Mechanical Properties of Temporary Anchorage Device

Objectives: Recently, with the increased need of bony anchorage in the orthodontic treatment, many different designs of mini-screws and mini-plates were developed and applied in the clinics. Mini-screws have restrictions in diameter and length, an optimal design of the shape is important for sufficient primary stability. The purpose of the study is researching how geometrical designs of mini-screws will affect the insertional torque, stability, and stress distribution, combined with finite element analysis (FEA), and applying to the development of new systems in the future.

Methods:  We build up a standardized equipment, method and procedures of mechanical and finite element analysis for three types of commercial mini-screws including (1) Bending test. (2) Torsion test. (3) Insertion moments test. (4) Pullout test and (5) Removing torsion test. All mechanical tests were performed in artificial bone with homogenous density to remove the variability associated with bone.

Results: (1) Increase of thread depth, taper of the core and the circles of taper part will result in larger insertion torque and provide better retention stability. (2) Exaggerated increase of thread depth, taper of the core or the circles of taper will decrease the resistance to pullout. (3) FEA is coincident with the mechanical test, and could be a good tool for detecting the stress and strain distribution of the mini-screws (4) There is a positive correlation between insertional torque and resistance to pullout.

Conclusions: Modification of the mini-screw design can substantially affect the mechanical properties. The finite element method is an effective tool to identify optimal design parameters and allow for improved mini-screw designs.