CAD-FEA modeling and fracture resistance of bilayer zirconia crowns manufactured by the rapid layer technology

Abstract In the RLT (Rapid Layer Technology), veneering ceramic and framework are fabricated by computer-aided design/computer-aided manufacturing (CAD/CAM) and then cemented to obtain the restoration. This study aimed to evaluate the effect of the thickness of veneering ceramic manufactured by the RLT technique on the fracture resistance (FR) of bilayer crowns with zirconia frameworks. Twenty zirconia frameworks and twenty feldspathic posterior crowns with two different veneering ceramic occlusal thicknesses (1mm=TF1; 2mm=TF2) were manufactured using CAD/CAM system. The specimens were luted to an epoxy resin abutment with resin cement and mechanically cycled (200N and 4.5×105 Pa, 37°C, 2×106 cycles, 3Hz). The FR test was performed (10kN, 0.5mm/min), and the specimens were analyzed in a stereomicroscope. For the stress analysis (finite element analysis, FEA), a 10kN load was equal to the in vitro test, and the principal stress was evaluated. The FR data were analyzed by Student’s t-test and Weibull's analysis. The thickness influenced the FR of bilayer crowns. The FR was higher in the TF2 than in the TF1 group. The TF2 group presented the highest characteristic strength compared to the group TF1. The predominant type of failure was delamination. The FEA showed higher stress concentrations below the loading application point at the veneering cement interface in the 1-mm-thick model. The bilayer crowns manufactured using the approach of 2mm of veneering ceramic promoted higher FR compared to the group with 1mm veneering ceramic. Also, the FEA showed that the veneer ceramic thickness has an effect on stress distribution in zirconia-based bilayer crowns.

摘要 在快速分层技术（Rapid Layer Technology，RLT）中，饰面瓷与基底冠通过计算机辅助设计/计算机辅助制造（CAD/CAM）制备后经粘接制得修复体。本研究旨在评估采用RLT技术制备的饰面瓷厚度对氧化锆基底双层冠断裂抗力（fracture resistance，FR）的影响。研究采用CAD/CAM系统制备了20个氧化锆基底冠与20个长石质后牙冠，分为两组，饰面瓷咬合面厚度分别为1mm（记为TF1组）与2mm（记为TF2组）。将试件通过树脂水门汀粘接至环氧树脂基台上，并进行机械循环加载（载荷200N、循环压力4.5×10^5 Pa、环境温度37℃、循环次数2×10^6次、加载频率3Hz）。随后开展断裂抗力测试（最大加载载荷10kN、加载速率0.5mm/min），并通过体视显微镜对试件进行检测分析。针对应力分析环节，采用有限元分析（finite element analysis，FEA），设置与体外试验一致的10kN加载载荷，对主应力进行评估。采用t检验与威布尔分析对断裂抗力数据进行统计学分析。结果表明，饰面瓷厚度对双层冠的断裂抗力存在显著影响：TF2组的断裂抗力显著高于TF1组，且其特征强度亦优于TF1组。试件的主要失效模式为分层脱落。有限元分析结果显示，1mm厚度组模型的饰面瓷-粘接剂界面加载点下方存在更高的应力集中。相较于1mm饰面瓷厚度组，采用2mm饰面瓷制备的双层冠可获得更高的断裂抗力；同时，有限元分析结果表明，饰面瓷厚度会对氧化锆基双层冠的应力分布产生影响。