Effect of Dental Chair Backrest Inclination on Mandibular Position Obtained with a Virtual Interocclusal Record at the Maximum Intercuspal Position: A Pilot Clinical Study

Background: This pilot clinical study aimed to evaluate the mandibular position at the maximum intercuspal position (MIP) obtained with a virtual interocclusal record using an intraoral scanner across different dental chair backrest inclinations, in order to establish a protocol for future larger studies. Methods: In this single-center pilot study, 10 fully dentate participants underwent maxillary and mandibular full-arch intraoral scans. Virtual interocclusal records at MIP were obtained with an intraoral scanner at three dental chair backrest inclinations (90, 120, and 180 degrees). Conventional vinyl polysiloxane interocclusal records were made at the same inclinations. All scans were printed into physical models and mounted using the conventional records. Digitized casts and intraoral scans were superimposed onto a single common reference, the virtually mounted 90-degree scan, using metrology software. Because all deviations were expressed relative to that reference, the virtual 90-degree condition is zero by construction and was excluded as a data level from inferential models. Separate linear mixed-effects models were fitted for each translation and rotation axis, with participant as a random intercept and recording method, inclination, and their interaction as fixed effects; p values were Holm-adjusted across the six axes. Equivalence was explored with two one-sided tests against margins of ±0.5 mm and ±1.0 degrees; because these margins were specified post hoc, the equivalence analysis is exploratory. Results: Chair inclination did not detectably affect either recording method: no inclination main effect or method × inclination interaction survived multiplicity correction, and virtual records at 120 and 180 degrees differed by no more than 0.05 mm and 0.08 degrees. Recording method did differ. Conventional records were displaced relative to virtual records by 0.27 mm along the Y axis (95% CI 0.19 to 0.34; Holm-adjusted p < 0.001), in the same direction in 9 of 10 participants. The corresponding X-axis offset was 0.23 mm (95% CI 0.06 to 0.40) and did not survive multiplicity correction in the primary analysis (Holm-adjusted p = 0.060). The exploratory equivalence criterion was met, that is, the 90% confidence interval fell entirely within the post hoc equivalence margin, for 11 of 18 method × inclination comparisons, rising to 16 of 18 in the sensitivity analysis. Conclusions: In this pilot sample, no statistically detectable effect of dental chair backrest inclination between 90 and 180 degrees on the recorded mandibular position at MIP was observed for either recording method in healthy, fully dentate individuals; virtual and conventional records met the exploratory post hoc equivalence criterion in most rotational axes but showed a small, systematic translational offset of approximately 0.23 to 0.31 mm, most clearly along the Y axis. These preliminary findings require confirmation in a larger, prospectively powered and randomized equivalence trial with a prospectively defined equivalence margin.