Is Shear The New Peak Plantar Pressure?
In a provocative discussion of the potential impact of shear forces in the high-risk diabetic foot, this author offers pertinent biomechanical insights and offers suggestions for minimizing the detrimental effects of shear forces in this patient population.
Since the work of Paul Brand, MD, the link between mechanical pressure and the formation of foot ulcers in diabetic patients with neuropathy has been widely accepted.1 Many types of mechanical force have been linked to the development of foot ulcers. Most commonly, we think of ground reactive forces, those forces that are perpendicular to the bottom surface of the foot during ambulation. However, mechanical forces are much more complex.
In order to control mechanical forces, the clinician must consider all of the attributes of those forces. In addition to the actual magnitude of load applied to the foot, one can also control the area over which the force is applied. Pressure, defined as pounds per square inch, is dependent on both the magnitude of pressure and the area over which it is applied. Furthermore, the speed of loading and unloading is widely variable. Tissues deform in response to the mechanical pressure applied and the magnitude of deformation is called strain. The rate of deformation is described as the strain rate.
In addition to the rate of tissue deformation, the response of tissues to mechanical forces also depends on the amount of time that the load is applied. It is apparent that the complex nature of mechanical forces makes controlling them a difficult proposition. However, there is still another force to consider: shear.
What is it about shear that makes it different from other types of mechanical forces? Any mechanical force that is not exactly perpendicular to the tissues where it is applied contains some component of shear. In other words, every force vector has a vertical component and a horizontal component. The horizontal force component is known as shear.
While vertical forces typically cause compression of the tissues, shear forces cause shear between the layers of the tissues, and tend to tear and separate them. In some cases, this results in blister formation and breakdown of the fibers that tether the layers of fat and collagen together. More importantly, depending on the depth, the separation of layers can tear apart the delicate vascular tissues that nourish the skin, resulting in deep, ischemic conditions.
The separation of layers has many ramifications that further increase the danger of foot ulcers in people with diabetes. Bacteria within an ulcer can easily spread between the layers of tissues, extending more deeply into the body, following damage caused by shear. Fluid accumulated between the layers of skin can also harbor bacteria, is not easily removed and can lead to maceration.
Understanding The Biomechanical Impact Of Shear Forces
Shear forces do not represent a new factor in the development of foot ulcers. The question is how we control them in order to prevent the development or deterioration of foot ulcers.
Standard multi-density insoles that are commonly used in diabetic shoes and various offloading devices are designed to reduce vertical ground reactive forces, and more evenly distribute mechanical forces over a larger area of the foot. The compliant surface of the insole matches the contours of the plantar surface of the foot — increasing the surface contact — and reduces pressure (again, defined as pounds per square inch). However, the reduction and dispersion of vertical ground reactive forces do not necessarily reduce shear, which can be characterized by the foot shifting fore and aft. The end result is an insole device that is capable of eliminating some components of the force vector but not others.