The prosthesis must be able to firmly hold down the portion of the rubber dam that is distal to the problem molar.

The prosthesis must be able to firmly hold down the portion of the rubber dam that is distal to the problem molar, against the strong dislodging forces exerted by the taut dam in this area. Solving this problem is akin to designing a distal extension partial denture that resists displacement from the edentulous distal area. The resistance forces of the clamp should be close to the origin of the rubber dam’s displacing forces; the ideal location is the next tooth mesial to the problem molar. This maximizes the mechanical advantage of the clamp’s resistance forces.

In addition, the prosthesis should be extended so that it covers the occlusal surface of a tooth mesial to the clamped tooth, thus forming a rest (or indirect retainer) on that tooth. Placing a rest on a tooth mesial to the clamped tooth shifts the fulcrum around which the prosthesis rotates, so that the fulcrum is farther away from the resistance forces of the clamp. This further increases the mechanical advantage of the lever arm and ensures the stability of the prosthesis against the displacing forces of the taut dam.

The prosthesis is made of pink denture base material that must be thick enough to prevent breakage at various stress points within the prosthesis.

FABRICATION OF THE PROSTHESIS
Obtain a diagnostic model. The clinician should obtain a diagnostic model of the patient’s dentition that includes the problem molar. Using petroleum jelly, lubricate the gingiva and clinical crowns of the molar on the model and at least two teeth mesial to it. The clinician should place a rubber dam on the model to act as a spacer, with the prosthesis fabricated over it. A spacer improves the adaptability and stability of the prosthesis, since it accommodates the thickness of the rubber dam in the patient’s mouth. If the prosthesis is made without it, the dam may prevent the prosthesis from seating snugly against the teeth and soft tissues. It also may be more difficult for the clamp to engage the undercut of the tooth mesial to the problem molar, and undue stress may be put on the prosthesis. This may lead a frustrated clinician to push down too hard on the clamp, exerting heavy forces that could break the prosthesis.

Adapt denture base material. The clinician should adapt the light-cured denture base material to the stone model, taking care to avoid undercuts in the model. The material should cover the gin-giva of the molar and at least two teeth mesial to it. In addition, it should surround, but not cover, the clinical crown of the molar. On the second tooth mesial to the problem molar, the clinician should cover the occlusal surface to form a rest, which is continuous with the buccal and lingual portions of the denture base material.

The material forms a continuous gingival ring around the problem molar, the tooth just mesial to it (which will be clamped) and the tooth just mesial to the clamped tooth (which will hold the rest). The buccal and lingual portions of the prosthesis do not have definitive dimensions; rather, they should be shaped closely to the gingiva for stability. They also should be thick enough to withstand the forces on the prosthesis.

The rest also does not have a definitive dimension, but, like the rest of the material, should be thick enough to prevent breakage of the cured prosthesis under the strong forces of the rubber dam. To maximize the stability of the prosthesis, the clinician should ensure that the rest fully covers the occlusal surface of the tooth.

Clamp the tooth mesial to the molar. Before light-curing, the clinician should clamp the tooth just mesial to the molar, so that the wings of the clamp imprint onto the denture base material. This will enable the clamp to fit snugly and predictably onto the cured prosthesis and will improve the stability of the prosthesis. After the clamp is placed, but before curing, the practitioner should make sure that the holes in the clamp wing are not plugged with the denture base material; otherwise, the clamp forceps may not be able to grasp the clamp once the prosthesis is cured. The practitioner also needs to make sure that no undercuts exist within the material that may make it impossible to remove the clamp after the material has cured.

Cure the material. Cure the material in a light-curing unit, with the clamp in place. After the material has partially hardened, the clinician may remove the clamp to allow the material covered by the metal to be light-cured.

Remove the prosthesis from the light-curing unit, and cure further if any soft spots are found. It is critical to test the strength of the prosthesis, and to practice placing it on and removing it from the model. After removing it from the model, the clinician should place a rubber dam on the teeth in the problem area, lay the prosthesis over the dam and anchor it by clamping the tooth mesial to the molar, while avoiding use of excessive force. Next, tug the rubber dam material distal to the problem molar, using a degree of force comparable to the dislodging force of the dam in the patient’s mouth.

If light dislodging forces dislodge or break the prosthesis, it may have to be remade. The prosthesis can be made thicker to strengthen it, or it may incorporate more than one clamp or more than one rest to improve its retention against the dislodging forces of the rubber dam. To reduce these forces, the clinician may attempt to slacken the rubber dam distal to the problem molar.

Achieve isolation. The clinician uses the prosthesis to achieve isolation in one of two basic ways. He or she may punch holes in the rubber dam to expose the tooth with the rest (tooth no. 4), the clamped tooth (tooth no. 3) and the problem molar (tooth no. 2) (Figure). Assuming that the punched-out dam will hold onto the clinical crown of the problem molar, the clinician then places the dam on the teeth and lays the clamped prosthesis over it. Alternatively, if the clinical crown of the molar cannot grasp the rubber dam hole, the clinician may use a sharp blade to cut away (on a model or in the patient’s mouth) the portion of the rubber dam covering the clinical crown. This alternative procedure may not result in 100 percent isolation, but it will enable the clinician to better manage the problem.

CONCLUSION
This article describes how to make a novel and simple custom prosthesis that can enable clinicians to achieve dry-field isolation of distal molars with short clinical crowns. Although the procedure is somewhat technique-sensitive, the prosthesis can be made within minutes. It also can be modeled in wax, rather than in denture base material, and cast in metal in a dental laboratory. This prosthesis expands the possibilities for isolation in procedures involving the placement of amalgam and resin-based composite restorations, as well as in endodontic procedures, for which isolation is the standard of care.

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