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Breakdown ahead? Time to replace insoles

By Cary Groner

Despite research suggesting diabetic foot orthoses may need replacing only once a year, practitioners say there are a number of clinical reasons why an individual patient might need new insoles more often.

Joanne Paton, MSc, a lecturer in podiatry at the University of Plymouth in the U.K., studied 43 patients with diabetic neuropathy who were given insoles constructed of a 3-mm medium EVA shell covered with 6 mm of poron. From issue until six months, average insole depth decreased by roughly 0.3 mm under the first metatarsal head and 0.5 mm under the heel. However, those numbers didn’t change much from six months to one year, and peak plantar pressure reduction was about the same at issue, six months, and one year. The results were presented in September 2010 at the International Foot and Ankle Biomechanics (i-FAB) congress in Seattle.

Although it may seem counterintuitive that a decrease in insole depth would not be correlated with less effective pressure control, Paton said several factors could explain such results.

“If reduction in peak pressure is associated with an increase in total [weight-bearing] surface area, as some suggest, then you would not expect insole compression to have a substantial effect on function measured that way,” she commented. “However, that is not to say the performance characteristics of the insole have not changed in some other way, [and] maybe we just weren’t measuring that change. I suspect material compression would alter the dampening characteristics of the insole, which may still be an important performance characteristic.”

Although Paton’s findings suggest that insoles may not need to be replaced for a year, clinicians suggest that an evaluation of the individual patient is crucial.

“With the diabetic foot, we are looking at people with Charcot deformity and other problems,” said Dennis Janisse, CPed, an assistant professor of physical medicine and rehabilitation at the Medical College of Wisconsin.  “Aging may be a factor, and they may have prominent metatarsal heads. These patients’ feet are changing all the time. If we check them at four months and the foot has changed, they may need a new insole to adapt to the new shape.”

David Levine, DPM, CPed, who practices in Frederick, MD, agreed that consideration of the individual is paramount.

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“Someone with a bony prominence in the middle of the plantar surface of the foot is going to break the orthosis down fairly quickly, whereas an elderly, petite woman who doesn’t walk a lot may not break it down much at all,” he said. “The breakdown is something I can use, however, because I can see where the pressure is and modify the device to accommodate it.”

Levine said that young active people, obese patients, and those with extremely pronated feet break insoles down more quickly than others.

“If a patient has neuropathy,” he added, “they should come in every three or four months, because they won’t have the feedback to know if the shoe fits, if the insole is wearing out, or if there is an area of pressure that needs to be assessed.”

Materials make a difference too, of course. Baylor University researchers published an article in 2007 reporting that only two of several combinations tested—medium plastazote plus nickelplast and nickelplast plus poron—were associated with similar peak pressures at both pretesting and after 100,000 cycles.

“We shouldn’t be saying orthoses need to be replaced once, twice, or three times a year, because they degrade by repetitive stress,” said David Armstrong, DPM, MD, PhD, a professor of surgery at the University of Arizona College of Medicine. “We have to count the number of repetitive stress events, multiply that by the vertical and shear stress on that material, then create an individualized protocol for insole replacement.”

Armstrong believes that, long term, the most promising approach will involve the use of “intelligent insoles” constructed of materials that signal when they are about to degrade.

Sponsored by an educational grant from Amfit

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