A 3-dimensional finite element model of the human foot and ankle for insole design

OBJECTIVE: To investigate the effect of material stiffness of flat and custom-molded insoles on plantar pressures and stress distribution in the bony and ligamentous structures during balanced standing. DESIGN: A 3-dimensional (3-D) finite element model of the human ankle-foot complex and a custom-molded insole were developed from 3-D reconstruction of magnetic resonance images and surface digitization. The distal tibia and fibula, together with 26 foot bones and 72 major ligaments and the plantar fascia, were embedded in a volume of soft tissues. SETTING: Computational laboratory in a rehabilitation engineering center. PARTICIPANT: A healthy man in his mid twenties (weight, 70 kg). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Foot-support interfacial pressure, von Mises stress in bony structures, and strain of the plantar fascia were predicted using the finite element model. RESULTS: A custom-molded, soft (Young modulus, E=0.3 MPa) insole reduced the peak plantar pressure by 40.7% and 31.6% at the metatarsal and heel region, respectively, compared with those under a flat, rigid (E=1000 MPa) insole. Meanwhile, a 59.7% increase in the contact area of the plantar foot was predicted with a corresponding peak plantar pressure increase of 22.2% in the midfoot. CONCLUSIONS: The finite element analysis implies that the custom-molded shape is more important in reducing peak plantar pressure than the stiffness of the insole material.