Quantification of the Influence of Prosthetic Ankle Stiffness on Static Balance Using Lower Limb Prosthetic Simulators

Abstract

After a transtibial amputation, the prosthetic foot aims at replacing the missing ankle joint. Due to alteration of proprioception and mobility, the static balance of amputees is challenging. The stiffness of most of the usual prosthetic feet cannot adapt according to the situation. Thus, the control of the user’s balance is closely related to the ankle stiffness value. The aim of this study is to evaluate both the impact of the ankle stiffness and the visual system on static balance. In order to avoid bias relative to different levels of residual proprioception among individuals, the study has been carried out on healthy subjects wearing lower limb prosthetic simulators under each foot. This configuration could be considered as a relevant model to isolate the effect of the stiffness. Eleven subjects wearing prosthetic feet with different modules were asked to remain as static as possible both with open eyes (OE) and closed eyes (CE). The center of pressure (COP) displacements and the joint angles range of motion (ROM) were experimentally assessed. The length of the major axis of the COP 95% confidence ellipse was projected on the antero-posterior direction (AP range). Linear regression models of the AP range and joint angles ROM as a function of the situation (OE and CE) and of the normalized ankle stiffness were created. A one-way analysis of variance test was performed on the model of the AP range. Linear regression coefficients and 95% confidence intervals (CI) were calculated between the AP range and the normalized ankle stiffness and between the joint angles ROM and the normalized ankle stiffness both in OE and CE. This study confirmed that static balance decreases when ankle stiffness decreases. The results also showed that a visual system alteration amplifies more significantly the decrease of static balance of people wearing prosthetic feet and has no significant influence on non-amputated subjects. The slope of the linear regression for the AP range according to the normalized ankle stiffness was equal to −9.86 (CI: −16.03, −3.69) with CE and −2.39 (CI: −4.94, 0.17) with OE. Both the normalized ankle stiffness and the visual system had a significant impact on the AP range (pvalue<0.05). The ankle stiffness is an interesting parameter as it has a high impact on the gait and on the static balance of the users and it must be controlled to properly design prosthetic feet.

Transfemoral prosthetic simulators versus amputees: ground reaction forces and spatio-temporal parameters in gait

“…The first purpose is to use it as a tool in the research and development of prosthetic components [7,9]. For example, TFSims have been used in research to evaluate design parameters in transfemoral prostheses, such as the effects of recognition error on volitional control as well as safe and confident use of prostheses [11], knee flexion strategy [10], ankle stiffness [6] and durability of prosthetic components [12]. In addition, TFSims have been used in the development of transfemoral prosthetic components [13][14][15][16] and their myoelectric control mechanisms [17,18].…”

“…Clinicians and engineers have been developing not only prostheses for individuals with amputation but also prosthetic simulators for non-disabled individuals. The prosthetic simulators are expected to assist in understanding the gait of individuals with amputation and in facilitating the research and development of prosthetic components, particularly in early-stage prototyping [ 5 , 6 ]. The transfemoral prosthetic simulator (TFSim, also referred to as a bypass device/prosthesis [ 7 ]) is a prosthetic device that is fitted on one of the lower limbs of a non-disabled individual to temporarily simulate the gait of a person with unilateral TFAmp (electronic supplementary material, video S1).…”