by Cary Groner

Clinicians have known for many years that uneven plantar pressures, combined with the loss of sensitivity caused by peripheral neuropathy, are associated with foot ulcers in diabetes patients. Ulcers often occur in different areas than peak pressures do, however. Shear forces, caused by pressure differentials in the foot, may be more to blame than plantar pressures per se.

Shear is notoriously hard to measure, though, and researchers don’t always agree on how to define it. But better understanding of these forces may lead to improved prevention and treatment of ulcers and their grim sequelae, such as osteomyelitis and amputation.

“We can create a topographical map of vertical pressure on the foot, but shear stresses don’t always correlate with vertical stresses,” said David Armstrong, DPM, PhD. Armstrong is a professor of surgery at the University of Arizona College of Medicine. “It’s the equivalent of podiatric dark matter: we believe it’s important but we can’t measure it well. In fact, shear stress is probably more important than vertical stress because it occurs twice per step instead of once.”

According to Armstrong, plantar pressure offloading is a zero-sum game, because relieving pressure in one area increases it in others. As long as that additional pressure remains below a certain threshold, which is unique to each patient, the risk of ulceration drops and the potential for healing increases.

Michael J. Mueller, PT, PhD, a professor of physical therapy at Washington University in St. Louis, has done research suggesting that pressure gradients in the foot contribute to shear forces. As a result, he defines shear differently than some clinicians do.

“We’ve always considered shear as parallel to the surface of the foot, but our research shows forces perpendicular to the skin, just as pressure variables are,” he said. “If you have high stress in one location in the foot, and low stress in an adjacent location, that generates vertical shear. This reinforces, from a clinical standpoint, the need for even pressure distribution.”

This perspective helps explain why clinicians see more skin breakdown at the forefoot than at the heel, even though there is typically more pressure at the heel, Mueller added.

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Georgeanne Botek, DPM, medical director of the diabetic foot program at the Cleveland Clinic, concurs with these perspectives.

“When it comes to shearing, most of what we see isn’t necessarily on the most weight-bearing surface,” she said. “I have several patients who have more of a lateral or plantar-lateral skin breakdown, which indicates it is more related to shear than to plantar pressure.”

According to Botek, the phenomenon also occurs frequently at the hallux–interphalangeal joint, where significant potential shearing occurs.

“That’s the plantar-medial aspect of the foot, where there tends to be extra friction and pressure, and that’s where we see changes in the skin—thickening, hardening, and subsequent hemorrhaging,” she said.

Clinicians should be attentive to the potential for such problems in diabetes patients with complications, Botek emphasized.

“Once we have a diagnosis of peripheral neuropathy, it raises a red flag,” she said.

Even if the underlying concepts remain incompletely understood, insights regarding the significance of shear forces offer hope for better orthotic management of at-risk feet.

“You need to devise an orthotic that focuses on equal distribution of the stresses,” Mueller said.

The others agree. Armstrong is on the scientific advisory board of a company that has commercialized an insole that purports to reduce shear, and Botek has her own approach.

“We try to look at the insole material, and the pattern of the foot imprint, then work with the pedorthist to come up with a top cover that may prevent shearing,” she said. “We also have to accommodate the foot more laterally. After all, bad doctors end up treating complications; mediocre doctors treat conditions; but great doctors prevent the problems in the first place.”

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