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.
Department of Applied Mechanics, Indian Institute of Technology, Madras.
K M
KM
Patil
S
S
Jacob
D020342:Finite Element Analysis; D005528:Foot / Q000033:anatomy & histology / Q000473:pathology / Q000502:physiology; D016523:Foot Ulcer / Q000503:physiopathology*; D006801:Humans; D007918:Leprosy / Q000150:complications*; D008953:Models, Anatomic; D010243:Paralysis / Q000503:physiopathology; D013314:Stress, Mechanical; D013639:Tarsal Bones / Q000503:physiopathology*; D016138:Walking / Q000502:physiology