Posterior restoration rotational torque associated with cuspal incline angulation: A proof of concept

Off-axis, external forces with a moment arm on posterior restorations have not been investigated in experimental studies.

The purpose of this proof-of-concept study was to determine the interaction between occlusal force directed toward cuspal incline angulations with variations in base width and preparation vertical heights. Torque forces on a single crown restoration in simulated premolar and molar tooth forms were calculated for 3 different rotational axes.

Trigonometric calculations were made to determine the amount of torque generated in a simulated-crown restoration in premolar and molar tooth forms. Restorations with different cuspal incline angulations were loaded with an off-axis force of 200 N. This force was applied to 5 different cuspal incline angulations in both tooth forms at varying preparation heights. Right triangles were used to enable trigonometric computations of the resulting moment arms that accompanied the 3 rotational axes.

The total torque values were calculated with a range from 7.5 to 372.8 Ncm. The highest levels of torque were generated in the 5-mm-high molar tooth form with a rotational axis located within the root form, perpendicular to the 45-degree cuspal incline. In general, large moment arms were generated with steep cuspal incline angulations and mid-root axis locations; the lowest torque values in all cuspal incline angulations were found in the rotational axis locations at the crown finish line. The torque values at the rotational axis finish line location were found to be greatest in the largest vertical tooth form height category (5 mm) in both tooth model sizes.

The crown restoration cuspal incline angulations, vertical preparation heights, base widths, and rotational axis locations all played a role in the torque force levels generated, probably influencing restoration stability.