Intraoperative evaluation of acetabular cup position during anterior approach total hip arthroplasty: are we accurately interpreting?

Introduction: Intraoperative radiographic evaluation during total hip arthroplasty (THA) has shown to improve the accuracy of acetabular component placement, however, differences in interpretation based on radiographic technique has not been established. This study aims to determine if differences exist in the interpretation of acetabular component abduction and anteversion between different radiographic projections. Methods: 55 consecutive direct anterior THAs in 49 patients were prospectively enrolled. Target anteversion and abduction was defined by the Lewinnek zone. Fluoroscopy was used to direct acetabular component placement intraoperatively. After final cup implantation, fluoroscopic posterior-anterior hip and pelvis images were obtained for analysis. After completion of the procedure, an anterior-posterior plain pelvis radiograph was obtained in the operating room. Acetabulum component abduction and anteversion were postoperatively determined using specialised software on each of the 3 image acquisition methods. Results: Average acetabular cup abduction for intraoperative fluoroscopic posterior-anterior hip (FH), intraoperative fluoroscopic posterior-anterior pelvis (FP), and postoperative, standard, anteroposterior pelvis radiographs (PP) was 40.95 degrees +/- 2.87 degrees, 38.87 degrees +/- 3.82 degrees and 41.73 degrees +/- 2.96 degrees respectively. The fluoroscopic hip and fluoroscopic pelvis tended to underestimate acetabular cup abduction compared to the postoperative pelvis (p < 0.001). Average acetabular cup anteversion for FH, FP, and PP was 19.89 degrees +/- 4.87 degrees, 24.38 degrees +/- 5.31 degrees and 13.36 degrees +/- 3.52 degrees respectively. Both the fluoroscopic hip and fluoroscopic pelvis overestimated anteversion compared to the AP pelvis, with a 6.38 degrees greater mean value measurement for FH (p < 0.001), and an 11 degrees greater mean value measurement for FP (p < 0.001). Conclusions: Fluoroscopic technique and differences between radiographic projections may result in discrepancies in component position interpretation. Our results support the use of the fluoroscopic posterior-anterior hip as the choice fluoroscopic imaging technique.