Comparison of the accuracy of direct and indirect three-dimensional digitizing processes for CAD/CAM systems – An in vitro study

Abstract

Purpose: To compare the accuracy (trueness, precision) of direct and indirect scanning CAD/CAM methods.

Methods: A master cast with prepared abutments and edentulous parts was created from polymethyl methacrylate (PMMA). A high-resolution industrial scanner was used to create a reference model. Polyvinyl-siloxane (PVS) impressions and digital impressions with three intraoral scanners (iTero, Cerec, Trios) were made (n = 10 for each) from the PMMA model. A laboratory scanner (Scan CS2) was used to digitize the sectioned cast made from the PVS impressions. The stereolithographic (STL) files of the impressions (n = 40) were exported. Each file was compared to the reference using Geomagic Verify software. Six points were assigned to enable virtual calliper measurement of three distances of varying size within the arch. Methods were compared using interquartile range regression and equality-of-variance tests for precision, and mixed-effects linear regression for trueness.

Results: The mean (SD) deviation of short distance measurements from the reference value was −40.3 (79.7) μm using the indirect, and 22.3 (40.0) μm using the direct method. For the medium distance, indirect measurements deviated by 5.2 (SD: 111.3) μm, and direct measurements by 115.8 (SD: 50.7) μm, on average; for the long distance, the corresponding estimates were −325.8 (SD: 134.1) μm with the indirect, and −163.5 (SD: 145.5) μm with the direct method. Significant differences were found between the two methods (p < 0.05).

Conclusions: With both methods, the shorter the distance, the more accurate results were achieved. Virtual models obtained by digital impressions can be more accurate than their conventional counterparts.