| Background:Wedge-shaped defect is a common clinical dental hard tissue diseases. Mainly occure in the cervical region of tooth which can often cause dentin sensitivity, some deep wedge-shaped defects may lead to pulp infection and even tooth horizontal crown fracture. At present, etiology studies of wedge-sharped defects indicates that there are three mechanisms that could cause the formation of Wedge-sharped defect: erosion, abfraction and abrasion. In recent years,some scholars pointed out that occlusal forces could couse stress concentration at the cervical region of the tooth, long-term periodic tensile-compression stress induced by occlusal loading may cause microcrack in enamel or dentin,which could reduce the resistance to mechanical wear and chemical corrosion of dental hard tissue. So stress fatigue in cervical region of tooth has been regarded as an intrinsic factor that could cause the formation of wedge-shaped defect.In process of the formation of wedge-shaped defect, the role of lateral forces have received a number of scholars concerned about. When the mastication is not ideal, for example, malocclusion, occlusal forces do not correspond with the longitudinal axis of the tooth. in this situation, tooth would subjected to lateral forces, which could cause the tooth to band. Many studies have shown that a lateral force causes banding of the tooth and disrupt the bonds between hydroxyapatite crystals, eventually the wedge-shaped defect occurred.At present the clinical treatment of wedge-shaped defect is mainly restorative treatment, while the shedding rate of restorative material after treatment is relatively high. Considering the role of occlusal factors, especially lateral forces in the process of wedge-shaped defects formation, It can be inferred that solely filling therapy without eliminated causative factor will inevitably affect the therapeutic effect Occlusal adjustment is necessary after filling therapy, lateral forces received by tooth will be relieved after appropriate occlusal adjustment, and this could reduce the stress endured by filling material and adjacent tooth hard tissue in oral functional status, thus raise the preservation rate of filling material.Objective:This topic from the biomechanical point of view, used human maxillary first premolar as the research object, established two three-dimensional finite element models, the first one is maxillary first premolar standard model and the other is post-treatment wedge-shaped defect model, in the process of establishing the two models, CT scanning and three-dimensional finite element analysis technology were used together. After the establishment of the two models, stress distribution were analyzed under loadings of different directions. From this topic we further explored the role of occlusal factors in the formation of wedge-shaped defects, and provided theoretical basis for the etiologic research, prevention and treatment method of wedge-shaped defect.Materials and Methods1. Establishment of the three-dimensional finite element model of maxillary first premolar:finite element analysis methods is a wildly used analysis methods in biomechanics research at present because of its high degree of simulation and accuracy. We used a human maxillary first premolar with standard shape, after the shape measurement and paraffin embedding, we get 32 2D sectional pictures of this tooth from spiral CT scans (0.6mm slice thickness). Based on these pictures, we established an three-dimensional finite element model of maxillary first premolar which would be used as the standard model in subsequent research.2. Biomechanic research of the etiology of wedge-shaped defect:on the standard model, we applied loadings(100N) at the center of occlusion surface central fossa,lingua slope of buccal cusp and buccal slope of lingua cusp; with the loading directions of parellel to the tooth's long axis,perpendicular to cusp slope and 20°inclined to the horizontal plane, all the different loading mode above were used to simulate the tooth under different occlusal forces. After the loaded step we analyzed the stress distribution condition of the buccal-cervical region of standard model, tested the validity of this model and analyzed the role of occlusal forces, especially lateral forces, in the formation of wedge-shaped defects.3. Biomechanical analysis of post-treatment wedge-shaped defects:we modified the established standard model into post-treatment model, applied loadings of same spots and directions as step 2, and analyzed the stress distribution condition of filling material unit.Results:1. Stress analysis of standard model of maxillary first premolar showed that when the loading direction was parallel to the tooth long axis, stress concentration region can be seen on the load spot of occlusion surface, uniform stress distribution can be seen on the other parts of the model. When the loading direction was oblique to the tooth long axis, stress concentration occurred at the buccal cervical region of the model, as the angle of loading inclination increased, the stress concentration became more pronounced.2. Stress analysis of post-treatment model showed that when the loading direction was parallel to the tooth long axis, filling material unit subjected to relative small stress and when the loading direction was oblique to the tooth long axis, stress endured by filling material unit was significantly increased. as the angle of loading inclination increased, the increase of stress became more pronounced.Conclusions:1. Lateral forces could cause the stress concentration at the buccal cervical region of tooth, and as the lateral forces increase, the stress concentration became more pronounced. This indicates that occlusal forces, especially lateral forces, played an important role in the formation of wedge-shaped defects.2. After the restorative treatment of wedge-shaped defects, lateral forces could make the filling material unit subjected to obvious stress effect, and the stress would became more pronounced as the lateral forces increased. This indicates that in clinical therapy process, after the restorative treatment of wedge-shaped defects, occlusal adjustment is necessary. We could reduce the inclination of cusp from occlusal adjustment to relieve the lateral forces subjected by tooth, thus the stress load in filling material unit would eased and the shedding rate would decrease correspondingly.
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