[Functional range of motion of the hip joint]

The functional mobility of a joint represents the range of motion healthy individuals require to fulfill everyday life tasks. Oscillation angle corresponds to the entire range of motion that can be achieved by the joint. Wedge opening and direction are the characteristic features. We describe the characteristics of functional mobility of the hip joint in healthy subjects.

Hip motion was analyzed in twelve healthy subjects aged 22 to 25 years. The three dimensional analysis used the Motion Analysis System (Motion Analysis Corporation, Santa Rosa, CA) at a frequency of 60 Hz. MatLab software was used to modelize a prosthesis and determine the oscillation angle and its direction as a function of implant position and head-to-neck ratio. After determining the hip center for each individual subject, the range of motion necessary to complete a task was given by the maximal angle along each anatomic axis needed to reach a given position in comparison with the resting position. The following tasks were studied: sit to stand motion, lifting weight from a squatting position, reaching the ground with both legs abducted in extension, walking, ascending and descending stairs, getting on a bicycle, sitting cross-legged, cutting toenails. Whether or not the task could be achieved with the prosthetic conformation was then determined.

Each task was described as a combination of motion in the three anatomic axes. Lifting weight from a squatting position combined flexion (110 degrees), abduction (9 degrees) and external rotation (18 degrees) with a standard deviation of 9 degrees. For a given task, only a few combinations of femoral and acetabular orientations were compatible with completion of that task. Combining the motions required for several tasks diminished the possible orientations for prosthetic positioning.

Analyzing the motion required for these tasks shows the maximal range of motion involved in each direction. There was very little variability among healthy subjects. These results are in agreement with other values determined with other methods. Compensatory mechanisms used by disabled people to complete different tasks were not taken into consideration. The effects of changing either the head-to-neck ratio or implant position are discussed in relation to completion of a given task.