Biomechanics Research On The Micro-Implant Embedded Loading And Enhance Anchorage By Micro-Implant In Distalizing Molar

Objective 1. To establish a three-dimensional finite element model of micro-implant and bone, and to study the influence of different embedded directions on the biomechanical characteristics at micro-implant-bone interface, so as to help the clinical application of micro-implant. 2.To establish four three-dimensional finite element models of distalizing maxillary first molar and anchorage micro-implant. Appraise micro-implant anchorage action, and study the influence of ditalizing molar with the maxillary second molar extracted and different healing extraction phases. Evaluating therapy effect of the method for enhance anchorage and decrease the resistance of molar distal to support a biomechanical basic for applying orthodontic forces in clinic.Method 1. I-DEAS finite element analysis software was used to setup micro-implant and bone finite element model. The micro-implant was embedded into the bone with 30°angle. Keep the orientation of force and the embedded angle fixed, make the embedded point as the center of a circle, the projective line of micro-implant in the surface of bone as the radius revolved in 180°to perform 5 different embedded directions, including 0°, 45°, 90°, 135°and180°.A simulated orthodontic force of 200g was loaded parallel to the surface of bone in embedded region. The stress and displacement distribution at micro-implant-bone interface were analyzed. 2. Transfer the Spiral CT images into the MIMICS and I-DEAS software to obtain four three-dimensional finite element models. Model one contained half maxilla and dentition of seven teeth from central incisor to second molar, simulated the traditional oral anchorage of maxillary second molar existed. Model two based on the Model one with one micro-implant embedded with 30°angle into maxillary alveolar bone between second premolar and first molar. The embedded point was 2 mm under the root tip of the second premolar. Keep the micro-implant parallel to the dental root, micro-implant was connected with the first premolar by ligature wire. Model two simulated micro-implant anchorage of maxillary second molar existed. Model three and four were based on the Model one with second molar extracted, simulated the traditional oral anchorage of maxillary second molar extracted. Based on the different healing phases of extraction, the extraction sockets in Model three was void, also simulated the clinical healing extraction phase (1-week extraction phase). The extraction sockets in Model four was filled up, simulated the bone healing extraction phase (12-week extraction phase).The brackets and archwire stimulated 0.020inch stainless steel added in all four groups. The incisors and canine and first premolar were ligated, 150g force of II elastic loaded in mesi-bracket of canine, the included angle between II elastic and archwire was 20°. Simulated 300g force of spring loaded between first premolar and first molar. The force of spring changed when the teeth moved. Observed the initial displacement of teeth and the maximum stress of periodontal ligament, analyzed the displacement graph of teeth and the stress graph of periodontal ligament, the distalizing effect of first molar in four groups were compared.Results 1. The change of stress and displacement on the groups are in the rational range, the embedded region of micro-implant and the cortical bone were stress-focused area. Von-mises stress and stretching stress and compressive stress have the same law of stress distribution at the micro-implant-bone interface, the tendency of stress distribution was decreasing. With the effect of retention, the stress of group 0°and 45°were smaller than other groups, the group 0°had the mini-stress distribution. The displacement of group 0°and 45°are significantly higher than group 90°, 135°and180°, the group 90°had the mini-displacement distribution. 2. The concentration of stress in Model one was more than other groups. The concentration of stress of premolar in Model two was uniform, The concentration of stress in Model three and four were similar. The tendency of teeth movement of first premolar and first molar in four groups were similar. The initial movement of first molar: Model three > Model two > Model four > Model one. The initial movement of first premolar: Model three < Model two < Model four < Model one.Conclusion 1. The micro-implant can safely loaded with 200g force parallel to the surface of bone in embedded region with different embedded directions. When choose the embedded direction of micro-implant, reducing the effect of retention are better for the stability of micro-implant. Multiple factors just like implant design, embedded condition and clinical operation must be considered. The clinicians should choose the suitable embedded direction on the basis of practical condition. 2. The effect of micro-implant enhance anchorage is visible. The extraction of second molar is helpful for distalizing first molar. Because of the different healing phases of extraction, the results of the traditional oral anchorage of maxillary second molar extracted is different to the micro-implant anchorage of maxillary second molar existed. Whether adopt micro-implant enhance anchorage or extract the second molar, the degree of malocclusion and patients demand should be considered.