Comparison of computational analysis with clinical measurement of stresses on below-knee residual limb in a prosthetic socket

Abstract

Interface pressures and shear stresses between a below-knee residual limb and prosthetic socket predicted using finite element analyses were compared with experimental measurements. A three-dimensional nonlinear finite element model, based on actual residual geometry and incorporating PTB socket rectification and interfacial friction/slip conditions, was developed to predict the stress distribution. A system for measuring pressures and bi-axial shear stresses was used to measure the stresses in the PTB socket of a trans-tibial amputee. The FE-predicted results indicated that the peak pressure of 226 kPa occurred at the patellar tendon area and the peak shear stress of 50 kPa at the anterolateral tibia area. Quantitatively, FE-predicted pressures were 11%, on average, lower than those measured by triaxial transducers placed at all the measurement sites. Because friction/slip conditions between the residual limb and socket liner were taken into consideration by using interface elements in the FE model, the directions and magnitudes of shear stresses match well between the FE prediction and clinical measurements. The results suggest that the nonlinear mechanical properties of soft tissues and dynamic effects during gait should be addressed in future work.

Introduction

A prosthetic socket is the primary interface between an amputated limb and its prosthetic socket. Knowledge of the biomechanics of load transfer at this interface would enable objective evaluation of prosthetic fit, and might advance socket design. The load distribution on the residual limb is an important consideration in prosthetic design.

During the last 30 years, many studies have been conducted in attempts to understand the load transfer between residual limbs and prosthetic sockets. Major clinical investigations have been undertaken to measure pressures [1], [2], [3], [4] and shear stresses [5], [6]. These experimental approaches can assess the pressures and shear stresses on the skin surface at specific sites. However, because of the complexity of the situation, it is not easy to understand socket interface mechanics by means of experimental measurements alone.

As a complement to clinical measurements, computational modelling based on finite element (FE) analysis has been identified as a potential method for prediction and evaluation of the load transfer between the residual limb and a socket [7]. The advantages of FE analysis is that it allows an examination of the stresses in the entire residual limb including the surface and internal tissues, to predict the load transfer prior to socket fabrication and to enable a full parametric analysis. In theory, FE analyses can give an approximation to any complicated engineering problem, while their accuracy depends on model establishment, simplifications and assumptions. In the last decade, FE models have been developed for the residual limb and prosthetic socket of both above-knee (AK) [8], [9], [10] and below-knee (BK) amputees [11], [12], [13], [14], [15], [16]. These models have attempted to find ways to better simulate the real situation, each approach with different emphases based on certain assumptions and simplifications.

Any computerized model requires validation by experimental verification. To validate the FE models for the residual limb and prosthetic socket in general, pressures obtained from FE analyses have been compared with those measured clinically [9], [11]. Because of the difficulties in measuring shear stress, there is little reported in the literature on comparison of shear stress, in spite of its importance. Sanders and Daly [14] compared pressures and shear stresses obtained from a linear FE analysis with those from experimental measurements. They found differences both in parameter values and predicted errors. These they believed resulted from (i) a tremendously simplified representation of the actual residual limb and prosthetic socket in their FE models (particularly the friction/slip condition), (ii) nonlinearities of the material properties of soft tissues and (iii) pre-stresses due to donning into a rectified socket.

In this present study, a 3D nonlinear FE model has been developed using real limb geometry and considering the friction/slip boundary conditions and socket rectifications. A triaxial transducer system has also been developed to measure pressures and shear stresses between the residual limb and the socket. The aim of this paper is to compare the stress results predicted by FE analyses with those measured clinically, in order to assess the advantages and limitations of our FE model.