Radiographic and non-invasive determination of the hip joint center location: effect on hip joint moments

Objective. To determine which of four non-invasive measures is most accurat e in locating the hip joint center. Background. The location of the joint centers must be accurately determined in three dimensions for calculation of the moments of force during gait. I t is not known which of the several non-invasive methods available for loca tion of the hip center is most accurate. Design. Hip center location was determined using standardized X-rays and fo ur non-invasive methods which utilized measured distances between bony land marks in 10 healthy subjects. Hip moments during gait were obtained from op tical tracking, force plate and anthropometric data. Results. The most accurate non-invasive method of locating the hip center w as by taking the midpoint of a line connecting the antero-superior iliac sp ine and the symphysis pubis and moving inferiorly 2 cm. Using this approach the hip center was located 0.7 cm medial and 0.8 cm superior to its true l ocation determined using the standardized X-rays. The 95% confidence interv al of the maximum error difference in moments measured between this method and the standardized X-rays ranged from -0.15 to 0.4 Nm/kg in the frontal p lane, -0.03 to 0.07 Nm/kg in the sagittal plane and -0.05 to -0.03 Nm/kg in the transverse plane. Conclusions. Locating the hip center based on the distance between the ante ro-superior iliac spine and the symphysis pubis is a valid technique for es timating the hip center in routine gait analysis.