However, there are some disadvantages:

– The cost is high compared with that of other materials.

– The material breaks when placed in areas of moderate stress.

– Some of the materials are difficult to repair.

– Bis-acryl material is excellent for single-unit PRs and can be used well by minimally experienced staff people, where allowed by law. Many clinicians make a wax impression, or a rigid addition-reaction silicone impression, of the tooth or teeth before preparing them and use these materials as a rapidly made matrix for the subsequent bis-acryl PR.

Polymethyl methacrylate. Acrylic resin had been the mainstay for PRs until bis-acryl made its entry. It still is useful for many situations. The most well-known brand is Jet (Lang, Wheeling, Ill.). The advantages of polymethyl methacrylate, or PMMA, are the following:

– high strength compared with most other materials;

– relatively good color stability over a few weeks;

– can be smoothed and polished;

– can be characterized;

– low cost;

– easily repaired.

Disadvantages are the following:

– High exotherm; the high temperature generated during the setting of PMMA can be traumatic to the dental pulp if not dissipated with cool water and air during the polymerization stage.

– Shrinkage is relatively high.

– The odor of the material is objectionable to many patients.

I prefer PMMA shells made in the office laboratory for long-span PRs. The PMMA shells can be made by competent dental assistants (where allowed by law). These shells are lined in the mouth with polyethyl methacrylate, or PEMA, as described below. The extra strength of these PRs allows moderate-term service (a few weeks to a few months), without the worry of the patient’s breaking the PR. Although PMMA provisional restorations were fabricated in the mouth and used without liners in the past, this concept is not used to a significant degree today, because of the high exotherm and odor of the PMMA.

Polyethyl methacrylate. Until recently, PEMA was the most popular material for PRs. It is similar to PMMA but has some positive and negative differences. Well-known brands are Trim II (Harry J. Bosworth Co., Skokie, Ill.) and Snap (Parkell, Farmingdale, N.Y.).

The advantages of PEMA are:

– lower exotherm than PMMA, but higher exotherm than bis-acryl resin;

– moderate strength;

– can be smoothed and polished;

– can be characterized;

– PEMA bonds to PMMA and is a good, clinically placed liner for a laboratory-made higher-exotherm PMMA;

– low cost;

– easily repaired.

PEMA disadvantages are:

– bodily discoloration over a two-week period;

– odor of the material is objectionable to some patients;

– weaker than PMMA.

PEMA can be used alone as a PR in the posterior portion of the mouth, but the bodily discoloration that occurs soon after placement makes it objectionable for situations needing PRs for more than a few days.

Microfill resin. These materials have been highly useful for PRs for inlays, onlays and some veneers. Standard restorative microfills can be used for PRs for veneers with great success. The only significant problem is that the PRs may look better than the final veneers. An example is Durafill VS (Heraeus Kulzer, Armonk, N.Y.). Light-curing modified microfill PR materials have been easy to use in inlay and onlay preparations. An example is Fermit N (Ivoclar Vivadent, Amherst, N.Y.).

The advantages of microfill resin as a PR are the following:

– They are light-cured.

– Restorative brands can be smoothed and polished to an excellent level.

– Putty-like consistency makes them easy to use.

– They have only slight exotherm.

The disadvantages of micro-fill are:

– high cost;

– brittleness limits their use to single-tooth restorations or to veneers.

Other types. Several other types of chemically set or light-curing resins are available, including ethyl vinyl methacry-late and urethane methacrylate. Thermoplastic resins are used by a small percentage of dentists. Polycarbonate crowns, lined with resin or just cemented with provisional cement, are still popular.

	THE BEST CHOICES FOR SPECIFIC SITUATIONS

TOP TYPES OF PROVISIONAL... THE BEST CHOICES FOR... SUMMARY

 
The following suggestions reflect my personal opinions based on thousands of restorations placed over many years.

Single crowns. In my opinion, the best choices are bis-acryl resin made in an impression matrix, or PEMA if the PR is planned to be used for only a short time, or if it is used in an area not requiring an optimum esthetic result. I recognize that many dentists use polycarbonate crowns for single crowns. However, I prefer to customize PRs by shaping and contouring the resin to the optimum anatomy and contour. At this time in dentistry, I do not see significant desirable characteristics in metal shells, although they are still used by some dentists.

Short-span fixed pros-theses (three unit). I prefer PMMA shells made in the laboratory by a dental assistant (where allowed by law). These shells, lined with lower-exotherm and better-fitting PEMA, make well-fitting, strong, color-stable PRs. PEMA can be used with success, but moderate exotherm and color degeneration are objectionable. Bis-acryl resin is acceptable for short spans with minimal occlusal stress expected, but they break easily in long-span fixed prostheses or in mouths with high-stress occlusions. Metal shells are not appropriate for PRs for fixed prostheses, because of unavoidable collapse of occlusion and changes in the spatial relationships of abutment teeth.

Long-span fixed pros-theses (more than three units). I prefer to use PMMA shells lined with PEMA for the strength reasons previously stated. Bis-acryl and PEMA are too weak for predictable service, unless the patient avoids chewing on the PR.

Inlays and onlays. Modified microfill resins make excellent PRs for short-term use of a few days. These restorations do not require cementation, and are easily made by competent dental assistants, where allowed by law. Bis-acryl, PMMA or PEMA PRs may be constructed, but they require cementation with provisional cement, and the cement is difficult to remove from the internal side of an inlay or onlay tooth preparation.

Veneers. Custom-made restorative microfill resin PRs are my strong choice, although they require a well-educated staff person or dentist to construct them. These PRs are color stable and smooth, but, as with all PRs for veneers, they are not strong because they are thin. Patients should be advised to be careful with them during the PR period. Bis-acryl PRs, connected among all of the prepared teeth, and provisionally cemented with resin-based cement (such as TempBond Clear, Kerr, a division of Sybron Dental Specialties, Orange, Calif.) allow acceptable service as well.

	SUMMARY

TOP TYPES OF PROVISIONAL... THE BEST CHOICES FOR... SUMMARY

 
Many options are available for PRs for teeth prepared for crowns, veneers, inlays, onlays or fixed prosthetic abutments. Dentists and dental assistants (where allowed by law) should know the physical properties of the various types of materials and should be able to select and place the appropriate type of PR based on the needs of specific clinical situations.

View larger version (139K): [in this window] [in a new window]   Dr. Christensen is co-founder and senior consultant of Clinical Research Associates, 3707 N. Canyon Road, Suite No. 3D, Provo, Utah 84604, and is a member of JADA’s editorial board. He has a master’s degree in restorative dentistry and a doctorate in education and psychology. He is board-certified in prosthodontics. Address reprint requests to Dr. Christensen.