There were an estimated 537 million adults (20-79 years old) globally with diabetes in 2021. The International Diabetes Federation predicted that this number will increase to 643 million by 2030 and 783 million by 2045 International Diabetes1. The majority (75%) of diabetic patients are 45 years old and older2. Diabetic foot ulcers (DFUs) are one of most serious complications for diabetic patients3. DFU issues affect one in three people with diabetes4, and the rapid increase in the prevalence of DFUs over the last few decades is a major challenge for healthcare systems around the world5.
The peak pressure value is defined as the highest pressure and frequently reported in plantar pressure studies6,7,8. Increases in peak pressure have been found to be particularly related to the development of DFUs due to the presence of peripheral neuropathy9,10. Both Types 1 and 2 diabetic patients are at risk of developing diabetic peripheral neuropathy due to uncontrolled or long-standing chronic hyperglycemia11. Many of the elderly with diabetes show an increase in peak pressure during gait12. The high levels of peak pressure show that the pressure has a linear regression relationship with DFUs in diabetic patients especially with peripheral neuropathy13. Due to higher magnitude and duration of mechanical stress within the foot during standing and walking, abnormal plantar pressure distribution on the soft tissues of the feet can result in soft-tissue breakdown and subsequent foot ulcers, limited joint mobility, and foot deformities in diabetic foot pathology11,14,15. Such deteriorations increase the risk of falls and disability, and cause a higher rate of mortality of the elderly with diabetes16. Therefore, evaluations of the peak pressure of the feet of diabetic patients have been commonly done to determine the effectiveness of footwear and foot devices (i.e., medical insoles, shoes, and orthoses) in relation to pressure offloading11,17. The effectiveness of diabetic footwear is determined by a significant reduction or lower peak pressure value18. Pressure-time integrals (PTIs) are also commonly reported and used in DFU studies3,11. The PTI is defined as the foot region under the peak pressure time curve6. The PTI is considered as an essential parameter of DFUs because it incorporates pressure and time factors18. Duan et al.3 showed that diabetic patients have a normal plantar blood flow response after walking at a low PTI stress, but impaired blood flow response after walking at a high PTI stress. This finding indicates that it is important for diabetic footwear products to prevent diabetic patients from walking with a stimulus with a high PTI.
Some clinical studies suggest that usage of suitable orthotic footwear or insoles that offer proper arch support can effectively offload plantar pressure19,20,21,22, which is an essential intervention to prevent the risk of DFUs23,24,25,26. Soft materials can readily be moulded for a custom fit to offer excellent wear comfort and cushioning of the feet27, while a composite structure constructed of soft and rigid materials have a superior performance in reducing the amount of plantar pressure28,29. In addition, the use of an arch support with an insole has been proven to offload plantar pressure when compared to an insole without an arch support for patients with diabetes30,31,32,33,34. The type of foam material used is also subjectively determined based on the experience of individual practitioners. Cellular polymer such as polyurethane foam (e.g., PORON®) is an open-celled material with excellent cushioning properties15,27,28, while ethylene-vinyl acetate (e.g., nora®) is moldable, resilient and elastic27 so that insoles that use this material can prevent callus formation and tissue damage because the material exerts less frictional force and shearing stress to the skin15,27,32,33,34,35; Polyethylene (e.g., Pe-Lite®) offers good cushioning and shock absorption36. Lo et al.35 tested the physical properties of those insole materials (e.g., polyurethane, ethylene-vinyl acetate, and Polyethylene),but lacking of practical clinical evidence for plantar pressure offloading. All of these materials are also available in a wide range of hardness, thickness and densities. For optimal foot protection, custom-fabricated orthotic insoles are typically made by using multiple layers of materials so as to provide the desirable amount of support and reduction of peak plantar pressure and PTI for diabetic patients15,28. However, the high incidence rates of ulcer recurrence and lower limb amputation suggest that current diabetic insole designs are not sufficiently catering to the needs of all diabetic patients.
Despite a large volume of biomechanical studies that have documented how insoles alleviate plantar pressures, there are large variations in the type of material used, shape, construction and properties of the insole and footwear conditions. Due to the lack of evidence in the literature, current understanding on the approach to selecting the type of insole material used to optimize the offloading performance is somewhat limited. The fabrication of insoles still greatly depends on repeated trials and error based on the experience of practitioners, and the availability of materials locally, rather than the foot conditions of patients and effectiveness of offloading. Hence, the aim of this study was to investigate the effectiveness of the above-mentioned soft insole materials (i.e., PORON® medical 4708 and Nora Lunalastik EVA) and the rigid insole materials (i.e., Nora Lunalight A fresh, and Pe-Lite) on offloading the plantar pressure of the elderly with diabetes during gait. The MPP and PTI of the dominant foot and four different plantar regions (i.e., toes, forefoot, midfoot, and rearfoot) were measured, and comparisons of MPP and PTI with or without the use of the different insoles were made. The influence of the type of insole material used on plantar pressure distribution was systematically investigated. Effectiveness in plantar pressure offloading by insole materials was determined in terms of significantly lower values of MPP or PTI, and vice versa. It was hypothesized that the contoured insoles constructed of different insole materials used in this study would help diabetic elderly to offload plantar pressure than barefoot during gait, the forefoot and rearfoot regions were hypothesized to be effectively offloaded by used insole materials when compared with barefoot.