Oral Health Group
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Development and Rationalization of the Ideal Tooth Preparation

July 1, 2006
by Adrian Jurim


It has been almost 20 years since I invented the etched porcelain laminate. Since then, I have observed, followed and participated in the evolution of “the prep” for this procedure. The purpose of this paper is to explain what does and doesn’t work when performing the etched porcelain laminate.

Composite bonding, a very noble and revolutionary concept, had catalyzed the evolution of esthetic dentistry. However, although its use was widespread, many dentists were not convinced that composite bonding was the ideal solution for anterior aesthetics. On a practicable level, a good percentage of dentists did not enjoy or feel secure with the responsibility of incrementally reconstructing teeth. This had previously been delegated to technicians, subject to the dentist’s approval, and many were uncomfortable starting from scratch.

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Additionally, though composites were and still are ideal far anterior repairs, the wear potential and possibility of discoloration made them less than perfect for extensive anterior alterations. Unlike its composite bonding counterpart, porcelain laminates fulfill all of the above esthetic requirements. For this reason and innumerable others, the development of the porcelain laminate was overwhelmingly welcomed and rapidly absorbed into the dental profession to become the standard bearer of esthetics in dentistry.

THE IDEAL PREP

Initially, when the porcelain laminate technique was developed, no one prepared teeth. The laminate concept was very new and the primary concerns rested on the stability of the bond and the proper thickness of the porcelain. However, as these problems were overcome, we realized the proper tooth preparation was essential for strength, gingival health and ideal aesthetics. Today, it is universally accepted that teeth must be prepared for best results.

From the vantage point of running a dental laboratory, I see numerous prepared teeth daily. Most of the cases, however, have not been properly prepared. Consequently, I am alarmed and saddened by how misinterpreted the prep is. The purpose of the following text is to explain in detail the proper means of preparing teeth for porcelain laminates.

I have done close to half a million laminates and the knowledge I have accumulated over the past 20 years is what I aim to share with you. However, to be able to talk about the “ideal prep”, we must first establish some crucial guidelines:

* sharp line angles will create stress points and eventually fracture the porcelain laminate;

* the porcelain laminate should not be thinner than 0.5mm;

* the porcelain should be 1.0mm thick in high stress bearing areas (incisal);

* gingival preparation is essential to allow the laminate a normal gingival emergence profile and prevent over-contouring;

* proximal preparation permits concealment of the interproximal porcelain margin and additional wraparound for greater bond strength. Furthermore, the increased thickness of the porcelain edges makes the laminate less likely to chip during insertion;

* incisal preparation yields increased porcelain thickness at the important stress bearing incisal area, permits a positive seat during cementation and hides incisal margins.

When beginning to prepare a tooth, I recommend the use of depth cutting burs to gauge the amount of tooth structure removed, assuming that the length and shade of the tooth does not need to be changed. Incisal reduction must be 1.0mm and should be perpendicular to the long axis of the tooth. Labial reduction should be 0.5mm and follow the labial tooth contour. Moreover, while the incisal labial has to be rounded, the lingual incisal should end in a sharp butt joint (Figs. 1 & 2).

Continuing with the preparation procedure, a deep chamfer at the gingival finishing line, should follow the contour of the soft tissue from mesial proximal to distal proximal.

The most common question asked is where to place this deep chamfer in relation to the soft tissue: subgingival, equagingival, or supragingival. The determining factor for this problem is the contrast between the shade of the prepared tooth and the shade of the finished restoration. For example, if the prepared tooth is a light shade like A1 Vita Lumin and the goal is to finish with the same shade, the gingival finishing line can be kept supragingival. In a situation where the starting shade is A2 Vita Lumin and the desired finished tooth’s shade is once again A1 Vita Lumin, then an equagingival placement of the finishing line is acceptable. Finally, in a situation where the prepared tooth’s shade is A4 Vita Lumin and the finished tooth’s shade is Al Vita Lumin, then the finishing line must be subgingival. Thus, it can be concluded that the greater the contrast between the starting and desired finishing shades, the further subgingival the finishing line should be placed on the prepared tooth. (Fig. 3)

PROBLEMATIC TOOTH PREPARATION

The most prevalent problems I have encountered in cases sent to my laboratory are those where the incisal labial has not been rounded off and instead sharp line angles have been left. This mistake creates a stress point in our new restoration that may fracture the laminate. Furthermore, it is impossible to achieve the ideal gradual transition of shade from the incisal to body color when there is a sharp line angle on the incisal labial. After the porcelain is bonded onto this type of preparation, the invariable result is a sharp demarcation of color at the transition area of body to incisal edge. This is demonstrated in Figure 4a, where light reflects off a mal-prepared tooth but travels through it at the incisal.

Figure 4b reveals how the rounded surface at the incisal labial reflects light in all directions, yielding an ideal transition of the shade from body color to incisal color.

Another common preparatory problem is demonstrated in Figs. 5a and b. Here, the incisal reduction is done in a sloping angle toward the lingual instead of being perpendicular to the long axis of the tooth. This creates an acute line angle that acts as a chisel within the porcelain laminate. The result is a certain fracture of the new restoration.

The faulty preparation pictured in Figure 6a does not follow the labial contour of the tooth. Consequently, the incisal labial was not adequately reduced by the minimal 0.5mm. Bonding the porcelain laminate to teeth prepared in this fashion forces the finished case further out labially then the original tooth, thus creating an unsatisfactory result (Fig. 6b).

Problems may also occur during lingual reductions, as evidence by the faulty preparations pictured in Figures 7a-c. The most common questions involving lingual reductions concern how far down and how far deep the reduction should be. Ideally, lingual reductions should be 0.5mm deep and should go down 0.5mm (Fig. 7a). A greater reduction creates a definite unyielding insertion path. If that reduction is NOT parallel to the gingival emergence profile, then a fracture of the porcelain laminate is unavoidable at insertion time. Another problem present in certain lingual reductions occurs when teeth are thinner than 1.5mm labio-lingually in the incisal third. Reducing the labial and lingual incisal on such teeth by 0.5mm leaves a thin sliver of tooth structure that cannot be rounded off (Fig. 7b). This is a stress point that will weaken the new restoration. When presented with such a situation, the tendency is to under-reduce the depth of the lingual overlap to 0.2mm instead of the required 0.5mm (Fig. 7b). Remember that the porcelain laminate cannot be made any thinner than 0.5mm. Thus, when the depth of the lingual preparation is 0.2mm and the thickness of the porcelain laminate is 0.5mm, the end result is a ledge of porcelain beyond the normal contour of the tooth (Fig. 7c). Dentists must then reduce the laminate with a rotary instrument to the lingual contour
of the tooth.

However, all that remains thereafter is 0.2mm of porcelain, such that a lingual portion of the restoration will break away after short time. For these reasons, cases involving teeth that are thinner than 1.5mm labio-lingually in the incisal third should be prepared as described in “The Ideal Prep” section of this paper, without any sort of lingual reduction.

With the myriad of potential problems surrounding lingual reductions, one may ask why the incisal must be reduced at all. The trouble with preparations lacking incisal length reduction occurs when a force is applied incisally after the porcelain laminate is inserted (Fig. 8). This junction is the weak link in the bonding chain. The great bond strength reported in all pertinent porcelain laminate studies is measured by pulling apart mass of porcelain from mass of composite resin (Fig. 8a). A peeling force can be introduced by applying a force at the incisal interference (Fig. 8).

However, since this peeling force is measured to be only 10 percent of the forces represented in Figure 5a, the weak interface at the incisal edge experiences small chipping of the porcelain shortly after the laminate is bonded into position. Compare this situation to that in Figure 8b where the incisal tooth structure is reduced by 1.0mm as specified by the ideal prep procedure. Here, the laminate benefits from the strength of the 1.0mm of solid porcelain and virtually no fractures are reported when a force similar to that pictured in Figure 8 is applied since it becomes a compression force and porcelain is at its absolute strongest in compression.

The same principal applies in the case of the incisal fracture in Figure 9. Many clinicians advocate rebuilding the fracture area in composite followed with a conventional tooth preparation and then bonding the laminate to complete the restoration. The danger here is that the flexure module of the composite and of the porcelain are substantially different. Consequently, when a force is applied in the incisal area, the composite always gives a little and the porcelain without a rigid support will break (Fig. 9a). Instead, I recommend that sharp edges of the fracture line are eliminated and the tooth be prepared as outlined earlier (Fig. 9b). The porcelain laminate fabricated for this type of restoration varies in thickness from 0.5mm on the labial to upwards of 6.0mm. of solid porcelain in the restored missing fractured area. The skill of the ceramist technician is highly tested here, but the benefit is well worth it. Applying the same incisal force as in Figure 9a, the strength of the solid porcelain will prevent compressive fracture.

Many cases involve restorations of 10 or even 12 teeth, extending deep into the posterior area. The buccal preparation of the upper posterior teeth is no different from what we presented thus far (Fig. 10a). On the occlusal surface, a deep chamfer is placed half way between the buccal

cusp tip and the central fossa. The depth of this chamfer is 0.5mm and the cut perpendicular to the occlusal incline angle. The preparation is completed with 1.00mm incisal reduction and then the rounding of the incisal buccal line angle (Fig. 10b). This becomes even more critical on the lower posterior teeth. The lower buccal cusps are functioning cusps and must have 1.00 mm of solid porcelain to support their activity. For aesthetic reasons, the occlusal preparation should be extended into the central fossa. (Figs. 10c & d)

Special consideration is necessary when preparing interproximally in cases where contact points are present and must remain so. These teeth are prepared with a 0.5mm deep chamfer interproximally and the finishing line should be brought lingually as far as possible without breaking the contacts (Fig. 11).

Particular attention must be paid to the reduction at the gingival interproximal. This so-called “dog leg” area {the transition area from buccal to gingival proximal} is frequently under-prepared. When this area is reduced by 0.2mm instead of 0.5mm, the finished case will look over contoured and impingement at the papilla is very possible. Also, when the proximal finishing line is not placed lingual enough, the junction between porcelain and tooth is noticeable, often making the case aesthetically unacceptable.

In cases where we have to change the proportion of the tooth (mesial and distal width) to achieve an aesthetic smile balance, contact points have to be opened.

When contact points are not present and we are working with diastemas, the finishing line concept changes considerably. In these situations, a straight wall is preferred rather than a chamfer. Observe in Figure 12 that on the same tooth the distal has a conventional chamfer finishing line while the mesial has a straight wall finishing line, where only the height of contour was reduced. For illustration purposes, Figure 13 shows the left central prepared correctly so it can be compared with the incorrectly prepared right central. The slice preparation on the mesial of the left central, where only the height of contour was reduced, allows the technician to bring the porcelain completely into lingual to a continuation of the lingual contour of the tooth. The right central was prepared incorrectly mesially as mentioned, with a chamfer instead of a slice preparation. As illustrated, the porcelain laminate cannot go past the sharp line angle created by this chamfer preparation and therefore cannot be brought into continuation with the lingual edge. This area thus becomes both an irritant to the tongue and a plaque trap, stressing the importance of adapting the straight line finishing wall in such instances.

CONCLUSION

The purpose of this article was twofold. First, I hoped to have disavowed the myth that tooth preparation is unnecessary with porcelain laminates. I and many others are greatly distressed by those who, for whatever reason, advocate little or no tooth preparation. I do not believe anyone has constructed more laminates than me over the past 20 years, and having seen nearly 500,000 units, I feel confident in stating unequivocally that preparation is essential for healthy tissue and aesthetically attractive laminates.

Second, I believed it necessary to stress that proper preparation is paramount to fully take advantage of porcelain laminates. Using the guidelines outlined here, I hope that the path to beautiful and healthy results is now easier to follow.

Adrian Jurim is the owner of Jurim Dental Studio, Inc., in Great Neck, New York.

Oral Health welcomes this original article.


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