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.

   In order to control shear forces, one must control the horizontal component of the force vector by preventing the foot from sliding forward and backwards, and from side to side. Shear forces are the forces that counteract the shifting of the foot on the insole. The force depends on motion and friction. The greater the friction, the greater the resistance is to the foot sliding on the insole. Additionally, one can stop the foot from shifting by counteracting inertia, the force which causes a body in motion to remain in motion. Therefore, the shear forces can be overcome with increases in friction or by locking the foot to the insole in order to overcome inertia.

   Both perpendicular and shear ground reactive forces may result in the formation of ulcers. The problem is that nearly all forces applied to the foot are complex and contain components of both vertical (perpendicular) and shear forces. Although both forces are present, they may result in different levels of damage, depending on the gait pattern, foot morphology and the characteristics of the interface between the foot and the ground.

   In order to reduce the damage, each force requires a different protective measure. Vertical forces diminish by increasing surface area, dissipating peak pressures and gradually decelerating the foot. Conversely, shear forces are more likely to be reduced by decreasing inertial effects of locomotion and balancing friction with the tolerances of the soft tissues to shear.

Keys To Reducing The Detrimental Effects Of Shear

Most physicians who treat people with foot ulcers are familiar with the benefits of a multi-density insole worn within a well-constructed shoe with a relatively stiff sole. However, what steps can clinicians take to reduce the detrimental effects of shear?

   • Fore/aft and side-to-side sliding of the foot upon the insole. This force is a result of inertial forces overcoming frictional forces, and can cause tearing and separation of tissues at the interface. One may consider several strategies to control this. First, a shoe upper that stops the foot from sliding is very helpful. For this reason, slip-on shoes are usually ineffective and shoes with laces or those that close with Velcro are usually preferred. In particular, shoes that come a little higher on the ankle may reduce slippage even further.

   Similarly, extra-depth shoes and shoes that do not fit properly can result in additional slippage between the foot and insole. Extra-depth shoes are particularly difficult because the depth is usually needed to prevent ulceration of hammertoes, but may exacerbate the problems of slippage on the ball of the foot.

   • Rapid changes in velocity with starting and stopping. When a patient walks quickly, there is greater inertia and the patient requires more friction to stop the foot from sliding, resulting in more shear between the shoe and foot. Consequently, there are clear benefits to reducing the amount of walking the patient does. Another way to reduce this shear is to ask the patient to walk more slowly. Reduced walking speed results in less inertia and less shear force on the foot.

   • 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.

In Conclusion

Simply put, shear is likely to play just as significant a role in the development of diabetic foot ulcers as peak plantar pressures. However, due to the difficulties associated with measuring shear, it is harder to know where the thresholds for injury exist. Remember that all mechanical forces consist of vectors that normally contain components in both the vertical and horizontal directions. If you understand the nature of the damage caused by each component, you can take precautions to offset these forces.

   Not all wounds require a total contact cast. However, if your current plan for reducing vertical loads is not resulting in wound healing, then consider the effect that shear forces may have as well. Utilize protective footgear that limits the excursion of the foot against the insole during walking. The patient can wear boots and fixed ankle walkers to limit motion in the ankle and lower leg.

   Finally, clinicians should reassess at each visit so no detrimental loads sneak through to impede the patient’s wound healing progress. Inspect the condition of the shoe and the insole to make sure patients are able to achieve the level of correction needed.

   Dr. Landsman is an Assistant Professor of Surgery at the Harvard Medical School. He is the Chief of the Division of Podiatric Surgery with the Cambridge Health Alliance in Cambridge, Mass. Dr. Landsman is a Fellow of the American College of Foot and Ankle Surgeons.

References
1. Yancey P, Brand P. The Gift of Pain: Why We Hurt & What We Can Do About It. Zondervan Publishing House, Grand Rapids, Michigan, 1993.
2. Coleman WC, Brand PW, Birke JA. The total contact cast. A therapy for plantar ulceration on insensitive feet. J Am Podiatry Assoc. 1984; 74(11):548-52.

   For further reading, see “Can Smart Orthotics Have An Impact In Preventing Ulceration?” in the June 2010 issue of Podiatry Today.