An in-shoe gait analysis device to measure the maximum shear stresses at the first metatarsal head

Abstract

Recent research indicates that the shear stresses acting on a diabetic's foot are one of the major mechanical contributors to the high incidence of ulceration experienced by these patients. These stresses together with direct pressure are thought to have an effect on blood flow occlusion elsewhere in the body. The reduced blood flow may relate in moderation to reduced tissue tolerance or repair capability or even in more severe cases to cell death. Repeated vascular occlusion in a normal person would produce a minor blister or a swollen area, but with a diabetic patient it has the ability to create large incisions and ulcers. This is because diabetic patients are unable to redistribute the load on their feet due to the lack of sensation in their lower extremities. This results in diabetes being the number one cause of all lower limb amputations and accounting for 50 to 70 % of all non-traumatic amputations in the U.S. In the same country, it accounts for $200 million a year in treatment costs directly related to diabetic foot infections. Quantifying the magnitude and duration of these shear stresses therefore has the potential to play a crucial role in assisting podiatrists and clinicians in their diagnosis and treatment of these patients. However these stresses have not been widely evaluated due to lack of suitable instrumentation for their measurement. A technique which has proven to be the most successful in measuring these stresses involves placing a discrete transducer inside a customised insole and fitting it to a patient's shoe. This report sets out to design a similar technique but with the use of a differently designed transducer. The validity of and confidence in the proposed transducer was established by assessing and comparing the results of the transducer under a series of controlled tests with the results of other transducers presented in the literature. To allow an accurate assessment of the transducer to be made, the tests which were performed on the transducer were controlled and conducted at a fixed walking speed. The computational theory used was based on the assumptions and equations of two dimensional plane strain for linear elastic isotropic homogeneous materials. The transducer is based on the principle that a shear angle is induced on a plane when a shear stress is applied to a plane continuous and orthogonal to it. This principle was adapted into the design of the transducer in the form of a square block of material, whose two orthogonal lateral surfaces were used to measure the shear stress applied to its top surface. The design of the transducer consists of a block of material, two laterally positioned rectangular strain rosettes and a circular base. The first series of tests conducted on the transducer were intended to verify and establish its material properties and characteristics. A model of the transducer was then constructed using the finite element package ABAQUS. Two Shape Factors – one for calibration purposes and the other for in-shoe testing - were generated for the transducer to allow for the effects of the geometrical inconsistencies present in its design to be accounted for. Without these Shape Factors the equations and assumptions of linear elasticity would not have been appropriate. A series of controlled pilot and analysis tests were then performed using the custom designed insole and transducer fitted to a diabetic shoe. The diabetic shoe was worn by a subject who performed the tests on a treadmill at a laboratory in the Sports Institute of South Africa.