Although the removal torque curves measured in the present study had similar shapes to one another, they were divided into two groups upon detailed observation, comprising a group of ST with parallel only, and a group of BL, TE, MK3, and MK4 having tapers and platforms. Since the thread contacts the artificial bone sequentially at the time of insertion, the torque curves showed the characteristics of each area. At the time of removal, since all threads come in contact with the artificial bone at first, the torque curve did not present the characteristics of the design until it reached the peak value.

However, it is estimated that when the thread begins to move subsequently, the difference in design of each area appeared in the torque curve. It seems that the change in torque after reaching the peak value at the time of removal is important information for predicting the influence of the change in primary stability occurring through instant load and the early load on secondary stability. The torque value that instantly decreased with a tapered implant was as small as 4–7 N · cm, and it is necessary to study this further in the future.

Comparison between IT and RT

The purpose of measuring and evaluating RT in the present study was to clarify whether the implant stability evaluated by IT can be guaranteed even immediately after insertion. In this study, RT was smaller than IT in the implants having tapered and/or platform areas and a significant difference was recognized, while in the design with only a parallel area, no significant difference was seen between IT and RT or RT was slightly greater than IT. Previous studies that measured both IT and RT include those using artificial bone, human bone, and animal bones. Among such studies, IT and RT were small in those using artificial bone, and RT was smaller than IT. Therefore, using IT to assess the primary stability of an implant revealed the need for certain adjustments.