A pilot study to assess the effectiveness of orthotic insoles on the reduction of plantar soft tissue strain

BackgroundPlantar ulcers pose a frequent serious complication in the neuropathic foot. Previous studies suggested that ulcer initiation occurs within the plantar soft tissue rather than on the plantar surface. This study investigated the effectiveness of different shaped silicone insoles on the reduction of both plantar soft tissue strain and pressure. The authors have found no previous experimental studies on the effectiveness of insole shape on reducing plantar soft tissue strain during standing.MethodsA custom molded silicone insole which allowed passage of ultrasound to the plantar surface of the foot was prototyped for this study. Soft tissue strain was computed from soft tissue thickness measured using ultrasound in five conditions: unloaded, barefoot, wearing a prefabricated silicone insole, wearing the custom molded silicone insole alone then with a metatarsal pad. Plantar pressure was measured for the same conditions.FindingsThe custom molded insole was found to significantly reduce soft tissue strain and plantar pressure relative to both the barefoot condition and the prefabricated insole under the second and third metatarsal heads. The metatarsal pad was found to significantly reduce soft tissue strain but not significantly affect plantar pressure.InterpretationA custom molded silicone insole can effectively reduce both soft tissue strain and plantar pressure and is thus preferable to a prefabricated insole. It is suggested that quantifying the reduction of soft tissue strain is an essential design requirement for orthotic insoles since plantar pressure may not be a sufficient indicator of the effectiveness of an insole in preventing ulcer initiation.