Finite element model-based evaluation of tissue stress variations to fabricate corrective orthosis in feet with neutral subtalar joint.

BACKGROUND: The subtalar joint position during static stance is a crucial determinant of the peak plantar pressures and forms ideal reference point for any intervention in foot-related problems for leprosy-affected patients. OBJECTIVES: The study pursued the hypothesis through a three-dimensional model that stress will be minimal in the distal joints of the foot when the subtalar joint is in neutral static stance position. STUDY DESIGN: Finite element model. METHODS: The computed tomography images of the feet for five patients suffering from Hansen's disease having no muscle weakness and joint restriction were acquired. The gray intensities corresponding to the bones of the foot from the computed tomography images were three-dimensionally reconstructed. The three-dimensional model of the human foot, incorporating the realistic geometry, and the material properties of the hard tissues were then analyzed using a finite element solver for the stress distribution on bones of the foot. RESULTS: The results demonstrate that the position of the calcaneum in the static stance position does contribute to the varying stress in the foot. CONCLUSION: The stresses in the bones of the foot are minimal while the subtalar is in neutral position; this position will be suitable for foot orthotic fabrication. Clinical relevance The clinicians, therapists, and podiatrists having less engineering skills can quickly assess the patient and get optimal results on the stress associated with the joints of the foot.

Mechanics of tarsal disintegration and plantar ulcers in leprosy by stress analysis in three dimensional foot models.

This paper describes three dimensional two arch models of feet of a normal subject and two leprosy subjects, one in the early stage and the other in the advanced stage of tarsal disintegration, used for analysis of skeletal and plantar soft tissue stresses by finite element technique using NISA software package. The model considered the foot geometry (obtained from X-rays), foot bone, cartilages, ligaments, important muscle forces and sole soft tissue. The stress analysis is carried out for the foot models simulating quasi-static walking phases of heel-strike, mid-stance and push-off. The analysis of the normal foot model shows that highest stresses occur at push-off over the dorsal central part of lateral and medial metatarsals and dorsal junction of calcaneus and cuboid and neck of talus. The skeletal stresses, in early state leprosy with muscle paralysis and in the advanced stage of tarsal distintegration (TD), are higher than those for the normal foot model, by 24% to 65% and 30% to 400%, respectively. The vertical stresses in the soft tissue at the foot-ground interface match well with experimentally measured foot pressures and for the normal and leprosy subjects they are the highest in the push-off phase. In the leprosy subject with advanced TD, the highest soft tissue stresses and shear stresses (about three times the normal value) occur in push-off phase in the scar tissue region. The difference in shear stresses between the sole and the adjacent soft tissue layer in the scar tissue for the same subject is about three times the normal value. It is concluded that the high bone stresses in leprosy may be responsible for tarsal distintegration when the bone mechanical strength decreases due to osteoporosis and the combined effect of high value of footsole vertical stresses, shear stresses and the relative shear stresses between two adjacent soft tissue layers may be responsible for plantar ulcers in the neuropathic leprosy feet.

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.