Cortical Thickness, Bone Density, and the Insertion Torque/Depth Integral: A Study Using Polyurethane Foam Blocks

Bone density and implant primary stability parameters have been introduced that are based on calculating (1) the average of the instantaneous torque needed to keep the rotation speed of a bone density probe constant while it descends into bone or (2) the integral of the instantaneous torque-depth curve at implant insertion (I), a quantity that is equal to the insertion energy multiplied by a constant. This study aimed to determine how these two quantities are affected by the presence and thickness of a cortical bone layer.

An instantaneous torque-measuring micromotor was used to measure the density of six double-layer polyurethane foam blocks mimicking different cortical/cancellous bone combinations. Twenty measurements per block were collected, averaged, and compared. The insertion torque and the integral (I) of the instantaneous torque-depth curve at implant insertion were recorded when 20 3.75 × 12-mm cylindrical implants were inserted in each of nine blocks, including three single-layer blocks simulating the absence of a cortical layer, under three final cortical (countersink) preparations: 4.0, 3.7, and 3.5 mm. The relationship between the insertion torque, the integral of the instantaneous torque-depth curve at implant insertion (I), cortical thickness, and the final diameter preparation were investigated with regression and best-fit slope analyses.

Bone density measurements showed that the average of the instantaneous torque at probing allowed differentiation of five of six different bone classes (hard-hard, hard-normal, hard-soft, normal-normal, normal-soft, soft-soft); the post hoc analysis of variance (ANOVA) comparisons were all statistically significant except for the hard-soft-normal-soft pair. The insertion torque and the integral (I) of the instantaneous torque-depth curve at implant insertion increased proportionally with cortical bone thickness (Pearson's r > 0.96 in all cases).

When the final preparation varied from 3.7 mm to 3.5 mm, the insertion torque-thickness plot slope did not change significantly, while that of the instantaneous torquedepth curve integral (I)-thickness plot did change, suggesting that the torque-depth curve at implant insertion integral (I) may detect the increase in implant stability consequent to slight anatomical changes or changes in the site preparation protocol better than the insertion torque when measuring the cortical bone layer stress while undergoing insertion. These findings concerning bone density and primary stability should be investigated further using different experimental settings. If confirmed, they might generate improvements in the predictability of implant and prosthetic rehabilitation outcomes.