15: INTERIM FIXED RESTORATIONS

15 INTERIM FIXED RESTORATIONSimage

KEY TERMS

Interim crowns or partial fixed dental prostheses (FDPs) are essential in prosthodontic therapy. The word interim means established for the time being, pending a permanent arrangement. Even though a definitive restoration may be placed as quickly as 2 weeks after tooth preparation, the interim fixed restoration must satisfy important needs of the patient and dentist. Unfortunately, temporary usually connotes laxity, and this may imply that requirements pertaining to the more permanent condition are ignored. If this connotation becomes a philosophy governing the interim phase of treatment, the clinical efficiency and treatment quality will be needlessly reduced. Experience has repeatedly shown that time and efforts expended in fulfilling the requisites of interim fixed restorations are well spent.

Because of unforeseen events (e.g., laboratory delays or patient unavailability), an interim fixed restoration may have to function for an extended period. On the other hand, a delay in placing the definitive restoration may be deliberate (e.g., because the etiologic factors of a temporomandibular disorder or periodontal disease must be corrected). Whatever the intended length of time of treatment, an interim restoration must be adequate to maintain patient health. Thus, it should not be casually fabricated on the basis of an expected short term of use.

Interim procedures also must be efficiently performed, because they are done while the patient is in the operatory and during the same appointment that the teeth are prepared. Costly chairside time must not be wasted, but the dentist must produce an acceptable restoration. Failure to do so results in the eventual loss of more time than was initially thought saved. For example, an inadequate restoration may lead to unnecessary repairs or to the need to treat gingival inflammation and remake the impression. Such problems can be avoided if the dentist thoroughly understands what is required of the interim restoration and makes the effort to meet these requirements.

REQUIREMENTS

An optimum interim fixed restoration must satisfy many interrelated factors, which can be classified as biologic, mechanical, and esthetic (Fig. 15-1).

Biologic Requirements

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

Table 15-1 FACTORS CONTRIBUTING TO PULP DEATH

Past Present (during fixed prosthodontic therapy)
Caries Preparation trauma
Operative dentistry Microbial exposure
Bruxism Desiccation
Periodontal surgery Chemical exposure
Prosthodontic therapy Thermal exposure

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.

Mechanical Requirements

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.

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).

Esthetic Requirements

The appearance of an interim fixed restoration is particularly important for incisors, canines, and sometimes premolars. Although it may not be possible to duplicate exactly the appearance of an unrestored natural tooth, the tooth contour, color, translucency, and texture are essential attributes. When necessary, esthetic enhancement procedures are available to create personalized details; however, because these are not routinely called for, they are addressed on page 500, after the discussion of cementation and repair.

How well a material matches the color of adjacent teeth initially is easily recognized as an essential requirement of prosthodontics. However, some resins discolor with time intraorally,7 and thus color stability (along with the propensity for stain accumulation) governs the selection of materials when a long period of service is anticipated.

The interim restoration is often used as a guide to achieve optimum esthetics in the definitive restoration. In complete denture prosthodontics, it is customary to have a wax evaluation so that the patient can respond to the dentist’s esthetic interpretation before the denture is processed. Many dentists consider this essential because of the frequency of patients’ requests for changes and the ease with which such changes can be made. When fixed prosthodontics is performed in the anterior oral cavity, it greatly influences appearance, and the patient should be given an opportunity to voice an opinion. Beauty and personal appearance are highly subjective and difficult to communicate verbally, and a facsimile prosthesis can play a vital role in the patient’s consideration of esthetics and the effect that the prosthesis has on his or her self-image. Obtaining the opinions of others whose judgment is valued is also important. An accurate interim restoration is a practical way of obtaining specific feedback for the design of a definitive restoration. Word descriptions alone are often too vague and frequently cause overcorrections, which are difficult to reverse in the definitive restoration. The interim restoration is shaped and modified until its appearance is mutually acceptable to dentist and patient. When this is achieved, an impression is made of the interim restoration (Fig. 15-9) and a cast is poured. This cast accompanies the fixed prosthodontic definitive cast to the laboratory, where the contours are duplicated. This process is more efficient when it begins with diagnostic waxing procedures. Involving the patient in decision making results in greater patient satisfaction.

MATERIALS AND PROCEDURES

Many procedures involving a wide variety of materials are available to make satisfactory interim restorations (Fig. 15-10). As new materials are introduced, associated techniques are reported, and thus there is even more variety. It is a helpful principle that all the procedures have in common the formation of a mold cavity into which a plastic material is poured or packed. Furthermore, the mold cavity is created by two correlated parts: one forming the external contour of the crown or FDP, the other forming the prepared tooth surfaces and (when present) the edentulous ridge contact area. The terms external surface form (ESF) and tissue surface form (TSF) are suggested for these mold parts. This terminology is used in the ensuing discussions.

External Surface Form

There are two general categories of ESFs: custom and preformed.

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).

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 reduction8,9 (Fig. 15-14).

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.

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).

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.

Cellulose acetate

Cellulose acetate is a thin (0.2- to 0.3-mm) transparent material available in all tooth types and a range of sizes (see Fig. 15-16A). Shades are entirely dependent on the autopolymerizing resin. The resin does not chemically or mechanically bond to the inside surface of the shell; therefore, after polymerization, the shell is peeled off and discarded to prevent staining at the interface. Removing the shell has the disadvantage of necessitating the addition of resin to reestablish proximal contacts.

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).

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.

Tissue Surface Form

There are two primary categories of TSFs: indirect and direct. A third category, indirect-direct, is the sequential application of these.

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:

1. There is no contact of free monomer with the prepared tooth or gingiva, which might cause tissue damage11 and an allergic reaction or sensitization.1215 One group of investigators16 reported a 20% incidence of allergic sensitivity in subjects previously exposed to a monomer patch test. The risk of sensitization in patients who are not allergic to monomer increases with the frequency of exposure. In allergic patients, an exposure to even small amounts of monomer usually causes painful ulceration and stomatitis (Fig. 15-20).
2. The procedure avoids subjecting a prepared tooth to the heat evolved from polymerizing resin. The exotherm charted in Figure 15-21 gives an indication of temperature increases with time for several materials under similar experimental conditions. Clinical simulation experiments17,18 have shown peak temperature increases of approximately 10° Celsius in the pulp chambers of prepared teeth upon which direct interim restorations had been made. That amount of temperature elevation is capable of causing irreversible damage to the pulp.19 The simulation experiments also indicate that temperature rise depends directly on the type and volume of resin present. Therefore, a directly made restoration with a large pontic is more likely to cause injury than one for a single crown (especially if the tooth is prepared conservatively). These studies also demonstrate that the heat-conducting properties of the ESFs significantly influence the maximum temperature reached. However, of importance is that peak temperatures were not reached until 7 to 9 minutes had elapsed18 (Fig. 15-22). For this and the practical reason that it must be drawn through the undercuts of adjacent proximal tooth surfaces, the resin should be removed at the rubbery stage of polymerization, which typically occurs 2 to 3 minutes after insertion in the mouth. In Figure 15-22, the temperature rise is negligible at 3 minutes, which suggests that thermal injury is easily avoidable.
3. The marginal fit of interim restorations that have been polymerized undisturbed on stone casts is significantly better than that of interim restorations that have been removed from the mouth before becoming rigid.20,21 The reasons for this are that (1) the stone restricts resin shrinkage during polymerization and (2) separating the resin from the tooth causes distortion. Directly made long-span or multi-abutment partial FDPs are likely to have unacceptable marginal discrepancies caused by shrinkage and distortion.
4. When a dimensionally stable elastomer impression is made to form the TSF,10 it can be retained for possible reuse with the ESF. This allows replacement restorations to be made without having the patient present. For example, if a patient calls to report a lost interim partial FDP, a replacement can be made at the dentist’s convenience before the patient arrives. This minimizes disruption of the office schedule and earns the appreciation of the patient. Whether using an elastomer TSF results in margins that fit as well as those obtained with a gypsum TSF is not known. The elastomer may not resist polymerization shrinkage as effectively as the gypsum.

Materials for Interim Fixed Restorations

While in a fluid state, the interim restorative materials fill the cavity formed by the external and TSFs; they then solidify, producing a rigid restoration.

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,2022 especially when the direct technique is used (Fig. 15-24). Also, the resins currently employed are exothermic and not entirely biocompatible.

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.

MATERIALS SCIENCE

William M. Johnston

The material used for fabrication of an interim restoration consists of pigments, monomers, filler, and an initiator, all combining to form an esthetic restorative substance. The pigments are incorporated by the manufacturer so that the set material appears as much like natural tooth structure as possible, with a variety of shades available. Although each of the other ingredients plays a role in the handling, setting, and final properties of the interim restoration, many important characteristics of the material are determined by the primary monomer. The ability of this monomer to convert to a polymer allows the material, after it has been formed as desired, to set into a solid that is durable enough to withstand the oral environment for the necessary interim period.

Depending on the brand, the most commonly used monomers are methyl methacrylate, ethyl methacrylate, isobutyl methacrylate, bisphenol A diglycidylether methacrylate (bis-GMA), and urethane dimethacrylate. Each of these, or combinations thereof, may be converted to a polymer by free radical polymerization, although the conversion process is never perfectly complete.

Free Radical Polymerization

The polymerization process invokes chemical, mechanical, dimensional, and thermal changes that affect the successful use of these materials in dentistry. Because monomers may be unpleasant or even harmful biologically, the chemical conversion of monomer to a biologically inert polymer is desirable. Also, if the polymerization process is not properly initiated or if it is prematurely terminated, the resultant restoration may not have adequate mechanical properties and may fail easily or quickly. However, because the density of the polymer is inherently and often substantially greater than that of the monomer, a dimensional contraction occurs during polymerization. The polymerization reaction is exothermic, which causes the material to become hot before it loses its fluidity, and so an additional contraction occurs on cooling of the restoration. If a direct technique is being used, the heat of reaction can cause irreversible damage to nearby pulpal tissues, which may already have been thermally insulted during cavity preparation.

Jan 17, 2015 | Posted by in Prosthodontics | Comments Off on 15: INTERIM FIXED RESTORATIONS
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