Is Shear The New Peak Plantar Pressure?

Adam Landsman, DPM, PhD, FACFAS

   • Emphasize sock wearing to help control the interface between the foot and the shoe. The interface between the foot and the shoe is the barrier to friction. If the skin tries to move upon the insole, there is only so much sliding that can occur before the skin catches and tears either the insole or the skin itself. Socks add a small layer of cushioning. More importantly, though, they allow for a little give in the interface.

   Furthermore, socks can wick away moisture, which can infiltrate the skin and the ulcer, as well as increase the friction within the skin. Socks with no seams reduce the friction by eliminating a rigid barrier in the foot/insole interface.

Can We Measure Shear Forces?

When we assess a patient for the presence of neuropathy, we utilize devices such as the Semmes-Weinstein monofilament, which applies 10 g of pressure to a very small area of the foot. At best, this is a crude measurement that helps the clinician to predict which patients have peripheral neuropathy that is severe enough to eliminate protective sensation.

   Of course, by now, it is apparent the simple mechanical load is just the tip of the complex array of forces that strike the foot. The rate and duration of load application certainly play equally critical roles. However, these measures are much more difficult to perform and usually require instrumented insoles that can record the spatial distribution along with the change in load over time.

   Shear forces are even more difficult to measure while walking, particularly using an in-shoe device. Usually, motion analysis laboratories measure shear forces with force plates. These devices are mounted to the floor and shift almost imperceptibly when one walks across them. Most of the time, these devices can measure shear and vertical forces at very high speeds in order to illustrate the temporal changes as well. However, measurement of shear in the shoe is impractical due to the lack of rigidity in the shoe as well as the constraints on motion.

What We Can Learn From The Total Contact Cast

In this article, we have introduced a variety of mechanical forces and discussed not only the direction of force (i.e. shear versus vertical loads) but also the rate of application, duration, and acceleration or deceleration. When it comes to managing this complex array of forces, the clinician is generally most successful when he or she minimizes nearly all of these forces. Dr. Brand and his associates noticed a long time ago that the total contact cast was capable of achieving the complex level of containment required to manage these widely varied loads.2

   Consider the various attributes of the total contact cast and how it works to help ulcers to heal.

   • Total contact. The “namesake” of the total contact cast really revolves around the fact that the cast distributes the mechanical loads over the entire plantar surface of the foot as well as the sides of the foot, ankle, and the entire circumference of the leg. In effect, the surface area of the bottom surface of the foot expands to the sides, ankle and leg, thereby dramatically reducing the pressure to any given site.

   • Securing the ankle. By securing the ankle in the rigid cast, one can use the entire leg to reduce inertia. Essentially, the snugness of the cast at the ankle stops the patient’s foot from migrating fore and aft, side-to-side, thereby reducing shear.

   • Total weight of the cast limits activity. The fact is that the weight of the cast itself is a great deterrent to fast walking. Simply put, it just is not easy to get around in the cast. However, by slowing the speed of walking, the rate of tissue deformation and the duration of load are reduced.

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