Effects of Screw Design on the Mechanical Properties of Mini-Implants

Mini-implants have been used as orthodontic temporary anchorage and have gained popularity due to their versatility. Although mini-implants remain clinically stable, they extrude and tip under orthodontic loading. As the orthodontic mini-implants have restriction in diameter and length, an optimal design of the shape is important to warrant sufficient primary stability. Objectives: The purpose of this study is to investigate the influence of different mini-implants design factors including the thread depth, the degree of tapering, and the taper length on insertional torque, pull-out strength, stiffness, and screw displacement before failure. Methods: Four types of mini-implants with different design parameters were custom-manufactured and tested mechanically. All mechanical tests were performed in artificial bone with homogenous density to remove variability associated with bone. Results: The results revealed that core/external diameter ratio (thread depth) is an important determinant in optimizing mini-implant performance. Increasing the core diameter of the uppermost threads to create a tapered core design and longer tapered core also improve mechanical properties. Conclusion: In conclusion, modification of the mini-implant design can substantially affect the mechanical properties. In clinical situation when cortical bone is thin, a mini-implant with a longer tapered portion would warrant better primary stability.