Mechanical evaluation of orthodontic mini-implants of different lengths

Purpose: The current study sought to evaluate the influence of the length of the mini-implant on its mechanical properties.

Materials and methods: A total of 405 mini-implants from the same manufacturer were divided into 3 groups (n = 135) according to whether they were 6, 8, or 10 mm in length. An insertion torque test was performed using minipig osseous tissue of different cortical thicknesses (1 mm to 6 mm). To assess the maximum fracture torque during insertion, the mini-implants were inserted into swine femoral cortical bone. A digital torque gauge was positioned perpendicularly to the bone surface to measure this parameter. To evaluate the deformation and subsequent fracture due to a bending force, the mini-implants were subjected to loading orthogonal to their length at a speed of 0.5 mm per minute.

Results: The insertion torque increased with increasing screw length and increasing cortical bone thickness. There were no statistically significant differences between the different implant lengths with respect to the insertion torque force required for fracture (P > .05) or the forces required for deformation (P > .05) and fracture (P > .05) when submitted to flexion.

Conclusions: Increasing the length of the screw does not increase its mechanical strength, but can efficiently reinforce the initial stability of mini-implants.