Effective en-masse retraction design with orthodontic mini-implant anchorage: a finite element analysis.

INTRODUCTION: The strategic design of an appliance for correcting a bialveolar protrusion by using orthodontic mini-implant anchorage and sliding mechanics must take into account the position and height of the mini-implant, the height of the anterior retraction hook and compensating curve, and midline vertical traction. In this study, we used finite element analysis to examine effective en-masse retraction with orthodontic mini-implant anchorage and sought to identify a better combination of the above factors.

METHODS: Base models were constructed from a dental study model. Models with labially and lingually inclined incisors were also constructed. The center of resistance for the 6 anterior teeth in the base model was 9 mm superiorly and 13.5 mm posteriorly from the midpoint of the labial splinting wire. The working archwires were assumed to be 0.019 x 0.025-in or 0.016 x 0.022-in stainless steel. The amount of tooth displacement after finite element analysis was magnified 400 times and compared with central and lateral incisor and canine axis graphs.

RESULTS AND CONCLUSIONS: The tooth displacement tendencies were similar in all 3 models. The height of the anterior retraction hook and the placement of the compensating curve had limited effects on the labial crown torque of the central incisors for en-masse retraction. The 0.016 x 0.022-in stainless steel archwire showed more tipping of teeth compared with the 0.019 x 0.025-in archwire. For high mini-implant traction and 8-mm anterior retraction hook condition, the retraction force vector was applied above the center of resistance for the 6 anterior teeth, but no bodily retraction of the 6 anterior teeth occurred. For high mini-implant traction, 2-mm anterior retraction hook, and 100-g midline vertical traction condition, the 6 anterior teeth were intruded and tipped slightly labially.