Half the occlusal reduction is completed. Note the functional cusp bevel. The occlusocervical height of the buccal surface is not reduced at this stage. D, Occlusal reduction completed. E, After guiding grooves are placed in the lingual surface of the tooth parallel to the proposed path of placement, the proximoaxial and linguoaxial reductions are initiated. Simultaneously, a smooth and even-width cervical chamfer is created. F, When the axial reduction of the first half is considered acceptable, the other half can begin. G, Proximal grooves are placed perpendicular to the prepared surface, and the buccal wall of each groove is flared to leave no unsupported enamel. The proximal flares are connected with a narrow contrabevel. After rounding of the line angles, the preparation is complete. H, The interproximal clearance relative to adjacent teeth extends cervically as well as near the occlusal aspect of the buccal flares of the proximal grooves.
Upon the completion of occlusal reduction, a clearance of at least 1.5 mm should exist on the functional cusp and at least 1.0 mm on the nonfunctional cusp and in the central groove. Simultaneously, the tooth should be prepared so that the restoration displays a minimum of metal, with preservation of the buccal wall outline.
Fig. 10-6 Recommended minimum clearances for reduction of a partial veneer crown preparation. Slight hollow grinding of the lingual incline of the buccal cusp results in an acceptable clearance with the least display of metal. Also, the final restoration retains the normal contours of the cuspal ridge, so that incident light is not reflected and the restoration is less evident.
Fig. 10-8 A, Upon completion of the proximal axial reduction, a groove is placed perpendicular to the prepared surface. B, Note that some unsupported tooth structure remains at the cavosurface angle. C, After the buccal wall of the proximal groove is flared, no unsupported tooth structure remains. Note: It is important to anticipate in advance the influence of the buccal extent of the proximoaxial reduction (A) on the ultimate location of the margin (C).
Fig. 10-10 Because of the rotary instrument’s taper, the proximal groove is deeper near the occlusal table (A). The floor of the groove should be flat and smooth. Often the proximal chamfer extends slightly cervically to the floor of the groove. If only minimal difference exists, as in B, the cervical margin adjacent to the groove can be beveled. The recommended occlusocervical height for a proximal groove is 4 mm.
Fig. 10-11 The 90-degree angle between the lingual walls of the proximal grooves and the axial walls resists lingual displacement. Because the buccal aspect of the grooves has been adequately flared, no unsupported tooth structure remains.
Depending on available access, it may be feasible to complete the flaring with the same rotary instrument that was used to place the groove (Fig. 10-12). However, removing the last lip of unsupported tooth structure with a chisel is often a better option, because this minimizes the risk of damage to the adjacent tooth.
Fig. 10-12 A, Initial preparation of the mesial proximal groove. Note that the carbide is oriented parallel to the path of placement as dictated by the lingual surface of the tooth. B, Initial flaring has resulted in elimination of most unsupported tooth structure. C, Hand or rotary instruments are used to refine these proximal flares and remove all unsupported enamel.
If additional bulk is needed to ensure rigidity of the restoration, it can be provided with an occlusal offset. This V-shaped groove extends from the proximal grooves along the buccal cusp. It is not usually necessary for posterior partial veneer crowns but is essential for the structural durability of anterior partial veneer crowns. This is described in detail on p. 301.
The principles used in a premolar preparation also apply for a maxillary molar (Figs. 10-15 and 10-16). However, some additional leeway may exist for groove placement because more tooth structure is present on molars than on premolars. Also, because of their less prominent position in the dental arch, molars are less visible. As a result, the mesioproximal flare can sometimes be extended onto the buccal surface without spoiling the esthetics.
The seven-eighths crown preparation (Fig. 10-17) includes, in addition to the surfaces covered by the three-quarter crown, the distal half of the buccal surface. Therefore, the mesial aspect of this preparation resembles that for a three-quarter crown; the distal aspect resembles that for a complete crown. The mesial half of the buccal tooth surface remains intact and is protected by a narrow contrabevel or chamfer similar to the one used in the three-quarter crown preparation. A distal groove may be placed, although generally this is not necessary. A groove in the middle of the buccal surface is placed parallel to the path of placement. Distal to this groove, the buccal surface is reduced in two planes, cervical and occlusal; the cervical plane parallels the path of placement, and the occlusal plane follows the normal anatomic contour. The lingual surface of the tooth also is reduced in two planes, and functional cusp bevels are incorporated.
Fig. 10-17 The maxillary molar seven-eighths crown preparation. A, Occlusal depth grooves. On the lingual of the mesiobuccal cusp, they are identical to grooves for any functional cusp. On the buccal, note their difference from grooves placed on the triangular ridges. The mesial groove becomes shallower as it approaches the cuspal ridge; the distal extends through the cuspal ridge. B, Mesial half of the occlusal reduction is completed. Normal occlusal form can be recognized in the reduced area. C, Occlusal reduction completed. D, Distal half of the axial reduction completed. This is comparable to the preparation for a complete cast crown. The rotary instrument is moved parallel to the guiding grooves placed in the lingual tooth surface. E, Mesial half of the axial reduction completed and a proximal groove placed. F, The buccal groove, with flaring of the mesial groove. Note the monoplane of the flare, extending from the deepest portion of the groove to the cavosurface angle. G, A contrabevel connects the mesial flare with the buccal groove. The mesial wall of the buccal groove is smooth and has a 90-degree cavosurface angle, leaving no unsupported enamel.
Fig. 10-18 The seven-eighths crown preparation. Note that adequate clearance has been established. From this perspective, it is evident why little or no flaring is necessary for the buccal groove, as opposed to the considerable flaring needed for the mesial groove.
Mandibular partial veneer preparations (Fig. 10-20) are made more often on premolars than on molars. They differ from maxillary molar three-quarter crown preparations in two respects: (1) Additional retention is required because of the shorter crown lengths of mandibular teeth. This can b/>