The Influence of Seating Load on Internal and Marginal Fit of a Fixed Dental Prosthesis: An In Vitro Study

This study aimed to investigate how axial seating loads and shear loads influence the marginal and internal fit of a six-unit fixed dental prosthesis (FDP), using the replica technique.

Preparation for a six-unit FDP was made on plastic teeth, and a titanium FDP was constructed using CAD/CAM. A master epoxy cast of the abutment teeth was fabricated. The fit of the FDP was evaluated using the impression replica technique. Seating loads at 20, 35, 50, and 70 N in an axial direction and shear loads at 50 N were applied. The replicas were sectioned, and the silicone film representing cement gap was measured.

The results showed that seating loads of 20 N gave the largest cement gaps, and seating loads of 35 N had a great range. At seating loads of 50 N, smaller cement gaps could be seen. When testing with 70 N the cement gaps did not become any smaller. Testing with shear loads resulted in larger cement gaps compared to testing without shearing at 50 and 70 N.

Within the limitations of the present in-vitro study, the following conclusions can be drawn. Seating loads below 50 N are insufficient to provide an acceptable fit of a six-unit FDP, whereas seating loads exceeding 50 N do not markedly improve the fit. The presence of shear loads affects the fit negatively. Further studies are needed to evaluate whether the results are applicable on other prosthetic constructions and in clinical settings.