Development of plantar pressure relieving orthotic insoles for people with diabetes

Abstract

Orthotic insoles are commonly used in the treatment of the diabetic foot to prevent ulcerations. The insoles are normally custom-made to provide an optimum fit and reduce and/or redistribute pressure on different plantar regions. The multi-layered design of the insoles not only accommodates the foot shape and provides comfort, but also maximizes the function of pressure relief for the diabetic foot. Nonetheless, the insole making process is highly complex, time-consuming and error-prone due to the large variations in the taking of the geometric shape of the foot, fitting of the insoles, and even the selection of foam materials or composites for fabrication. Up to now, there has been limited knowledge about the effects of foot and insole geometric determination and foam material selection on the performance of orthotic insoles in the control of plantar pressure. To fill the knowledge gaps in the traditional process of insole fitting, this project aims to propose a quantitative method to measure foot and insole geometry by adopting a simple portable 3D desktop scanner that is non-contact and non-invasive to capture images. In this study, the 3D geometry shape of the foot and insole, and deviations between the foot geometry and its interface conformity with custom-fabricated insoles are first examined from a clinical practice perspective. Accompanied with an in-shoe pressure measurement system, the effects of the foot and insole geometry in relation to foot-orthosis interface pressures at different plantar regions are identified. To enhance the understanding of the key properties and end-uses of orthotic insoles for the diabetic foot, the physical and mechanical properties of currently used insole materials are investigated. Methods that test for the important properties of force reduction and insole-skin friction and shear, as well as comfort of the fabrication materials have been developed. To quantify the overall performance of the insole materials, a novel performance index that combines the test results of various materials has also been established. The properties and performance of various insole materials are formulated based on the performance index. To improve the performance of plantar pressure reduction and perceived comfort, new insoles made of weft-knitted spacer fabrics are developed. The implications of the spacer fabricated insoles on plantar pressure distribution by using lower limb electromyography (EMG) and through perceived comfort in daily life activities are examined. The potential use of spacer fabric in orthotic insoles for the design and development of orthotic insoles for patients with foot problems has been identified. In consideration of the influence of the physical and mechanical properties on the pressure relieving performance of insoles, a finite element (FE) model of the insole is developed by using ABAQUS software. By inputting Young’s modulus under compression, Poisson’s ratio and the shear modulus of materials, as well as foot-insole interface pressure from the plantar pressure measurement system, regional displacement and compressive stress distribution across the insole surface are simulated in respect to various material combinations. Hence, the optimal insole combination in response to various foot problems can be determined with minimal trial and error. This study herein not only can improve the insole fit, but also optimizes the functions of the insole so as to enhance the effectiveness of foot orthotic treatment in pressure relief, thus preventing ulceration in the diabetic neuropathic foot.