Open-face plane orientation of a BioMedtrix biologic fixation acetabular cup can be accurately determined from a lateral radiograph.

OBJECTIVE: To develop and validate a method to calculate biologic fixation (BFX) acetabular cup open-face plane orientation based on measurements from a lateral radiograph. METHODS: The formulas describing the open-face plane orientation were derived using geometric arguments and trigonometry. To assess the method, an in vitro plastic model of a pelvis was implanted with a BFX acetabular cup. A lateral radiograph of the implanted model was obtained and used to calculate the cup open-face plane orientation using the derived method. Direct measurement of the open-face plane orientation was performed, and the calculated values were compared to the directly measured values. The procedure was repeated with the cup in a different orientation for each trial. RESULTS: 52 trials were performed. The derived calculation method demonstrated high repeatability and reproducibility, with intra- and interobserver intraclass correlation coefficients > 0.94. The direct measurement method demonstrated high reproducibility, with interobserver intraclass correlation coefficients > 0.98. The mean absolute angular difference between the calculated and directly measured orientation vectors was 2.8 ± 1.3°, with 95% of trials within 4.7°. CONCLUSIONS: The open-face plane orientation determined by the derived method agreed closely with the directly measured orientation. CLINICAL RELEVANCE: Knowledge of the BFX open-face plane orientation can inform decisions regarding cup revision, help identify complications associated with cup malpositioning, and allow the surgeon to improve their cup placement performance.