OBJECTIVETo investigate the biomechanics of J-hook headgear in En mass intrusion and retraction of maxillary anterior teeth and provide guidance for clinical treatment.

METHODSA three-dimensional finite element model of maxillary teeth, periodontium, straight wire appliance and maxillary bone was established in ANSYS 14.0. En mass retraction of anterior teeth with force of 1.5 N through J-hook headgear was stimulated. Force was applied mesial to lateral incisor in group A and distal to lateral incisor in group B. The force direction was 30° to the sagittal plane and 20° to 60° to the occlusal plane. Force direction to the occlusal plane was changed every 5° and 18 cases were calculated. Displacement of upper anterior teeth and stress distribution in the periodontium were analyzed.

RESULTSAs the degrees of force direction to the occlusal plane increased, the moving pattern of upper anterior teeth changed from clockwise rotation (lingual movement with intrusion) to bodily retraction and intrusion, and counter- clockwise rotation (intrusion with labial movement). With the force direction of 35° to occlusal plane applied mesial to lateral incisor or force direction of 45° to the occlusal plane applied distal to lateral incisor, bodily movement of upper anterior teeth without rotation was achieved.

CONCLUSIONSPlacement of J-hook mesial to lateral incisor enable orthodontists to maintain better en mass intrusion and retraction of upper anterior teeth. The direction of J-hook should be adjusted according to individual condition and treatment objective.