Rational Tooth Preparation for Porcelain Laminate Veneers
 
Chairside with David A. Garber, DMD
Compend Contin Educ Dent, Volume XII, Number 5.
 
In the last "Chairside" article (March 1991), we discussed "Porcelain Laminate Veneers - To Prepare or Not to Prepare." We came up with a consensus that tooth preparation is essential for predictable esthetic changes and long-term clinical efficacy. Our rationale for this enamel modification encompassed:

The modification to the tooth should be intraenamel whenever possible - but with the advent of the newer dentin adhesive systems, this factor may not be as critical as we perceived it to be some years ago.

The preparation will be addressed from both a biologic periodontal perspective and the technical laboratory requirements. That is, enamel preparation is required to ensure the prevention of changes in the underlying periodontium, and to meet the technician's requirement to rapidly and effectively produce esthetically fine porcelain laminate veneers on a regular basis.

The desired form of enamel modification (Figures 1A and 1B) can be viewed from four distinct aspects:

1. labial reduction
   
2. interproximinal extension
   
3. incisal modification
   
4. cervical definition

1. Labial Reduction
The labial reduction required is a minimum of 0.5 mm, a thickness of ceramic readily managed by ceramists. Gauging the amount of enamel reduction requires a series of depth cuts, and a particularly simple system is the three-tiered depth cutter. This instrument is merely stroked across the labial enamel surface from the mesial to the distal to develop threegrooves 0.5 mm deep, but limited in depth by the interspersed shank (Figure 2).

Deveneering
The remaining enamel (colored green in Figure 3) between the grooves must now be reduced to the level of the depth cuts over the entire labial surface.
The labial reduction should be directed toward two distinct facets:

1. the reduction of the remainder of the labial surface to the level of the depth cuts. This should be done with a coarse diamond to strengthen adhesion (see previous "Chairside" article) and improve light refraction.
   
2. a finish line contiguous with the free gingival margin should be developed with a microfine finishing diamond or appropriately shaped finishing carbide bur. This will facilitate an intimacy of fit between the ceramic and the underlying tooth surface, minimizing the composite at this critical interface. This can be accomplished simultaneously with a Two-Grit® diamond instrument (Figure 4), which will rapidly remove the enamel while creating the desired modified chamfer finish line.

The instrument is moved across the labial surface, following the curvature of the free gingival margin from the tip of the mesial interproximal papilla through the maximal curvature of the gingival and down to the tip of the distal interproxirnal papilla (Figure 5). This completes the labial reduction.

2. Interproximinal Extention
The interproximal extension is developed for two distinct reasons:

1. to improve the adhesion of the laminate to the underlying tooth surface
   
2. to hide the interface between the new veneer in the desired shade of porcelain and the discolored tooth, in a nonvisible area.

To increase bonding strength, the veneer should have adhesive bonds at right angles to the possible direction of displacement. The displacement tends to be in a labial direction, and bonds at right angles can be developed only by extending the cut interproximally where there is sufficient thickness of enamel (Figure 6). This should extend two thirds of the way from the buccal towards the lingual. The preparation is performed with the same rotary diamond Two-Grit® instrument, extending the margin within the confines of the interdental space and ensuring that the junction of the restoration and the tooth will be lingual to the soft tissue of the interproximal papilla.

The interproximal reduction also develops a framework of added strength across the interproximal areas of the tooth and the labial incisal edge to support the otherwise friable laminate (Figure 7).

3. Incisal modification
The labial reduction and interproximal extension thus far completed are now modified at the incisal edge. The form of this enamel modification depends on two factors.

1. whether the clinician intends to increase the length of the tooth or maintain the existing length.
   
2. the buccolingual width of the incisal edge.

No Increase In Crown Length
If the desired esthetic change is not in tooth form, but only in color, it may not be necessary to overlap the incisal edge of the tooth, particularly if there is sufficient buccolingual width. In this situation, the modification will incorporate the removal of an additional 0.5 mm to 1 mm of porcelain across the incisal tip (Figure 8). There is some clinical evidence that not overlapping the incisal edge is the largest cause of failure in laminates. This is probably a valid observation, if the incisal edge is not sufficiently thick to develop the desired edge modification. If it is too thin buccolingually, overlapping the incisal edge into the lingual surface would be prudent.

Increase In Crown Length
If it is desired, for any reason, to increase the length of the crown by adding to the incisal aspect, it is important to overlap the incisal edge (Figure 9A). This involves a slight flattening of the incisal edge to accentuate the buccolingual width, thereby increasing the available bonding surface. The incisolabial line angle must be rounded, thereby also increasing the area available for bonding and avoiding a sharp angle from which to propagate fracture. This preparation is then extended onto the lingual surface to develop a mini-chamfer (Figures 9A and 9B).

In the previous article, we discussed the need to develop bonds at right angles to the direction of potential displacement of the tooth. If the mandibular incisor traverses the lingual aspect of the maxillary tooth in a protrusive movement, the initial direction of displacement is incisally. The adhesive bonds at right angles to this are developed within the lingual chamfer. As the mandibular incisor reaches the incisal edge of the maxillary tooth, it progresses anteriorly. Bonds at right angles to this direction of force are found on the flatness of the incisal edge and are aided by encompassing this edge in porcelain.

It is important not to terminate the incisal preparation in situations where excursive movements of the mandible will cause shearing stresses at the junction of porcelain and tooth surface (Figure 10). The lingual chamfer is critical to avoiding the placement of a butt joint directly on the horizontal incisal edge. If there is no overlapping lingual chamfer, and only a butt joint, protrusive movements of the mandible will tend to torque the incisal porcelain off the underlying tooth surface, with a resultant fracture, usually at the junction of the labial and incisal planes (Figure 10).

A particularly convex labial surface of any tooth can complicate the process of overlapping the incisal edge because of the problems with developing a path of insertion. When not overlapping the incisal edge, the path of insertion is usually from the labial towards the tooth surface in a lingual direction.

When overlapping the incisal edge, the path of insertion is, of necessity, from the incisal towards the apical. If the labial convexity is such that a single path of insertion becomes difficult, it is possible to accentuate the curvature of the incisal line angle to rotate the laminate into position about the incisal edge. The laminate will be positioned on the incisal edge and then rotated into position on the labial surface of the tooth.

4. Cervical Definition
The desired position for the finish line of the veneer is just within the confines of the gingival sulcus. Unless a very dramatic color change is desired, there is no reason to hide the interface between the laminate and the tooth subgingivally. In most situations, it is possible to achieve a blend in color from the tooth surface to the porcelain restoration that is not noticeable, even if it ends supragingivally.

The finish line is developed with the tip of the Two-Grit® diamond or a fine finishing bur. A narrow-gauge gingival retraction cord is placed in the sulcus for 1 to 2 minutes to displace the tissue apically and laterally. This cord can be impregnated with an astringent, such as aluminum sulfate, to facilitate the retraction process. Because there should be no bleeding at this stage, leaving the cord for only 1 to 2 minutes is sufficient to create the gingival displacement. If there is bleeding for any reason, such as when extending somewhat deeper to include or incorporate an old restoration into the preparation, it may be necessary to leave the cord in place for a total of 7 to 10 minutes to facilitate hemostatis.

The process of first delineating the original position of the free gingival margin with the finish line, and then displacing the tissue, leaves this finish line at least temporarily supragingival. This provides for:

1. the ability to refine and define the finish line under direct vision. The concept is to now develop a smooth, harmonious finish line for easy adaptation of the porcelain.
   
2. access for the diamond instrument, so that there is no gingival trauma. The process of finishing the margin with the fine grit tip of the Two-Grit® diamond finishing bur moves it just within the confines of the sulcus, without compromising the dentogingival unit or changing the subgingival contour of the tooth. This finish line must be in a position where it is readily available for the finishing procedures when placing the porcelain veneer.

The only time that it is desirable to go further subgingivally is when preparing extremely dark, tetracycline-stained teeth for veneers. In these situations, the staining is darkest in the cervical region, where there is a diminished thickness of enamel to cover the stained dentin. In fact, in this situation, it may be necessary to remove slightly more tooth - although the tooth gets darker as this is done - to provide for an adequate thickness of porcelain to neutralize the underlying color.
The configuration of the finish line should be a modified chamfer, at least 0.4 mm thick. Although feather-edged preparations and rounded shoulders have been suggested, the modified chamfer appears to result in the least dimensional change during firing of the porcelain.

The modified chamfer finish line should provide for:

1. an adequate thickness of porcelain for strength without overcontouring.
2. enamel preparation exposing the enamel rods at the correct angle for increased bonding strength during the luting process
3. a definitive seat for positioning the laminate accurately on the tooth.
4. an easily distinguishable finish line in impressions and during laminate fabrication
5. a smooth, finished tooth surface and harmonious form for accurate adaptation of the porcelain to the preparation.

This four-stage process for veneer preparation will enable the ceramist to effectively produce a color - corrected, accurately fitting porcelain restoration, thereby minimizing the potential for causing untoward soft-tissue reactions. It will also allow you to effectively develop the esthetic changes your patients desire.

Next article: One of the major controversies currently discussed in the dental literature and in continuing education lectures throughout the world of dentistry is: Do dental lasers work? If so...for what? Are they any sort of panacea? Seemingly knowledgeable clinicians are making a variety of claims. We'll look into this controversy next time.

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