Multiple analyses on mechanisms of root fracture in endodontically treated teeth

拔除的上颌前磨牙采用纤维桩、预制金属桩或铸造金属桩核进行修复。无桩核修复的全冠预备牙作为对照。所有牙齿均采用全覆盖冠修复。以0.5 mm/min的十字速度对修复牙施加90度垂直或45度斜向载荷，记录断裂载荷和断裂方式。分别从垂直方向和斜向方向对修复牙施加正弦循环载荷进行疲劳断裂试验。使用阶梯法计算每次修复的疲劳极限。在竖向载荷条件下，铸造金属桩核修复牙的断裂载荷最大。所有类型的桩核系统修复的牙齿骨折都在根的中间部分扩展，包括桩的顶端。在倾斜载荷条件下，预制金属桩修复牙的断裂载荷明显小于其他组。纤维桩组三分之二的骨折在颈椎区域内传播，而其他组大多数骨折延伸到根中部以外。不同修复方式的牙齿在静态断裂抗力试验中呈现出相同的疲劳断裂抗力规律。纤维桩和金属桩修复牙的疲劳极限分别为垂直载荷下静态断裂载荷的73%和倾斜载荷下的48%和46%。综上所述，在垂直和斜向载荷作用下，纤维桩复合树脂核与全铸造冠的组合对剩余牙体结构的保护效果最好。

Extracted human upper premolars were restored with a fiber post, prefabricated metallic post or cast metallic post-core. Teeth with full crown preparations without post-core restorations served as a control. All teeth were restored with full coverage crowns. A 90-degree vertical or 45-degree oblique load was applied to the restored teeth with a crosshead speed of 0.5 mm/min, and the fracture loads and mode of fracture were recorded. Fatigue fracture tests were conducted by applying sinusoidal cyclic loads to restored teeth from vertical or oblique directions. Fatigue limits for each restoration were calculated using the staircase approach.Under the condition of vertical loading, the fracture load of teeth restored with the cast metallic post-cores was greatest among the groups. All fractures in teeth restored with all types of post-core systems propagated in the middle portions of roots, including the apices of the posts. Under the condition of oblique loading, the fracture load of teeth restored with pre-fabricated metallic posts was significantly smaller than that in other groups. Two-thirds of fractures in the fiber post group propagated within the cervical area, while most fractures in other groups extended beyond the middle of the roots. Fatigue fracture resistance of teeth with different restoration methods presented identical patterns in static fracture resistance tests. Fatigue limits of teeth restored with fiber and metallic posts presented 73% of static fracture loads under vertical loading, and 48 and 46% under oblique loading.From the results of the present investigations, it was concluded that under conditions of vertical and oblique loadings, the combination of a fiber post and composite resin core with a full cast crown is most protective to the remaining tooth structure.