Pulpal protection

An interim fixed restoration must seal and insulate the prepared tooth surface from the oral environment to prevent sensitivity and further irritation to the pulp. A certain degree of pulp trauma is inevitable during tooth preparation because of the sectioning of dentinal tubules (Fig. 15-2). In health, each tubule contains the cytoplasmic process of a cell body (the odontoblast), whose nucleus is in the pulp cavity. Unless the environment around the exposed dentin is carefully controlled, adverse pulp effects can be expected.¹ In addition, the pulp health of a tooth requiring a cast restoration is likely to be compromised before and after preparation (Table 15-1). In severe situations, leakage can cause irreversible pulpitis, with the consequent need for root canal treatment.²

Fig. 15-2 Pulp trauma and exposure of the dentinal tubules from tooth preparation.

Table 15-1 FACTORS CONTRIBUTING TO PULP DEATH

Periodontal health

To facilitate plaque removal, an interim fixed restoration must have good marginal fit, proper contour, and a smooth surface. This is particularly important when the crown margin is placed apical to the free gingival margin.³ If the interim fixed restoration is inadequate and plaque control is impaired, gingival health deteriorates.⁴

The maintenance of good gingival health is always desirable, but it has special practical significance when fixed prosthodontics is undertaken. Inflamed or hemorrhagic gingival tissues make subsequent procedures (e.g., impression making and cementation) very difficult. The longer the interim fixed restoration must serve, the more significant any deficiencies in its fit and contour become (Fig. 15-3). When gingival tissue is impinged upon, ischemia is likely to develop. This can be detected initially as tissue blanching. If it is not corrected, a localized inflammation or necrosis develops.

Fig. 15-3 An interim restoration should have good marginal fit, proper contour, and a smooth surface finish. A, The properly contoured interim restoration. Smoothly continuous with the external surface of the tooth. B, Overcontouring. Irregular transition from the restoration to the root surface and inadequate marginal adaptation. These contribute to plaque accumulation and an unhealthy periodontium.

Occlusal compatibility and tooth position

The interim fixed restoration should establish or maintain proper contacts with adjacent and opposing teeth (Fig. 15-4). Inadequate contacts allow supraeruption and horizontal movement. Supraeruption is detected at the evaluation appointment, when the definitive restoration makes premature contact. It is possible to correct this in the operatory, but the effort is time consuming and often leads to a restoration with poor occlusal form and function. Horizontal movement results in excessive or deficient proximal contacts. The former require tedious chairside adjustment; the latter involve a laboratory procedure to add metal or ceramic to the deficient site. In spite of these efforts, proximal crown contours are distorted. This, along with a resulting root proximity (Fig. 15-5), impairs oral hygiene measures.

Fig. 15-4 Proper occlusal and proximal contacts promote patient comfort and maintain tooth position.

Fig. 15-5 A missing proximal contact allows tooth migration. The resulting root proximity may necessitate surgical or orthodontic correction to allow impression making.

Prevention of enamel fracture

The interim fixed restoration should protect teeth weakened by crown preparation (Fig. 15-6). This is particularly true with partial coverage designs in which the margin of the preparation is close to the occlusal surface of the tooth and could be damaged during chewing. Even a small chip of enamel makes the definitive restoration unsatisfactory and necessitates a time-consuming remake.

Fig. 15-6 The interim restoration must protect the tooth. Fracture of a tooth after the impression phase delays treatment and jeopardizes restorability.

Function

The greatest stresses in an interim fixed restoration are likely to occur during chewing. Unless the patient avoids contacting the prosthesis when eating, internal stresses are similar to those occurring in the definitive restoration. The strength of polymethyl methacrylate resin is about one-twentieth that of metal-ceramic alloys,⁵ which makes fracture of the interim fixed restoration much more likely. Fracture is not usually a problem with a complete crown as long as the tooth has been adequately reduced. More frequently, breakage occurs with partial-coverage restorations and partial FDPs. Partial-coverage restorations are inherently weaker because they do not completely encircle the tooth.

A partial FDP must function as a beam in which substantial occlusal forces are transmitted to the abutments. This creates high stresses in the connectors,⁶ which are often the site of failure. To reduce the risk of failure, connector size must be increased in the interim restoration in comparison with the definitive restoration (Fig. 15-7). Greater strength is achieved by reducing the depth and sharpness of the embrasures. This increases the cross-sectional area of the connector while reducing the stress concentration associated with sharp internal line angles. The biologic and sometimes the esthetic requirements place limits on just how much larger connectors can be made. To avoid jeopardizing periodontal health, they should not be overcontoured near the gingiva (Fig. 15-8). Good access for plaque control must have high priority.

Fig. 15-7 The connectors of an interim fixed dental prosthesis are often purposely overcontoured. A, In the anterior region, the degree of overcontouring is substantially limited by esthetic requirements. B, In the posterior region, esthetics is less restrictive, but overcontouring still must not jeopardize the maintenance of periodontal health.

Fig. 15-8 In this mesiodistal section, an overcontoured connector impinges on the gingiva. Pressure ischemia and poor access for plaque removal promote gingivitis.

In some instances, cast metal or heat-processed resin interim restorations can spare the practitioner and the patient inconvenience, lost time, and the expense of remaking a restoration (Box 15-1).

Displacement

If irritation to the pulp and tooth movement are to be avoided, a displaced interim restoration must be recemented promptly. An additional office visit is usually required, which results in considerable inconvenience for both patient and dentist. Displacement is best prevented through proper tooth preparation and an interim restoration with a closely adapted internal surface. Excessive space between the restoration and the tooth places greater demands on the luting agent, which has lower strength than regular cement and thus cannot tolerate the added force. For this and for biologic reasons, unlined preformed crowns should be avoided.

Removal for reuse

Interim restorations often need to be reused and so should not be damaged when removed from the teeth. In most instances, if the cement is sufficiently weak and the interim restoration has been well fabricated, it does not break upon removal.

Custom

A custom ESF is a negative reproduction of either the patient’s teeth before preparation or a modified diagnostic cast. It may be obtained directly with any impression material. Impressions made in a quadrant tray with irreversible hydrocolloid or silicone are convenient. The higher cost of addition silicone may be offset by its ability to be retained for possible reuse at any future appointment. Accurate reseating of the ESF is easier and the mold cavity produces better results if thin areas of impression material (as may be found interproximally or around the gingival margin) are trimmed away (Fig. 15-11). The moldable putty materials are popular because they can be used without a tray and are easily trimmed to minimum size with a sharp knife. Also, their flexibility facilitates subsequent removal of the polymerized resin (Fig. 15-12).

Fig. 15-11 Shortening proximal projections of the impression material facilitates complete reseating of the ESF. Note that excess impression material palatally and facially has been trimmed away with a sharp knife for this reason. The anterior sextant tray shown was selected because it adequately captures the teeth adjacent to the proposed interim restoration.

Fig. 15-12 A, One of the flexible silicone putties suitable for making external surface forms. B, The putty form has been spread apart. Note the completed resin interim restoration in place, to demonstrate the degree of putty flexibility.

A custom ESF can be produced from thermoplastic sheets, which are heated and adapted to a stone cast with vacuum or air pressure while the material is still pliable (Fig. 15-13). This produces a transparent form with thin walls, which makes it advantageous in the direct technique because of its minimum interference with the occlusion. It is filled with resin, placed in the mouth, and fully seated as the patient closes into maximum intercuspation. Little additional effort is required to adjust the occlusal contacts. The thinness of the material may also be a disadvantage in the direct technique, however. The material is a poor dissipater of the heat released during resin polymerization, and so care must be taken to remove it from the mouth before injury can occur. A thermoplastic ESF has other uses in fixed prosthodontic treatment, in both the clinical and the laboratory phase; for example, it can be helpful in evaluating the adequacy of tooth reduction^8,9 (Fig. 15-14).

Fig. 15-13 A, Inexpensive system for producing external surface forms from thermoplastic sheets. B, After heating, the sheet is formed with reusable putty and finger pressure applied over a stone cast. C, More expensive system incorporating an electric heating element and a vacuum source. D, Trimmed polypropylene external surface form. Note the detail that can be captured with this material.

Fig. 15-14 A, The thinness and transparency of these external surface forms (ESFs) allow their use directly as tooth-reduction guides both in and out of the mouth. B, Tooth reduction may be assessed by using the ESF to mold alginate over the prepared tooth. When the alginate is set, the ESF is removed, and a periodontal probe is pushed through the alginate for measurements at desired locations.

(B, Courtesy Dr. T. Roongruangphol.)

Transparent sheets are available in cellulose acetate or polypropylene of various sizes and thicknesses; a 125 × 125 mm sheet of 0.5-mm (0.020-inch) thickness is recommended for making interim restorations. Polypropylene is preferred because it produces better surface detail and is more tear resistant. Better tear resistance makes initial removal from the forming cast less tedious and enables the ESF to be used more than once.

Although thermoplastic sheets have a number of advantages, a wide variety of other materials and methods can be used successfully. For example, some practitioners favor baseplate wax because it is convenient and economical (see Fig. 15-10B).

Preformed

Various preformed “crowns” are available commercially. On their own, they rarely satisfy the requirements of a interim restoration, but they can be thought of as ESFs rather than as finished restorations and thus must be lined with autopolymerizing resin. Most crown forms need some modification (internal relief, axial recontouring, occlusal adjustment) in addition to the lining procedure (Fig. 15-15). When extensive modification is required, a custom ESF is superior because it is less time consuming. Preformed crowns are generally limited to use as single restorations, because it is not feasible to use them as pontics for partial FDPs.

Fig. 15-15 A, The time necessary to modify this particular preformed crown outweighs the advantages it might provide. Were a custom external surface form available, it would be more efficient and more economical. B, The excessively tapered internal lingual wall of this preformed crown requires grinding in order to accommodate a properly prepared tooth. The stone cast in the lower portion of the illustration duplicates the internal surface of the preformed crown.

Materials from which preformed ESFs are made (Fig. 15-16) include polycarbonate, cellulose acetate, aluminum, tin-silver, and nickel-chromium. These are available in a variety of tooth types and sizes (Table 15-2).

Fig. 15-16 A, Preformed anterior crown forms: polycarbonate (left) and cellulose acetate (right). B, Preformed posterior crown forms: aluminum shell (left), aluminum anatomic (center), and tin-silver anatomic (right).

Table 15-2 PREFORMED CROWNS

Polycarbonate

Polycarbonate (Fig. 15-17) has the most natural appearance of all the preformed materials. When properly selected and modified, it rivals in appearance a well-executed porcelain restoration. Although available in only a single shade, this can be modified to a limited extent by the shade of the lining resin. Polycarbonate ESFs are supplied in incisor, canine, and premolar tooth types.

Fig. 15-17 Polycarbonate crowns. Available in maxillary and mandibular incisor, canine, and premolar shapes.

Aluminum and tin-silver

Aluminum (Fig. 15-18) and tin-silver are suitable for posterior teeth. The most elaborate crown forms have anatomically shaped occlusal and axial surfaces. The most basic and least expensive forms are merely cylindrical shells resembling a tin can (see Fig. 15-16B).

Fig. 15-18 Aluminum anatomic crowns. Available in a variety of sizes and shapes. The manufacturer has produced two maxillary and four mandibular shapes for the left and right side of the mouth, each in six sizes.

The nonanatomic cylindrical shells are inexpensive but require modification to achieve acceptable occlusal and axial surfaces. It is more efficient to use crowns that have been preformed as individual maxillary and mandibular posterior teeth. Care must also be taken to avoid fracturing the delicate cavosurface margin of the tooth preparation when a metal crown form is fitted. This is a greater risk if adaptation is carried out directly by having the patient forcefully occlude on the crown shell. The edge of the shell can engage the margin and fracture it under biting pressure. An even greater risk occurs when the crown has a constricted cervical contour. Tin-silver crowns are deliberately so designed (see Fig. 15-16B). This highly ductile alloy allows the crown cervix to be stretched to fit the tooth closely. Direct stretching on the tooth is practical only where feather edge margins are used. For other margin designs, cervical enlargement should be performed indirectly on a swaging block, which should be supplied with the crown kit.

Nickel-chromium

Nickel-chromium shells (Fig. 15-19) are used primarily for children with extensively damaged primary teeth. In that application, they are not lined with resin but are trimmed, adapted with contouring pliers, and luted with a high-strength cement. They may be applied to secondary teeth but are more suitable for primary teeth, where longevity is less critical. Nickel-chromium alloy is very hard and thus can be used for longer-term interim restorations.

Fig. 15-19 Nickel-chromium anatomic crowns. These are available in an array of sizes and shapes, including ones for the primary teeth, with straight and contoured axial surfaces.

Indirect procedure

An impression is made of the prepared teeth and ridge tissue and is poured in quick-setting gypsum or polyvinyl siloxane.¹⁰ The interim restorations are fabricated outside the mouth. This technique has several advantages over the direct procedures:

Fig. 15-20 Labial (A) and gingival (B) ulcerations after brief polymethyl methacrylate monomer exposure.

Fig. 15-21 Heat generated during resin polymerization. Under nonclinical experimental conditions, the temperature rises are severe. Sevriton (a polymethyl methacrylate resin) produced significantly higher temperatures than did the others represented. This is useful information for selecting resins to be used intraorally, although under clinical conditions the differences may be insignificant.

(Redrawn from Braden M, et al: A new temporary crown and bridge resin. Br Dent J 141:269, 1976.)

Fig. 15-22 These exotherms (time in minutes) are derived from a simulated clinical procedure for making a single crown with silicone putty as the ESF. A thermocouple probe in the pulp chamber of an extracted tooth was used to measure temperature changes. Initial readings reflect the cooling effect of room-temperature resin mixtures. For all three classes of resins tested, the temperatures did not exceed 35°C until more than 6 minutes had elapsed.

(Redrawn from Tjan AHL, et al: Temperature rise in the pulp chamber during fabrication of provisional crowns. J Prosthet Dent 62:622, 1989.)

Direct procedure

The patient’s prepared teeth and gingival tissues (in the case of a partial FDP) directly provide the TSF, and so the intermediate steps of the indirect technique are eliminated. This is convenient when assistant training and office laboratory facilities are inadequate for efficiently producing an indirect restoration. However, the direct technique has significant disadvantages: potential tissue trauma from the polymerizing resin and inherently poorer marginal fit. Therefore, the routine use of directly formed interim restorations is not recommended when indirect techniques are feasible.

Indirect-direct procedure

In this technique the indirect component produces a “custom-made preformed ESF” similar to a preformed polycarbonate crown. In most cases, the practitioner uses a custom ESF with an underprepared diagnostic cast as the TSF. The resulting mold forms a shell that, after tooth preparation, is lined with additional resin (the patient serving as the TSF). This last step is the direct component of the procedure. Another method of creating the shell eliminates the need for an indirect TSF. It is accomplished by painting monomer liquid into the ESF and carefully sprinkling or blowing resin powder on it. The thickness of the resin shell is difficult to control with this technique, however, and may result in the need for time-consuming corrective grinding.

The indirect-direct approach has these advantages:

However, even with the diagnostic cast method, adjustments are frequently needed to seat the shell completely on the prepared tooth. This is the chief disadvantage of the indirect-direct procedure.

Ideal properties

The characteristics of an ideal interim material are as follows:

Currently available materials

As yet, an ideal interim material has not been developed. A major problem still to be solved is dimensional change during solidification. These materials (Fig. 15-23) shrink and cause marginal discrepancy,^20-22 especially when the direct technique is used (Fig. 15-24). Also, the resins currently employed are exothermic and not entirely biocompatible.

Fig. 15-23 Currently available interim materials. A, A poly(methyl methacrylate) resin. B, A poly(R′ methacrylate) resin. C, Microfilled composite resins with AutoMix delivery system. D, Light-cured resins: a microfilled urethane-dimethacrylate (left) and a light-cured polyethyl methacrylate.

Fig. 15-24 With ideal axial wall convergence, a 2% reduction in crown diameter results in a comparatively high marginal discrepancy.

The materials can be divided into four resin groups:

The properties of these resins are compared in Table 15-3. The overall performances of the groups are similar, with no material being superior in all categories. Choosing a material should be based on optimally satisfying the requirements or conditions crucial for the success of the treatment. For example, materials with the least toxicity and least polymerization shrinkage should be chosen for a direct technique. Alternatively, when a long-span prosthesis is being fabricated, high strength is an important selection criterion.

Table 15-3 RANKED CHARACTERISTICS OF REPRESENTATIVE PROVISIONAL RESTORATION RESINS

Initiation

Free radical polymerization begins with the formation of a free radical, a process called activation, and the subsequent combination of this free radical with a monomer. Free radicals are formed by the decomposition of a chemical (the initiator), with the method of decomposition dependent on the nature of the initiator. Possible initiators include benzoyl peroxide and camphoroquinone.

Benzoyl peroxide decomposes to free radicals at approximately 50°C or higher in a process called thermal activation. Because the heating of some monomers to temperatures near 100°C can cause them to vaporize, with subsequent formation of porosity in the resultant polymer, excessive temperatures should be avoided during the early stages of thermal activation. Thermal activation results in greater contraction on cooling than is obtained with other activation methods and therefore is usually avoided for interim restorations.

Benzoyl peroxide also decomposes to free radicals when catalyzed by a tertiary amine, and this process is called chemical activation. Chemical activation occurs when the activator, initiator, and monomer are mixed together, and so these materials are usually supplied separately, the monomer and activator in one container and the initiator and filler in another. Proper mixing is necessary to prevent voids. Because chemical activation requires intimate contact of the chemical activator with the initiator, this activation method is not as efficient as thermal activation. Inefficient activation of the initiator results in more residual monomer and less color stability of the restoration, inasmuch as unreacted benzoyl peroxide can cause color changes. However, because benzoyl peroxide is decomposed by both thermal and chemical activation, increased temperature can enhance its decomposition in a chemically cured system and does not increase contraction if the restoration initially undergoes chemical setting. Heating a recently set restoration in 100°C water promotes greater polymerization efficiency and removes any unconverted monomer, which might cause a sensitivity reaction in a patient susceptible to monomer irritation.

Camphoroquinone decomposes to free radicals in the presence of both an aliphatic amine and blue light energy, and this process is called visible-light activation. Light-activated materials have two advantages: (1) the ingredients can be mixed by the manufacturer with little porosity, and (2) working time is infinitely long because no setting occurs if the material is kept in a dark environment. The limitation of this method is the depth to which visible light can penetrate (less for darker materials). Whenever possible, the activation illumination should be directed toward the center of the restoration from all surfaces. Also, for darker materials, the exposure time should be longer.

Propagation

After its onset, the polymerization process continues by including more monomer molecules in the growing molecular chain. It is important that the material be allowed to set undisturbed because, during this phase, defects can easily result if the material is jostled. During propagation, (1) the setting material undergoes an increase in density, causing contraction, (2) the exothermic heat of reaction may cause a substantial increase in temperature, with subsequent increased contraction, and (3) other physical properties (e.g., rigidity, strength, and resistance to dissolution) increase.

Termination

Because of the randomness of position of the growing chains, it is possible that some of them might combine and thereby terminate the growth process. This type of termination cannot be avoided, although it is desirable to have termination only after polymerization of all the monomer has occurred. Termination may also result from the reaction with eugenol, hydroquinone, or oxygen; therefore, contact with these substances must be avoided or at least minimized when possible.

Filler

Although the primary properties of an interim restorative material are determined by the monomer or monomers involved, a decrease in the less desirable setting and mechanical properties is accomplished mainly through the filler. An increase in filler content reduces the relative amounts of exothermic heat and contraction while increasing the strength of the set material. However, too much filler can lead to insufficient handling characteristics before setting, and this impedes mixing and shaping, as well as introduces porosity in the set restoration. For light-activated systems, the amount of filler is determined by the manufacturer; for the other systems, it is desirable to incorporate as much filler as possible without interfering in the handling or manipulation characteristics of the material.

Laboratory armamentarium (Fig. 15-26)

Fig. 15-26 Basic laboratory armamentarium. A, Assorted small items appearing on the accompanying list. B, Pressure-curing vessel. C, Cast trimmer. D, Dental lathe and dust collector. E, Ultrasonic cleaner and liquid detergent.

Additions to clinical armamentarium

Step-by-step procedure

Fig. 15-27 Subgingival margins often require tissue displacement before an adequate impression can be made. Alginate in a disposable tray produces an economical and satisfactory impression. After treatment for infection control, the impression is poured in quick-set plaster to create the tissue surface form (TSF).

Additions to laboratory armamentarium

Step-by-step procedure

The setting of plaster can be accelerated by shaking dry powder with the water before mixing (1 tsp of powder in 30 mL water²³). Alternatively, a commercially available quick-setting plaster may be used.

Fig. 15-28 A, After trimming, the indirect tissue surface form (TSF) is mated with the external surface form (ESF) to verify accurate passive indexing. B, With this accomplished, the forms are separated, and the TSF is completely coated with a resin-gypsum separating medium (brushed on).

Fig. 15-29 A polymer syringe with a widened orifice (2 mm diameter) is useful for filling the external surface form (ESF). To avoid entrapping air, it is best to begin at one end and progress slowly to the other, keeping the syringe tip in contact with the expressed resin.

Fig. 15-30 A, The resin-filled external surface form placed on the tissue surface form. B, Rubber bands around the mold assembly and located over adjacent unprepared teeth. This avoids distorting the external surface form. C, The assembly being placed into a pressure vessel filled with warm water. D, The resin cures for 5 minutes under 0.15 MPa (20 psi) pressure.

Fig. 15-31 External surface form removed.

Fig. 15-32 Tissue surface form reduced to attain the final shape of the restoration. A, Bulk reduction on a cast trimmer. B, Sectioning and removal of pontic-contact areas with a carborundum disk. C, Linguogingival surface of the pontic shaped with a tapered bur. D, An abrasive disk (7/8-inch diameter, garnet) is excellent for creating proper embrasure form. It must be carefully oriented parallel to the desired contour so that overtrimming at the margins is avoided. E, The contoured restoration.

Fig. 15-33 The restoration before try-in.

Evaluation

The interim partial FDP should be evaluated in the patient’s mouth for proximal contacts, contour, surface defects, marginal fit, and occlusion. Deficient proximal contacts, imperfections in contour, or surface defects can be corrected by adding resin through the bead-brush technique (Fig. 15-34; see Fig. 15-70).

Fig. 15-34 Proximal contact added by the bead-brush technique. When the resin reaches the doughy stage, the restoration is set on the prepared tooth to form the contact.

Fig. 15-70 Bead-brush technique for repairs. A, Monomer liquid is painted on the surface of the thoroughly cleaned restoration to which resin will be added. B, The brush is dipped in monomer and briefly touched to the powder, forming a small bead on the tip. C, The bead is touched to the repair site, and the brush handle is rolled to deposit it. Bead placement continues in this manner until the desired contour is achieved. To prevent excessive porosity, the unset resin should be painted lightly with monomer until hard.

Unacceptable marginal fit can be corrected as for custom indirect-direct partial FDPs (p. 487, steps 3 to 9), provided that the patient has no history of allergy to monomer. If occlusal correction is needed, the restoration is marked with articulating ribbon and adjusted with a 12-fluted tungsten carbide finishing bur rotating at high speed with copious air-water spray to prevent the resin from melting (Fig. 15-35). Adequate intraoral evacuation and eye protection are essential.

Fig. 15-35 Intraoral adjustment of occlusal contacts.

Additions to laboratory armamentarium (Fig. 15-36)

Fig. 15-36 Additions to the basic laboratory armamentarium for the indirect-direct procedure: the diagnostic tissue surface form (TSF) and the polypropylene external surface form (ESF).

Step-by-step procedure

Fig. 15-37 Preparations involved in making the articulator-mounted diagnostic cast. A, Conservative depth-orientation grooves. B, Placement of supragingival cavosurface margins.

Fig. 15-38 The prepared cast is duplicated by means of an alginate impression. This creates the indirect tissue surface form. Quick-set plaster is used.

Fig. 15-39 Creating a custom external surface form (ESF) from a diagnostic waxing. A, The diagnostically waxed articulated casts. Patterns should satisfy biologic, mechanical, and esthetic requirements. B, If a thermoplastic ESF is desired, the completed waxing must be duplicated in stone.

Fig. 15-40 Proper relationship between the external surface form (ESF) and the tissue surface form (TSF). If it is necessary to remove any cast artifacts to correct the relationship, this should be done before the separating medium is applied.

Fig. 15-41 The completed custom-preformed external surface form (ESF). This is the end product of the indirect component of the indirect-direct technique.

Addition to clinical armamentarium

Step-by-step procedure

Fig. 15-42 The custom-preformed external surface form (ESF) fully seated over the prepared teeth. Note the marginal discrepancy on each abutment. The tip of the periodontal probe easily fits into the space, which will be filled by a direct lining procedure.

Fig. 15-43 Lining the custom-preformed external surface form (ESF). This is the direct component of the indirect-direct technique. A, Oral tissues are protected with petrolatum. B, Vent holes help to eliminate trapped air. C, Abutment retainers filled with lining resin. D, The restoration completely seated. (The amount of resin at the margins is controlled by covering or uncovering the vent holes.)

Fig. 15-44 Removal of excess after the lining resin hardens. A, Margins marked with a sharp soft lead pencil. B, Gross resin excess is quickly removed. (Margins must be avoided.) C, The final axial contours, connectors, and marginal fit are perfected with an abrasive disk rotating toward the margin to prevent debris from obscuring the pencil line. Note the orientation of the disk, parallel to the desired final contour.

Accurate trimming to the margins can be simplified by holding the disk parallel to the desired final contour. A paper-thin extension remaining beyond the marked margin is an indication that the contour is correct and the cavosurface margin fully covered. Often this flash can be easily peeled away from the margin with the fingers (Fig. 15-45).

Fig. 15-45 Flash at the margin of a restoration whose axial surface was contoured with proper disk orientation.

(Courtesy Dr. R.E. Kerby.)

Fig. 15-46 Occlusal contacts of the completed restoration are checked and adjusted before polishing. Note that the units are splinted together for increased resistance to dislodgment during an anticipated lengthy treatment period.

(Courtesy Dr. R. Liu.)

Complete crowns

Single-unit complete crowns or splinted crowns may be made directly or indirectly in accordance with the basic procedures described for partial FPDs. Because pontics are not involved, creating an ESF is simpler. Diagnostic procedures are not required unless extensive coronal changes are planned. For example, extensive changes are usually required when the occlusal vertical dimension is increased. If diagnostic procedures are not needed, an alginate impression of the crown or crowns before tooth preparation should be adequate to serve directly as the ESF or indirectly when a cast has been poured in another impression material.

Onlays and partial veneer crowns

The technique for making onlay and partial veneer interim restorations is similar to that for making custom single crowns. However, the interim restorations are more easily distorted during handling because of the conservative tooth preparations that interrupt the continuity of the axial walls. Thus, the direct method mandates extra care in separating the resin from the tooth. Significantly better results can be expected with the indirect procedure.

Two other points merit mention. When the polymerized resin is trimmed to the margin, it is advisable to leave an excess of resin at the occlusal cavosurface margin (see Fig. 15-9). This helps prevent fracture of enamel, which is likely to result from the lesser strength of resin in comparison with metal. Second, if lining is needed, an occlusal vent hole is not necessary because the shape of these restorations provides an adequate escape way for trapped air and excess resin.

Inlays

Inlays present the problem of being small and difficult to handle, especially during trimming. Making interim restorations requires a number of modifications.

Step-by-step procedure

1. For a two- or three-surface inlay, apply the matrix band and wedges as in preparation for condensing a Class II amalgam restoration. The wedges should be placed with firm pressure so that when the band is removed, proximal contact is reestablished. The band must seal all aspects of the proximal cavosurface margins.

2. Using petrolatum on a small cotton pellet, lightly coat all sides of the cavity preparation and the matrix band.

3. Make a handle to remove the resin by placing one end of a 2- to 3-cm length of unwaxed dental floss in the preparation cavity.

4. Mix a small amount of poly-R′ methacrylate, and when it can be kneaded like bread dough, mold a small cone of it on the end of an amalgam condenser.

5. Lightly condense the resin into the cavity, being careful not to force it past the matrix into an undercut. Immediately remove as much occlusal excess as possible with a sharp spoon excavator.

6. Monitor the polymerization by light probing with a hand instrument. When the resin reaches the late rubbery stage, remove it by tugging the floss handle with cotton roll forceps along the path of withdrawal (Fig. 15-47).

7. Place the resin in a cup of warm water (37°C) for 5 minutes.

8. Mark the margins with a sharp pencil and trim away whatever flash may be present.

9. Return the cured resin to the cavity preparation and adjust the occlusion, using marking film and a slow-speed handpiece. (Take extreme care to avoid removing tooth structure.) Leave the floss handle in place as long as it does not interfere with adjustments of the occlusion.

10. Remove the adjusted interim restoration with the floss handle and put it aside where it may be found easily after impression making for the definitive inlay.

11. Clean and dry the cavity preparation and place a thin coat of interim cement on the cavity walls. Immediately insert the interim restoration.

12. When the cement is set, remove the excess with an explorer and a spoon excavator. Carefully cut off the floss handle with the scalpel blade.

Fig. 15-47 A floss handle facilitates removal of an inlay resin interim restoration during late rubbery stage.

Additions to clinical armamentarium

Step-by-step procedure

If the material is difficult to control, placement and curing may be accomplished in stages.

It is also possible to form the veneers indirectly by creating a TSF and an ESF, as was recommended for the partial FDP interim restoration. The indirect method may be more efficient if multiple veneers are being made.

Additions to clinical armamentarium

Step-by-step procedure

Fig. 15-48 Crown selection. A, Measuring the mesiodistal width of the space with dividers. B, Appropriate crown size for the measured space.

Fig. 15-49 Crown length adjustment. A, Incisocervical height required for the completed restoration. B, Measurement transferred to the crown. C, Cervical portion of the crown adjusted to duplicate the curvature of the cavosurface margin.

Fig. 15-50 A, Cervical portion of the crown trimmed until the length and axial inclination are correct. B, If necessary, internal surfaces are adjusted for proper orientation of the crown.

Fig. 15-51 Lining the adjusted shell. A, Protection with petrolatum. The shell is filled with resin (B) and is seated (C) when the resin does not slump after a peak is formed with the tip of an explorer. D, Excess resin is immediately removed after the crown has been positioned.

Fig. 15-52 A, When the resin has reached the rubbery stage, the crown is removed and placed in warm water (37°C). Hot water must not be used, because it increases resin shrinkage. However, warm water is not recommended for polymethyl methacrylate resin, because excessive shrinkage makes the marginal fit unacceptable. B, After about 5 minutes in warm water, the resin should be rigid enough for marking the margins. C, Initial removal of the excess lining resin is accomplished with a coarse garnet disk.

Fig. 15-53 A, A considerable amount of lingual reduction may be called for. If it is only minor, it can be accomplished intraorally. B, To increase efficiency and ensure patient comfort, any bulk reduction should be accomplished extraorally. C, Finalized lingual contour that promotes gingival health and allows access for oral hygiene. Note the more natural contour of the left central incisor than that of the interim crown on the right central incisor.

Fig. 15-54 A, A rag wheel and pumice are used before polishing with compound. Note the parallel orientation of the wheel to the crown’s axial surface at the point of contact (arrow). The crown should be positioned so that the wheel rotates from the surface toward the margin. B, An explorer and dental floss are used to carefully remove all excess luting agent. C, Overpolishing results in a deficient mesial contact (arrow). The bead-brush technique is recommended for correcting a small inadequacy.

Additions to clinical armamentarium (Fig. 15-55)

Fig. 15-55 Additions to the basic clinical armamentarium for aluminum crown forms.

Step-by-step procedure

Fig. 15-56 Aluminum crown selection and modification. A, Mesiodistal dimension of the space. B, Appropriate crown size, nearest this measurement. C, Contouring pliers to make small size modifications. This is frequently unnecessary.

Fig. 15-57 A, Cervical portion of the crown trimmed to proper length. B, Smoothing the cut edge to prevent gingival injury.

Fig. 15-58 The patient is instructed to bite on the shell after the length has been adjusted. Note the occlusal indentation and gingival blanching (arrows). Additional shortening should be done where the blanching occurs.

Fig. 15-59 A, Protection with petrolatum. B, The adjusted shell is filled with lining resin and seated to just short of its final position after the resin has lost its sheen. C, The final position is determined by the patient’s closing into maximum intercuspation. Excess resin is immediately removed.

Fig. 15-60 A, Backhaus forceps provides definite purchase of the shell for controlled removal. B, After 5 minutes in warm water, the margin is marked with a pencil. C, A coarse garnet disk is recommended for initial contouring of the axial surfaces. This usually necessitates partially removing the aluminum. After the overcontoured aluminum has been ground away, a fine garnet disk is used to finalize the axial contours (including the marginal areas). Again, disk orientation is important to establishing a straight emergence profile and well-adapted margins.

Fig. 15-61 A, Proper contouring of the axial walls exposes lining resin in the cervical area. Note the indentations in the shell from the Backhaus forceps. B, Final occlusal adjustment removes the anodized gold finish, but this is of no concern.

Fig. 15-62 Adding proximal contacts to aluminum crowns. A, Contacts absent in this lined crown (arrows). B, The metal in the contact area is ground away to expose the underlying resin. The bead-brush technique is then used for correcting the deficiency. C, Crown after resin addition to the mesial surface. Further contouring with a disk is recommended to improve the gingival embrasure form.

Additions to clinical armamentarium

Step-by-step procedure

5. When the resin loses its surface gloss, place the wire in the post space and seat the ESF over it (Fig. 15-64).

Fig. 15-63 Interim post preparation. A, Wire marked so that the bend is made at the correct level. When in position, the wire must not interfere with the external surface form. B, A 180-degree or greater bend in the wire to resist displacement in the lining resin.

Fig. 15-64 A, Wire in the post space just before placement of the filled external surface form (ESF). A gauze throat pack is recommended to protect the patient from aspirating or swallowing the wire. B, Filled ESF seated.

Precautions must be taken to protect the patient from swallowing or aspirating the wire.

Fig. 15-65 The completed interim post and crown restoration. Note the unusual mesial contour (which is the result of a mesiobuccal root amputation).

Ideal properties

Desirable characteristics of an interim luting agent are as follows:

Available materials

Of the currently available materials (Fig. 15-66), zinc oxide–eugenol (ZOE) cements appear to be the most satisfactory. Zinc phosphate, zinc polycarboxylate, and glass ionomer cements are not recommended because their comparatively high strength makes intentional removal difficult. Using high-strength cements frequently damages the restoration or even the tooth when removal is attempted and can make seating of the definitive restoration difficult. Weaker ZOE cements provide for easy removal, allowing the restoration to be reused when additional service is needed. ZOE also has an obtundent effect on the pulp in addition to its acceptable sealing properties.²⁴ Unfortunately, however, free eugenol acts as a plasticizer of methacrylate resins. It has been shown to reduce surface hardness²⁵ and presumably strength. New resin applied over polymerized resin previously in contact with free eugenol results in softening²⁶ of the added resin, making linings or repairs unsuccessful. The R′ methacrylates are severely affected by free eugenol. Methyl methacrylates are affected moderately, and the composites are only slightly softened. These adverse effects have stimulated the marketing of interim luting agents without eugenol, but in the studies cited, the mere presence of eugenol in a cement was not enough to cause adverse effects. It appears that unreacted or free eugenol must be present to cause problems. Therefore, when using products that contain eugenol, the dentist must be sure that the correct proportions are blended. Whether free eugenol is necessary to provide an obtundent effect on the pulp remains a question.

Fig. 15-66 A noneugenol and a eugenol interim luting agent.

In situations in which the tooth preparation lacks retention, a span is great or long-term use is anticipated, or parafunction exists, it may be desirable to use a higher strength cement. A good compromise would be reinforced ZOE; another might be eugenol-free zinc oxide, which has slightly greater strength than cements containing eugenol.²⁷ Conversely, sometimes minimum strength is desired, as with temporary placement of the definitive restoration. (Its removal may be needed to refire the porcelain.) Petrolatum can be mixed with equal parts of the interim cement base and catalyst to reduce the cement’s strength by more than half. When the definitive cement planned is a resin luting agent, non-eugenol cements are recommended for the interim restoration because of the adverse effect of eugenol on bond strength.

Armamentarium (Fig. 15-67)

Fig. 15-67 Interim restoration luting armamentarium.

Step-by-step procedure

Most interim luting agents are supplied as a two-part system (Fig. 15-68).

Fig. 15-68 Luting procedure. A, The external surface is lightly coated with petrolatum to aid removal of the set luting agent. B, Careful placement of the luting agent seals the margins and reduces the clean-up effort. C, The restoration is seated with firm finger pressure, or (for posterior restorations) the patient may bite on a cotton roll. D and E, An explorer is used to remove excess and to probe the sulcus gently for remnants. F, The proximal contact areas and sulcus are cleaned with dental floss (a knot will help remove excess cement), followed by copious irrigation with the air-water syringe.

Cement remnants left in the sulcus have an irritating effect on the gingiva and may cause severe periodontal inflammation with possible bone loss. Therefore, the sulcus must be carefully checked and irrigated with the air-water syringe.

Armamentarium

The interim restoration is removed when the patient returns for placement of the definitive restoration or for continued preparation. Fracture of the prepared tooth or foundation must be avoided. Risk of this can be minimized if removal forces are directed parallel to the long axis of the preparation. The Backhaus or a hemostatic forceps is effective for obtaining sound purchase on a single unit (Fig. 15-69). A slight buccolingual rocking motion helps break the cement seal.

Fig. 15-69 Backhaus towel clamp forceps provide positive purchase on interim restorations. For maximum control, occlusal finger pressure is applied directly to the tines.

Damage can occur when an FDP is being removed. If one abutment retainer suddenly breaks loose, the other can be subjected to severe flexure stresses when the FDP acts as a lever arm. Care must be exercised to remove the prosthesis along the path of placement. Sometimes it is helpful to loop dental floss under the connector at each end of the FDP.

Step-by-step procedure

A fractured or damaged interim restoration can be easily repaired with resin, added directly by using the bead-brush technique (Fig. 15-70).

Contour

Diagnostic waxing provides the ultimate control of contour, and using a shade guide before tooth preparation gives the operator some control of color. If contour and color are well controlled, most interim restorations are very acceptable to excellent in appearance. Routinely achieving this requires attention to detail and skill. Although listed third, translucency can be a significant appearance element for patients with unabraded teeth.

Color

Whereas some resin manufacturers use only general color descriptors (light, medium, dark) for their products, most cross-reference their colors to popular shade guides for porcelain or denture teeth. However, even when cross-referenced, manufacturer and material differences make shade matching inaccurate. Better control of color is obtained by using a custom shade guide. This can be easily made by casting the resins into an elastomeric putty mold of an extracted incisor crown. Combining two or more existing hues in known proportions can create a wider selection of shades, or resin-coloring tints are an option.

Custom color effects that simulate intrinsic and extrinsic stains, cracks, or hypocalcification of adjacent teeth may be added to interim restorations with the help of paint-on stain kits (Fig. 15-71). These are best applied quickly, and overmanipulation should be avoided because it causes streaking and surface roughness. Under optimum conditions, the surface should have a glazed appearance similar to that of porcelain. Thickening of stains as a result of the evaporation of solvent is a common problem hampering manipulation. Another problem with the paint-on colorants is their poor resistance to abrasion. Loss of the pigments in high-abrasion areas produces an unattractive mottled effect.

Fig. 15-71 This interim stain kit contains violet, blue, yellow, orange, brown, white, and gray paint-on colorants to create custom effects and a clear material used to form a glazed translucent surface. The liquids are formulated to dry quickly, which requires that they be kept covered until immediately before use. A thinner and brush cleaner are provided.

Translucency

Coronal translucency is determined by the type and amount of enamel present. At the incisal edge of an unworn anterior tooth where there is no dentin in the light path, that region often takes on a blue or gray hue, which comes about from the dark oral cavity. This effect is most pronounced with enamel that scatters very little light as a result of the absence of pigments or opacifying mineralization (e.g., fluorosis). Although less obvious, the translucent appearance of enamel is observable over the entire incisal or occlusal third of the crown. Thus, when it is readily visible in adjacent teeth or when a more realistic appearance is desired, translucency can be simulated in the interim. The procedure requires two resins, one colored to match the body and one to match the enamel of the tooth. Some manufacturers produce enamel or incisal shades that may be used without modification. When these are not available or when variation is needed, clear resin powder may be mixed with a smaller fraction of the “body” powder to produce the desired translucency.

Two procedures can be followed to create a translucent effect. In the first, which is more difficult to control, the enamel color resin is carefully bead-brushed onto the occlusal or incisal surface of the ESF and tapered to end at the middle or cervical third. The tendency to flow where it is not wanted is controlled in part by the orientation of the ESF with regard to gravity and in part by manipulation with the brush tip. When the desired distribution of enamel color resin is achieved, a disposable syringe is loaded with body color resin and the ESF is immediately filled, to avoid disruption of the enamel color resin. The TSF is then positioned in the ESF, and normal procedures are followed (Fig. 15-72).

Fig. 15-72 The layering of translucent resin and dentin-shaded resin allows a more realistic appearance of the premolar and canine interim restorations. They serve as removable dental prosthesis (RDP) abutments and are splinted together to help resist dislodgment.

In the second procedure, the enamel color resin is allowed to polymerize on the ESF without adding body color resin or the TSF. The rigid enamel veneer is removed from the ESF and trimmed to occupy only the space intended for enamel. It is important to check that the ESF and TSF can be mated without interference from the in-place veneer. With the veneer in place, monomer liquid is painted on it and the body color resin is added. The TSF is then inserted, and standard procedures are followed for the remainder of the restoration. The timing for this procedure is less crucial than for the first one described and may be better suited to practitioners with less experience. A disadvantage is that sometimes there is a more obvious demarcation between the enamel and the body resins.

Texture

With practice, texture effects require only a small amount of time, but in some cases they contribute significantly to the overall appearance of the interim restoration. These effects are most important for maxillary anterior teeth adjacent to teeth with well-defined lobes, imbrication lines, or developmental defects.

Developmental lobes are best simulated in wax during the final stage of the diagnostic waxing. To produce a natural effect, it is crucial to avoid making grooves that are straight or sharp edged or have uniform cross-sections. Rather, the simulation should have a gentle crescent shape, with softening of the edges and slight varying of the cross-section by burnishing with the largest diameter waxing wire. If a polypropylene sheet is used to form the ESF, these subtle details can be reproduced in the resin.

Placement of developmental defects is best accomplished in the resin just before pumice and rag wheel finishing. Depending on their size and definition, these features may be made with a sharp-edged inverted-cone green stone rotating parallel to the occlusal plane and touched briefly to the resin. Often the defects are most noticeable in the cervical third of the tooth, but an adjacent tooth is the best guide for determining their distribution.

Imbrication lines may be simulated with a coarse diamond rotary instrument rotating slowly and moved across the facial surface from proximal to proximal. This reduces the surface reflectance of the resin after it is finished and polished. However, as with all texture effects, overfinishing obliterates these lines. Care must be taken to monitor the finishing by rinsing pumice from the surface and drying it. A completely smooth and highly polished interim restoration may be excellent for plaque control but not esthetically compatible with the adjacent teeth. The debate as to which is more important can probably best be settled by consulting with the patient to determine his or her needs.