Reliability of lithium di-silicate and monolithic zirconia ceramics with different thickness cemented to a dentin analogue material

The study assessed the reliability of disk-shaped specimens made with two monolithic zirconia (5Y-TZP and 3Y-TZP) and a Lithium Disilicate, with thicknesses of 0.5 mm, 1.0 mm, and 1.5 mm cemented on an epoxy-resin substrate, through step-stress accelerated life testing (SSALT). Step-stress accelerated life testing (SSALT) comprises 18 specimens distributed into three profiles: mild (n=9), moderate (n=6), and aggressive (n=3). Each profile started at 300 N and finished at 3,000 N. The mild profile was designed with 200 N load increase at each 20,000 cycles up to 320,000 cycles; the moderate profile was designed with a 250 N load increase each 15,000 cycles up to 195,000 cycles; the aggressive profile had 300 N load increase at each 10,000 cycles up to 110,000 cycles. The data were analyzed at 90% two-sided confidence interval using the software Synthesis 9, Alta Pro, Reliasoft, Tucson, AZ, USA. 3Y-TZP 1.5mm was the only group to have specimens that survived. The beta value (β) was ˂1 for 3Y-TZP, suggesting early damage in the 7 specimens that failed; β ≈ 1 for Lithium Disilicate 1.0 mm and 1.5 mm, 5Y-TZP 1.0 mm and 1.5 mm, 3Y-TZP 0.5 mm and 1.0 mm suggested stability of failure rate over time; β ˃ 1 for Lithium Disilicate 0.5 mm and 5Y-TZP 0.5 mm suggested increased failure rate over time. Lithium Disilicate ceramic demonstrated high reliability irrespective of thickness as well as 3Y-TZP 1.5 mm and 0.5 mm, but reliability was significantly reduced in 3Y-TZP 1.0 mm (86%). No significant difference was observed among the 3 thicknesses of 5Y-TZP groups. Lithium Disilicate 0.5 mm and 3Y-TZP 1.0 mm groups were not different from 5Y-TZP 0.5mm and 1,5 mm. The contour plot of Weibull modulus (m) versus characteristic strength, using load at failure during fatigue, showed that group 3Y-TZP 1.5 mm presented the highest load at failure followed by 3Y-TZP 0.5 mm. There was no difference among the Lithium Disilicate groups. The 5Y-TZP 1.0 mm showed the lowest values, followed by 5Y-TZP 0.5 mm, 5Y-TZP 1.5 mm and 3Y-TZP 1.0 mm which were not different from each other. In summary, 5Y-TZP presented lower reliability than the other ceramics, particularly when compared to all Lithium Disilicate groups and 3Y-TZP 0.5 mm and 1.5 mm. There was significant intra-group reduction in reliability and characteristic strength in intermediate thickness (1.0 mm) of 3Y-TZP and 5Y-TZP. Fatigue behavior of Lithium Disilicate ceramic was not markedly affected by thickness.本研究通过步进应力加速寿命试验（step-stress accelerated life testing, SSALT），评估了三种牙科陶瓷材料制成的圆盘状试样的可靠性：两种单块氧化锆（monolithic zirconia，5Y-TZP、3Y-TZP）及二硅酸锂玻璃陶瓷（Lithium Disilicate）。所有试样厚度分别为0.5 mm、1.0 mm及1.5 mm，均粘接于环氧树脂基底（epoxy-resin substrate）之上。 步进应力加速寿命试验共纳入18个试样，分为三类应力剖面：轻度组（n=9）、中度组（n=6）及重度组（n=3）。所有剖面的加载均从300 N起始，最终加载至3000 N。其中，轻度剖面的加载方案为每20000次循环递增200 N，直至总循环数达320000次；中度剖面为每15000次循环递增250 N，直至总循环数达195000次；重度剖面为每10000次循环递增300 N，直至总循环数达110000次。 本研究采用美国亚利桑那州图森市Reliasoft公司的Synthesis 9及Alta Pro软件，以90%双侧置信区间对试验数据进行统计分析。 试验结果显示：仅3Y-TZP 1.5 mm组的试样全部存活，未发生失效。对于3Y-TZP组，其威布尔形状参数β值小于1，提示7个失效试样出现了早期损伤；二硅酸锂1.0 mm、1.5 mm组，5Y-TZP 1.0 mm、1.5 mm组，以及3Y-TZP 0.5 mm、1.0 mm组的β值约等于1，表明其失效发生率随时间保持稳定；二硅酸锂0.5 mm组与5Y-TZP 0.5 mm组的β值大于1，提示其失效发生率随时间呈上升趋势。 二硅酸锂玻璃陶瓷无论试样厚度如何，均表现出较高的可靠性；3Y-TZP 0.5 mm组与3Y-TZP 1.5 mm组同样具备优异可靠性，但3Y-TZP 1.0 mm组的可靠性显著下降（留存率为86%）。5Y-TZP组的不同厚度试样之间未观察到显著可靠性差异。二硅酸锂0.5 mm组与3Y-TZP 1.0 mm组的可靠性，与5Y-TZP 0.5 mm、1.5 mm组无统计学差异。 以疲劳失效载荷为表征的威布尔模量（Weibull modulus）与特征强度（characteristic strength）的等高线图显示：3Y-TZP 1.5 mm组的失效载荷最高，其次为3Y-TZP 0.5 mm组；二硅酸锂各组的失效载荷无显著差异。5Y-TZP 1.0 mm组的失效载荷最低，其次依次为5Y-TZP 0.5 mm、5Y-TZP 1.5 mm组，而3Y-TZP 1.0 mm组与后三者均无统计学差异。 综上，5Y-TZP组的可靠性低于其余两种陶瓷材料，尤其是与所有二硅酸锂组、3Y-TZP 0.5 mm及1.5 mm组相比差异显著。3Y-TZP与5Y-TZP组在中间厚度（1.0 mm）时，组内可靠性及特征强度均出现显著下降。二硅酸锂玻璃陶瓷的疲劳失效行为未受试样厚度的显著影响。