Material Selection for Diabetic Custom Insoles: A Systematic Review of Insole Materials and Their Properties

ABSTRACT Introduction Diabetic foot ulceration is one of the most significant causes of infection and amputation in neuropathic patients. Most often, the plantar surface of the foot is affected. Offloading the foot by custom-molded insoles is considered a crucial step in prevention of diabetic foot ulcers, although the most beneficial choice of materials is not well documented. This review focuses on identifying the most suitable materials and material compositions for offloading the diabetic foot. Materials and Methods A systematic review was performed with the aid of a clinical librarian on September 9, 2020. The review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) guidelines. Studies were included based on the following eligibility criteria: interventions in terms of material selection/properties with outcomes reported as either pressure reduction and/or ulcer prevention. Bench studies were also included. At each level of selection, records were reviewed by two independent reviewers. Clinical studies were appraised using the ROBINS-1 tool. Results Through database search, 1131 records were identified and an additional eight records were identified through references and other sources. After removal of duplicates, 927 records were screened, which resulted in 74 full-text records assessed for eligibility. Thirty-seven studies were included in the qualitative synthesis. Twelve studies examined material properties by bench testing. Five studies were based on finite element analysis. Nineteen clinical studies on material parameters were identified. Six studies supported the use of moderately deformable materials for insole production. Six studies showed better performance of combination material insoles in offloading and under sustained loading. Five studies concluded that increasing the thickness of insoles increased the offloading capabilities. Conclusions The diversity of studies and study designs to date precludes the ability to recommend one type of material over the other. Still some general considerations can be made. For solo-material insoles, moderately deformable materials are preferable, balancing the need for offloading with the need for durability. Increasing the thickness may increase the longevity of the insoles. Combining materials of different properties seems to enhance both the offloading capability and durability of insoles. Clinical Relevance When using a single-material design, moderately deformable materials are preferable. Combining materials of different properties enhances both the offloading capability and durability of insoles.