The Effects of Polycentric Knee Design and Alignment on Swing Phase Gait Parameters: A Simulation Approach
Authors: Sudeesh S. , Sujatha S. , Shunmugam M.S. 
Published: August 2021

ABSTRACT

Introduction
Typically, polycentric prosthetic knees have been designed with a focus on improved performance during stance. This study analyzes the influence of geometric aspects such as linkage dimensions of the polycentric knees and anterior-posterior (A/P) alignment on the swing phase of walking. Such a study would enable the development of polycentric knees with improved performance during both stance and swing phases.

Methods
A simulation-based approach was used to analyze the influence of geometric aspects on swing dynamics. A newly developed four-bar knee called IITM polycentric knee (IPK) was used as a baseline prosthesis. First, the swing was simulated with the baseline prosthesis. The IPK in the lower limb was then replaced with the linkage dimensions of four other commercial polycentric knees (Ottobock's 3R36, 3R55, and 3R70 and Ohio Willow Wood's Pendulum knee) for the simulation of swing phase. Effect of A/P alignment was analyzed by translating the socket adapter of the IPK along the A/P axis in the baseline prosthesis, which has the effect of moving the knee and foot in the opposite direction. A maximum of 20 mm translation of the socket adapter was applied on either side of its base location in the lower-limb model. The gait parameters from the simulations and the centrode of different polycentric knees were then analyzed to understand how linkage dimensions and A/P translations of the knee and foot affect the swing dynamics.

Results
All four-bar knees without extension assists extended to less than 6° before heel contact, which is significantly lower than the 24° obtained for a single-axis knee in a previous study. The swing performances of the different four-bar knees studied here were found to depend on their centrode. The instant centers (ICs) of the 3R55 and IPK, which were comparatively located more superior to the anatomical knee center during swing, enabled better knee extension and foot clearance than other knees. A/P translation of the knee and foot significantly affected the foot clearance. A 20 mm anterior translation increased toe and heel clearance by 1.6 cm and 1.4 cm, respectively, when compared with a 20 mm posterior translation.

Conclusions
This study shows that polycentric knees, which generally perform better than single-axis knees with improved stance stability and ease of push-off, can also perform well during swing in terms of knee extension before heel contact and foot clearance throughout swing. The extended moment arm for the weight of the shank and foot due to the superior location of the IC in polycentric knees is the reason for the improved swing performance of polycentric knees. Appropriate placement of the socket adapter, which affects A/P alignment, is crucial so that foot clearance during swing is not compromised. The insights obtained from the study would be useful in developing simpler polycentric knees, which can extend completely without any extension assist if proper alignment is used as recommended.

Clinical Relevance
The study provides better understanding of the effects of polycentric knee placement and the need for an extension assist during the swing phase of walking, which could help clinicians improve the selection of appropriate knees and their alignment for prosthesis users.