Internal and marginal discrepancies associated with stereolithography (SLA) additively manufactured zirconia crowns

Abstract

Statement of problem

Stereolithography (SLA) additive manufacturing (AM) technologies can be selected to fabricate zirconia crowns; however, the internal and marginal discrepancies associated with these new technologies remain unclear.

Purpose

The purpose of this in vitro study was to measure and compare the marginal and internal discrepancies of milled and AM zirconia crowns by using the silicone replica technique.

Material and methods

An implant custom abutment was manufactured and scanned by using a laboratory scanner (CARES Software; Institut Straumann AG). An anatomic contour crown was digitally designed, and the standard tessellation language (STL_C) file was obtained. The STL_C file was splinted into 2 pieces, simulating the parts of the crown that would replace the enamel (STL_G1 file) and dentin (STL_G2 file) structures. Three groups were determined: anatomic contour zirconia milled (CNC group), AM anatomic contour zirconia (AM group), and AM splinted zirconia (SAM group). For the CNC group, the STL_C file was used to manufacture milled (CARES zirconium-dioxide crown; Institut Straumann AG) zirconia specimens. For the AM group, the STL_C file was used to additively fabricate (CERAMAKER 900; 3DCeram Co) the zirconia (3DMix ZrO₂ paste; 3DCeram Co) specimens. For the SAM group, the STL_G2 file was selected to AM (CERAMAKER 900; 3DCeram Co) the zirconia (3DMix ZrO₂ paste; 3DCeram Co) specimens. Ten specimens per group were manufactured. The silicone replica technique was used to measure the marginal and internal discrepancies. The cement gap was measured on images captured by using a digital microscope at ×100 magnification. For the internal gap, 50 measurements were made for each specimen, and for the marginal gap, 25 measurements were made for each specimen. The normality test, Shapiro-Wilk test, was conducted. The results indicated that the distributions were not normal; therefore, nonparametric Kruskal-Wallis H and pairwise Mann-Whitney U-tests were used to analyze the data. The Spearman correlation coefficient was used to determine the correlation between marginal and internal discrepancies in all 3 groups.

Results

Significant differences were found in marginal and internal discrepancies among the groups. The CNC group had the least marginal and internal discrepancies compared with the AM and SAM groups. The SAM group had significantly lower values for marginal and internal discrepancies than the AM group. The AM group showed the highest marginal and internal discrepancies. The CNC group had a weak correlation coefficient of 0.13 (P=.046), the AM group had a moderate correlation coefficient of 0.32 (P<.001), and the SAM group had a nonsignificant correlation coefficient of 0.12 (P=.051).

Conclusions

CNC and SAM groups had clinically acceptable marginal and internal discrepancies, while the AM group had a clinically unacceptable marginal and internal crown discrepancies. Furthermore, a weak correlation was encountered between the marginal and internal discrepancies measured in all groups.