BACKGROUND: The learning curve of total hip arthroplasty (THA) using the anterior approach in the supine position is considered to be long owing to limited surgical field visualization. This study aimed to investigate the learning curve of a trainee's cup placement using computed tomography (CT)-based navigation, focusing on identifying the number of cases required to achieve proficiency. METHODS: This retrospective cohort study included 112 hips that underwent total hip arthroplasty using CT-based navigation via anterolateral approach in the supine position by a trainee surgeon who had performed fewer than 20 prior THA cases. The absolute differences in cup alignment between postoperative measurements and intraoperative records, and differences in three-dimensional position between postoperative measurements and preoperative plan were assessed using the Mann-Whitney U test. A receiver operating characteristic (ROC) curve was used to determine the cutoff point for achieving accuracy. RESULTS: The absolute differences were 2.4°±2.5° (inclination) and 2.4°±2.0° (anteversion), and 1.5 mm ± 1.3 mm (coronal plane) and 1.4 mm ± 1.2 mm (axial plane). The ROC curve identified 20 cases as the cutoff point for proficiency, with significant improvement in cup inclination accuracy. Multivariate analysis revealed that the first 20 cases (odds ratio, 10.4
95% confidence interval, 3.1-34.5) were a predictive risk factor for inaccurate cup alignment. No cup revisions or dislocations occurred. CONCLUSIONS: This is the first study to identify the learning curve of cup placement using CT-based navigation via an anterior approach by an inexperienced trainee surgeon. Proficiency in cup placement was achieved after 20 cases using CT-based navigation.