3D Modeling And FEA At Lower Residual Limb/Prosthetic Socket Frictional Interface

The prosthetic is the main means for individuals with lower-limb to restoring activity ability and appearance. When patients wear prosthetic, the contact interface between the residual limb and prosthetic socket is the only channel to load transfer between the amputee stumps and prosthetic. The reasonableness of its design is directly related to the adaptation of the prosthesis and the security and comfort degree of patients. Therefore, the study on contact stress state and tribological behavior of the interface between the residual limb and prosthetic socket have important theoretical and practical significance for prosthesis interface optimization designing and improving the quality of the lives of people with disabilities.Making lower residual limb/prosthetic socket system as the research object, based on the CT tomography data, using the reverse technology and finite element method, the three-dimensional finite element model of the bone, soft tissue, and prosthesis were established. The stress、 slippage and the maximum critical parameters of the residual limb skin in a gait cycle were calculated by applying the load of five typical phases:Heel Strike, Foot Flat, Mid Stance, Heel Off and Toe Off respectively. By using a UMT-II tribometer to simulate lower residual limb/prosthetic socket interface, in vivo tribological behavior of residual limb skin was investigated under the critical parameters in the gait cycle. Integrating the finite element analysis and tribological experiment, the main work and conclusions are as follows:1. The contact interface between residual limb and prosthetic socket mainly transmits normal stress and shear stress, and the shear stress is caused by friction. The residual limb is supported by normal pressure and friction force collectively. The existence of friction and the interface slip will cause damage to the residual limb skin. However, the normal pressure increases with the friction decreasing, which result the more severe damage to soft tissue. So the friction is controlled in a reasonable value is the key to design the functional and comfortable prosthetic socket.2. Based on the data of CT scanning, using reverse engineering modeling method, the geometric model of residual limb/prosthetic socket system was reconstructed. The model can capture the real bone or soft tissue on the geometry morphology and anatomy, so the superiority of the technology route in biological tissue modeling can be confirmed.3. The finite analysis results show that the maximum normal stress and shear stress in the five typical phases all occurred at the position of patellar tendon. Patellar ligament, muscle group, the lateral tibia, the media tibia were the main stress points of the residual limb, the curves of the stress of each part in athe gait cycle present a bimodal butterfly. Heel Off is the most unfavorable phase in the gait cycle. The maximum normal stress of it was384.3KPa and shear stress was102.1kPa. With the normal pressure decreasing, the friction force reduced and the friction coefficient increased. In the high stress zone, such as patellar ligament, static friction was dominant, while in the low stress area, dynamical friction appeared on the interface, and39.5kPa was the critical pressure value to cause relative sliding on the contact interface. The friction coefficient was maximum at the stress points just began to slip, which induced the most serious friction damage to the skin. The finite element results show that the maximum node displacement of residual limb surface was4.365mm and the maximum relative slip was2.137mm.4. Frictional experiment results show that the larger the load, the greater the friction force and the smaller the friction coefficient, and the larger the skin elastic deformation, the greater the degree of adhesion. The friction behavior of residual limb skin was all in a adhesion state when the normal load was7N and17N respectively. The finite element method can be used to predict the distribution and the size of pressure on the interface, but there were some differences compared with experimental results in terms of interface friction state. It still needs further study.