Teeth with intact crowns rarely split or fracture, despite decades of cyclic loading and occasional unexpected overload. This is largely attributed to the presence of dentine, since cracking and fracture of enamel have been frequently reported. Dentine is similar to bone, comprising mineralised collagen fibres as a main constituent. Unlike cortical bone, however, where microcracking and damage arrest are essential for re/modelling and healing, dentine can neither remodel nor regenerate. This raises questions regarding the evolutionary benefits of toughening, leading to uncertainty whether cracks actually appear in dentine in situ. Here we study the notion that circumpulpal dentine is usually protected against, rather than damaged by severe overloads, and consequently it is not much more massive or stronger than it needs to be. To answer this, we examined hydrated teeth still within whole jawbones of freshly-slaughtered skeletally mature pigs, mechanically loaded until fracture. Force displacement curves, optical and electron microscopy combined with 3D microstructural analysis by conventional micro-computed tomography (μCT) revealed mostly brittle fracture paths in circumpulpal crown dentine. Once overload cracks reach this mass of dentine they propagate rapidly along straight paths often parallel to the enamel flanks of the oblong shovel shaped premolars. We find infrequent signs of active toughening mechanisms with minimal crack diversion, ligament bridging and microcracking. When such toughening is seen, it mainly appears in softer dentine in the root, or near the dentine-enamel-junction (DEJ) in mantle dentine. We observed shear bands in overloaded circumpulpal dentine, due to mutual gliding of large cracked segments. These shear bands are formed as periodic arrays of rotated dentine fragments. The 3D data consistently demonstrate the importance of the layered tooth structure, containing a stiff outer enamel shell, a soft sub-DEJ interlayer and a stiff circumpulpal dentine bulk, for deflecting cracks from splitting the tooth.

中文翻译：

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破坏全牙冠牙本质：过载后普遍骨折模式的 3D 观察

尽管有数十年的循环加载和偶尔意外的过载，但具有完整牙冠的牙齿很少裂开或断裂。这主要归因于牙本质的存在，因为经常报道牙釉质开裂和断裂。牙本质类似于骨骼，以矿化胶原纤维为主要成分。然而，与皮质骨不同的是，微裂纹和损伤抑制对于重塑/建模和愈合至关重要，牙本质既不能重塑也不能再生。这提出了关于增韧的进化益处的问题，导致裂纹是否真的出现在原位牙本质中的不确定性。在这里，我们研究了一个概念，即牙髓周围的牙本质通常受到保护，而不是受到严重过载的损害，因此它不会比它需要的更重或更坚固。要回答这个问题，我们检查了新鲜屠宰的骨骼成熟猪的整个颚骨中的水合牙齿，机械加载直到骨折。力位移曲线、光学和电子显微镜结合传统显微计算机断层扫描 (μCT) 的 3D 显微结构分析显示，牙髓周围牙本质中大部分为脆性断裂路径。一旦超载裂纹到达这些牙本质，它们就会沿着通常平行于椭圆形铲形前磨牙的牙釉质侧面的直线路径快速传播。我们发现很少有裂纹转向、韧带桥接和微裂纹的主动增韧机制的迹象。当看到这种增韧时，它主要出现在牙根中较软的牙本质中，或在地幔牙本质中的牙本质-牙釉质连接处（DEJ）附近。我们在过载的牙周牙本质中观察到剪切带，由于大裂纹段的相互滑动。这些剪切带形成为旋转牙本质碎片的周期性阵列。3D 数据一致地证明了分层牙齿结构的重要性，包含坚硬的釉质外壳、柔软的亚 DEJ 夹层和坚硬的牙周牙本质块，用于偏转因牙齿分裂而产生的裂缝。