Background: To describe and validate a novel mixed-reality (MR) navigation system for cup positioning in trapeziometacarpal arthroplasty and to assess its accuracy and reproducibility compared with the conventional freehand technique. Methods: A trackerless holographic navigation software for Microsoft HoloLens 2 was developed to provide real-time holographic feedback of anteversion and inclination during guidewire insertion. Two in vitro studies were performed. First, geometric trapezoidal models (n = 80) were used, in which an orthopaedic resident and an expert hand surgeon inserted Kirschner wires freehand and with navigation. Second, anatomical trapezium bio-models (n = 20) were used, with an expert surgeon performing freehand (n = 10) and navigated (n = 10) insertions. Postoperative CT quantified angular deviation from planned targets. Results were analysed using Student's t-test and Bartlett's test. Results: In geometric models, the pooled total mean absolute error was 6.99° (SD = 4.92°, 95% CI 3.47-10.52) for the freehand technique and 0.65° (SD = 0.26°, 95% CI 0.47-0.84) for MR guidance, with a mean paired difference of 6.34° favouring navigation (p = 0.002). Anteversion and inclination errors were significantly lower, and variability was reduced (Bartlett p < 0.01). No significant differences were observed between operators under MR guidance. In anatomical bio-models, MR guidance significantly reduced the overall mean angular error from 9.10° to 0.68° (p = 0.0004), while both anteversion and inclination errors were also significantly lower than with the conventional freehand technique. Conclusions: In this in vitro study, MR navigation substantially improves the accuracy and reproducibility of guidewire placement for trapeziometacarpal cup positioning compared with the freehand technique, while reducing variability and operator dependence. Clinical studies are required to determine its impact on intraoperative workflow and patient outcomes.
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