Effects of prosthetic foot forefoot flexibility on gait of unilateral transtibial prosthesis users

INTRODUCTION Although a large number of prosthetic foot types are commercially available, clinically useful objective evidence to guide their prescription is scarce [1-3]. At the same time, the field of prosthetics is changing rapidly. Many new devices are introduced each year and many leave the market, making it is difficult for clinicians to choose the best possible components for their patients with amputations. This problem is certainly true of prosthetic ankle-foot systems. The rapid change in the available foot types supports the need for an improved core understanding of prosthetic foot properties and their effects on user function. Earlier studies have attempted to determine differences between groups of commercially available prosthetic feet to aid in understanding and prescribing these systems [4]. However, the feet in these groups normally have many mechanical differences, which makes linking a functional difference to a specific prosthetic foot feature difficult. Also, many of the feet that have been tested in the past are no longer commercially available or have been altered by their manufacturers through periodic design changes. A more controlled approach is needed to broaden our core understanding of prosthetic foot features and their effects on function of the prosthesis user. Prosthetic feet have many features, including energy storage and return, hysteresis, effective foot length (anterior moment arm), and general stiffness properties. Many of these properties are not independent--e.g., stiffness and hysteresis relate to the amount of energy storage and return possible with the foot. This interdependence of mechanical properties causes difficulty in understanding which properties lead to specific gait deviations when they occur. Controlled studies that attempt to change only one feature at a time are needed to help build a core knowledge of these features and their effects on amputee gait. One area in which a more controlled approach to research is needed is in the study of prosthetic ankle-foot flexibility and its effects on gait. Previous studies have reported decreased sound-limb loading in unilateral transtibial prosthesis users when they used the Flex-Foot, a J-shaped prosthetic foot with flexibility along its length from the socket to the end of the foot (Ossur; Reykjavik, Iceland). These studies also measured an increased "ankle" range of motion (ROM) on the prosthetic side when subjects used the Flex-Foot [5-7]. One may presume causality between these findings and conclude that prosthetic ankle-foot systems offering more flexibility will reduce sound-limb loading. However, this idea may be problematic if taken to extremes. For example, prosthetic feet that have extreme levels of forefoot flexibility may not allow adequate forward progression of the center of pressure of the ground reaction force (GRF) during late stance on the prosthetic side, thus decreasing the prosthetic foot's effective foot length. A person using a prosthetic foot with a short effective foot length may experience a reduced ankle moment on the prosthetic side and a "drop-off effect during load transfer from the prosthesis to the sound limb. Possible consequences of the drop-off effect include shorter step lengths on the sound side, increased vertical GRFs (VGRFs) on the sound limb during initial stance phase, and decreased VGRFs on the prosthetic limb during the end of stance phase. Hansen et al. altered the effective keel length of an experimental prosthetic foot and observed a drop-off effect when subjects used prosthetic feet with smaller effective foot lengths, including increased loading on the sound side [8]. Prosthetic feet that are overly flexible likely will not allow forward progression of the GRF under the foot, yielding similar results as found when the keel structure is shortened (Figure 1). The purpose of this study was to investigate the effects of forefoot flexibility on gait of unilateral transtibial prosthesis users using an experimental solid-ankle foot in which the forefoot flexibility feature could be easily altered without changing other properties of the feet. …