Effect of Marker Ball Positioning on Templating Accuracy in Hip Arthroplasty: A Closed-Loop Audit

Introduction Positioning of external calibration devices (ECDs) in anteroposterior (AP) pelvic radiographs influences magnification correction and therefore the accuracy of digital templating in hip arthroplasty. Orthopaedic departments rely on radiology teams to accurately position ECDs on pre-operative radiographs, but due to multiple factors, placement is not always satisfactory, which may incorrectly guide selection of the hip prosthesis size. This closed-loop audit assessed the accuracy of templating femoral head size in patients with intra-capsular neck of femur (NOF) fractures before and after an instructional intervention for radiology staff on optimal marker ball placement. Methods A closed-loop retrospective audit was performed at York District Hospital, UK. Data were collected from September to December 2023 and March to May 2024. Inclusion criteria were trauma patients undergoing hip arthroplasty who had a templated pre-operative pelvic radiograph via TraumaCad software (Brainlab, Munich, Germany). The agreed audit standard was that 100% of pre-operative pelvic radiographs should contain a correctly positioned marker ball. The marker ball is a metal sphere, attached to a stand, used whilst obtaining a pelvic radiograph, as a calibration reference to accurately determine the true size and magnification of anatomical structures on the radiograph; this is used by the TraumaCad software to assist the template. Mean absolute head sizes were compared between digitally templated head sizes and implanted head sizes via intra-operative prosthetic stickers. Following the first cycle, results were presented at the departmental audit meeting, and a teaching session on marker ball positioning was delivered to radiology staff. The second cycle was conducted post intervention, and mean differences between templated and actual head sizes in both cycles were compared using a parametric unpaired t-test, with significance set at p < 0.05. Results In the first cycle, 52 patients met the audit inclusion criteria, of whom 50 (96.2%) had pre-operative pelvic radiographs with a fully visible marker ball. These 50 cases were included for accuracy analysis. The mean difference between templated and implanted head size was 2.82 mm (range: -5 to +11 mm); 24 patients (48%) received a larger head size than templated, 18 (36%) a smaller size, and six (12%) received the same size. Post intervention, 43 patients met the inclusion criteria in the second cycle. Of these, 37 (86.0%) had a fully visible marker ball and were included for analysis. The mean difference between templated and implanted head size decreased to 1.51 mm (range: -4 to +7 mm); 16 patients (43%) had larger actual head sizes, 12 (32%) had smaller, and nine (24%) had identical sizes. Statistical comparison demonstrated a significant improvement in templating accuracy between audit cycles (p = 0.008). However, despite the improvement in accuracy of head size selection, compliance with fully visible marker head balls in the radiograph decreased from 96.2% to 86.0%. Conclusion This quality improvement project demonstrates that a targeted educational intervention can significantly enhance the accuracy of radiological marker ball placement, leading to improved templating precision in hip arthroplasty.