| ObjectiveIn recent years, with the development of the Material Science, more and more restorative materials were used in the clinician. As these materials have different mechanical behavior, the cavity designs of teeth preparation are different. Therefore, only the suitable cavities could make the restored teeth better. Whether the restoration of the distal - occlusal class II cavities will be succeed depends on many factors, of which the biomechanics factors are the most important.Finite Element Method ( FEM) is an analysis method that is contacted with modern computer technology. After Thresher and Farash developed FEM into the dentistry simultaneously in 1973, FEM has been being concerned with bioengineering problems in dentistry. In this study, the three - dimensional finite element analysis model of a mandibular second molar with the different inter - proximal height of the distal - occlusal class fl cavities were developed, and when the cavities were rilled with silver amalgam or composite resin,the stresses in the teeth and the restoration complex were analysed by using FEMsoftware.Materials and Methods1. Generating the FEM model: We pick out one probably intact extracted mandibular second molar as the specimen tooth, whose crown length , width and height are close to the normal mean value of the named same teeth. A point was chose as the work coordinate origin in occlusal central fossa of the tooth, and all the three - dimension coordinates of the main autopsy symbol point around the tooth surface were measured out and put into computer. Then the three - dimension FEM model of the tooth was produced.2. Preparation of the distal - occlusal class II cavities: The standard distal - occlusal class II dovetail cavities were prepared in the FEM model. Its dovetail width was the half width of the distance from buccal to lingual cusp, and its gorge width was the one - third width of that distance . Its depth of the occlusal cavity was 2mm. The height of inter -proximal wall was 2mm, 3mm, 4mm respectively.3. Applying the forces: The tooth roots were regarded as the stable points, then 100N uniform load was applied to the same four points on the 1mm * 1mm surface area in the occlusal central fossa of the restored tooth vertically download or 30 degrees angled to vertical axis or 45 degrees, directed from medial to distal action.4. The Unigraphics NX software, Patran and Nastran FEM analysis softwares were used in this research.5. The mechanic parameters; Please check in the table 1.Table 1 Mechanic parameters of tooth and restorative materialsResults1. When 100N vertical and lateral forces were loaded on the teeth with distal - occlusal class II cavities whose inter - proximal wall height was different, the shiftness of composite resin restoration complex was greater than that of silver - amalgam restoration complex; the Von Mises of restoration complex were smaller in silver - amalgam than in composite resin, besides the situation in lateral load that was applied to the restored tooth with 2mm inter - proximal wall height; the maximum compressive stresses and the Von Mises of restored teeth with cavities filled with composite resin were higher significantly than those filled with silver - amalgam. Nevertheless, the maximum tensile stresses of which were lower.2. The stress distribution on the tooth surface was even under the vertical load. But the angles of the loading were greater; the stress concentration was more uneven. And the stress concentration was a-round the cervical area of the tooth.3. The stresses were concentrated on the gorge and around the bottom of the dovetail restoration complex under the vertical load, andwhen the angles of loading were greater; the stresses were more concentrated distinctly on the gingival wall of dovetail.4. The stresses concentration were around the dovetail gorge, bottom and lateral walls of the restored teeth under the vertical and lateral load, furthermore, the increasi...
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