Prosthetic Knee Stability during the Push-off Phase of Walking - Experimental Findings

Most of the energy needed for ambulation is generated during the double support phase of walking. Knee flexion during push-off is crucial to maintain the walking velocity. Since users of an above-knee prosthesis have to stabilize the knee with the hip muscles, and regular knee mechanisms are not stable during flexion, this may cost a large effort. This paper deals with experimental findings of walking with different types of knees. The results indicate that for walking with a more stable knee, the symmetry increases and the net hip moments of force, required to stabilize the knee, reduce. The mechanical work however, performed at the hip joint at the prosthetic side, remains about equal. INTRODUCTION With the present Itnee-hinges, there is no or only a very limited possibility to make a stable knee flexion during the stance phase. None: of the knee mechanisms allow for a functional knee flexion during push-off. Since no muscles cross the knee in an above knee prosthesis, the prosthetic user has to stabilize the knee with the hip musculature, which may cost a large effort. The knee flexion during push-off is crucial to maintain the walking velocit-y. The knee instability is one of the reasons that users of an above-knee prosthesis in general walk at a lower speed and with a smaller time duration of the prosthetic stance phase [ 11. Numerical simulations of walking revealed that an implicitly more stable construction of the knee would yield a larger walking velocity and an increased symmetry. This resulted in the design of the UTKnee, a four-bar knee mechanism with an inverted (as compared to the regular knees) trajectory of rnomentary points of rotation (figure 1). The purpose of this paper is to compare the effect of different knee designs on prosthetic gait, especially the effect on the work performed at the hip joint to stabilize the knees. Figure I : Protorype of the UTKnee, posterior view. METHODS Three different knees were tested on a single healthy subject (male, 26 years, 83 kg, 1.83 m) with an above-knee prosthesis. Analysis of the kinematic and dynamic gait parameters has been done by measurements with a Vicon video system. An average cycle was calculated in each session from 10 walking trials at a comfortable speed. In each session, a different type of knee was fitted in the prosthesis. With the exception of the knee, the prosthesis remained identical. Three knee-types were used: A single axis knee with an adaptable (in anterior-posterior direction) center of rotation, a regular four-axial knee which the subject normally used and a UTKnee with adaptable polar curves varying from setting 1 (largest polar curve) to setting 4 (smallest polar curve). The average cycle for each session was analyzed using an inverse dynamics method and a segments model containing 8 segments: Pelvis, upper legs, lower legs, feet and a HAT segment for head, arms and trunk [ 2 ] . RESULTS One of the most significant parameters is the push-off time with the prosthetic leg. Normally this double support time is shorter for the prosthetic leg than for the sound leg. This is of the subject (figure 2). also the case for the measurement with the normal prosthesis 0-7803-3811-1/97/$10.00 WEEE 471 18th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Amsterdam 1996 2.6.1: Locomotion