The biomechanical effect of different spring extension assist mechanisms in mechanical 4-bar polycentric prosthetic knees for unilateral above/through-knee amputees.

Background: Passive prosthetic knees incorporate extension assist mechanisms to enhance the swing phase of the gait cycle for persons with above/through-knee amputations. In conventional polycentric knees using a 4-bar linkage, the extension assist mechanism connects one of the bars to the body of the prosthesis. However, this design introduces a singularity in the spring action delivery, causing the spring to push the knee into flexion beyond a certain angle. In contrast, polycentric prosthetic knees with an extension assist mechanism that links the upper and lower parts of the knee eliminate the singularity feature, ensuring that the knee is pushed toward extension throughout the full range of motion.

Objective: This study aims to investigate the effects of different spring extension assist mechanisms in polycentric knees on the walking capacity of unilateral above/through-knee amputees.

Study design: Repeated measures.

Methods: Gait data were collected at self-selected and fast speeds from 8 unilateral above/through-knee traumatic amputees using a 10-camera motion capture system. Participants trialed 2 different polycentric prosthetic knees with and without the singularity feature, both coupled with a solid-ankle-cushioned-heel prosthetic foot. They underwent gait analysis with both prosthetic knees, and a comparison analysis was conducted to examine the trial conditions.

Results: The symmetry between the prosthetic and intact knee flexion angles was higher for the design without singularity by 5.2% (p = 0.025) at self-selected speeds and by 7.7% (p = 0.003) at fast speeds. Also, peak prosthetic knee flexion angles were lower for the design without singularity by 9.4° (p < 0.001) at self-selected speeds and by 9.6° (p = 0.012) at fast speeds.

Conclusions: The extension assist mechanism without singularity improved the symmetry between the intact and prosthetic knee flexion angles by preventing excessive prosthetic knee flexion in the swing phase of the gait cycle.