The Effect of Cyclic Loading On Fracture Strength of 3D Printed Zirconia Crowns and CAD/CAM Zirconia Crowns

Abstract

<strong>Objectives:</strong> This study aimed to evaluate and compare the marginal gap using two different methods and the internal fit of 3D printed and zirconia crowns and evaluate the effect of cyclic mechanical loading on the fracture resistance of 3D printed zirconia crowns in comparison to milled zirconia crowns. <br /> <strong>Materials and methods:</strong> Monolithic zirconia crowns (n=30) were manufactured using subtractive milling (group M) and 3D additive printing (group P). Concerning measuring the marginal gap and the internal fit, 10 crowns per each group were measured. The marginal gap was measured at 60 points using vertical marginal gap technique (VMGT). On the other hand, the silicone replica technique (SRT) was used to evaluate the internal fit and was divided into 4 groups: marginal gap, cervical gap, axial gap, and occlusal gap where the thickness of light impression was measured at 16 references. The numerical data was tested for normality using Shapiro-Wilk's test. They were found to be normally distributed and were analyzed using an independent t-test. For the fracture resistance test, 6 samples of each group were fractured under static loading while the other 6 samples were subjected to cyclic loading for 1.2 million cycles before being subjected to static loading until fracture. Scanning electron microscope (SEM) fractographic analysis was carried out on fractured fragments of representative samples. 3Y-TZP zirconia crowns (n=20) were manufactured using subtractive milling (group M) and 3D printed (group P). The marginal gap was measured at 60 points using vertical marginal gap technique (VMGT). On the other hand, the silicone replica technique (SRT) was used to evaluate the internal fit and was divided into 4 groups: marginal gap, cervical gap, axial gap, and occlusal gap where the thickness of light impression was measured at 16 references. The numerical data was tested for normality using Shapiro-Wilk's test. They were found to be normally distributed and were analyzed using an independent t-test. <br /> <strong>Results: </strong> In the VMGT, group P had significantly higher mean marginal gap values of 80 ± 30 µm compared to group M = 60 ± 20 µm (p<0.001). In SRT, the marginal gap of group P (100 ± 10 µm) also showed significantly higher values compared to group M (60 ± 10 µm). The internal fit showed significant differences between the tested groups, with the exception of the axial gap. The mean value of the breaking strength of group M was 1890 N without cyclic loading and 1642 N after cyclic loading and was therefore significantly higher than that of group P (1658 N and 1224 N respectively). <br /> <strong>Conclusions: </strong> 1- Although milled crowns showed better results, the 3D-printed zirconia crowns provide clinically acceptable results in terms of marginal adaptation and internal fit. Both VMGT and SRT are reliable methods for evaluating the marginal gap. 2- In terms of fracture resistance, 3D-printed crowns can withstand long-term masticatory forces without cracking or failure.