Three-dimensional foot modeling and analysis of stresses in normal and early stage Hansen's disease with muscle paralysis.

The models of the foot available in the literature are either two- or three-dimensional (3-D), representing a part of the foot without considering different segments of bones, cartilages, ligaments, and important muscles. Hence, there is a need to develop a 3-D model with sufficient details. In this paper, a 3-D, two-arch model of the foot is developed, taking foot geometry from the X-rays of nondisabled controls and a Hansen's disease (HD) subject, and taking into consideration bones, cartilages, ligaments, important muscle forces, and foot sole soft tissue. The stress analysis is carried out by finite element (FE) technique using NISA software for the foot models, simulating quasi-static walking phases of heel-strike, midstance, and push-off. The analysis shows that the highest stresses occur during push-off phase in the dorsal central part of the lateral and medial metatarsals, the dorsal junction of the calcaneus, and the cuboid and plantar central part of the lateral metatarsals in the foot. The stresses in push-off phase in critical tarsal bone regions, for the early stage of HD with muscle paralysis, increase by 25-50% as compared with the control foot model. The model calculated stress results at the plantar surfaces are of the same order of magnitude as the measured foot pressures (0.2-0.5 MPa). The high stress concentration areas in the foot bones indicated above are of great importance, since it is found from clinical reports that in some subjects with pathogenic decrease in the mechanical strength of the bone from HD, these areas of bone are disintegrated. Therefore, this investigation could possibly provide an insight into the factors contributing to disintegration of tarsal bones in HD.