| Maxillary protraction appliance (MPA) is a classic removable appliance which is often used in the early clinical treatment of Class III malocclusion with maxillary deficiency or retraction. It has been popular among orthodontists all over the world since it appeared and extensively used in the clinical medicine with the development of related mechanical effects and biomechanics. The main function principle is the reconstruction of craniofacial sutures with heavy traction and stimulating vertical and sagittal growth of maxilla, so as to achieve the purpose of correcting Class III malocclusion with maxillary hypoplasia or retraction. Delair mask is one of the common clinical protraction which has generating and shaping effects on the maxilla with anchorage of forehead and chin which can have a larger resistance.Recently, the domestic and foreign scholars’study on biomechanical principles of protraction are mainly focus on the continuous improvement of the three-dimensional finite element modeling method of craniomaxillofacial complex, stress distribution in the maxilla with traction and the effect of protraction cooperate with rapid maxillary expansion (RME). However, there is still no related report for the biological effect study of the reaction force. As we know, the anchorage is an important concept in orthodontic area. Every applied force has an equal and rightabout force acting on the anchorage sites. By the same token, Delair mask has the reaction force on the forehead and chin when it is used in the treatment of maxillary protraction. The anchorage site of chin will transfer the reaction force to the temporomandibular joint (TMJ) indirectly which may cause internal stress reaction in TMJ. What biomechanical changes will cause by different values of the reaction force? Whether need we to establish a concept of safety using range of mechanical force? This study attempts to establish a three-dimensional finite element model (3-D FEM) of craniomaxillofacial complex including TMJ zone and analyse the stress distribution and displacement changes in TMJ zone and jaw caused by anchorage site of chin. It will provide the theory basics to the Delair mask with selection of traction force values and avoid the oriatrogenic injury of TMJ.Purpose:The study is to establish a 3-D FEM of craniomaxillofacial complex including TMJ zone with good biomechanical characteristics by the spiral CT with original DICOM data and related softwares. To analyse the change rules of stress and displacement of each node in TMJ zone and jaw by the reaction force in craniofacial 3-D structure with the simulation model of maxillary protraction device according to the reaction principle.Methods:1. A male healthy subject was choosed to obtain a two dimensional image of craniomaxillofacial complex with original data by CT scanning and store in standard of DICOM 3.0 digital imaging and communication in medicine. A 3-D FEM of craniofacial structure including TMJ zone, maxilla and mandible was established with continuous, linear, homogeneou and isotropic linear elastic material by using Mimics 10.01、Magics 12.01、MSC.Marc 2005 and other data analysis softwares.2. Boundary conditions were simulated by using MSC.Marc 2005 software and then the simulating force model was established according to the reaction principle of MPA. Different nodal forces from 3N to 6N with a 37 degree angle from the occlusion plane were loaded at the gnathion of the lower jaw. Each 0.5N was a gradient and 7 kinds of working conditions were established in total.3. Stress distribution and changes of displacement in the jaw and TMJ including fossa and condylar were measured and the von Mises value of nodes in different parts under every working condition were calculated. Then the line graphs and the stress and displacement distribution nephograms were draw and the results were analyzed.Results:1. A 3-D FEM of craniomaxillofacial complex including TMJ zone with good biomechanical characteristics was obtained. The final model was meshed with 110770 entity elements and 28740 nodes. The cranio-maxillary model was meshed with 76892 entity elements and 20387 nodes, and the mandibular part was meshed with 33878 entity elements and 8353 nodes. Mesh generation of the model was meticulous and the geometric similarity was good. It could be rotated to different angle and cut or extracted with any structure to analyse and the mechanical characteristics were reflected in a high level of accuracy.2. At the different values of applied force with the same angle, maximum stress was the position where the stress was loaded on the mandible and the rigid fixation of maxilla.3. At the different values of applied force with the same angle, the stress variation law of each node was unanimous. The stress concentration of the maxilla was in the glenoid fossa and the stresses in the glenoid fossa were the greatest in the back slope of the articular eminence. The stress concentration of the mandible was on the top and the neck of the TMJ. The region of the highest stresses was in the anterior slope of the condyle. The stress in articular fossa and the head and neck of condylar increasesed with loading force. When the loading force was greater than 5N, the stress concentration trend of related nodes increased significantly.4. Meanwhile, this model appeared a tiny variation of displacements which increased with loading force. Along with the increasing distance of the force loading site, displacements decreased gradually. The maxillary displacements from the cranial base and occlusion plane to the parietal lobe decreased gradually. Moreover, the mandibular displacements from the front to the rear was gradually reduced, and it appeared clockwise rotation.Conelusions:1. Using the method of CT scanning to obtain original DICOM data, and constructing a 3-D FEM of craniomaxillofacial complex including TMJ zone with help of Mimics 10.01, Magics 12.01 and MSC.Marc 2005 softwares is feasible. The 3-D FEM proves to be a reliable and convenient procedure for biomechanical study of MPA.2. MPA makes impact force on TMJ zone and jaw and the values of stress and deformation become heavier with the applied force. At the same time, the mandibular apppears clockwise rotation and the shape of chin may change.3. It is indicated that in the clinical application of Delair mask with skeletal Class III malocclusion, we should try to use smaller traction value (less than 6N) without sacrificing the good effect in order to protect the healthy growth of TMJ and jaw. In addition, it will be not suitable for the patients of Class III malocclusion with high angle. |
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