MODELING CRUTCH COMPENSATION OF HIP ABDUCTOR WEAKNESS AND PARALYSIS

Abstract

Hip abductor weakness or paralysis is prevalent in the half a million cases of low level spinal injuries in the United States alone. Crutches are often used as an ambulatory aid by individuals with this type of permanent disability. This study investigates whether using a crutch with a wide stance, as opposed to a conventional vertical stance, returns the hip rotation and pelvic obliquity to a more normal range of motion for individuals with weak or paralyzed hip abductors.

An inverse dynamics six link model of the body with ten degrees of freedom and a forward dynamics six link model with six degrees of freedom were used to simulate the swing and stance phases of gait with hip abductor weakness/paralysis while using either compensatory motions (hip hiking or lateral displacement of the torso) or crutches. The forward dynamics model characterizes the effect of hip abductor weakness on the gait kinematics hip rotation and pelvic obliquity. The model also characterizes the effect of compensatory motions and crutch use on gait with paralyzed hip abductors. The inverse dynamics model calculates the time varying body weight that must be supported on a contralateral crutch to achieve normal gait kinematics even with paralyzed hip abductors.

The forward dynamics model predicts that hip abductor paralysis reduces the range of pelvic obliquity and increases the range of hip rotation. The model also predicts that compensatory motions and crutch use restore the range of motion of hip rotation and pelvic obliquity in gait with paralyzed hip abductors to more normal. The inverse dynamics model predicts that the portion of body weight that must be supported on a crutch for normal gait kinematics with paralyzed hip abductors is lowered by using a wide crutch stance.

This study suggests that contralateral crutch use replaces the need for the compensatory motions hip hiking and lateral displacement of the torso while restoring the range of hip rotation and pelvic obliquity to more normal ranges in an individual with weak or paralyzed hip abductors. Furthermore, angling the crutch side-to-side restores the range while supporting less body weight on a contralateral crutch.