Verification of an improved hip joint center prediction method.

In motion analysis, the hip joint center (HJC) is used to define the proximal location of the thigh segment and is also the point about which hip moments are calculated. The HJC cannot be palpated; its location must be calculated. Functional methods have been proposed but are difficult to perform by some clinical populations. Therefore, regression methods are utilized, but yield large errors in estimating the HJC location. These prediction methods typically utilize the anterior and posterior superior iliac spines, where excessive adipose tissue makes correctly locating difficult. A new regression method (Hara) utilizes leg length and has been shown to improve HJC location in cadavers and less error than previous pelvic based regression methods, such as those proposed by Harrington et al. This study compared the accuracy of the HJC location calculated with both of the Harrington methods and the Hara method. The coronal knee angle was calculated for each method using a static motion analysis trial, and compared to the tibiofemoral angle measured on a gold standard digital full-leg coronal radiograph. This study demonstrated that the Hara method was more accurate than either of the Harrington methods. The mean error between the gold standard x-ray measurement and the motion analysis calculation for the Harrington (stepwise and LOOCV), the Harrington (linear regression), and Hara regression methods, respectively were 6.0°, 4.0°, and 1.8°. Accurately modeling the HJC is critical for data interpretation and patient care. This study confirmed that the Hara HJC regression method is valid in an in-vivo setting.