Impact of reference body position on linear and 3D accuracy of intraoral scans

PURPOSE: Intraoral scanners (IOS) are widely used in digital implant workflows, and their accuracy is typically evaluated in vitro using reference bodies to establish a coordinate system. While the impact of scanning strategy and evaluation methods on accuracy is well documented, the potential effect of the spatial position of the reference body itself on measured transfer accuracy has not been systematically investigated yet. This study aimed to evaluate whether the position of a reference body within a master model affects linear and three-dimensional (3D) accuracy outcomes of intraoral scan data. METHODS: A partially edentulous maxillary implant master model with four implants (FDI #16, #14, #25, and #26) and two perpendicular cuboid-shaped reference bodies was scanned ten times each using Trios 4 and Primescan AC IOS. Each dataset was evaluated twice by aligning it either to reference body 1 (REF-1) or reference body 2 (REF-2), generating paired measurements from identical scans. Linear distances between implant-abutment interface points and 3D deviations from the reference bodies were calculated. Trueness and precision were assessed according to ISO 5725-1, and paired statistical comparisons were performed. Differences between REF-1 and REF-2 were evaluated by paired t-tests for trueness and variance-related tests for precision, with a significance level set at α = 0.05. RESULTS: For both scanners, significant differences in trueness were observed between REF-1 and REF-2 for several linear distances and three-dimensional deviations (p < 0.05), despite identical scan datasets. Reference-dependent effects were more pronounced for three-dimensional deviations than for linear measurements. For selected distances, significant differences in precision were also detected (p < 0.05). CONCLUSIONS: The position of the reference body within a model significantly influences measured linear and 3D implant accuracy, independent of the IOS system used. Alignment reference position represents an independent source of systematic bias in digital accuracy assessments. These findings should be considered when designing, interpreting, and comparing accuracy studies in digital implant dentistry.