Development of a biomechanical model of the interface between the residual limb and prosthesis for trans-femoral amputees
Prosthetic socket fitting is achieved by the prosthetist applying artisan techniques which are skill dependent and of subjective nature. This study investigates the use of finite element (FE) modelling techniques to predict the biomechanical behaviour at the residual limb/socket interface for the purpose of obtaining a quantitative evaluation of socket fit. Three dimensional FE models of the residual limb of trans-femoral amputees were generated based on geometrical data obtained using a mechanical digitizer and magnetic resonance (MR) imaging techniques. The inter-segmental loadings at the amputee's hip during standing and walking were applied to the FE models. These were measured with the aid of force platforms and infrared cameras. The material characteristic is introduced to the FE models were obtained by testing the residual limb's soft tissue with a computer controlled mechanical indentor. The FE models were validated by comparing predicted and measured pressures at the inter face between the residual limb and the socket. The majority of the FE prediction erred within 70% of the measured values. Detailed internal geometry of two trans-femoral amputees' residual limb in its natural shape and wearing quadrilateral and ischial containment type sockets was studied using MR imaging techniques. At the ischial level, the maximum difference in cross sectional area between the muscles of the sound limb and the residual limb was approximately 62%. The difference in muscles' size can be attributed to muscle atrophy in the residual limb or an increase in the muscle bulk in the sound limb. At similar level, the cross sectional area of the rectus femoris in the residual limb was reduced by as much as 68% from its natural shape when wearing the quadrilateral socket. Based on the acquired MR images,a two dimensional FE model of a transverse section 30 mm below the ischium was modelled. The model incorporated the interface characteristics between the muscles and intermuscular tissues. The maximum stress was recorded inside the residual limb near muscles/intermuscular tissue interface and at muscles/bone interface. The FE models generated have shown the potential of predicting stresses and deformation at the residual limb.