Do people who load their feet differently need insoles that have different stiffness?

Background: Plantar pressure reduction is an important aspect of diabetic foot management. However little information exists about the optimum cushioning properties of materials used in diabetic footwear as insoles/foot-beds. Numerical analyses have indicated that optimizing the material properties of footwear materials can improve their ability to reduce pressure.

Aim: To investigate if the optimal insole stiffness would vary based on patients’ body mass (BM) in people with diabetic neuropathy.

Method: Custom PU foams were produced using different ratios of chemical components to achieve a range of different stiffness. Uniform thickness (400 mm × 400 mm × 10 mm) foam sheets were produced with shore-A hardness between 3 and 45 and average(±stdev) increments of 5(±3). Standardized compression tests were performed for all 10 custom materials as well as for 3 commercially available foam materials used in diabetic footwear. Plantar pressure was measured during balanced standing on all custom material sheets for 4 diabetic neuropathic volunteers: 2 with BM of 49 kg ± 1 kg and 2 with BM of 73 kg ± 2 kg.

Results: The maximum compressive force for 50% compression of the commercially available foams was similar to custom foams with 11–28 shore-A hardness. Peak plantar pressure was minimised for materials with shore-A hardness 6 and 11 in subjects with BM of 49 kg ± 1 kg and 73 kg ± 2 kg respectively. In all cases using softer or stiffer material (by 1 shore hardness increment) increased pressure by 24% ± 26% and 32% ± 34% respectively.

Conclusions: Careful selection of insole/foot-bed stiffness can improve the pressure reduction capacity of diabetic footwear. Optimum material stiffness increased with the BM of the volunteers.