Three-Dimensional Finite Element Investigation On The Retraction System Of Upper Incisors

Background:The position of upper incisors has striking effect on the aesthetic appearance of the face and on the function of the stomatognathic system. Retraction of the upper incisors as part of orthodontic therapy represents a fundamental phase of treatment. Different retraction systems are adopted by different clinical orthodontic appliances and the retraction system can be mainly divided into two categories: continuous archwire retraction system and segmented arch retraction system. The biomechanics differ in different retraction system and the results may also differ under different loading conditions in the same retraction system . So it is of vital importance for orthodontist to learn the biomechanics of all the retraction systems. The biomechanical effect on the retraction of upper incisors have been previously investigated by means of holographic interferometry, photoelastic technique and magnetic sensing system. But the former model constructed could hardly simulate the whole characteristics of retraction system. Three-dimensional finite element method is an efficient measure to investigate the orthodontic tooth movement, but most former studies focus on the single tooth or parameter analysis. The more complex three-dimensional finite element model of the retraction system of upper incisors have never been constructed previously.Objective:The first step of this study is to construct a three-dimensional finite element model of maxilla composed of maxillary bone, alveolar crest and the upper teeth. Then the more complex FEA model containing bracket, archwire and retraction hook even micro-implant inserted in the posterior alveolar crest were constructed to investigate the different biomechanics between continuous archwire retraction system using sliding mechanics and segmented arch retraction system. Furthermore, the position of center of resistance of upper 6 anterior teeth will be calculated and compared with the results of other investigations with other method.Method:1. A macerated human skull was scanned with spiral CT and the slices were imported into MIMICS software. Then the three-dimensional geometrical model of maxilla containing all upper teeth was constructed .All the teeth were relocated in the ABAQUS software .Then the model was meshed and the three-dimensional finite element model was constructed at last.2. Based on the above three-dimensional finite element model, the model of brackets and archwire was added according to the formula Y=2.115×10-5X4+1.248×10-2X2-1.289.Then the model of micro-implant was constructed and was inserted into the area between second premolar and first molar. The initial displacement of anterior upper teeth were calculated when the heights of retraction hook were 1mm, 4mm, 7mm, 10mm and the insertion heights of implant were 4mm and 8mm respectively.3. The center of resistance of anteriors upper six teeth was investigated under retraction force system using three-dimensional finite element method. The relationship between the center of resistance and the center of rotation was also investigated. The results were compared with the former studies using other biomechanical method.Results:1. A three-dimensional finite element model of maxilla with upper teeth was constructed. The constructing method was efficient, easy to grasp and with high geometrical simulation level.2. According to the FEA analysis, the anterior upper teeth tend to tip more lingually with a lower retraction hook height and a higher hook will cause more labial displacement under retraction force system. The higher hook added additional labial torque to the main archwire.3. The center of resistance of anterior upper six teeth under retraction force system was located 5.1mm above alveolar crest. The anterior upper teeth will rotate clockwise under retraction force lower than this level and rotate counter-clockwise when the force is higher than this level. The force through this level will result bodily movement.Conclusions:The method to construct a complex maxilla three-dimensional finite element model is improved by using the MIMCS and ABAQUS software and the simulation level is also improved by the new method.The sliding biomechanical effect under retraction force of micro implant-straight wire system could possibly help to learn the biomechanical nature of such system. The results also reveal some side effect of such system.The position of center of resistance of FEA investigation differs in some extent of degree from other studies resulting from other biomechanical method. The results should be analyzed with care taking the model-constructing process into account.