Development of knowledge-based criteria for designing foot orthoses

Abstract

Diabetes especially for those with peripheral neuropathy are susceptible for developing neuropathic ulcers on the plantar foot surface, which frequently lead to hospitalization and limb amputations. One of the major causes of diabetic ulceration and plantar foot pain is thought to be the presence of abnormally high plantar foot pressures. Increasing evidence suggests that diabetic feet and painful foot syndrome can be successfully resolved or relieved with a proper foot orthosis, helping to correct ankle-foot abnormalities and to relieve and redistribute elevated plantar pressures. Numerous in vivo experimental studies have been directed to analyze the performance of specific orthosis in terms of subject comfort and disability level, plantar foot pressure relief or redistribution and its functional role in correcting pathological gait and providing good foot support. Owing to the complexity of the ankle-foot structures and experimental difficulties, most studies focused on subjective assessments, gross joint motions and plantar pressure distribution between the foot and supports. The rationale behind the functional role of a foot orthosis on the load distribution and stabilizing ability relies mainly on subjective views, interfacial pressure measurements, or gross motion tracking. In order to provide a supplement to the experimental inadequacy, many researchers had turned to the computational methods in search of more clinical information. Computational modeling, such as the finite element (FE) method can be an adjunct to experimental approach to predict the load distribution between the foot and different supports, which offer additional information such as the internal stress and strain of the ankle-foot complex. The FE analyses can allow efficient parametric evaluations for the outcomes of the shape modifications and other design parameters of the orthosis without the prerequisite of fabricated orthosis and replicating patient trials. Existing FE models of the foot or footwear in the literature were developed under certain simplifications and assumptions including a simplified or partial foot shape, assumptions of linear material properties, infinitesimal deformation and linear boundary conditions without considering friction and slip. Although several 3D foot models were developed recently to study the biomechanical behaviour of the human foot and ankle, a geometrically detailed and material realistic 3D FE model of the human foot and ankle specialized for footwear or orthotic design has not been reported. In this study, a 3D FE model of the human foot and ankle was developed from 3D reconstruction of Magnetic Resonance (MR) images from the right foot of a male adult subject. The developed FE model, which took into consideration the nonlinearities from material properties, large deformations and interfacial slip/friction conditions consisted of 28 bony structures, 72 ligaments and the plantar fascia embedded in a volume of encapsulated soft tissue. Parametrical studies were conducted to investigate the biomechanical effects of tissue stiffness, muscular reaction, surgical and orthotic performances on the ankle-foot complex. Experimental measurements on cadavers and on the subject who underwent the MR scanning were obtained to validate the FE predictions in terms of plantar pressure distribution, foot arch and joint motion, plantar fascia and ligamentous strains under different simulated weightbearing and orthotic conditions of the foot. The parametrical analyses showed that increasing soft tissue stiffness led to decreases in total contact area of the foot-support interface and pronounced increases in peak plantar pressure at the forefoot and rearfoot regions. Decreasing the stiffness of plantar fascia would reduce the arch height and increase the strains of the plantar ligaments. In addition, surgical releases of partial and the entire plantar fascia increased the strains of the plantar ligaments and intensified the stresses in the midfoot and metatarsal bones. The FE predictions showed that both the weight on the foot and Achilles tendon loading resulted in an increase in tension of the plantar fascia with the latter showing a two-times larger straining effect. Above all, unloading the posterior tibial tendon was found to increase the arch deformations and strains of the plantar ligaments especially the spring ligament. The parametrical analyses performed on the foot orthosis showed that the custom-molded shape was a more important design factor in reducing peak plantar pressure than the stiffness of the orthotic material. Besides the use of an arch-supporting foot orthosis, the insole stiffness was found to be the second most important factor for peak pressure reduction. Other design factors contributed to a less pronounced role in peak pressure reduction in the order of insole thickness, midsole stiffness and midsole thickness. Further investigations on the biomechanical effect of different types of foot orthosis can be used to refine the design principles of orthosis in the CAD/CAM process in terms of appropriate shape and material of the orthosis in order to fit specific functional requirements of the subject and individual foot structure. The ultimate goal of the developed computational system is to establish the knowledge-based criteria to provide systemic guidelines for clinicians to prescribe and fabricate an optimized orthosis to maximize the functions of the foot orthoses as well as the subjects' comfort and gait performance.