Histomorphometric evaluation of cortical bone surrounding mini-screw: Why is the insertion torque critical for primary stability?

Abstract

Objective: Mini-screws have widely been used as a temporary anchorage device. We previously reported the importance of insertion torque for primary implant stability. However, it remains unclear whether or not the destruction of cortical bone surrounding the mini-screw depends on the magnitude of the insertion torque, and whether or not the destroyed bone can be repaired around titanium alloy mini-screws. Materials and methods: Self-drilling type titanium alloy mini-screws were placed into the rat maxillary bone. Insertion torque and screw diameter were varied and their effect on primary stability of mini-screw was evaluated. Bone remodeling around the mini-screw and the ratio of osteoid and calcified bones was examined using Villanueva bone staining on each section. Moreover, during the experimental period, the tensile strength of the implanted screw was measured. Results: Irrespective of the diameter of micro-implants which were implanted with 5-10 N.cm insertion torque were in close contact with the cortical bone, whereas the cortical bone was broken when implants were inserted with 15-20 N.cm insertion torque. While using the 15-20 N.cm insertion torque, the screw surface did not exhibit close contact with the cortical bone and osteoid was increased at 56 d after implantation. Furthermore, tensile strength of the bone in the 15-20 N.cm insertion torque group remained lower than in the 5-10 N.cm group through an experiment period. Conclusions: Our findings emphasized that we should use an optimal insertion torque at 5-10 N.cm for mini-screw implantation to establish primary stability.