Compensatory mechanism involving the knee joint of the intact limb during gait in unilateral below-knee amputees

Abstract

This study evaluated the asymmetry of knee kinetics during uncomfortable gait induced by prosthesis misalignment to further demonstrate the compensatory function of the knee joint of the intact limb during gait. Three-dimensional gait analysis including knee kinematics and kinetics at the beginning of stance phase was conducted in 15 healthy subjects and 17 unilateral trans-tibial amputees (TTA) walking at self-selected speed in three conditions of prosthetic alignment: initial alignment (IA); initial alignment altered either by 6° of internal rotation (IR) or by 6° of external rotation (ER) applied on the pylon. Patients reported best comfort of gait in IA condition and discomfort mainly in IR condition. Maximum knee flexion and knee total work at power phases K0–K2 were significantly higher in intact limbs compared to prosthetic and control limbs. In intact limbs, these variables had significantly higher values (+10–35%, p < 0.05) in IR condition than IA condition whereas these were not altered across conditions in prosthetic limbs. In trans-tibial amputees, inducing uncomfortable gait by internally rotating the prosthetic foot did not alter the knee kinetics of the prosthetic limb, which suggests a protective mechanism. Knee kinetics of the intact limb did alter, which suggests a compensatory mechanism.

Introduction

Unilateral trans-tibial amputees (TTA) are asymmetrical in their gait. They spend less time in stance on their prosthetic limb and more time on their intact limb [1], [2], [3]. They load their prosthetic limb less than their intact limb [1], [4], [5], [6], [7] and their intact limb more than able-bodied persons during natural cadence walking [8]. This reduction in both the load intensity and duration of the prosthetic limb during gait may be related to a protective motor strategy [9], [10] since the soft tissues of the amputated limb are not suitable for load-bearing [11]. At the beginning of stance, the knee extensor moment and related work during the power phases K1 and K2, which contribute to stability and shock absorption of the weight accepting limb [12], [13], are reduced in the prosthetic limb [1], [9], [14] and increased in the intact limb [15] compared to able-bodied persons. These knee kinetics patterns suggest a protective mechanism for the prosthetic limb. However, they also suggest a compensatory mechanism involving the intact limb and particularly the knee joint [15]. A compensatory mechanism can be demonstrated if the related deviations are further increased in response to more constraining conditions. Increased loading of the intact limb in response to higher walking speed in TTA further supported the presence of a compensatory mechanism [10]. However, loading asymmetry did not increase significantly after inducing a 10° of frontal or sagittal plane prosthetic misalignment in TTA [5]. Walking with prosthetic misalignment may be less demanding on lower limb dynamics than walking at high speed. Knee kinetics has not been tested in response to any prosthetic misalignment and may be more sensitive than the loading pattern.

In this study we assessed knee dynamic variables to demonstrate the compensatory function of the knee joint of the intact limb in response to prosthetic transverse plane misalignment. The foot progression angle, which is defined as the angle between the line of progression and the longitudinal axis of the foot, has been reported to influence knee dynamics during gait [16]. Therefore, the prosthetic foot transverse plane alignment was altered to induce ipsilateral discomfort. We hypothesized that uncomfortable gait induced by alteration of the prosthetic foot progression angle would be associated to an increase in knee total work at the beginning of stance phase, not on the prosthetic limb but on the intact limb, suggesting protective and compensatory mechanisms, respectively.