Is Shear The New Peak Plantar Pressure?

Adam Landsman, DPM, PhD, FACFAS

   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.

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