The effects of a controlled energy storage and return prototype prosthetic foot on transtibial amputee ambulation

Abstract

The lack of functional ankle musculature in lower limb amputees contributes to the reduced prosthetic ankle push-off, compensations at other joints and more energetically costly gait commonly observed in comparison to non-amputees. A variety of energy storing and return prosthetic feet have been developed to address these issues but have not been shown to sufficiently improve amputee biomechanics and energetic cost, perhaps because the timing and magnitude of energy return is not controlled. The goal of this study was to examine how a prototype microprocessor-controlled prosthetic foot designed to store some of the energy during loading and return it during push-off affects amputee gait. Unilateral transtibial amputees wore the Controlled Energy Storage and Return prosthetic foot (CESR), a conventional foot (CONV), and their previously prescribed foot (PRES) in random order. Three-dimensional gait analysis and net oxygen consumption were collected as participants walked at constant speed. The CESR foot demonstrated increased energy storage during early stance, increased prosthetic foot peak push-off power and work, increased prosthetic limb center of mass (COM) push-off work and decreased intact limb COM collision work compared to CONV and PRES. The biological contribution of the positive COM work for CESR was reduced compared to CONV and PRES. However, the net metabolic cost for CESR did not change compared to CONV and increased compared to PRES, which may partially reflect the greater weight, lack of individualized size and stiffness and relatively less familiarity for CESR and CONV. Controlled energy storage and return enhanced prosthetic push-off, but requires further design modifications to improve amputee walking economy.

Introduction

One in 190 Americans is currently living with the loss of a limb and the incidence of amputation is on the rise (Ziegler-Graham, MacKenzie, Ephraim, Travison, & Brookmeyer, 2008). Two-thirds of these amputations are of the lower limb, which often leads to limitations in functional mobility, an array of co-morbidities of the intact and residual limb joints (Gailey et al., 2008, Struyf et al., 2009) and persistent pain and discomfort (Ehde et al., 2000). The absence of plantar-flexor musculature significantly affects amputee gait by reducing ankle push-off power of the prosthetic limb, likely leading to compensations at other joints and increased loading of the intact limb compared to non-amputees (Gitter et al., 1991, Seroussi et al., 1996, Winter and Sienko, 1988). These alterations in gait may subsequently contribute to the increased metabolic cost of amputee walking compared to non-amputee walking at the same speeds (Genin et al., 2008, Waters and Mulroy, 1999) as well as contribute to the reduced self-selected walking speed, cadence and stride length (Gitter et al., 1991, Robinson et al., 1977, Torburn et al., 1990, Waters et al., 1976, Winter and Sienko, 1988).

Energy storage and return (ESR) prosthetic feet were designed to address reduced prosthetic limb push-off work by returning mechanical energy absorbed from mid to late stance, rather than dissipating it through viscoelastic deformation as with non-ESR prosthetic feet. ESR feet result in amplified peak fore-aft ground reaction forces in the prosthetic limb (Lehmann et al., 1993, Powers et al., 1994), increased prosthetic foot power and work generated (Barr et al., 1992, Gitter et al., 1991) and decreased vertical ground reaction forces in the sound limb during weight acceptance (Lehmann et al., 1993, Perry and Shanfield, 1993, Powers et al., 1994, Snyder et al., 1995) compared to more traditional prosthetic feet (i.e., SACH foot). Subjectively, many amputees prefer ESR feet (Romo, 1999) and perceive an increase in activity, velocity and stability with less pain and skin problems than with non-ESR feet (Hafner, Sanders, Czerniecki, & Fergason, 2002). However, in several studies the metabolic cost of walking (Hafner et al., 2002, Lehmann et al., 1993, Torburn et al., 1990, Versluys et al., 2009, Waters et al., 1976), preferred walking speed and gait symmetry (from Barth et al., 1992, Grabowski et al., 2010, Hsu et al., 2006, Torburn et al., 1990) have not been shown to change significantly for amputees wearing ESR feet compared to more traditional designs. The general consensus is that despite subjective preference, current ESR feet demonstrate little objective improvement in overall walking performance (Hafner et al., 2002, van der Linde et al., 2004).

Increased prosthetic ankle push-off work may improve amputee gait by minimizing compensations at other joints (Au et al., 2009, Houdijk et al., 2009, Seroussi et al., 1996). Simple walking models suggest that much of the mechanical work required for ambulation is determined by the amount of energy dissipated in the transition between steps, as the center of mass (COM) is redirected by the new stance limb (Donelan et al., 2002, Kuo et al., 2005). The appropriate magnitude and timing of trailing leg push-off is thought to reduce the collision of the leading leg, which may lead to a decrease in total COM work (Kuo, 2002, Ruina et al., 2005). Traditional prosthetic feet cause a disruption in this cycle by reducing the positive work performed by the trailing prosthetic leg at push-off, disproportionately increasing dissipation in the leading intact leg and increasing metabolic energy cost of amputee walking (Houdijk et al., 2009). Therefore, a prosthesis with increased push-off work could potentially reduce this dissipation, reduce metabolic cost and improve amputee gait.

A Controlled Energy Storage and Return (CESR) prototype prosthetic foot (Collins & Kuo, 2010), which can capture and store some of the collision energy normally dissipated at foot contact and then transfer it to the forefoot just prior to toe-off, may increase prosthetic push-off work, reduce energy loss at collision of the intact leg and reduce metabolic cost of gait. In a study of non-amputees wearing a simulator boot, a CESR foot prototype increased both average ankle push-off power and COM push-off work, decreased intact limb COM collision work, and reduced metabolic cost by 9% compared to a conventional prosthetic foot (Collins & Kuo, 2010), suggesting the potential for similar benefits in amputees.

The aim of this study was to test for biomechanical and metabolic effects of the same CESR prototype foot on transtibial amputee gait compared to a common conventional foot and to each subject’s prescribed foot. We tested whether amputees wearing this CESR prototype would exhibit: (1) increased prosthetic foot push-off power and work, (2) increased prosthetic limb COM push-off work, (3) decreased intact limb COM collision work, (4) decreased total COM work, and (5) reduced energetic cost of walking.