Principles of Tooth Preparation

Iatrogenic damage to an adjacent tooth is a common error in dentistry. Even if a damaged proximal contact area is carefully reshaped and polished, it remains more susceptible to dental caries than the original undamaged tooth surface (Fig. 7.2), presumably because the original surface enamel contains higher fluoride concentrations and the polished layer is more prone to plaque retention.⁴ Sound tooth preparation technique avoids and prevents damage to adjacent proximal surfaces.

A metal matrix band placed around the adjacent tooth for protection may be helpful; however, the thin band can be perforated and the underlying enamel still damaged. The preferred method is to use the proximal enamel of the tooth that is being prepared for protection of the adjacent structures. Teeth are 1.5 to 2 mm wider at the contact area than at the cementoenamel junction. Therefore, a thin, tapered diamond or tungsten carbide rotary instrument can be passed through the proximal contact area (Fig. 7.3) while leaving a slight “lip” or “fin” of enamel without resulting in excessive tooth reduction or necessitating undesirable angulation of the rotary instrument. The latter situation, tipping the diamond unnecessarily away from the adjacent proximal surface, is a common clinical error.

Placing preparation finish lines subgingivally takes great planning and care lest the periodontal soft tissues becomes red, inflamed, or recede (see Chapter 5). Naturally, a poor periodontal response at the gingival margin of a crown can adversely impact the esthetic result.

Damage to the soft tissues of the tongue and cheeks can be prevented by careful retraction with an aspirator tip, mouth mirror (Fig. 7.4), or flanged saliva ejector. Great care is needed to protect the tongue when the lingual surfaces of mandibular molars are being prepared.

Great care is also needed to prevent pulpal injuries during fixed prosthodontic procedures, especially when significant amounts of tooth structure are being removed. Pulpal degeneration that occurs many years after tooth preparation has been documented.5 Extreme temperatures, chemical irritation, or microorganisms can cause an irreversible pulpitis,⁶ particularly when they occur on freshly sectioned dentinal tubules. Prevention of pulpal damage necessitates selection of techniques and materials that reduce the risk of injury while teeth are prepared.⁷

Tooth preparations must account for the geometry of the pulp chamber. Pulp size can be evaluated on a radiograph and decreases with age. Up to about age 50, it decreases more so occlusocervically than faciolingually. Average pulp dimensions have been related to coronal contour⁸ and are presented in Table 7.1 and Fig. 7.5. Literature reports 5.7% to 33.8% of teeth prepared for a compete crown or fixed partial denture (FPD) retainers succumb to pulpal degeneration and require endodontics.^9–12 Vital pulp survival in teeth restored with single metal-ceramic crowns was more favorable than teeth prepared for FPD retainers.¹² Pulp tissue in maxillary anterior teeth prepared for metal-ceramic FPD has a higher risk of necrosis than for any other tooth type.¹² This is not a contradiction for crowns or FPDs but illustrates the importance of careful case selection, meticulous delivery of patient care, and constant improvement of one’s clinical skills.

Crown contours have been long debated with little scientific evidence. Gingival inflammation is commonly associated with crowns and fixed dental prosthetic abutments that have excessive axial contours, probably because it is more difficult for the patient to maintain plaque control around the gingival margin (Fig. 7.18).⁵⁹ Successful preparations provide a balance between sufficient space for the development of anatomically correct axial contours and conservation of tooth structure to preserve a durable abutment. The junction between the restoration and the tooth must be smooth and free of any ledges or abrupt changes in direction.

In most circumstances, a crown should duplicate the profile of the original natural tooth that it is restoring.60,61 Natural anterior teeth have facial mean subgingival emergence angles of 9.93 ± 5.68 degrees to the long axis of the tooth and 14.34 ± 8.44 degrees at the lingual.⁶² As the contours migrate supragingivally, they slightly increase on the facial and lingual 11.13 ± 7.92 degrees and 15.58 ± 9.16 degrees respectively.⁶² If an error is made, a slightly undercontoured, flat restoration is better because it is easier to keep free of plaque; however, increasing proximal contour on anterior crowns to maintain the interproximal papilla⁶³ (see Chapter 5) may be beneficial from an esthetic perspective. A general guideline is to match the contours of the contralateral tooth. Following this guideline, one would need to impress the complete arch so that these contours are communicated to the laboratory. Sufficient tooth structure must be removed to allow the development of correctly formed axial contours (Fig. 7.19), particularly in the interproximal and furcation areas of posterior teeth, where periodontal disease often progresses with serious consequences.

Whenever possible, the margin of the preparation should be supragingival. Restorative margins placed within the periodontal sulcus are known as subgingival margins. Subgingival margins of indirect restorations have been identified as a factor in gingival inflammation, bleeding on probing, and recession of the periodontium (see Chapter 5).^64–70 Supragingival margins are easier to prepare accurately without trauma to the soft tissues and facilitate impression making or optical capture. They can usually also be situated on hard enamel, whereas subgingival margins are often on dentin or cementum.

Advantages of supragingival margins include the following:

Subgingival margins (Fig. 7.20), however, are indicated if any of the following conditions are present:

Axial contour modification is indicated: for example, to provide an undercut to provide retention for a removable partial denture clasp (see Chapter 21). To start the emergence profile of a crown more apical to close gingival black triangles Average dimensions for clinical crown height and sulcus depth in young healthy adults are provided in Fig. 7.21.

The interface between a restoration and the tooth is always a potential site for recurrent caries because of dissolution of the luting agent and inherent surface roughness. The more precisely the restoration is adapted to the tooth, the lower is the risk for recurrent caries or periodontal disease.71 Although a precise number for acceptable marginal gap width is not known, opinion papers have established a range from 40 to 120 μm.⁷² A skilled technician can routinely make castings that fit to within 10 μm⁷³ and porcelain margins that fit to within 50 μm,⁷⁴ provided that the tooth was properly prepared. A well-designed preparation has a smooth and even margin. Rough, irregular, or “stepped” junctions between tooth and restoration greatly increase overall margin length and substantially reduce the adaptation accuracy of the restoration (Fig. 7.22).^75,76 The clinical significance of preparing smooth margins cannot be overemphasized. Time spent obtaining smooth margins makes the subsequent steps of tissue displacement, impression making, laboratory communication, die formation, waxing, and finishing much easier and ultimately results in longer-lasting restorations. Smooth, accurately placed preparation finish lines are particularly important when restorations are fabricated with a computer-aided design and computer-aided manufacturing (CAD-CAM) process.⁷⁷ Preparations need to consider the shape and diameter of the burs that mill the ceramic restorations. Preparations with sharp line angles result in restorations with over-milled intaglio surfaces. Over milling naturally leads to large internal cement gaps.

Many fixed dental prostheses depend on the geometric form of the preparation and then are supplemented with adhesion. To understand the theory of mechanical retentive forms, it is helpful for one to first understand traditional luting agents. Zinc phosphate, a traditional luting agent, is nonadhesive (i.e., they act by increasing the frictional resistance between tooth and restoration). Interestingly, zinc phosphate is also water soluble. The grains of the luting agent prevent two surfaces from sliding, although they do not prevent one surface from being lifted from another. This is analogous to the effect of particles of sand or dust within machinery: They do not adhere specifically to metal, but they increase the friction between sliding metal parts. If sand or dust gets into a mechanical camera or watch, the increase in friction can effectively jam the mechanism. To demonstrate the success of mechanical retention, zinc phosphate, and the highest level of clinical precision of cast gold restorations, a retrospective clinical evaluation of 1314 cast gold restorations by Dr. R. V. Tucker was performed. Over a period of service that ranged from 1 to 52 years, data showed an overall failure rate of 4.6% with a survival rate of 95.4% (Fig. 7.33).¹¹⁴

Traditional luting agents are effective only if the restoration has a limited number of paths of placement (i.e., the tooth is shaped to restrain the free movement of the restoration). The relationship between a nut and a bolt is an example of restrained movement (Fig. 7.34): The nut is not free to move in just any direction; it can move only along the precisely determined helical path of the threads on the bolt.

The relationship between two bodies, one (in this case, a tooth preparation) restraining movement of the other (a luted restoration), has been studied mathematically and is known in analytical mechanics as a closed lower pair of kinematic elements.¹¹⁵ In fixed prosthodontics, a sliding pair is the only pair that has relevance. It is formed by two cylindrical^a surfaces constrained to slide along one another. The elements are constrained if the curve that defines the cylinder is closed or shaped to prevent movement at right angles to the axis of the cylinder (Fig. 7.35).

A tooth preparation is cylindrical if the axial surfaces are prepared by a cylindrical rotary instrument held at a constant angle. The fixed curve of the mathematical definition is the gingival margin of the preparation, and the occlusoaxial line angle of the tooth preparation should be a replica of the gingival margin geometry. The curve of a complete crown preparation is closed, whereas the grooves of a partial crown preparation prevent movement at right angles to the long axis of the cylinder. However, if one wall of the complete crown preparation is overtapered, it is no longer cylindrical, and the cemented restoration is not constrained by the preparation because the restoration will have an increased number of pathways for withdrawal. Under these circumstances, the cement particles tend to lift away from rather than slide along the preparation, and the only retention is a result of the cement’s limited adhesion (Fig. 7.36).

Taper

Taper is defined as the convergence of two opposite-facing external walls of a crown preparation as viewed in a given plane (e.g., taper between a mesial wall and a distal wall, or between a buccal wall and a lingual wall of a crown preparation). The extension of those planes forms an angle described as the angle of convergence. Theoretically, maximum retention is obtained if a tooth preparation has parallel walls. However, it is neither desirable nor practical to prepare a tooth this way with current techniques and instrumentation because (1) some convergence is desirable to allow escape of excess luting agent during seating of the crown and (2) slight undercuts are often present in preparations that are too cylindrical and prevent the restoration from seating.

An undercut on a complete crown preparation is defined as any irregularity in the wall of a prepared tooth that prevents the withdrawal or seating of a wax pattern or crown. Such is the case when divergence is inadvertently created between opposite-facing external axial walls, or wall segments, in a cervico-occlusal direction (Fig. 7.37A). In other words, if the cervical diameter of a tooth preparation at the margin is narrower than at the occlusoaxial junction (reverse taper), it is impossible to seat a complete crown of similar geometry (see Fig. 7.37A and B). Undercuts can be present whenever two axial walls face opposite directions (see Fig. 7.37C). Thus, the mesial wall of a complete crown preparation can be undercut in relation to the distal wall, or the buccal wall can be undercut in relation to the lingual wall, and the mesiobuccal wall can be undercut in relation to the distolingual wall. In a partial veneer preparation, in accordance with the same principle, the lingual wall of a proximal groove can be undercut in relation to the lingual wall of the preparation, but the buccal wall of the same groove cannot be undercut in relation to the lingual axial preparation wall; either of these walls may, however, restrict the number of directions in which a restoration can be placed on the preparation in relation to the other.

A slight convergence, or taper, is clinically desirable in complete crown preparations. As long as this taper is small, the movement of the luted restoration will be effectively restrained by the preparation and will have what is known as a limited path of placement. As taper increases, so does the number of paths of placement. An increase in the number of paths of placement decreases the tooth preparations retention form.

The relationship between the degree of axial wall taper and the magnitude of retention was first demonstrated experimentally by Jørgensen116 in 1955. He cemented brass caps on Galalith cones of different tapers and measured retention with a tensile-testing machine. The relationship was found to be hyperbolic, with retention rapidly becoming less as taper increased (Fig. 7.38), although the relationship was no longer hyperbolic when the internal surfaces of the caps were roughened. The retention of a cap with 10 degrees of taper^b was approximately half that of a cap with 5 degrees. Similar results have been reported by other workers.^117–119

Selection of the appropriate degree of taper for tooth preparation involves compromise. Too small a taper may lead to unwanted undercuts; too large leads to a lack of retention. The traditionally recommended convergence between opposing walls has been 6 degrees. The 6 degrees of convergence was developed between two opposing axial walls of 3 degrees of convergence to a path of removal chosen by a clinician. However, to give perspective, the traditional 6-degree convergence form was recommended to optimize retention for zinc phosphate cement.120 Being able to recognize this angle is important (Fig. 7.39). It is necessary to be able to quickly quantify the approximate angle of convergence between preparation walls. To evaluate the angle from an occlusal perspective, it is recommended to close one eye to judge the relative convergence of all involved axial walls.¹²¹ When both eyes are used to evaluate taper, axial walls that are divergent (undercut) up to 10 degrees cannot be detected from a typical viewing distance of 25 to 45 cm (10 to 20 inches).¹²¹ Since most individuals are quite skilled at discerning subtle differences in height and width, but less effective in terms of our depth perception, evaluating preparation convergence from the buccal or lingual is often straightforward. Since adjacent teeth can partially obstruct the view of the preparation silhouette from mesial or distal, most will evaluate convergence from as many different directions as conveniently feasible.

It is not necessary to deliberately tilt a rotary cutting instrument to create a taper because this invariably leads to overpreparation. Rather, teeth are easily prepared with a rotary instrument of the desired taper held at a constant orientation. The tapered rotary instrument is moved through a cylindrical path as the tooth is prepared, and the taper of the instrument should produce the desired axial wall taper on the completed preparation. In practice, many dentists experience difficulty consistently avoiding excessively tapered preparations, particularly when preparing posterior teeth with limited access.122–124 Comprehensive review of the dental literature suggests that clinically acceptable taper for a complete crown preparation may range from 10 up to 20 degrees.¹¹¹ It should be noted, however, that angles of convergence in excess of 10 degrees will result in reduced dentin thickness between the pulpal tissues and the axial walls of the prepared tooth.

Clinicians have a tendency to overtaper preparations. Abutments for fixed dental prostheses tend to be prepared with greater taper than do single crown preparations.¹²¹ The visual quantification skill is low enough that some may need additional training.¹²⁵ One technique that is useful intraorally is the use of a dental bur of known taper to survey the tooth preparation. Another technique is a total occlusal convergence template of known angles to overlay on to tooth preparations to evaluate the amount of taper (Fig. 7.40).

Some authorities recommend the routine use of secondary retention features like grooves, boxes, or even pin holes to supplement the primary retention of crown preparations. Theoretically, they can reduce the incidence of restoration displacement. But it is unclear, however, whether accurate alignment of secondary retentive features is achieved more easily than restricted axial wall convergence. Skillfully prepared axial walls at a minimal convergence are mechanically desirable for primary retention while having a very conservative tooth structure. Furthermore, one research article evaluated the dentin thickness with different amounts of axial taper. In regard to maxillary first premolar, when the axial reduction was preformed with a margin width of 1.2 mm and the axial walls converged at 20 degrees, the proximity to the pulp ranged from 0.3 to 1.8 mm of dentin thickness.126 Dentin is a porous material that delivers protection to the dental pulp. An axial wall of 0.3 mm in thickness raises a clinical concern of creating irreversible pulpitis. Twenty degrees of axial wall convergence may be acceptable for a large molar but is a questionable practice on teeth with smaller diameters. Secondary retentive features may be useful to supplement the primary retention form when retreating teeth that have previously been prepared and over tapered. In the case of a maxillary first premolar with 20 degrees, there may not be enough dentin thickness for a groove without getting a pulp exposure. Since 1955, adhesive resin cements have been developed that have increased the retention of crowns. For some restorations, research suggests that the selection of a resin cement over a water-based luting agent may have more clinical relevance to enhance crown retention than does preparation taper.^127,128

Type of preparation

Different types of preparation have different retentive values that correspond fairly closely to the total surface area of the axial walls with restricted taper, as long as other factors (e.g., preparation height) are kept constant. Thus, the retention of a complete crown is more than double that of partial-coverage restorations (Fig. 7.41).¹³²

Adding grooves or boxes (Fig. 7.42) to a preparation with a limited path of placement does not markedly affect its retention because the surface area is not increased significantly. However, where the addition of a groove limits the paths of placement, retention is increased.^133,134

When the internal surface of a restoration is very smooth, retentive failure occurs not through the cement but at the interface between the luting agent and the restoration. Under these circumstances, retention is increased if the restoration is roughened or grooved.135–137 Metal castings are most effectively prepared by airborne-particle abrading the fitting surface with 50 μm of alumina. This should be done carefully to avoid abrading the polished surfaces or margins. Airborne-particle abrasion has been shown¹³⁸ to increase in vitro retention by 64%. Similarly, acid etching of the fitting surface of restorations can improve retention with certain luting agents.

Failure rarely occurs at the interface between the luting agent and the tooth. Therefore, deliberately roughening the tooth preparation hardly influences retention and is not recommended because roughness adds to the difficulty of subsequent technical steps in crown fabrication such as imaging, impression making, milling, and waxing (see Chapters 14 and 18).

Retention is affected by both the type of casting alloy and any core or buildup material that is present on the axial walls of the crown preparation. The clinical significance of laboratory testing results has yet to be confirmed by longer-term clinical studies, but it appears that the more reactive the alloy is, the better adhesion there is with selected luting agents. Therefore, base metal alloys are better retained than are less reactive metals with high gold content.139 The effect of adhesion to different core materials also has been tested, with conflicting results. In one laboratory study,¹⁴⁰ researchers examining adhesion between luting agents and core materials found that the luting agent adhered better to amalgam than to composite resin or cast gold. However, when other researchers¹⁴¹ tested crowns for retention, they found higher values with the composite resin cores than with amalgam cores. The differences may have resulted from dimensional changes of the core materials, although the clinical implications of this finding are not clear.

Some patients can develop enormous biting forces. Gibbs and colleagues153 described one individual (Fig. 7.45) who had a biting force of 4340 N (443 kg).^c Although this is considered extraordinary, historically, restoration designs attempted to withstand forces approaching such magnitude. In one laboratory study,¹³⁰ a complete crown cemented on a nickel-chromium test die was found to be capable of withstanding more than 13,500 N (1400 kg)—a far greater force than would occur in the mouth—before becoming displaced (Fig. 7.46).

In a normal occlusion, biting force is distributed over all the teeth; most of it is axially directed. If an FPD is carefully made with a properly designed occlusion, the load should be well distributed and favorably directed (see Chapter 4). However, if a patient has a biting habit such as vaping or parafunctional movements, it may be difficult to prevent fairly large oblique forces from being applied to a restoration. Consequently, the successful tooth preparation and restoration must be able to withstand considerable oblique forces, as well as the normal axial ones, and it has been argued that from a clinical durability perspective, adequate resistance form may be more crucial than overall preparation retentiveness.^154,155

Resistance is a function of the relationship between axial wall taper, preparation diameter, and preparation height. Consider the diameter of a tooth as a radius of an imaginary circle. The radius represents a pathway a crown would theoretically pivot off of a tooth. If the arch of the radius interfered with the axial wall of its tooth preparation, the crown would have resistance (Fig. 7.47). If the arch of the radius did not interfere with the axial wall of its tooth preparation, the crown would not have resistance and rotate off the preparation (Fig. 7.48). Resistance decreases as taper or diameter increases or as preparation height is reduced.¹⁵⁶ The relationship between preparation height, or diameter, and resistance to displacement is approximately linear.¹⁵⁷

Preparation taper of 5 to 22 degrees has been suggested as being within a clinically acceptable range.158,159 However, at the higher end of this range, the tipping resistance of both cemented and uncemented cast restorations is inadequate but increases significantly as taper is reduced.^158,160

Short tooth preparations with large diameters were found to have very little resistance form.¹⁶¹ In general, molar teeth require more parallel preparation than do premolar or anterior teeth to achieve adequate resistance form.¹⁶¹ A 3-mm preparation height provides adequate resistance if taper is restricted to 10 degrees or less,¹⁶⁰ but additional height is necessary as tooth diameter increases. On molar crown preparations in which many more preparations are observed that lack resistance,¹⁶² minimal preparation wall height should thus be in the range of 3.5 to 4 mm. A fairly simple way to quantify this at chairside is to evaluate whether the height-to-width ratio of a preparation is 4:10 or greater. If so, resistance is probably adequate.

Hegdahl and Silness¹⁶² analyzed how the areas that provide resistance form change as the geometry of the tooth preparation is modified. They demonstrated that increasing preparation taper and rounding of axial angles tend to reduce resistance; pyramidal preparations thus have greater resistance than do conical ones. Secondary retentive grooves or boxes placed in healthy tooth structure are particularly effective in enhancing the resistance form of crown preparations because these shorten the theoretical radius of rotation which results in a greater chance of interfering with rotational movement (tipping) of the crown and thereby subject additional areas of the luting agent to compression (Fig. 7.49). Therefore, the resistance form of an excessively tapered preparation can be improved by adding such grooves or boxes. As an alternative, pinholes can be prepared to achieve the same effect by making use of dentin that surrounds the pin.

Preparation modifications appear not to be used as often¹⁵⁷ as clinical failure data suggest they should be.¹⁵⁵

A partial-coverage restoration may have less resistance (Fig. 7.50) than a complete crown because it has no buccal resistance areas. In this case, resistance is provided by proximal boxes or grooves (Fig. 7.51) and is greatest if the groove, box walls, or both are perpendicular to the direction of the applied force. Thus, U-shaped grooves or flared boxes provide more resistance than do V-shaped ones.¹³² On short preparations, the reverse scenario can apply: Short complete crown preparations may lack resistance, where proximal grooves, in comparison, will then result in better resistance on a partial veneer crown. In general, ideal grooves and boxes should be prepared so that their walls are located in a healthy tooth structure. Placing proximal grooves as close as possible to the location of the original proximal contact offers the opportunity to maintain as much dentin as possible because this is where the tooth has its greatest mesiodistal dimension. However, the proximal axial walls are typically shorter in the occlusogingival dimension than the buccal and lingual axial walls because of the scalloped shape of the periodontium. As a result, buccal and or lingual retentive grooves are generally longer than mesiodistal grooves. An in vitro study reported that mesial-distal grooves delivered better resistance to dislodgment than buccal-lingual grooves.¹⁶⁰ However, the study only looked at forces applied in a buccal direction. Whereas it is reasonable to assume that most loads resulting in crown dislodgment are applied from either a buccal and lingual direction since some bracing of the restoration occurs as a function of stabilizing proximal contacts, it is not possible to declare an absolute optimal location for retentive grooves. Each tooth has to be individually assessed for its resistance form in all horizontal directions. The weakest area of the tooth preparation will have the longest radius of rotation for a crown to pivot off. Once the longest radius has been identified, a retentive groove can be placed in the axial wall that will shorten the radius of the specific rotation (see Figs. 7.48 and 7.49).

Similarly, in order to more effectively enhance resistance, grooves that are placed in excessively inclined preparation walls should be prepared to greater depth in their cervical aspect than in their occlusal aspect. Taper restriction in the cervical aspect of an excessively tapered crown preparation has been shown to enhance resistance more effectively than do grooves that are prepared flush into excessively inclined preparation walls because the number of paths of removal is reduced.163

Resistance to deformation is affected by physical properties of the luting agent, such as compressive strength and modulus of elasticity.164 To satisfy American Dental Association/American National Standards Institute specification no. 96 (International Standards Organization specification no. 9917), the compressive strength of zinc phosphate cement must exceed 70 MPa^d at 24 hours (Fig. 7.52).^165–168 Glass ionomer cements and most resins have higher compressive strength, whereas polycarboxylates have values similar to those of zinc phosphate.¹⁶⁴

Increasing temperature has a dramatic effect on the compressive strength of luting agents, particularly weakening reinforced zinc oxide–eugenol cement (Fig. 7.53). An increase from room temperature (23°C) to body temperature (37°C) halves the compressive strength of reinforced zinc oxide–eugenol cements, and a rise in temperature to 50°C (equivalent to hot food) reduces the compressive strength by more than 80%.¹⁶⁹ Equivalence testing of more modern luting agents has not been reported.

Zinc phosphate cements have a higher modulus of elasticity than do polycarboxylate cements, which exhibit relatively large plastic deformation.170 This may account for the observation that the retentive ability of polycarboxylate cement is more dependent on the taper of the preparation than is the retention with zinc phosphate cement.¹⁷¹

In the selection process of choosing a durable luting agent, clinicians have a choice in how the dental cement is delivered and mixed. Cements come in a liquid-powder or dual paste form and can be hand mixed, auto-mixed through convenient dispensing tips, or triturated in prepackaged capsules (see Chapter 30). Although the decision process of luting agent selection may be driven by factors such as mixing ease and cleanup time, the method of luting agent mixing and delivery can affect resulting physical properties as well. One thermocycle study investigated the long-term aging effect of monolithic zirconia crowns cemented on natural teeth with powder-liquid and paste-paste forms of the same resin-modified glass ionomer. Interestingly, the retention of the zirconia crowns luted with the dual paste method required less than half of the removal force than the crowns cemented with the same commercial resin-modified glass ionomer cement that was a powder-liquid mix.¹⁷²

The factors that affect the resistance to displacement of a cemented restoration are summarized in Table 7.5.

Table 7.5

Factors Influencing the Resistance of Cemented Restorations

Composite luting agents shrink upon polymerization and deliver a pre-compressive force to bonded ceramics.173 On thermal loading, the luting composite also expands and generates tensile forces within ceramic restorations.¹⁷⁴ When bonded feldspathic ceramic restorations fit well (i.e., a gap width of approximately 100 μm) the luting composite tends to protect the ceramic against thermal expansion.¹⁷⁴ Lithium disilicate and zirconia restorations have also been shown to mechanically benefit from adhesive luting.^175,176 Laboratory research reports that for posterior ceramic occlusal veneers, there is no significant difference in fracture resistance between 1 mm and 2 mm of lithium disilicate ceramic when bonded to tooth structure (Fig. 7.55).¹⁷⁷

Largely according to empirical data, metal restorations require a minimum alloy thickness of about 1.5 mm over functional cusps (buccal in the mandible, lingual in the maxilla). The less stressed nonfunctional cusps can be protected with less metal (1 mm is adequate in most circumstances) for a strong and long-lasting restoration. Occlusal reduction should be as uniform as possible, following the cuspal planes of the teeth; this ensures that sufficient occlusal clearance is combined with the preservation of as much tooth structure as possible. In addition, an anatomically prepared occlusal surface (Fig. 7.56) gives rigidity to the crown because of the “corrugated effect”¹⁷⁸ of the planes.

When teeth are misaligned or overerupted, the occlusal surface needs to be prepared with the thickness requirements of the eventual restoration in mind. For example, a supra-erupted tooth may need considerably more than 1.5 mm of reduction to establish adequate clearance so that optimal occlusal form and the appropriate plane can be reestablished and adequate restoration thickness ensured (Fig. 7.57). Diagnostic tooth preparation and waxing are helpful in determining the correct tooth reduction. A practical approach is to reshape the supra-erupted tooth on the diagnostic cast to the desired occlusal plane that is anticipated. Diagnostically, the opposing teeth and, if necessary, the target tooth itself can be waxed to final form. An external matrix is then fabricated over the diagnostic endpoint from a suitable elastomeric putty. After sectioning, this can be used intraorally as a reduction guide to ensure that optimal, yet conservative tooth reduction is achieved (Fig. 7.58).

Ideally, cervical enamel should be conserved to maintain structural integrity of the tooth and for bonding. When finish lines need to be extended apically, tooth reduction must provide sufficient room for the bulk of restorative material at the margin to prevent distortion. For example, as discussed before, one disadvantage of the feather edge margin preparation is that the resulting thin layer of gold is not as strong as the comparatively thicker restoration of a chamfer margin preparation. When teeth have been prepared with increased taper, however, it is advisable to reduce margin width in order to maintain adequate dentin thickness between the axial preparation wall and the pulpal tissues.179

Quantitatively, the amount of reduction in the cervical part of a preparation is a function of the restorative material selected. For gold castings or high-strength anatomic contour zirconia and bonded lithium disilicate, 0.3 to 0.5 mm is adequate; for bilayered ceramic and metal-ceramic crowns, 1 to 1.2 mm of facial shoulder or chamfer margin width is desirable but not easily achieved on small teeth or teeth with large pulps. Lower strength ceramic crowns can be fabricated successfully on shoulder margin preparations with a margin width of 0.8 to 1.0 mm, although minimum dimensions may increase for certain materials and fabrication techniques.¹⁸⁰ With the advent of CAD-CAM restorations, the limitations of the milling systems used may influence specific reduction requirements. Not all milling machines are equal. The selection of CAD-CAM laboratory equipment or a laboratory that uses CAD-CAM technology should be weighed carefully against the biological criteria discussed at the beginning of this chapter.

The grooves and ledges incorporated in a partial-coverage preparation provide essential strengthening for such castings that yet remain inherently weaker than their complete-coverage counterparts; in particular, the classic anterior pinledge retainer benefited from the beamlike reinforcement that resulted from the incorporation of ledges in the tooth preparation design (Fig. 7.59).

Although type I and type II gold alloys (see Chapter 22) are satisfactory for intracoronal cast restorations, they are too soft for crowns and fixed dental prostheses, for which type III or type IV gold alloys (or an appropriate low-gold alternative) are chosen. These are harder, and their strength and hardness can be further increased by heat treatment.

Metal-ceramic alloys with high noble metal content have a hardness equivalent to that of type IV gold alloys, whereas nickel-chromium or cobalt-chromium alloys are considerably harder yet. These may be indicated when large forces are anticipated, as with a long-span FPD, although their use presents certain challenges (see Chapter 19). Even the stronger alloys need sufficient bulk if they are to withstand occlusal forces.

If there is to be sufficient bulk of porcelain for appearance and metal thickness for strength, adequate reduction of the facial surface is essential. The exact amount of reduction depends to some extent on the physical properties of the alloy used for the substructure, as well as on the manufacturer and the shade of the porcelain. A good color match for some restorations in older individuals typically requires a slightly greater porcelain thickness than is needed in younger patients. A minimum reduction of 1.5 mm is typically required for optimal appearance. Adequate thickness of porcelain (Fig. 7.62) is needed to create a sense of color depth and translucency. Shade problems are frequently encountered in maxillary incisor crowns at the incisal and cervical thirds of the restoration, where direct light reflection from the opaque layer can make the restoration very noticeable. Because opaque porcelains generally have a shade different from that of body porcelains, they often need to be modified with special stains in these areas (see Chapter 24).¹⁸¹

With very thin teeth (e.g., mandibular incisors), it may be impossible to achieve adequate tooth reduction without exposing the pulp or leaving the tooth preparation severely weakened. Under these circumstances, a less-than-ideal appearance may have to be accepted.

The labial surfaces of anterior teeth should be prepared for metal-ceramic restorations in two distinct planes (Fig. 7.63). If they are prepared in a single plane, the reduction in either the cervical or the incisal area of the preparation is insufficient.

The extent of proximal reduction is contingent on the exact predetermination of the location of the metal-ceramic junction in the completed restoration. The proximal surfaces of anterior teeth look most natural if they are restored at the incisal edges, without metal backing. This allows some light to pass through the restoration in a manner similar to what occurs on a natural tooth (Fig. 7.64). Obviously, if the restoration is part of an FPD, the need for connectors makes this impossible.

The patient’s smile is observed as part of the initial examination (see Chapter 1). It is important to record which teeth and which parts of each tooth are exposed. Patients with a high lip line, which exposes considerable gingival tissue, present the greatest problem if complete crowns are needed. Where the root surface is not discolored, appearance can be restored with a metal-ceramic restoration with a supragingival porcelain labial margin (see Chapter 24). If the patient has a low lip line, a supragingival margin may be placed because the restorative interface is not seen during normal function

However, it cannot be assumed that the patient will be happy with a supragingival margin just because it is not visible during normal function. Some patients have reservations about exposed restorative interfaces, and the advantages of such supragingival margins must be carefully explained before treatment.

Metal collars can be hidden below the gingival crest, although there is some discoloration if the gingival tissue is thin. Successful margin placement within the gingival sulcus requires care to ensure that inflammation and recession, with resulting metal exposure, are avoided or minimized. The periodontium must be healthy before the tooth is prepared. If periodontal surgery is needed, the sulcular space should not be eliminated completely; rather, a postsurgical depth of about 2 mm should be the objective. Sufficient time should be allowed after surgery for the periodontal tissues to stabilize. Wise¹⁸² found that the gingival crest does not stabilize until 20 weeks after surgery (see Chapter 5).

Margins should not be placed so far apically that they encroach on the attachment; extension to within 1.5 mm of the alveolar crest leads to bone resorption.¹⁸³ Subgingivally placed margins should follow the scalloped contour of the free gingival margin, being further apical in the middle of the tooth and further incisal interproximally. A common error (Fig. 7.65) is to prepare the tooth so that the margin lies almost in one plane, with exposure of the collar labially and irreversible loss of bone and papilla proximally.

Precise placement of the proximal margins (particularly the mesial, generally more visible, margin) is crucial for the esthetic result of a partial-coverage restoration. The rule is to place the margin just buccal to the proximal contact area, where metal is hidden by the distal line angle of the neighboring tooth and yet provides adequate access to the tooth-restoration interface for plaque control. Tooth preparation angulation is critical and should normally follow the long axes of posterior teeth and the incisal two-thirds of the facial surface of anterior teeth. If a buccal or lingual tilt is given to the tooth preparation, the likelihood that metal will be visible increases significantly (Fig. 7.66).

The distal margin of posterior partial-coverage restorations is less visible than the mesial margin. In this area, it is often advantageous to extend the preparation farther beyond the contact point for easier preparation and finishing of the restoration and to facilitate access for oral hygiene.

If the facial margin of metal is correctly shaped (Fig. 7.67), it does not reflect light to an observer. As a result, the tooth appears to be merely a little shorter than normal and not as though its buccal cusp is outlined in metal. If the buccal margin is skillfully placed so as to follow the original cuspal contour, the appearance of the final restoration is acceptable.

When mandibular partial cast crowns are made, metal display is unavoidable because the occlusal surface of mandibular teeth can be seen during speech. A chamfer margin, rather than a beveled margin, is recommended for the buccal margin because it provides a greater bulk of metal around the highly stressed functional cusp (Fig. 7.68). If the appearance of metal is unacceptable to the patient, a metal-ceramic restoration with porcelain coverage on the occlusal surface can be made.

Anterior partial-coverage restorations can be fabricated to show no metal (Fig. 7.69), but their preparation requires considerable care. The facial margin is extended just beyond the highest contour of the incisal edge but not quite to the incisolabial line angle. In this case, the metal protects the tooth from chipping but is not visible.

For all but the most straightforward prosthodontic treatment plans, a diagnostic waxing procedure (Fig. 7.72) should be performed. This is done on diagnostic casts and helps determine optimal contour and occlusion of the eventual prosthesis. The procedure is of particular benefit if the patient’s occlusal scheme or anterior (incisal) guidance requires alteration (Fig. 7.73). Once the teeth are prepared consistent with the needs identified during the diagnostic waxing, interim restorations can be created from the waxing. At a subsequent appointment before making a definitive impression, the interim restorations are removed and the thickness of the restorations and interim cements are evaluated to verify that there is appropriate prosthetic space for the specific definitive restoration(s) (Fig. 7.74).

Each step of a tooth preparation should be carefully evaluated with direct vision or indirectly with a dental mirror. Alignment of multiple abutment teeth can be problematic, and use of the mirror helps superimpose the image of adjacent abutment teeth. To evaluate complex preparations, the dentist should make an alginate impression and pour it in fast-setting stone. A dental surveyor (Fig. 7.75) can then be used to precisely measure the axial inclinations of the tooth preparation. Making such an impression may appear to take unnecessary time; however, the information obtained often saves time in subsequent procedures by identifying problems that can then be addressed immediately. For tooth preparation, the contra-angle handpiece can be used for both measuring and cutting. This is done by concentrating on the top surface of the turbine head, which is perpendicular to the shank of the rotary instrument. If the top surface is kept parallel to the occlusal surface of the tooth being prepared, the rotary instrument is automatically in the correct orientation (Fig. 7.76). To prevent undercuts or excessive convergence during axial reduction, the handpiece must be maintained at the same angulation. The correct taper is imparted by the diamond instrument. Keeping the turbine head at its correct angulation initially is often most effectively done by supporting it with a finger of the opposite hand.