The effect of cortical bone thickness on the stability of orthodontic mini-implants and on the stress distribution in surrounding bone.

Cortical bone thickness (CBT) was evaluated at mini-implant placement sites in 65 orthodontic patients and was found to be directly proportional to the success rate of the mini-implant. The success rate of the mini-implant was significantly greater at sites with CBT> or =1.0mm. To examine the biomechanical effects of CBT, finite element models were made for CBT from 0.5 to 1.5mm, at 0.25-mm intervals. Cortical bone models without cancellous bone were constructed to examine the biomechanical influence on cortical bone after cancellous bone resorption. CBT influenced the stresses in the cancellous bone, but could not directly influence the stresses in the cortical bone. For CBT<1mm, the cancellous bone models exhibited von Mises stresses exceeding 6MPa, and the cortical bone models without cancellous bone showed von Mises stresses exceeding 28MPa. Greater CBT values were associated with higher mini-implant success rates. This morphometric study and mathematical simulation verify that a clinical CBT threshold of 1mm improves the success rate of mini-implants.