The reason why a simulation test for only cancellous bone without cortical bone was performed in the present study has already been described. It was reported that bone density and the ratio of cortical bone and cancellous bone have influence on the primary stability of an implant and that higher primary stability is achieved with thread, even at the slightest level, binding to cortical bone rather than being surrounded by only cancellous bones. Therefore, it is expected that torque will rise at the end of the torque curve in the cortical bone region and that the torque will further grow by a synergistic effect with factors that increase the torque, such as a taper or platform of an implant. In the simulation experiments in this study, quantitative measurements were successfully performed by extracting only the effects of implant designs and by using a uniform pseudo bone without cortical bones. Sufficient torque is needed for primary stability of an implant, although the risk that excessive compressive force acts on the bone to cause bone resorption and further bone necrosis has been pointed out. To avoid such a situation, it is necessary to find a balance between local bone resorption and the torque, and Meredith recommended insertion torque values of 25–30 N · cm. The torque value and torque rising rate according to the design of implant bodies obtained in the present study enabled estimation of the part of bones generating retention and identification of the part giving compressive force to bones. This will allow us to clarify the relationships among the design of an implant, the value of the torque generated by the implant, and the compressive force to the bones.

In the torque-duration curve at the time of insertion, the characteristics of the implant design are well shown. It is presented as a straight line with a moderate gradient in the parallel thread area, a quadratic curve-like curve in the tapered area, and a hyperbola-like curve in the platform area. The torque rise rate was 2.14 N · cm/s for the initial area, 0.36 for the parallel area, 2.33 for the tapered area, and 2.65 for the platform area. The torque-duration curves at the time of removal were classified into tapered implants with the peak magnitude as the maximum torque value and straight implants with a maximum torque value greater than the peak magnitude. The RT of the implants having tapered or platform areas was significantly smaller than the corresponding IT, while the RT of the straight implants was the same as or slightly greater than the corresponding IT.