下颌第一磨牙牙体缺损全冠修复的应力分析

目的：观察下颌第一磨牙根管治疗术后不同程度的远中牙合面（distal oeclusal简称DO）牙体缺损，在银汞合金或复合树脂修复后再用全冠修复的应力分析，了解牙体缺损程度对牙体抗折力的影响，比较两种修复DO洞是否有不同的影响。方法：通过三维激光扫描、三维造型设计建立下颌第一恒磨牙髓室壁未破坏的最小、中等、最大以及髓室壁部分破坏的最大远中黯面牙体缺损的三维有限元模型。观察4种模型4种载荷下牙本质Mohr应力分布，比较牙本质最大Mohr值。结果：在最大、垂直、斜向载荷下，两种修复体牙本质最大Mohr值均明显小于牙本质拉伸极限强度。在水平载荷下，两种修复体牙本质最大Mohr值均有急剧增加。银汞合金、全冠修复应力面积普遍大于复合树脂、全冠修复应力面积。牙本质最大Mohr值总的趋势复合树脂、全冠修复的大于银汞合金、全冠修复的。结论：在正常咬合下，下颌第一磨牙（36、46）DO牙体缺损髓室壁牙体组织未破坏，牙体预备体颊、舌侧厚度不少于1mm；髓室壁牙体组织部分破坏，牙体预备体舌侧厚度至少1mm、颊侧厚度至少2．5mm，临床可考虑用复合树脂、银汞合金修复再加全冠修复。在存在磨牙症情况下，应采取减轻黯力的措施。对较大面积牙体组织DO缺损且破坏到髓室壁的病例以复合树脂加全冠修复为好。

The stress analysis on filling and full crown restoration of the structure defect of the first mandibular molar

AIM:To observe the influence of different level of distal occlusal teeth defect of the mandibular first molars after root canal therapy on the teeth resistance which were filled with amalgam or composite resin and restored with NiCr metal crown;and to compare if the two different filling materials had different effects.METHODS:3-dimensional finite element models of the minimum, middle and maximum distal occlusal structure defect without the pulp chamber wall involved and the maximum distal occlusal structure defect with the pulp chamber wall involved were built by the advanced 3-dimensional laser scanning and 3-dimensional computer assisted design technique. The Mohr strength theory was applied for comparing the maximum Mohr stress value of the dentin and stress concentration areas in four models under four different loads.RESULTS:Under the maximum, vertical and inclined loads,the maximum Mohr stress values of the dentin of the two kinds of restorations were obviously smaller than the stretch limit of the strength of the dentin.Under horizontal direct load,the maximum Mohr stress values of the dentin of the two kinds of restorations increased remarkably.The stress area in the model with amalgam filling and full crown restoration was generally larger than that in the model with composite resin filling and full crown restoration.The maximum Mohr stress value of the dentin of the model with composite resin filling and full crown restoration was greater than that of the model with the amalgam filling and full crown restoration.CONCLUSION:Under normal masticatory force,when the distal occlusal teeth defect of the mandibular first molars does not involve the pulp chamber wall and the thickness of buccal and lingual wall of the tooth preparation is at least 1 mm,and when the distal occlusal teeth defect involves the pulp chamber wall and the thickness of lingual wall is at least 1 mm and the thickness of buccal is at meast 2.5 mm,the restorations of composite resin,amalgam filling and full crown can be considered to be a proper restoration in clinic. Under the condition of bruxism,the measures of decreasing the bite force should be applied. For the case of bigger distal occlual teeth defect and the defect involving the pulp chamber wall,the composite resin filling and the full crown restoration are relatively better than amalgam filling and full crown restoration.