Issues in handling impression materials.

Implant restorations require extreme accuracy since the implants are rigidly fixed. Although impression materials can be highly accurate, small inaccuracies are acceptable because the periodontal ligament allows for tooth movement. This luxury is not available with implants. A problem is the rotation of direct impression copings and distortion of interabutment positions in the master cast. Wee investigated the amount of torque required to displace a coping in eight impression material groups and the positional accuracy of the materials. The initial examination of torque narrowed the acceptable materials to a medium viscosity polyether, a high viscosity addition silicone, and a medium viscosity polysulfide-condensation silicones were eliminated. The results showed that the polyether and addition silicone produced casts with minimal distortion. Finally, it always is prudent to read the manufacturer's recommendations for use of impression materials. Catalyst-to-base ratios vary, as do working and setting times, depending on exact compositions. Impression materials also have various degrees of compatibility with stone, particularly the hydrocolloids, and it is advisable to use the recommended stone. Adhesives should be applied at least 15 minutes prior to loading the impression material. If at all possible, use of a rigid tray is recommended and impressions should be poured in the dental office; this minimizes the error produced at the beginning of the fabrication process. Elimination of errors early in fabrication of a prosthesis help prevent their magnification and result in a better-fitting restoration, requiring minimal adjustment in the end.