Objective

To understand the stress development in porcelain-veneered zirconia (PVZ) and porcelain-veneered lithium disilicate (PVLD) crowns with different veneer/core thickness ratios and cooling rates. To provide design guidelines for better performing bilayer restorations with the aid of Viscoelastic Finite Element Method (VFEM).

Methods

The VFEM was validated by comparing the predicted residual stresses with experimental measurements. Then, the model was used to predict transient and residual stresses in the two bilayer systems. Models with two different veneer/core thickness ratios were prepared (2:1 and 1:1) and two cooling protocols were simulated (Fast: ∼300 °C/min, Slow: ∼30 °C/min) using the heat transfer module, followed by stress analysis in ABAQUS. The physical properties of zirconia, lithium disilicate, and the porcelains used for the simulations were determined as a function of temperature.

Results

PVLD showed lower residual stresses than PVZ. The maximum tensile stresses in PVZ were observed in the cusp area, whereas those in PVLD were located in the central fossa. The 1:1 thickness ratio decreased stresses in both layers of PVZ. Slow cooling slightly decreased residual stresses in both systems. However, the cooling rate effect was more evident in transient stresses.

Significance

Slow cooling is preferable for both systems. A thinner porcelain layer over zirconia lowers stresses throughout the restoration. The different stress distributions between PVZ and PVLD may affect their failure modes. Smaller mismatches in modulus, CTE, and specific heat between the constituents, and the use of low T_g porcelains can effectively reduce the deleterious transient and residual tensile stresses in bilayer restorations.

中文翻译：

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解释双层氧化锆和二硅酸锂冠临床骨折的残余应力：一项 VFEM 研究

客观的

了解具有不同饰面/芯厚度比和冷却速率的瓷贴面氧化锆 (PVZ) 和瓷贴面二硅酸锂 (PVLD) 冠的应力发展。借助粘弹性有限元法 (VFEM) 为更好地执行双层修复体提供设计指南。

方法

通过将预测的残余应力与实验测量值进行比较来验证 VFEM。然后，该模型用于预测两个双层系统中的瞬态和残余应力。制备了具有两种不同单板/芯厚度比（2:1 和 1:1）的模型，并使用传热模块模拟了两种冷却方案（快速：~300°C/min，慢速：~30°C/min） ，然后在 ABAQUS 中进行应力分析。氧化锆、二硅酸锂和用于模拟的瓷器的物理性质被确定为温度的函数。

结果

PVLD 显示出比 PVZ 更低的残余应力。PVZ 的最大拉应力出现在牙尖区域，而 PVLD 的最大拉应力位于中央颅窝。1:1 的厚度比降低了 PVZ 两层的应力。缓慢冷却略微降低了两个系统中的残余应力。然而，冷却速率效应在瞬态应力中更为明显。

意义

两种系统都优选缓慢冷却。氧化锆上较薄的瓷层可降低整个修复过程中的应力。PVZ 和 PVLD 之间不同的应力分布可能会影响它们的失效模式。成分之间的模量、CTE 和比热更小的不匹配，以及使用低T _g瓷可以有效减少双层修复体中有害的瞬态和残余拉伸应力。