Thread Pitch Variant in Orthodontic Mini-screws: A 3-D Finite Element Analysis

Orthodontic miniscrews are widely used as temporary anchorage devices to facilitate orthodontic movements. Miniscrew loosening is a common problem, which usually occurs during the first two weeks of treatment. Macrodesign can affect the stability of a miniscrew by changing its diameter, length, thread pitch, thread shape, tapering angle and so on. In this study, a 3-D finite element analysis was done to show the effect of thread pitch variant on the stress distribution pattern of the screw-cortical bone interface. While orthodontic forces were applied, stresses were usually concentrated at the first thread of the screw in contact with the cortical bone. The cortical bone provided a significant percentage of stability compared to the trabecular bone against orthodontic forces. Therefore spongy bone was removed from the finite element analysis. The changes of maximum von Mises stresses were shown on the charts. The results showed that stresses decreased with decrease in thread pitch, but they increase when thread pitch becomes less than a certain value. The pattern of stress distribution differed when the stresses were increased. The results are beneficial for the design of an ergonomic dual miniscrew, with better properties than the commercially available miniscrews and based on the results, a new dual miniscrew is recommended.