Measuring acetabular component version after THA: CT or plain radiograph?

BACKGROUND Although cross-table lateral radiographs are commonly used to measure acetabular component version after THA, recent studies suggest that CT-based measurement is more accurate. This has been attributed to variations in pelvic tilt, pelvic rotation, and component inclination. Furthermore, it has been suggested, based on limited data, that even with ideal positioning of the cross-table lateral radiograph, CT remains the more accurate modality. QUESTIONS/PURPOSES We determined whether appropriately positioned cross-table lateral radiographs could provide accurate measurements of acetabular component version, and compared accuracy and reliability of measurements from modified cross-table lateral radiographs with those from standard cross-table lateral radiographs and CT. METHODS We implanted 27 Sawbones pelves with an acetabular cup using computer navigation. CT, an AP view of the pelvis, and cross-table lateral and modified cross-table lateral radiographs were performed for each specimen. For the modified cross-table lateral radiograph, the beam angle varied based on the cup inclination as measured on an AP view of the pelvis. Two independent observers measured acetabular component version and inclination. We calculated intraobserver and interobserver reliabilities for each method and compared these with values obtained from navigation presuming that to be the standard for judging accuracy. RESULTS Interobserver and intraobserver reliabilities were greater than 0.95 (95% CI, 0.904-0.999) for all measurements. Correlation with navigated values was 0.96 or greater (95% CI, 0.925-0.998) for all methods. Although CT had the highest correlation with navigated values, the correlations for the modified cross-table lateral and cross-table lateral radiographs were similar. CONCLUSION CT allows for accurate measurement of acetabular component version; however, when properly positioned, cross-table lateral radiograph-derived measurements are similarly accurate. CLINICAL RELEVANCE Our results support the use of plain radiographs to obtain important measurements after THA.