饰瓷对磨牙二硅酸锂玻璃陶瓷冠疲劳失效与结构可靠性的影响

Influence of veneer application on failure behavior and reliability of lithium disilicate glass-ceramic molar crowns

目的探讨二硅酸锂玻璃陶瓷冠的疲劳失效机制，以期为临床提供参考。方法制作二硅酸锂玻璃陶瓷磨牙单层和双层冠共26个（每组13个），粘接树脂代型后进行滑动接触疲劳实验，对未失效试样进行抗折破坏实验，记录断裂载荷并进行两独立样本t检验；计算断裂载荷值的Weibull模数以分析二硅酸锂玻璃陶瓷单层和双层冠的结构可靠性；采用体视显微镜与扫描电镜观察失效模式。结果单层和双层冠断裂载荷分别为（2071．23±397．05）和（1483．41±327．87）N，两组差异有统计学意义（P〈0．001）。单层和双层冠断裂载荷值的Weibull模数分别为6．15和5．54，两组差异无统计学意义（P〉0．05）。起源于加载头下方斜嵴中央处的全层断裂是二硅酸锂玻璃陶瓷单层和双层冠失效的主要模式；起源于饰一核瓷界面缺陷处的裂纹扩展是双层冠失效的另一主要模式。结论饰瓷能降低磨牙二硅酸锂玻璃陶瓷冠的疲劳断裂载荷，但对其结构可靠性无明显影响；提高饰瓷的力学性能及规范与改进饰瓷制作工艺能减少二硅酸锂玻璃陶瓷冠的失效。

Objective To evaluate the influence of veneer application on failure behavior and reliability of lithium disilicate glass-ceramic （LDG） crowns of maxillary first molar, and thus to reveal the failure mechanism of bilayered LDG crowns. Methods Twenty-six LDG maxillary first molar crowns were fabricated in a dental laboratory using IPS e. max Press or IPS e. max Press/Ceram. The crowns were randomly assigned into two groups （ with or without veneer application ） with thirteen in each group. The crowns were cemented on composite resin dies. After storage in water for one week, the sliding-contact fatigue test was performed by sliding the steatite ceramic ball indenter （6 mm in diameter） from central fossa up to the lingual surface of disto-buccal cusp, cyclic loaded 1 200 000 times with a weight of 100 N at 2 Hz with a fatigue chewing simulator. Survived specimens were subjected to single-load-to-fracture testing using a steatite ceramic ball of 6 mm in diameter at a cross-head speed of 0. 5 mm/min in a universal testing machine. Fracture load values were recorded and analyzed with t test. Weibull modulus was calculated to evaluate structure reliability. Fractographic analysis was carried out to determine fracture modes of the failed specimens by a stereomicroscope and a scanning electron microscope （SEM）. Results Statistical analysis results indicated a significant difference of the fracture load values between monolithic group [ （2071.23 ± 397.05 ） N] and bilayered group [ （1483.41 ± 327.87） N] （P 〈 0. 001 ）. Monolithic and bilayered groups present similar Weibull modulus （95% confidence interval） as 6. 15 （5.15±7. 15） and 5.54 （4. 01 ± 7.08 ） respectively, with no significant difference （ the confidence bounds overlapped with each other）. Bulk fracture initiating from the middle of oblique ridge of the first maxilla molar was the primary failure mode of monolithic/bilayered LDG crowns. Crack propagation initiated from core-veneer interfacial defects wasanother major failure mode of bilayered all-ceramic crowns. Conclusions Veneer application has some influence on fatigue failure of LDG crowns, but shows no effect on structure reliability. Accumulated damage combined with tensile stress concentration on the surface of veneer layer and defects within core-veneer interface lead to initiating of cracks. The mechanical property of veneering materials should be increased, and procedure of veneer application should be standardized and improved in order to reduce the failure rate of LDG molar crowns.