Trueness and precision of 3D-printed versus milled monolithic zirconia crowns: An in vitro study

Purpose: To compare the trueness and precision of 3D-printed versus milled monolithic zirconia crowns (MZCs). Methods: A model of a maxilla with a prepared premolar was scanned with an industrial scanner (ATOSQ (R), Gom) and an MZC was designed in computer-assisted-design (CAD) software (DentalCad (R), Exocad). From that standard tessellation language (STL) file, 10 MZCs (test) were 3D-printed with a Lithography-based Ceramic Manufacturing (LCM) printer (CerafabS65 (R), Lithoz) and 10 MZCs (control) were milled using a 5-axis machine (DWX-52D (R), DGShape). All MZCs were sintered and scanned with the aforementioned scanner. The surface data of each sample (overall crown, marginal area, occlusal surface) were superimposed to the original CAD file (ControlX (R), Geomagic) to evaluate trueness: (90-10)/2, absolute average (ABS AVG) and root mean square (RMS) values were obtained for test and control groups (MathLab (R), Mathworks) and used for analysis. Finally, the clinical precision (marginal adaptation, interproximal contacts) of test and control MZCs was investigated on a split-cast model printed (Solflex350 (R), Voco) from the CAD project, and compared. Results: The milled MZCs had a significantly higher trueness than the 3D-printed ones, overall [(90-10)/2 printed 37.8 pm vs milled 21.2 pm; ABS AVG printed 27.2 pm vs milled 15.1 pm; RMS printed 33.2 pm vs milled 20.5 pm; p = 0.0000051, at the margins [(90-10)/2 printed 25.6 pm vs milled 12.4 pm; ABS AVG printed 17.8 pm vs milled 9.4 pm; RMS printed 22.8 pm vs milled 15.6 pm; p= 0.0000111 and at the occlusal level [(90-10)/2 printed 50.4 pm vs milled 21.9 pm; ABS AVG printed 29.6 pm vs milled 14.7 pm; RMS printed 38.9 pm vs milled 22.5 pm; p = 0.0000051. However, with regard to precision, both test and control groups scored highly, with no significant difference either in the quality of interproximal contact points (p = 0.355) or marginal closure (p = 0.355). Conclusions: Milled MZCs had a statistically higher trueness than 3D-printed ones; all crowns, however, showed high precision, compatible with the clinical use. Clinical significance: Although milled MZCs remain more accurate than 3D-printed ones, the LCM technique seems able to guarantee the production of clinically precise zirconia crowns.