Comparison of hip center estimation using in-vivo and ex-vivo measurements from the same subject

OBJECTIVE: To evaluate the accuracy of locating the hip center (HIPC) by comparing in-vivo estimations to posthumous HIPC measurement in the same subject. BACKGROUND: Numerous techniques have been used to locate the HIPC in living subjects and in cadavers. There are no published reports, however, which have compared measurements of the HIPC obtained in-vivo and ex-vivo for the same subject. DESIGN AND METHODS: Bilateral hip centers (HIPC) were estimated for an elderly male patient seven times over 32 months in a gait laboratory using a technique which combines kinematic data and static pointing. The patient's pelvis was recovered posthumously and analytical photogrammetry used to estimate the HIPC locations. Published methods which scale anatomical measurements to locate the HIPC were compared to our ex-vivo and in-vivo measurements. RESULTS: HIPC precision was 0.94 cm for the in-vivo measurements (N = 7 trial days) and 0.15 cm for the ex-vivo measurements (N = 6 photogrammetry trials). The resultant error between ex-vivo and in-vivo estimates of HIPC position was 1.17 cm. Resultant errors in locating the HIPC using published anatomical scaling factors ranged between 1.48 cm and 5.37 cm. CONCLUSIONS: Although we have been unable to compare in-vivo and ex-vivo HIPC measurements for more than one subject, results of our analysis suggest that using anatomical scaling to locate the HIPC may result in unsatisfactory subject-specific estimates of the HIPC.