Metabolic cost and mechanical work during walking after tibiotalar arthrodesis and the influence of footwear

Abstract

Background

This study examined metabolic energy cost and external mechanical work for step-to-step transitions after tibiotalar arthrodesis, and the effect of MBT rocker bottom shoes.

Methods

Oxygen uptake, forceplate and kinematic data were recorded in 18 controls and 15 patients while walking at a fixed speed of 1.25 m/s in three walking conditions: barefoot, normal walking shoes and MBT rocker bottom shoes. Metabolic energy cost, external mechanical work, and the roll-over shape of the ankle–foot complex were analyzed.

Findings

Tibiotalar arthrodesis leads to higher metabolic energy cost during walking. During step-to-step transitions positive work during push-off with the impaired ankle was decreased but negative work during collision was not affected. The roll-over shape of the ankle–foot complex did not differ between groups and shoe conditions. However, both in patients and controls rocker bottom shoes did lead to decreased positive work at push-off and increased negative work at collision and consequently higher metabolic energy cost of walking.

Interpretation

External mechanical work for step-to-step transitions is not different between patients and controls and could not account for the higher metabolic energy cost in patients. Apparently, patients adopt a different walking strategy that limits step-to-step transition cost but nevertheless induces a higher metabolic energy cost. Despite restricted ankle movement, patients retain a normal roll-over shape of the ankle–foot complex. MBT shoes do not affect roll-over shape and appear to have a counterproductive effect on step-to-step transition cost and walking economy.

Introduction

An ankle arthrodesis is a surgically achieved bony fusion of the tibiotalar joint, immobilizing the joint. Previous research has shown that immobilization of the tibiotalar joint alters the gait pattern (Wu et al., 2000, Beyaert et al., 2004). Patients adopt a walking speed that is 84% of normal walking (Waters et al., 1988) and stance time on the impaired side is reduced with 2.7% compared to the contralateral side (Beyaert et al., 2004).

Normalized for walking speed, oxygen consumption of patients with an ankle arthrodesis is suggested to increase 10% compared to healthy subjects (Waters et al., 1988). The reason for this increase has not been established until now. Recent insights predict that the increase in the energy cost of walking could be a direct consequence of impaired ankle function. Based on the double inverted pendulum model of walking, Kuo (2002) demonstrated that the energy cost of walking is for a substantial part determined by the amount of work needed for step-to-step transitions. Step-to-step transitions involve the redirection of the velocity of the bodies' center of mass (COM) between steps. At heel-strike of the leading leg the velocity of the COM is directed downward and needs to be redirected upward. The redirection of COM velocity goes at the expense of mechanical energy: at heel-strike energy is dissipated and consequently positive work is required to restore the energy loss to be able to transit to the next step (Fig. 1). Theoretically, energy loss at heel-strike is minimized when positive work is generated just before or at heel-strike through a powerful plantar flexion in the trailing limb (Donelan et al., 2002a, Donelan et al., 2002b). In patients with an immobilized tibiotalar joint powerful plantar flexion is limited and consequently positive work needs to be generated elsewhere, presumably around the hip during midstance, which is less efficient (Donelan et al., 2002a). Previous research on patients after lower limb amputation (Houdijk et al., 2009a) and total ankle arthroplasty (Doets et al., 2009) has shown that lack of plantar flexion power indeed leads to increased collision and concomitantly increased mechanical work needed for step-to-step transitions.

Apart from the timing and origin of positive work generation, the amount of work needed for step-to-step transitions is also influenced by the roll-over shape of the ankle-foot complex. Adamczyk et al. (2006) found that in healthy people with an immobilized ankle (by a cast) step-to-step transition cost was mediated by the rocker shape of the foot. The metabolic energy cost of walking increased when the radius of the rocker deviated from 0.3 times leg length. A decreased range of motion (ROM) after ankle arthrodesis not only reduces power output at the ankle, but probably also deteriorates the roll-over shape of the ankle–foot complex. Restoring the roll-over shape of the ankle–foot complex in patients after tibiotalar arthrodesis, using special footwear, might therefore reduce energy loss at heel-strike despite reduced plantar flexion power at the ankle. This could result in a decreased metabolic energy cost.

The purpose of the present study was first to examine the effects of impaired ankle function due to ankle arthrodesis on metabolic energy cost of level walking. Second, to examine whether ankle arthrodesis leads to increased external mechanical work needed for step-to-step transitions as predicted by the double inverted pendulum model. Finally, to study the effect of rocker bottom shoes on external mechanical work and metabolic energy cost in patients with an ankle arthrodesis.