The importance of shear stress in ulcer development
Advanced biomechanical measurements are finally moving from the laboratory to clinical practice, with the relevance of in-shoe pressure measurements in ulcer prevention described in the most recent International Working Group on the Diabetic Foot (IWGDF) Guidance. These recommendations mainly focus on peak pressure or force, as these can be measured in preventative devices and using commercially available equipment. However, plantar pressure and force cannot explain all ulcer development. One aspect that gains more attention in research that may further explain some of the variation is shear stress.
Shear stress is the resultant of movement of a foot over the antero-posterior (forward-backward) and mediolateral (left-right) axes, which is different from the peak pressure measured along the vertical axis. Shear may be understood by the chainsaw comparison: a running chainsaw may easily cut the branch of a tree, while continuously applying pressure alone (i.e.: pressing the chainsaw against the tree) will not break this branch. With new sensors, this can now be measured; either on a plate on which participants need to walk barefoot, or using in-device sensors that need to be taped to the foot.
Three articles have recently been published by Dr. Yavuz and colleagues from Texas, US. In their first publication, they compare ulcer locations with peak pressure and peak shear locations. Although they include only eight participants, they find interesting variations: some ulcers are located at the site of peak pressure, some at the site of peak shear, and some at neither. This may be partly explained by the findings of their second study, where they show that the highest foot skin temperature is often found at the location of peak shear, rather than peak pressure. These high temperatures may signal tissue breakdown preceding the ulcers developing at these locations. They follow these studies with a new publication, where they show (in again only a small group of participants) higher peak pressure and peak shear in people with diabetic foot ulcers compared to people without a history of foot ulceration. More information, for example on the location of the peak pressure or peak shear, is unfortunately missing. However, the point to be taken from these studies is that peak shear is an interesting variable that may account for ulceration, and needs to be investigated further next to peak pressure.
A new study from Japan, however, did not find differences in peak pressure or peak shear between feet of people with diabetic neuropathy with or without callus at the metatarsal heads, when measured in the preferred footwear of the participants. Small significant differences are found at some locations when the ratio between pressure and shear is calculated. The authors explain this by stating that no callus will be formed when high pressure is accompanied by low shear. However, not only are the differences are small, the sensitivity and specificity values found are mediocre at best. Caution is needed when interpreting these results, and the bold statements of the authors are not backed by their results. The ratio between shear and pressure may be an interesting feature to investigate, but its value is far from proven.
In conclusion, shear stress is an interesting variable, but there is much that we still do not know with regard to its role in ulcer development, and its direct application to clinical practice is uncertain. Whereas peak pressure has been investigated in large cohorts and preventative intervention trials, shear stress has still only been studied in small groups of patients, with varying results. This will hopefully change in the future, when shear pressure measuring equipment may become widely available.