Mechanical Properties of Temporary Orthodontic Devices

Objectives: The primary stability of a temporary orthodontic device is critical, since most temporary orthodontic device failures occur at an early stage. As temporary orthodontic devices have restrictions in diameter and length, an optimal design of the shape is important for sufficient primary stability. The objective of this study was to investigate the influence of various temporary orthodontic devices design factors, including thread depth, degree of taper, and taper length on insertion torque, pullout strength, stiffness, and screw displacement before failure.
Methods: Finite element analyses were conducted first for identification of optimal design parameters. Four types of temporary orthodontic devices with different design parameters were then custom manufactured and tested mechanically. All mechanical tests were performed in artificial bone with homogenous density to remove the variability associated with bone.
Results: Finite element results showed that, for temporary orthodontic devices with a fixed external diameter of 2 mm, a thread length of 9.82 mm, and a pitch of 0.75 mm, those with greater thread depths, smaller taper degrees, and shorter taper lengths generated higher maximum stresses on the bone and thread elements. These temporary orthodontic devices also had larger relative displacements. Maximum pullout resistance was attained with a core/external diameter ratio of 0.68. All mechanical results were compatible with the findings in the finite element analyses.
Conclusions: Modification of the temporary orthodontic device design can substantially affect the mechanical properties. The finite element method is an effective tool to identify optimal design parameters and allow for improved temporary orthodontic devices designs.