10: THE PARTIAL VENEER CROWN, INLAY, AND ONLAY PREPARATIONS

10 THE PARTIAL VENEER CROWN, INLAY, AND ONLAY PREPARATIONSimage

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.

PARTIAL VENEER CROWNS

Several types of partial veneers exist: for posterior teeth, three-quarter, modified three-quarter, and seven-eighths crowns; for anterior teeth, three-quarter crowns and pinledges.

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.

Preparation

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.

Posterior Partial Veneer Crown Preparations

Maxillary premolar three-quarter crown

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.

image

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.

Occlusal reduction

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.

3. Place three depth grooves on the lingual incline of the buccal cusp. Initially, these should be kept somewhat shallow as they approach the buccal cusp ridge (see Fig. 10-3B). In the area of occlusal contact, the groove should be about 0.8 mm deep so that there will be at least 1 mm of clearance after finishing.
5. Assess the amount of occlusal clearance in maximum intercuspation and in all excursive movements of the mandible (Fig. 10-5). Grinding a small concavity on the incline of the buccal cusp may help obtain sufficient clearance while maintaining the original occlusocervical dimension of the buccal tooth surface (Fig. 10-6).

Axial reduction

9. Carry the diamond into the proximal embrasure and reduce the proximal wall (see Fig. 10-3E and F). For proper reduction of the axial tooth surface, it is important to understand the factors that determine correct positioning of the proximal groove. A proximal groove is placed parallel to the path of placement. Normally, unsupported tooth structure remains on the buccal side of the groove, and this side is flared to remove it. Figure 10-8 illustrates the relationship among the initial axial reduction, groove placement, and location of the cavosurface angle where the flare meets the intact buccal wall. The cavosurface angle is especially significant when a tooth is prepared for a partial veneer that should display a minimum of metal; the farther to the buccal the margin is, the more gold is visible. A subtle but extremely important variable that determines the final location of the cavosurface angle is the apical extension of the preparation. As the cervical chamfer extends closer to the cementoenamel junction, more axial tooth structure is removed. Consequently, the deepest portion of the groove (its pulpal wall) is located slightly closer to the center of the tooth. This results in a flare that can extend farther onto the facial or buccal surface than desirable. Marking the location of the intended facial flare on the tooth with a pencil before initiating the proximoaxial reduction is helpful. The intersection of this mark with the reduced occlusal surface is a convenient reference point.
10. Stop the proximal reduction well short of the pencil mark and usually slightly short of breaking the proximal contact (Fig. 10-9). The resulting flange should be parallel to the linguoaxial preparation, with the chamfer placed sufficiently cervical to provide at least 0.6 mm of clearance with the adjacent tooth and the axial wall allowing for a proximal groove of at least 4 mm of length occlusocervically (see Fig. 10-3F).

Groove placement

Preparation of the proximal grooves is best done with a tapered carbide bur.

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.

Maxillary molar three-quarter crown

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.

Maxillary molar seven-eighths crown

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.

Mandibular premolar modified three-quarter crown

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/>

Only gold members can continue reading. Log In or Register to continue

Jan 17, 2015 | Posted by in Prosthodontics | Comments Off on 10: THE PARTIAL VENEER CROWN, INLAY, AND ONLAY PREPARATIONS
Premium Wordpress Themes by UFO Themes