Minimum required length of orthodontic microimplant: a numerical simulation and clinical validation

This study aimed to determine the minimum required length of microimplants (MIs) to prevent excessive micromotion during MI healing that can lead to MI failure.

Hypothesizing that the implantation depth of MI in cancellous bone (ID_cancel) is the key to the control of micromotion during MI healing, we numerically investigated the minimum ID_cancel required to maintain MI micromotion to below the threshold (30 μm) that would threaten MI survival. Twenty MI and bone models were built using MIs of 4 lengths and bone specimens with 5 different cortical bone thicknesses to create ID_cancel in the 0.5-5.5 mm. Then, applying a horizontal force of 1.5 N on the MI head, we calculated the micromotion (peak and average MI micromotions) and determined the minimum ID_cancel. A clinical test was performed to verify the numerical result by placing 160 MIs in the posterior maxilla and mandible.

A strong correlation (r²= 0.694) was found to exist between ID_cancel and MI micromotion. A minimum of 2.5 mm of ID_cancel was needed to maintain the level of MI micromotion (peak micromotion) <30 μm threshold. The 6-month survival rate of MI was strongly correlated with ID_cancel (r²= 0.744) and decreased sharply when ID_cancel was ≤2 mm.

The minimum lengths of MIs to provide the minimum ID_cancel of 2.5 mm required to promote successful MI healing in the posterior maxilla and mandible are 5.2 and 6.5 mm, respectively.