Passive fit evaluation of implant superstructures by analyzing the accumulated screw tightening torque

Implants, particularly restorations with screw-retained superstructures, must have a stable passive fit. Clinical fit evaluations are often subjective. Therefore, this study investigated the feasibility of identifying misfits and evaluating the fit based on the screw-tightening torque.

Screw-retained monolithic zirconia superstructures supported by two implants and several master models were prepared. A passive-fit model was created by connecting the implant body to the superstructure and embedding it in dental stone. Vertical misfit models were created by placing titanium membranes with thicknesses of 50, 100, 150, and 200 µm between the superstructure and implant body. In each model, the screw-tightening torque on each side was measured at 0.05-s intervals up to 25 N cm. The torque for the second screw was divided into initial, middle, and final rotation phases, and the results under different conditions were compared.

No vertical gaps were observed at the joints in the passive-fit model, whereas vertical gaps were observed in the misfit models. No significant differences between the accumulated torque on each side were observed in the passive fit group. By contrast, significant differences between the accumulated torque on each side were observed in the misfit groups. When the instantaneous torque was divided into three phases based on time, the samples with misfits of 150 and 200 µm showed high values in the middle and final rotation phases.

Passive fit and misfit can be evaluated numerically and objectively by measuring the torque and accumulated torque during the installation of screw-retained superstructures.