Effects of thread shape on strength and stability of dental mini-screws against orthodontic forces

Abstract

Orthodontic mini-screws are short-term anchorage implants using widely to assist orthodontic mechano-therapies. Mini-screw loosening from the host bone during the first weeks of treatment is their most frequent problem, which is called weakness in the stability, as well. Applying changes in the macro-design of the mini-screws can considerably solve their problem in the stability. In this study, a 3-D finite element analysis (FEA) is done in order to improve the thread shape aspect in designing mini-screws. In this optimized design, a few parameters are considered. First, the upper angle of threads tried to be close to the right angle in order to provide more stability. Second, the sharpness of the threads wanted to be kept because convenience in implantation is helped by sharpness in thread apex, and it is necessary to reduce tissue trauma. Another essential parameter needing to be taken into consideration is the limited stress which bones can tolerate, and it causes to be cautious about stresses amounts. In order to reach this goal, moderate changes in thread angles were done while an axial load was applied to the mini-screw. Finally, the triangular thread shape of the mini-screw was optimized, and one specific shape was considered as the best shape for the given dimensions. These results are valuable to follow the same instruction and steps in order to optimize the design of other mini-screws to ergonomic ones. Also, the obtained relationship between the thread angles facilitates designing a modified thread in order to have less exerted stress on the mini-screw.