Torque control of the maxillary incisors in lingual and labial orthodontics: a 3-dimensional finite element analysis.

INTRODUCTION: Lingual orthodontics has developed rapidly in recent years; however, research on torque control variance of the maxillary incisors in both lingual and labial orthodontics is still limited, especially studies with 3-dimensional finite element methods. Thorough understanding of the biomechanical differences of incisor torque control during lingual and labial orthodontic treatment is critical for the best results.

METHODS: A 3-dimensional finite element model of the maxilla and the maxillary incisors was made with 98,106 nodes, 71,944 10-node solid elements, and 5236 triangle shell units. Horizontal retraction force, vertical intrusive force, and lingual root torque were applied to simulate labial and lingual orthodontic treatment. Then the distribution of the stress-strain (maximum and minimum principal stresses; maximum and minimum principal strains) in the periodontal ligament, the total displacement, and the vector graph of displacement of the nodes of the maxillary central incisor were analyzed and compared between labial and lingual orthodontics.

RESULTS: Loads of the same magnitude produced translation of the maxillary incisor in labial orthodontics but lingual crown tipping of the same tooth in lingual orthodontics. This suggests that loss of torque control of the maxillary incisors during retraction in extraction patients is more likely in lingual orthodontic treatment.

CONCLUSIONS: Lingual orthodontics should not simply follow the clinical experience of the labial techniques but should increase lingual root torque, increase vertical intrusive force, and decrease horizontal retraction force properly to achieve the best orthodontic results.