October 2010

Insoles stand test of time

In the moment: Footcare

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Six months may be too soon to replace

By Jordana Bieze Foster

Diabetic insoles may not need to be replaced as frequently as previously thought, according to research from the U.K. presented in September at the second International Foot and Ankle Biomechanics (i-FAB) conference.

An earlier study found that insoles lose their ability to offload peak plantar pressures after six months and therefore recommended that they should be replaced at that time. However, the current study suggests that although the cushioning materials in an insole do compress significantly during the first six months of wear, peak plantar pressures are not significantly affected for at least six more months.

“Insole replacement should be assessed at 12 months,” said Joanne S. Paton, PhD, a lecturer in podiatry at the University of Plymouth, who presented her group’s findings at the i-FAB conference.

The six-month recommendation came from German researchers at the University of Magdeburg, who assessed the durability of insoles made from a combination of ethylene-vinyl-acetate (EVA), polyethylene foam, elastomere, and silicone in 81 patients with type 2 diabetes. They found that use of the insoles reduced maximum plantar pressure for the whole foot by 39% initially, but the degree of pressure reduction gradually decreased over time to 32% of baseline at six months and 20% at one year. That study was published in the April 2001 issue of Diabetes Medicine.

Paton and her colleagues studied 43 patients with diabetic neuropathy who wore EVA insoles covered with 6 mm of Poron. In addition to plantar pressures, they also assessed changes in insole depth over time as a reflection of material compression.

They found that insole depth decreased by about 0.5 mm under the central heel seat and about 0.3 mm under the first metatarsal head, but that compression did not change significantly between six and 12 months. Although the mean peak pressures did increase slightly over time, the amount of peak pressure reduction was not significantly different at six month or 12 month follow-up than at the time of insole distribution.

Patient compliance with insole wear did not appear to have affected the results. Of the 43 patients, 18 were determined to be fully compliant, in that they wore their insoles for at least seven hours per day, seven days a week for the 12 months. When the fully compliant patients were analyzed separately, changes in compression were still limited to the first six months, and mean peak plantar pressure reductions were still maintained for 12 months.

However, Paton noted that even the patients who were not fully compliant reported wearing their insoles for at least five hours per day, five days per week, which may explain the lack of differences between groups.

“It also may be that activity level does not correlate with patient compliance,” Paton said.

Previous research also suggests that compression of diabetic insoles may depend on the specific materials used for insole construction. Investigators from Baylor University compared the response of six different orthotic material combinations to 100,000 cycles on a materials testing machine designed to model the bony prominences of the foot that are at high risk for ulceration in patients with diabetes.  Specifically, the researchers analyzed elastic deformation, the percentage of compression a material experiences with respect to its original thickness, as well as plantar pressures.

They found a trend for decreased elastic deformation over time for four of the six combinations tested. Three of those material combinations also demonstrated a trend for increased plantar pressures with increased testing. However, two combinations (medium plastazote plus nickelplast and nickelplast plus Poron) were not associated with demonstrable changes in peak pressures.  Those findings were published in the August 2007 issue of Foot & Ankle International.

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