The occlusal precision of milled versus printed provisional crowns

The aim of this study was to compare the occlusal precision of computer-aided-design/ computer-assisted-manufacturing (CAD/CAM) milled versus 3D printed polymethylmethacrylate (PMMA) temporary prosthetic crowns , starting from the same digital CAD design.

The study sample included 34 patients presenting 34 premolars in need of prosthetic rehabilitation: a total of 68 temporary crowns were manufactured, 34 of which milled and 34 printed. Immediately after manufacturing, the milled and printed provisionals were scanned with a desktop scanner (E1, 3Shape) to obtain STL files, that were superimposed to the original CAD design in order to identify the occlusal trueness (Analysis A). A second occlusal comparison was performed by scanning both kind of provisional after being placed intraorally with Trios scanner (3 Shape); intraoral scans were obtained in order to compare STL files of provisionals before and after occlusal adjustments (Analysis B).  The occlusal trueness was identified at three reference points, P1 (vestibular cusp), P2 (palatal / lingual cusp), P3 (central fossa). The statistical analysis was performed using the R 3.4.3 statistical software (The R Foundation for Statistical Computing), with a significance level of p <0.05.

Overall, the printed crowns showed lower occlusal differences than the milled crowns, when compared to the CAD design file, with a statistically significant difference in P1 (difference of 0.025 ± 0.046 mm), P2 (difference of 0.027 ± 0 0.044 mm) and P3 ( difference of 0.018 ± 0.050) for Analysis A (p <0.05). In the Analysis B the direct comparison between the degree of average difference between the pre-adjustment and the post-occlusal adjustment of the milled and printed crowns shows that the printed crowns have lower occlusal mean difference values in all three points compared to the milled ones (difference of 0.146 ± 0.273 mm in P1, 0.285 ± 0.360 mm in P2 and 0.257 ± 0.277 mm in P3).

Within the limitation of this study, the data obtained showed a better occlusal surface dimensional accuracy of the 3D printed provisional crowns, when compared to the milled ones. Comparing the results obtained, it is possible to assume that the intraoral scans also had a contribution to occlusion, beside the manufacturing method. 3D printing can be successfully applied for manufacturing temporary PMMA crowns.