15 INTERIM FIXED RESTORATIONS
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.
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
|Past||Present (during fixed prosthodontic therapy)|
|Operative dentistry||Microbial exposure|
|Periodontal surgery||Chemical exposure|
|Prosthodontic therapy||Thermal exposure|
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.
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.
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.
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.
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.
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.
Fig. 15-9 A, This interim fixed dental prosthesis established anterior guidance and pontic form before work on the definitive restoration was begun. (Note the facial cavosurface margin of the mandibular second premolar covered by the interim restoration to protect it from damage.) B, The definitive restoration closely matches its predecessor in form and function.
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.
Fig. 15-10 Although there are many variations, molds used in making interim restorations consist of an external surface form (ESF) and a tissue surface form (TSF). Direct techniques entail use of the patient’s mouth directly as the TSF. A, Indirect technique: ESF, an alginate impression; TSF, a quick-set plaster cast. B, Direct technique: ESF, a baseplate wax impression; TSF, the patient. C, Direct technique: ESF, a vacuum-formed acetate sheet; TSF, the patient. D, Direct technique: ESF, a polycarbonate preformed shell; TSF, the patient. E, Indirect-direct technique: ESF, a custom preformed three-unit fixed dental prosthesis shell (maxillary right central incisor to canine) made indirectly; TSF, the patient. F, Indirect technique: ESF, a silicone putty impression; TSF, a quick-set plaster cast of the preparations.
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 reduction8,9 (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).
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).
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 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 (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 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.
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. 15-21 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.)
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.
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.
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.
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.
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.
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.
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 conta/>