Finite Element Simulation of Frontal Plane Adaptation Using Full-Foot, Split-Toe, and Cam-Linkage Designs in Prosthetic Feet
Authors: Maitland Murray E. , Allyn Katheryn J. , Ficanha Evandro M. , Colvin James M. , Wernke Matthew M. 
Published: March 2021

ABSTRACT

Introduction
People with lower-limb amputation have problems with balance, falls, residual limb symptoms, functional mobility, and the need for cognitive attention during gait. Insufficient prosthetic foot frontal plane adaptability may be partly responsible. Theoretically, prosthetic foot design should allow smooth forward motion of the body mass with minimal correction despite initial ground-foot contact geometry. The purpose of this study is to determine if a cam-linkage articulation, designed for frontal plane prosthetic foot adaptability, improves forward motion on cross-slopes compared with single (full-foot) or double (split-toe) cantilever spring designs, using finite element simulation.

Materials and Methods
Model construction (material data, geometry, and mesh) and simulations were performed using Ansys LS-Dyna. The cam-linkage mechanism was based on a crossed four-bar linkage mechanism to provide frictionless frontal plane rotation. The cam-linkage was placed in series with a full-foot or a split-toe prosthetic foot and either locked or unlocked during stance phase of simulated gait on level surfaces and 15-degree cross-slopes. The simulation was initiated with foot flat and a proximal mass having a forward velocity of 1 m/s. Rotations and translations were calculated for the proximal mass and mediolateral contact forces at the proximal pylon–body mass connection and were calculated to determine the theoretical effect of the frictionless cam-linkage prosthetic foot on the socket interface. Primary outcomes were lateral deviation, vertical displacement, frontal plane rotation, change in forward velocity (percentage of initial) of the body mass, and maximum and average forces at the pylon–body mass connection.

Results
During the stance phase of gait on the 15-degree cross-slope, with the cam-linkage locked, the split-toe reduced lateral displacement compared with the full-foot prosthetic foot by 33%. The frictionless cam-linkage decreased lateral displacement by 67% for the full-foot and 50% for the split-toe variations. Vertical displacement and forward velocity were increased by the split-toe locked variation (7.4 mm and 3%, respectively) and also increased with the cam-linkage (7.6 mm and 1.5%, respectively). Frontal plane rotation was reduced with full-foot, frictionless cam-linkage compared with other variations. The frictionless linkage reduced average mediolateral forces at the pylon–body mass connection by approximately one-third compared with a locked linkage with either a full-foot or split-toe variation.

Conclusions
This finite element simulation study shows that a frictionless cam-linkage with frontal plane adaptability integrated into a prosthetic foot improved mediolateral forces, displacement, and frontal plane angular rotation of the proximal pylon compared with a locked linkage during the stance phase of gait when in contact with a 15-degree cross-slope. These findings may have implications for people using lower-limb prostheses if future studies demonstrate improved gait over uneven ground, or reduced pain, skin breakdown, and the work of walking.