Shear, Pressure, and the Diabetic Foot

Marty Carlson, the founder and co-owner of Tamarack and I were looking through some old seminar materials on neuropathic ulcers that Marty had. The National Hansen’s Disease Center, then located in Carville, Louisiana, had conducted the seminar. Included in the materials was a 1983 article written by Dr. Paul W. Brand, the legendary physician who worked for twenty years at the Center. Dr. Brand did pioneering work on sensory neuropathy – the loss of sensation that people with diabetes and Hansen’s disease (Leprosy) often experience in their extremities. Dr. Brand was the first physician to determine it is primarily the loss of protective sensation that results in terrible wounds on the feet of people with diabetes or Hansen ‘s disease. As Dr. Brand discovered the loss of protective sensation results in the failure of patients to detect or appreciate the severity of mechanical loading on their feet. Therefore, mechanical loading – not the diseases themselves – directly cause the wounds.

As Dr. Brand elucidates in his article “the whole problem is one of mechanics, not of medicine.” Therefore “off-loading” the forces became critical to the prevention or healing of the wound.

What does off-loading mean? Most authorities today focus on pressure off-loading. While Dr. Brand thought pressure was important, it was not the most important force causing diabetic foot ulcers. Instead, Dr. Brand knew the importance of another force: shear. As Dr. Brand observed:

There are two types of force which occur on the sole of the foot, one is vertical force at right angles to the foot, which causes direct pressure on the tissues. The other is horizontal force, or shear stress, which is parallel to the surface of the foot and occurs in association with acceleration and deceleration. Of the two forces shear stress is more damaging than pressure.

Brand PW, Neuropathic Ulceration, the Star, Carville, Louisiana, May-June, 1983.

Today shear off-loading as well as pressure off-loading is becoming a focus in diabetic foot care. That’s a good thing.

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