Comparison of Shear Stress Acting in the Sockets of Typical Mobile and Immobile Ankle Prostheses Using Finite Element Analysis (FEA)

Major problems with below the knee prosthetics today involve both cost and mechanics.Current design lacks an ankle complex which disrupts not only the natural biomechanics of ambulation but also sitting.Without the necessary frontal plane motion (inversion and eversion) along with sagittal plane motion (plantarflexion and dorsiflexion), the current manual immobile prostheses for below the knee amputees the lower limb is unable to adapt appropriately to certain ground surfaces because they are rigid and do not allow for a normal gait.Therefore this causes excessive moment and shear force to occur and consequently compensation occurs at the stump and socket interface.This will later in turn cause complications of pressure sores and discomfort along the stump.In the biomechanics lab at Mercer University, students have been involved in several research projects and lab works relating to lower extremity biomechanics using gait analysis system and measure moment and reaction forces.The authors used Pro-E to model a mobile (full range of motion at the ankle level) and immobile (rigid and no motions occur at the ankle joint) prosthetic devices and analyzed the shear stress in the pylons and sockets with a constant body load of 1,000 lbs acting in the sockets.Aluminum is selected for the material property of the pylon and the socket.Results showed that the Von Mises stress to anterior aspect of the immobile ankle prosthesis was 1,567 psi compared to 131 psi for the mobile ankle prosthesis; the maximum shear stress acting on the immobile ankle prosthesis anteriorly was 905 psi verses 52 psi in mobile ankle prosthesis.The results also showed large amounts of reduction in stresses in lateral, medial, and posterior aspect of the mobile ankle prosthesis.