中文摘要：

目的：初步探讨磨牙牙冠中央区内部显微组织结构对其宏观抗弯强度的影响。方法：取人离体第三磨牙40个，用硬组织切片机分别切取其冠部中央釉牙本质交界区（DFj）以及牙本质浅层、中层、深层4个部位的牙片，制成1mm×1mm×10mm的梁型试件；从每个部位各取10个试件，通过三点弯曲试验分别检测各部位的抗弯性能，同时采用扫描电镜观察各试件断面处内部的显微组织结构，分析其与宏观抗弯性能的关系。结果：各测试部位的抗弯强度由高到低依次为牙本质浅层〉牙本质中层〉DEJ区〉牙本质深层，除DEJ区与牙本质中层无统计学差异（P〉0．05）外，其他各层间的差异均有统计学意义（P〈0．05）；扫描电镜观察结果显示，牙本质小管的密度从牙本质浅层至深层呈逐渐递增趋势；在DEJ区的釉质侧常有裂纹发生，并有部分胶原纤维从牙本质穿透进入釉质，从而使釉一牙本质界区形成相互渗透的连接形式。结论：牙本质的抗弯强度由浅层至深层呈逐渐递减趋势，牙本质小管密度越高，抗弯强度越小；牙本质和釉质交界区互相渗透的连接形式可能是导致DEJ区对裂纹扩展具有抑制作用的原因之一。

英文摘要：

AIM： To study the influence of the internal microstructure of molar crown central region on ben- ding strength. METHODS：dO extracted third molars were cut into pieces at the DEJ, superficial dentin, middle den- tin and deep dentin were respectively cut using a sliding microtome to make rectangular beams of 1 mm × 1 mm × 10 mm. The bending strength of 10 specimens at each tested region was measured by three point bending test. The mi- crostructure of the fractured specimens was observed by SEM, and the relationships between the microstructure and the bending strength was analyzed. RESULTS：The superficial dentin has the highest bending strength, followed by the middle dentin, the DEJ and the deep dentin（between DEJ and the middle dentin,P 〉 0.05, between each 2 parts of others, P 〈 0.05）. SEM observation showed that the dentin tubule density increased from the superficial dentin to the deep dentin. Crack was observed in most DEJ specimens in the nearby enamel. Connection of mutual penetration of collagen fibers from dentin into the enamel was in observed in DEj. CONCLUSION：The bending strength of dentin correlates negatively with the dentin tubule density. The penetration of collagen fiber in DEJ may be one factor that can restrain the crack propagation.