Effect of cutting flute geometry of orthodontic miniscrew on cortical bone microdamage and primary stability using a human bone analog

Abstract

Although orthodontic miniscrews were developed to provide reliable anchorage against orthodontic traction forces, they have pervasive low stability problems. Cutting flute geometry of the screw is considered to be a key factor; however, it has not been fully investigated. Therefore, the objective of our study is to evaluate the effect of cutting flute geometry on cortical bone microdamage.Human bone analogs, 1.0 mm thick, were prepared alongside custom‐made Ti6Al4V screws. These screws, with no flutes or modified flute geometries, came in lengths of 1.2, 2.4, and 5.0 mm, each with rake angles of 0° and 20°. Each screw was inserted into a bone sample and removed while measuring the maximum insertion torque (MIT) and the maximum removal torque (MRT). The bone microdamage was evaluated using cross‐sectional surface images of human bone analogs.MRT was significantly different in all pairwise comparisons among the three flute‐length groups in each rake angle, and the 5.0‐mm‐length groups demonstrated the lowest values. Similar tendencies were found in MIT and the bone microdamage, although significant differences were not observed in all the pairwise comparisons in the flute‐length groups in each rake angle. Interestingly, the two rake angle groups in each flute‐length exhibited significant differences in MIT and the bone microdamage, but not in MRT.From our investigation using custom‐made screws and human bone analog, cortical bone microdamage is affected by both flute geometries, whereas primary stability is affected by only the flute length.