Developing a framework for an adaptive transtibial prosthetic socket using FEA-based tissue injury risk estimation and generalised predictive control

Abstract

To perform daily activities, people with amputation depend on the socket for stability and proprioceptive feedback for control over their prosthetic. Sockets are bespokely fitted, rarely definitive, and require iterative, expensive replacement to accommodate residual limb changes. The socket is the primary load-bearing interface and user comfort is greatly linked to the quality of the socket fit. Poorly fitting sockets cause pain, limb tissue injuries, limited device usage, and potential rejection. Contact stresses at the socket-limb interface and strain of underlying soft tissues greatly determine user comfort and the risk of residuum tissue injury. Adjustable socket technologies exist, but are passive or semi-passive, entrusting responsibility of determining safe interface pressure levels solely on the user’s perception. This research entails a set of theoretical studies developing a framework for an automatically adjustable prosthetic socket system enabling estimation of residuum tissue injury risk for safe interface pressure modulation, within a control system structure.

Candidate methods for functional interface actuation were identified, and their design specifications and theoretical models developed and described. A comparative Concept- Design Failure Mode and Effects Analysis was performed, considering the limitations of the different actuation options for the adaptive socket system. This revealed that the probability of detection of some potential design weaknesses largely determines overall failure risk criticality among the actuation options. Also, mitigation measures to address high scoring risks should consider users with compromised sensory perception of discomfort or injury.

A study was performed using finite element modelling, to determine the effect of local socket stiffness changes on tissue strain and interface pressure, and between select anatomical regions. Minimal changes in compressive strain (< 2%) indicated negligible cross-effects between regions, and appropriate application of an uncoupled controller configuration for the multiple interface actuators. Application of representative prosthetic loading instances allowed estimation of interface pressure-tissue strain relationships at the actuator locations. These were used as training data to create surrogate models for each location for tissue injury risk assessment within the socket system control framework.

Generalised Predictive Control (GPC) was simulated for active interface actuation within estimated safe and functional limits. Optimisation of a cost function to minimise tissue injury risk by adaptive interface pressure control showed adequate dynamic performance. Feasibility of the GPC formulation to satisfy operational requirements, and its influence on actuation performance of the different actuators for prosthetic device usage in several scenarios was demonstrated. This research provides a systematic development platform for designing an adjustable prosthetic socket integrating dynamic monitoring and minimisation of sub-dermal residuum tissue injury risk with active adaptation of the interface pressure.

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Identifiers

ORCID for Florence M. Mbithi: orcid.org/0000-0002-6103-7996

ORCID for Alexander Dickinson: orcid.org/0000-0002-9647-1944

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