Effects of Prosthetic Foot Rocker Radius
on Gait of Prosthesis Users

Andrew Hansen, PhD and Steven Gard, PhD, Principal Investigators
Elizabeth Klodd, BS, Project Director

Co-Investigator:
Mark Edwards, CP, MHPE, Northwestern University Prosthetics-Orthotics Center (NUPOC)

Funded by: Department of Veterans Affairs Rehabilitation Research and Development, Merit Review Grant

Purpose

The purpose of this project is to determine if prosthetic feet made to mimic the able-bodied ankle-foot roll-over shape will provide any biomechanical or energetic advantage to their users during walking. 

Background

Prosthetic feet conform to effective rocker shapes, or roll-over shapes, during walking by bending under loads imposed by their users (see Figure 1).  The radius of curvature (R) that a prosthetic foot conforms to is a function of the user’s weight, their walking speed, and the material properties of the prosthetic foot.  Under similar loading conditions, a “soft” foot will conform to a smaller radius and a “stiff” foot will conform to a larger radius (see Figure 2).  Characteristics of the roll-over shape including the radius of curvature and the arc length can be used to quantify differences in prosthetic foot design. 

Previous studies comparing prosthetic foot types have most commonly examined ankle flexion during walking.  However, most prosthetic feet do not have articulations at the ankle.  The measured ankle flexion in these cases reflects a general degree of bending in the feet but is also dependent on placement of gait analysis markers on top of the foot.  However, roll-over shape incorporates the net effect of bending of joints and material deformations occurring between the rigid attachment surface of prosthetic ankle-foot components and the floor, providing an effective rocker shape (see Figure 1). 

The roll-over shapes of able-bodied ankle-foot systems can be measured and used for comparison with roll-over shapes of prosthetic ankle-foot devices.  Our lab has measured the roll-over shape of the able-bodied ankle-foot system under various conditions of level ground walking including a wide range of speeds, while carrying added loads, and when walking with shoes of different heel heights (Hansen et al, 2004; Hansen and Childress, 2004; Hansen and Childress, 2005).  In general, our group found that able-bodied persons adapt under conditions of level ground walking to maintain a roll-over shape with a radius approximately equal to 35% of their leg length.  This radius is between the values predicted by researchers studying walking with physical models (McGeer, 1990) and mathematical models (Gard and Childress, 2001).

Figure 1: Drawing illustrating the roll-over shape of a prosthetic ankle-foot system.  Typically, a prosthetic limb is made up of a prosthetic socket and a prosthetic foot connected by a pylon.  The roll-over shape is the effective rocker (or cam) shape that the prosthetic ankle-foot (and shoe) conforms to during walking.  The proposed work examines the effect of the roll-over shape radius (R) on oxygen cost and joint kinematics and kinetics during walking.

Figure 2: Drawings of three Shape&Roll Prosthetic Feet (S&R PF) with different radii are shown. For the proposed experiment, five S&R PF will be made with different numbers of cuts and placements of cuts, yielding five prosthetic feet with radii of 15%, 25%, 35%, 45%, and 55% of the user’s leg length.

Methods

We will measure the oxygen cost of users while walking with five prosthetic feet having different roll-over shape radii. Each participant will walk with the five different feet on a treadmill at the same self-selected speed.  We will also perform gait analyses on the subjects walking with the five different feet.  Body center of mass movements, joint kinematics, joint kinetics, ground reaction forces, and temporal-spatial parameters of gait will be analyzed to document changes that occur when using feet with different radii.

The proposed work takes a step toward determining if roll-over shape can be used as a prescription tool for prosthetic ankle-foot mechanisms.  Roll-over shape arc length has already been shown to affect loading on the sound limb (Hansen et al., 2006).  Future work will need to examine other factors important for prescription of prosthetic ankle-foot systems such as stiffness profiles of feet in producing their roll-over shapes and torsional properties of prostheses during movement.  Roll-over shapes of various prosthetic foot designs can be quickly and easily measured in a laboratory setting and then shared with clinicians, making their future use in prescription feasible.

References

Gard, S.A. and Childress, D.S. (2001). What determines the vertical displacement of the body during normal walking? J Prosthet Orthot, 13: 64-67.

Hansen, A. and Childress, D. (2004) Effects of Shoe Heel Height on Biologic Roll-over Characteristics During Walking. Journal of Rehabilitation Research and Development, Vol. 41, No. 4, 547-554.

Hansen, A. and Childress, D. (2005) Effects of Adding Weight to the Torso on Roll-over Characteristics of Walking. Journal of Rehabilitation Research and Development, Vol. 42, No. 3, 381-390.

Hansen, A. H., Childress, D. S., and Knox, E. H. (2004a). Roll-over shapes of human locomotor systems:effects of walking speed. Clin Biomech 19(4), 407-14.

Hansen, A., Meier, M., Sessoms, P., and Childress, D. (2006) The Effects of Prosthetic Foot Roll-over Shape Arc Length on the Gait of Trans-tibial Prosthesis Users. Proshetics and Orthotics International, Vol. 30, No. 3, 286-299.

McGeer, T. (1990). Passive Dynamic Walking. International Journal of Robotics Research, 9: 62-82.