Objective: To compare the effects of resin base and different retention depth on the fracture resistance of mandibular molars restored with nano-ceramic endocrowns.
Methods: Forty mandibular molars selected and randomly divided into 5 groups: ① The control group which was consisted of intact teeth, ② the non-resin base group, ③ the 2 mm retention depth group, ④ the 3 mm retention depth group, ⑤ the 4 mm retention depth group, respectively. After tooth preparation, in vitro root canal therapy was conducted, which was followed by endocrown design, production and adhesive of groups ②-⑤. All the samples were under load (N) of the universal mechanical testing machine after embedding. The fracture pattern of each sample was observed under stereomicroscope. Then the microstructure of the fracture surface was observed by scanning electron microscopy.
Results: The fracture loads of each group were respectively: the control group fracture load was (3 069.34±939.50) N; experimental groups: fracture load of (2 438.04±774.40) N for the group without resin base; fracture load of (3 537.18±763.65) N for the group with 2 mm retention depth. The fracture load of the retention depth 3 mm group was (2 331.55±766.39) N; the fracture load of the retention depth 4 mm group was (2 786.98±709.24) N. There was statistical significance in the effect of resin base and different retention depth on the fracture loads of molars restored with nano-ceramic endocrown (P < 0.05). Repairable fractures in each group were as follows: control group 2/8, non-resin base group 1/8, retention depth of 2 mm group 1/8, retention depth of 3 mm group 2/8, and retention depth of 4 mm group 0/8. The effects of the retention depth and the presence of resin base on the fracture resistance of the resin nano-ceramic endocrowns were statistically significant (P < 0.05). Scanning electron microscopy showed more arrest lines and small twist hackles on the fracture surface of the restorations with resin base (retention depths of 2 mm, 3 mm, and 4 mm), with cracks extending towards the root. In addition to the characteristics above, more transverse cracks parallel to the occlusal surface, pointing outwards from the center of the pulp cavity retention, were also observed on the fracture surface of the non-resin base restorations.
Conclusion: When molar teeth with nano-ceramic endocrowns are restored, resin base and the retention depth of 2 mm help the teeth to obtain optimal fracture strength.
目的: 比较有无垫底树脂和不同固位深度对树脂基纳米陶瓷髓腔固位冠修复下颌磨牙抗折能力的影响。
方法: 选择40颗完整离体下颌磨牙随机分为5组(①对照组：完整牙，②无垫底树脂组，③固位深度2 mm组，④固位深度3 mm组，⑤固位深度4 mm组)，对组②~组⑤进行根管治疗、牙体预备与髓腔固位冠设计制作和粘固。所有样本包埋后使用万能力学试验机测试断裂强度，使用体视显微镜观察样本折裂模式，扫描电镜观察修复体断裂面微观特征结构。
结果: 组①~组⑤样本断裂强度分别为(3 069.34±939.50) N、(2 438.04±774.40) N、(3 537.18±763.65) N、(2 331.55±766.39) N、(2 786.98 ±709.24) N。使用垫底树脂和不同固位深度对树脂基纳米陶瓷髓腔固位冠修复磨牙断裂强度的影响差异有统计学意义(P < 0.05)。组①~组⑤样本可修复型折裂分别为2/8、1/8、1/8、2/8、0/8。有垫底树脂时(固位深度2、3、4 mm)修复体断裂面上止裂线和细小的扭转扭曲较多，裂纹均向根方扩展。无垫底树脂修复体断裂面还可见较多平行于咬合面的横向裂纹，自髓腔固位体中心指向外侧。
结论: 使用树脂基纳米陶瓷髓腔固位冠修复磨牙时，进行树脂垫底和固位深度2 mm，其断裂强度最高。
Keywords: Endocrowns; Fracture resistance; Resin base; Resin nano-ceramic; Retention depth.