Accuracy of the functional method of hip joint center location: effects oflimited motion and varied implementation

Accurate location of the hip joint center is essential for computation of h ip kinematics and kinetics as well as for determination of the moment arms of muscles crossing the hip. The functional method of hip joint center loca tion involves fitting a pelvis-fixed sphere to the path traced by a thigh-f ixed point while a subject performs hip motions, the center of this sphere is the hip joint center. The aim of the present study was to evaluate the p otential accuracy of the functional method and the dependence of its accura cy on variations in its implementation and the amount of available hip moti on. The motions of a mechanical linkage were studied to isolate the factors of interest, removing errors due to skin movement and the palpation of bon y. landmarks that are always present in human studies. It was found that re ducing the range of hip motion from 30 degrees to 15 degrees did significan tly increase hip joint center location errors, but that restricting motion to a single plane did not. The magnitudes of these errors, however, even in the least accurate cases, were smaller than those previously reported for either the functional method or other methods based on pelvis measurements of living subjects and cadaver specimens. Neither increasing the number of motion data observations nor analyzing the motion of a single thigh marker (rather than the centroid of multiple markers) was found to significantly i ncrease error. The results of this study (1) imply that the limited range o f motion that is often evident in subjects with hip pathology does not prec lude accurate determination of the hip joint center when the functional met hod is used: and (2) provide guidelines for the use of the functional metho d in human subjects. (C) 2001 Elsevier Science Ltd. All rights reserved.