Comparison of trueness of cobalt chromium implant-supported frameworks fabricated using additive and subtractive manufacturing: An in vitro study.

STATEMENT OF PROBLEM: Studies comparing the fabrication trueness of different digital techniques for cobalt chromium (Co-Cr) frameworks in multiunit implant-supported fixed dental prostheses (ISFDPs) are scarce. PURPOSE: The purpose of this in vitro study was to compare the trueness of Co-Cr implant-supported frameworks fabricated using additive manufacturing (AM) and subtractive manufacturing (SM). MATERIAL AND METHODS: A resin block with 3 noncollinear abutment analogs was created to simulate the jawbone. A 5-unit implant-supported framework featuring a short mesial bar (10.6 mm) and a long distal bar (17.7 mm) was digitally designed and exported as a reference standard tessellation language (STL) file. Eight Co-Cr frameworks were fabricated by computer numerical control (CNC) milling and 8 by direct metal laser sintering (DMLS). All frameworks were scanned with a laboratory scanner to generate test STLs, which were then imported into a metrology-grade software program for 3D comparison with the reference STL. The deviations were quantified by using color maps and root mean square (RMS) values. Deviations in the bar connector were evaluated via cross-sectional measurements. Overall RMS, marginal RMS, and bar dimensional deviations were analyzed by using the Mann-Whitney U test, independent-samples t test, and linear mixed model, respectively (α=.05). RESULTS: For the overall RMS values, the median of the milling group was 67.5 µm [65.5, 68.7], which was significantly lower than that of the DMLS group (90.3 µm [82.1, 92.8], P=.010). Moreover, milled frameworks exhibited significantly higher marginal trueness than DMLS frameworks (P=.045), with mean ±standard deviation marginal RMS values of 68.8 ±9.2 μm and 80.3 ±11.5 μm, respectively. No significant main effect of fabrication method was observed for bar connector deviations (P=.211), while measurement surface (P<.001) and bar span (P=.002) significantly affected the deviations. Additionally, a significant interaction was found between fabrication method and measurement surface (P<.001). CONCLUSIONS: Co-Cr implant-supported frameworks fabricated by SM showed higher overall and marginal trueness than those fabricated by AM, while dimensional deviations in the bar connector were surface-dependent for both fabrication techniques.