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Objective: To investigate the most reliable stress or strain parameters in subject-specific finite element (FE) models to predict success or failure of orthodontic mini-implants (OMIs). Materials and Methods: Subject-specific FE analysis was applied to 28 OMIs used for anchorage. Each model was developed using two computed tomography data sets, the first taken before OMI placement and the second taken immediately after placement. Of the 28 OMIs, 6 failed during the first 5 months, and 22 were successful. The bone compartment was divided into four zones in the FE models, and peak stress and strain parameters were calculated for each. Logistic regression of the failure (vs success) of OMIs on the stress and strain parameters in the models was conducted to verify the ability of these parameters to predict OMI failure. Results: Failure was significantly dependent on principal strain parameters rather than stress parameters. Peak maximum principal strain in the bone 0.5 to 1 mm from the OMI surface was the best predictor of failure (R 2 5 0.8151). Conclusions: We propose the use of the maximum principal strain as a criterion for predicting OMI failure in FE models. (Angle Orthod. 2016;86:24-31.)

Factors responsible for the success or failure of orthodontic mini-implants (OMIs) in clinical settings are unclear. Failure of OMIs was found to be associated with increased maximum principal strain (MaxPN) when assessed using the subject-specific finite element (FE) modeling technique. The purpose of the present study was to identify factors that increase MaxPN and thereby predispose the OMI to failure. Using the FE method, MaxPN was calculated around 28 OMIs placed in orthodontic patients, 6 of which failed during the first 5 months. Sixteen potential risk factors related to patients or to OMI position were measured on computerized tomographic images or calculated in FE models. The impact of these factors on MaxPN was verified using regression analysis. Three factors were found to have significant nonlinear relationships with MaxPN: cortical bone quality, vertical angulation of the OMI, and proximity of the OMI to the tooth in the direction of force. In conclusion, failure of an OMI is a multifactorial problem, and position and angulation of the implant are among the affecting factors. Slight apical inclination and positioning at least 1 mm off the root in the direction of force may significantly decrease failure probability.

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