Assessment of the functional method of hip joint center location subject to reduced range of hip motion

Abstract

Motion analysis of the lower extremities usually requires determination of the location of the hip joint center. The results of several recent studies have suggested that kinematic and kinetic variables calculated from motion analysis data are highly sensitive to errors in hip joint center location. “Functional” methods in which the location of the hip joint center is determined from the relative motion of the thigh and pelvis, rather than from the locations of bony landmarks, are promising but may be ineffective when motion is limited. The aims of the present study were to determine whether the accuracy of the functional method is compromised in young and elderly subjects when limitations on hip motion are imposed and to investigate the possibility of locating the hip joint center using data collected during commonly studied motions (walking, sit-to-stand, stair ascent, stair descent) rather than using data from an ad hoc trial in which varied hip motions are performed. The results of the study suggested that functional methods would result in worst-case hip joint center location errors of 26 mm (comparable to the average errors previously reported for joint center location based on bony landmarks) when available hip motion is substantially limited. Much larger errors (∼70 mm worst-case), however, resulted when hip joint centers were located from data collected during commonly performed motions, perhaps because these motions are, for the most part, restricted to the sagittal plane. It appears that the functional method can be successfully implemented when range of motion is limited but still requires collection of a special motion trial in which hip motion in both the sagittal and frontal planes is recorded.

Introduction

Motion analysis often requires determination of the location of the hip joint center. The hip joint center is frequently used to establish segment-fixed coordinate systems in the thigh and pelvis and is commonly taken to be the point of application for the hip joint reaction force in inverse dynamic analyses. Errors in hip joint center location that are as small as 20–30 mm may lead to substantial inaccuracies in hip joint moment calculations (Holden and Stanhope, 2000; Kirkwood et al., 1999; Stagni et al., 2000) and model-based estimates of the moment arms of muscles that cross the hip (Delp and Maloney, 1993). Cappozzo (1984) described a functional method for hip joint center location in which a single center of rotation between the thigh and pelvis is determined from the relative motion of the two segments. This method is attractive for its ease of implementation and because hip joint centers are calculated from subject-specific kinematic data, but questions about its accuracy remain, especially when it is implemented under suboptimal conditions.

Leardini et al. (1999) evaluated the accuracy of the functional method for hip joint center determination by comparing its results to those obtained using radiographs from which the center of the femoral head was located. The authors reported that the functional method was accurate to within 13 mm in healthy subjects, but subjects with limited range of motion at the hip were not considered. Several investigators have suggested that the functional method may not be useful for subjects who can manage only limited hip motion (Bell et al., 1990; Kirkwood et al., 1999; Seidel et al., 1995). Previous work in our laboratory (Piazza et al., 2001) has suggested that neither limiting hip range of motion nor restricting thigh motion to a single plane in an anthropomorphic mechanical linkage results in substantial hip joint center location errors. The robustness of the functional method has not yet been demonstrated, however, in human subjects whose hip motion is limited. Establishing that functional methods are effective when available hip motion is limited is necessary if these methods are to be employed as part of the clinical motion analysis of patients with muscle contractures or painful conditions such as arthritis that restrict hip motion. If it is the case that accurate location of the hip joint center is possible using hip motions that are small in magnitude or that are confined to a single plane, it may also be possible to find the hip joint center by applying the functional method to common motions such as walking. Such an application would obviate the need for a special trial in which the subject is required to circumduct the hip and perform uncomfortably large abduction–adduction and flexion–extension.

The purpose of the present study was to assess the accuracy of hip joint center location performed using motion trials in which the range of hip motion in healthy subjects was limited. Limitations in both magnitude and direction of hip motion were considered. Accuracy was also evaluated for hip joint centers calculated using motion data collected during a variety of activities to determine if any of these activities might make suitable substitutes for the specialized hip motion trials suggested in previous studies of the functional method (Bell et al., 1990; Leardini et al., 1999; Shea et al., 1997). Motion data were collected as subjects walked, ascended stairs, descended stairs, and a rose from a seated position. Groups of young and older subjects were considered to test for age-related differences in the range of subject-selected hip motion during motion trials.