Hip joint center differences between dual fluoroscopy, functional hip, and coda - establishing a reference standard

Measurements of joint angles, moments, and forces may be highly sensitive to positional changes in the center of a joint. Due to its deep location, the human hip joint center (HJC) can be difficult to determine. Previous studies have defined the HJC relative to external landmarks using infrared motion capture coupled with regression equations or functional measurements. These methods may mask subtle, yet clinically important, differences in the HJC location. A more recent technique termed dual-fluoroscopy (DF) and model based tracking (MBT) affords direct visualization of bony anatomy. This technique has been validated for the hip joint to track three-dimensional bone motions in vivo with subdegree and submillimeter accuracy. Thus, use of the HJC calculated by DF may facilitate an improved understanding of how deleterious hip biomechanics occur in patients with hip pathoanatomy. The purpose of this study was to develop a method that would allow simultaneous measurement and comparison of the HJC location using DF and the two infrared motion capture methods. When the three measurements of the HJC for a subject were visualized on the associated hemi-pelvis, the HJC measured by DF qualitatively appeared to be the most centrally located with respect to the acetabulum (hip socket). The average (and standard error of the mean) Euclidean distance between the DF-defined HJC (reference standard) was 18.6 (3.4) mm and the regression equation technique and 11.4 (1.2) mm for the functional method across the eleven subjects. Currently, our group is expanding this work by measuring the HJC using DF imaging and MBT during dynamic activities. With this information, we hope to improve current regression equations for investigators who do not have DF imaging abilities.