| Objective: Anchorage control is the key factor that affects orthodontic treatment effect. In recent years, mini-implant that could provide firm and effective anchorage, especially have some advantages, has been widely used in Orthodontic treatment, including intrusion of elongated molars to provide repair space for jaw teeth, furthest withdrawal of protrusive anterior teeth for closing extraction space, mesiotranslation of the second molar, and so on. Some past cases that need surgery profited from implant and got satisfactory therapeutic effect.The size of mini-implant is small, which was used easily, but it also brings the shortcoming of insufficient stability. It has been reported that the clinical use success rate was above 80%, and the failure often occurred in the jaw area of posterior teeth. Mini-implants were implanted in alveolar bone between the two roots, because space is limited, the probability of mini-implant approaching to root in the embedded process was 27.1%, and the failure rate of the mini-implants that invaded the roots was 79.2%. Some research showed that the implant with contacting root significantly influenced the stability of the implant. However, Kim has different viewpoint, he believes that root proximity alone was not defined as a principle reason for mini-implant collapse. The amount of root contact area of a mini-implant is more significant for its stability. The real reason of implant with root proximity loosening was unclear. The implant loss occurred in 1 to 2 months after implant, but at this time, some implants had yet unload. So we assumed that apart from implant loads, dental physiological load also affected the stability of implant.Mini-implant loading way, because of the influence of orthodontic treatment purpose, may be a single load, may also be composite load, and the load directions were various. No previous study has examined that what is the effect of the different of load way in clinical to the stability of mini-implant proximity to the root. This study discussed the biomechanical influences of the bone around the mini-implant using finite element analysis when mini-implant was close to the root, and examined the stress and displacement distribution under physiological bite force and the different load ways in an attempt to explore the cause of high failure rate of mini-implant with root proximity and provide theoretical basis for clinical application.Methods:1 Equipment Computer: Dell precision Software: Mimics, Catia V5, Hyperworks 12.0, Abaqus6.13 2 Establish the finite element models of mandible with mini-implant 2.1 Set up the finite element model of mandible A healthy helper was scanned by CT and CT images were saved in DICOM. In Mimics, we split and partial smoothed the model, and set the periodontal membrane thickness of 0.25 mm, the three-dimensional finite element model was then created. Also STL file was generated and leaded into Catia V5 for point cloud reverse model processing. 2.2 Set up the finite element model of mini-implant According to the clinical commonly size: the length in the bone is 8 mm, diameter is 1.6mm, thread height is 0.3 mm, and the cutting edge thread apex angle of 60 degrees, the pitch is 0.6mm, 3-D implant model is established. 2.3 Assemble the implant-bone model 2.3.1 Implant location and angle: mini-implant was placed in the left side of the lower first molar 5mm from the top of alveolar crest with a 45-degree angle to alveolar bone surface. 2.3.2 The four models are established as follow Model 1: Mini-implant contact the root surface Model 2: Part of the screw thread embedded in the periodontal membrane, but did not contact with the root surface Model 3: Mini-implant contact surface of the periodontal membrane Model 4: Mini-implant is 1.0mm away from the adjacent periodontal membrane surface 3 Materials 3.1 Material properties Assuming that material and tissue are continuous, homogeneous and isotropic linear elastic material, the material deformation is small. 3.2 Solid Modeling Complete the assembly of the model of mini-implant and jaw with 3-D modeling function of Catia V5. 3.3 Mesh generation The three-dimensional model was imported into Hypermesh module in the finite element modeling software Hyperwork 12.0, and then was set up finite element mesh model and refined grids in the area where mini-implant was close to the root. 3.4 Parts connection Import the mesh finite element model into the Abaqus6.13, give material properties and set up corresponding interface. Set the implant-bone interface friction contact, allowing the relative sliding under the external force, and coefficient of friction μ=0.3. The others are set to blinding contact completely. 4 The way of orthodontic loading 4.1 Boundary conditions: all degrees of freedom constraints at the bottom of jaw underside. 4.2 Group situations LoadⅠ: The teeth are loaded alone with a load of 300 N was put on the first molar. LoadⅡ: Mini-implants are loaded separately from load A to load ABC LoadⅡ- A: It is parallel to the X-Y plane and slant upward at a 15-degree angle to the X-axis positive side, and the force is 200 g. Load Ⅱ-B: It is parallel to the X-Y plane and slant upward at a 15-degree angle to the X-axis positive side, and the force is 200 g. LoadⅡ-C: It is parallel to the X-Y plane and slant upward at a 90-degree angle to the X-axis positive side, and the force is 200 g. The loadⅡ-AB, loadⅡ-AC, loadⅡ-BC and loadⅡ-ABC were combined applications. Load Ⅲ mini-implant and the tooth are loaded at the same time With the teeth loaded at the same time, the mini-implants are loaded separately from the load A to the load ABC, and they expressed as Load Ⅲ-A,Load Ⅲ-B, Load Ⅲ-C, Load Ⅲ-AB, Load Ⅲ-AC, Load Ⅲ-BC, Load Ⅲ-ABC. 5 Computing and data collection The Hyperworks12.0 Hyperview module was used for viewing results, the stress and the displacement value were collected, the stress and displacement nephagram were capture, the stress and displacement distribution were analyzed.Results:1 The implant-bone models of various distances from mini-implant to the root were established successfully. 2 The distribution of stress and displacement in implant-bone interface when teeth were loaded alone 2.1 The von Mises stress in the implant-bone interface was mainly located in the cortical bone and the area in which the implant was close to root. 2.2 The maximum von Mises stress occurred in the cortical bone, and reduced quickly, but there is a second climax in the area where the mini-implant contacted with the root and embedded in the periodontal ligament. 2.3 The closer the mini-implant was to the root, the higher the stress and displacement in implant-bone interface was, and the greater the stress in the area where the mini-implant was close to the root. 2.4 The displacement in implant-bone interface decreased along with the implant long axis, but when the mini-implant was touching the root surface, the displacement in the body and tail of implant-bone interface increased significantly in a certain range. The displacement in the implant-bone interface was highest in Model 1, and lowest in Model 4. 3 The distribution of stress and displacement in implant-bone interface when mini-implant was loaded 3.1 Von Mises stress and displacement are mainly focused on the zone of cortical bone and reduced rapidly, and the stress in the area of cancellous bone is smaller. 3.2 In the same load way, the stress and displacement values were similar in different models. 3.3 In the same model, the highest von Mises stress and displacement peak value were observed under LoadⅡ-AC, and in Model 1,3,4, the lowest were observed under LoadⅡ-AB, and in Model 2, the lowest were observed under loadⅡ-C. 4 The distribution of stress and displacement in implant-bone interface when teeth and implant were both loaded 4.1 The von Mises stress in the implant-bone interface was mainly located in the cortical bone and the area in which the mini-implant was close to root. 4.2 The maximum von Mises stress occurred in the cortical bone, and there is a second climax in the area where the mini-implant was close to the root in Model 1 and Model 2. 4.3 Synergistic effect on the stress in implant-bone interface was observed when teeth and implant were both loaded. 4.4 In the same load way, the stress and displacement peak values were big but similar in the Model 1-3. However, the maximum stress and displacement peak values were clearly smaller in Model 4 than other Models. But in Model 4, the stress and displacement was not lower than others. 4.5 In Model 1-3, the difference in the von Mises stress and displacement peak values of various load ways was small, among them, the stress with Load Ⅲ-A was smallest, but the stress peak value with Load Ⅲ-AC was distinctly greater than others in Model 4. Others, the peak stress with Load Ⅲ-A was smaller than that with Load Ⅲ- B and the peak stress with Load Ⅲ-AC was smaller than that with Load Ⅲ- BC.4.6 The displacement in implant-bone interface decreased along with the implant long axis, but the displacement in the cancellous bone when mini-implant was touching the root surface was obviously higher than other models.Conclusions:1 The bite force can induce the stress concentration in implant-bone interface, and it was unfavorable to the stability of mini-implant with root proximity. The orthodontic force has little effect on the solidity of mini-implant. 2 The stress in implant-bone interface increased with the decreasing the space between the mini-implant and the root. 3 Synergistic effects on the stress in implant-bone interface were observed when teeth and implant were both loaded. 4 The different load ways have certain effect on the stress and displacement distribution of implant-bone interface. |
PDF Full Text Request