January 2012

Shear significance

In the moment: Footcare

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Study delves beyond plantar pressure

By Emily Delzell

Recent research confirms that peak shear and peak pressure points occur at different locations under the foot, shedding some light on the role of shear in diabetic foot ulceration.

Peak plantar pressure is a relatively poor predictor of diabetic foot ulcer site and risk, and a study e-published in the December Journal of Biomechanics suggests skin breakdown is a multifactorial problem requiring consideration of clinical and biomechanical factors—including both pressure and shear.

“Designing interventions based on pressure alone may not be enough, as our findings showed shear occurs at different locations and different times than pressure,’” said Samantha Stucke, study lead author and senior engineer at the Austen BioInnovation Institute in Akron, OH. “This knowledge will help diabetic patients and can hopefully apply to amputees, who also deal with skin breakdown.”

Stucke and colleagues collected shear and pressure data from 11 healthy volunteers (mean age 38 ± 14.5 years; 4 women) during three barefoot walking trials on a walkway fitted with a custom-built shear and pressure measurement system.

The system simultaneously captured shear and pressure data from the heel and forefoot. Participants started walking with their right foot, and took three steps; data were collected from their second step and from three vectors—vertical, anterior-posterior (AP), and medio-lateral (ML) shear.

Investigators found that shear forces working under the forefoot, where skin breakdown occurs more frequently than the heel, were much higher than those under the heel.

“Mechanical events causing shearing in the forefoot seem to be different from those that cause shearing under the heel,” said coauthor Brian Davis, PhD, vice president of the institute. “We have not yet shown that shear causes skin breakdown, but we found that shearing under the heel may be caused by the foot ‘landing and skidding’ on the ground, whereas in the forefoot, shear may be caused by tissue spreading radially as weight is applied.”

The mean peak pressure values in the heel and forefoot were 362 ± 106 kPa and 527 ± 123 kPa, respectively. The peak average shear acting on the forefoot was directed anteriorly (37.7 ± 7.6 kPa) while the minimum average shear was in the posterior direction (17.6 ± 5.7 kPa). For the heel, the peak average shear occurred posteriorly (21.2 ± 5kPa), and the minimum average shear, anteriorly (8.3 ± 2.8 kPa).

In all volunteers the location of peak shear in any direction diverged from that of peak pressure for both the heel and the forefoot. In the heel, the peak pressure site, on average, was 24.8 mm away from the peak anterior shear force, 17.37 mm from the posterior, 20.93 mm from the medial, and 22.94 mm from the posterior. In the forefoot, the peak pressure site, on average, was 22.79 mm away from the peak anterior shear force, 29.66 mm from the posterior, 24.26 mm from the medial, and 26.67 mm from the lateral.

Peak AP shear values also occurred at different times than peak pressure values. In the heel, peak AP shear preceded peak pressure 60.61% of the time; in the forefoot, peak AP shear succeeded peak pressure more than half the time (57.58%).

Investigators also looked at shear forces near the peak pressure location, measuring instances of bilateral outwardly directed forces, which indicates a “spreading” effect, and at the number of times shearing stresses were unidirectional and occurred before or after the peak pressure timepoint, which indicates “dragging.” In the forefoot, spreading occurred bilaterally (94% ML, 67% AP). In the heel, spreading occurred in the ML direction 82% of the time and dragging in the AP direction, 82% of the time.

“Shear distributions are complicated to assess,” Davis said. “They are sort of like cloud formations—many patterns continuously changing, some moving left, some right. Understanding what these patterns mean is a research area in its infancy.” –ED

Source: Stucke S, McFarland D, Goss L, et al. Spatial relationships between shearing stresses and pressure on the plantar skin surface during gait. J Biomech 2011 Dec 12. [Epub ahead of print].

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