| Objective: Malocclusion is the deformity of tooth, jaw and craniofacial which is formed in growth because of innate factors or acquired factors. The number of patients requiring orthodontics treatment has undergone a marked increase with the increasing demand in health and beauty. Anchorage control is critical to a successful orthodontic treatment. Anchorage is the device that can resist the anti-force during the anticipated tooth movement. The traditional methods for anchorage control are difficult to provide absolute stability, because of its deficiency in beauty and comfort. In recent years, mini-implants are becoming popular because of its small size, ease placement, providing stable skeletal anchorage and immediate loading. However, mini-implants still fail frequently. It is reported that the success rates for mini-implants were 70-91%. The stability of implant was influenced by many factors. Insertion position, insertion direction and load direction are the important factors on stability of mini-implants.With the development of materials science, recent research shows that implant-bone biocompatibility is the main factor which affects the stability of mini-implants. The stress is primarily located at the neck of implant under different loads. Excessive loading can lead to stress concentration and affect the implant-bone healing. The more bone quantity endured loading , the better distributed the stress. The bone quantity vary with the implant position and loading direction, but the researches in the relationship between implant position and load direction are rear. Four different models that differ in distance to alveolar ridge crest were established with three dimensional finite element. The purpose was to explore the relationship between the bone quantity endured loading and load direction, analyzing the stress distribution and displacement in bone influenced by orthodontics loading, thus to provide reference in clinics.Meterials and Methods:1 A simplified three-dimensional finite element model of mini-implant and bone was establish with ANSYS12.0 software.2 Establishing the mini-implant model, the diameter was 1.6mm and the length was 8mm.3 The implant was inserted in the inter-root gap between the second premolar and first molar. The distance from alveolar ridge crest to the insertion site were 3.0mm, 3.5mm.4.0mm and 4.5mm respectively.4 The four finite element models with the mini-implant inserted 45°tilted occlusally were loaded by a force of 200g mesiol-vertically and vertically respectively.5 The distributions of stress and displacement on the implant-bone interface were analyzed.Results:1 The four finite element models of mini-implants were established.2 The results showed that the peak stress occurred at the cervical bone margin adjacent to the implants on different loading conditions.3 The bone quantity endured loading showed significant influence on the stress distribution. The maximum stress values of the implant and their surrounding bone decreased when the implant far from alveolar ridge crest.4 With the same insertion position, the maximum stress values of the mini-implant loaded mesiol-vertically were less than the implant loaded vertically.Conclusions:1 The bone quantity endured loading showed significant influence on the stress distribution. The stress distribution of the implant and their surrounding bone was uniform when the implant was far from alveolar ridge crest. Increasing the bone quantity endured loading is beneficial to the stability of mini-implant.2 The loading direction affects stress distribution of the implant-bone interface. When implant embedded tilted occlusally, loading vertically is much more beneficial for the stability of mini-implant than loading mesiol-vertically.
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