Three-dimensional Finite Element Analysis Of Different Reactive Force Direction Of Maxillary Protraction On Temporomandibular Joint

Class III malocclusion and anterior crossbite are common clinical problems, which seriously affect the patients’appearance, function and mental health, if not treated timely, symptoms may continue to increase with age growth. In recent years, studies have shown that approximately 42% to 63% of Class III malocclusion were accompanied by the maxillary dysplasia.The best treatment mean for Class III malocclusion with maxillary hypoplasia was promoting the maxilla growth. Maxillary hypoplasia correction could achieve the purpose of improving the face type and simplifing adult orthognathic surgery.Currently, maxillary protraction appliance is one of the most effective methods used on clinic to promote maxilla growth. Since 1988, scholars at home and abroad were concentrated on the improvements of three-dimensional finite element model of cranial-maxillary complex establishment, and the stress distribution of traction in maxillary, however, the biomechanical principles study of protraction reactive force was still blank. In this study, we established and used a complete three-dimensional finite element model which included TMJ, imitated the force pattern of maxillary protraction appliance, directly exerted nodal forces on the chin, analysed the influence of reaction force of different protraction direction on the jaw and TMJ, to offer a scientific basis on treating Class III malocclusion more well and avoid injuring TMJ.OBJECTIVE:To analyze the influence of different reactive force direction of protractions on temporomandibular joint (TMJ) by establishing a three-dimensional finite element model (FEM) of craniomaxillofacial complex.Study Design:The CT image of the head of a healthy young male volunteer was obtained. With the help of Mimics software, we established a three-dimensional finite element model of craniomaxillofacial complex which included TMJ. The force pattern of maxillary protraction appliance was imitated. The force(5 N) was applied on the chin and the direction of force was from 22° to 49° relative to the occlusal plane. The displacement and stress distribution of TMJ were analyzed.RESULTS:l.The evident stress parts in the model were loaded parts, fixed parts on the top of the upper jaw and the contact area. The biggest stress points of mandible and maxillary appeared in the loading position-chin and rigid fixed surface, respectively, and that the contact area such as the glenoid fossa, condylar head, condylar neck, stress concentration was also more evident. After loading nodal force from different direction, the maximum stress generated in the loading position;2.The contact stress on the maxilla decreased with the angle of the force direction increased from 22° to 40°, and increased with the angle increased from 40° to 49°. The stress on the condyle decreased with the angle of the force direction increased. The stress on the condylar neck decreased initially and then increased with the angle of the force direction increased. Comprehensively, the stress was the smallest when the angle of the force direction was 40°. The clockwise rotation of the mandible was found when the angle of the force direction was smaller than 40°. The displacement value was relatively small when the angle was 40°.CONCLUSIONS:1. A three-dimensional finite element model of craniomaxillofacial complex which included TMJ was successfully established.2. Protraction appliance generated reaction force to the mandible and TMJ when made maxillary forward, such as the clockwise rotation of the mandible and chin deformation.3. Stress and displacement were relatively small when the angle of the force direction was 40° relative to the occlusal plane.