* ECPN * January/February 2001

Patient Support Surfaces
Influence of Support Surfaces on Physical Factors Pressure, Shear, and Friction

Eric Flam, PhD
NTL Associates, Inc., East Brunswick, New Jersey

The underlying physical causes of pressure ulcers are pressure, shear and friction.¹

Pressure

Excessive, sustained pressure compresses the soft tissues and blood vessels. When the blood vessels are sufficiently compressed, the soft tissues will be degraded by the resulting lack of blood supply, leading to ulcer formation. The support system must re-distribute the body weight to reduce the pressure at the sites of ulcer formation. The two means of doing this are:

*Use of static products that maintain a constant redistribution pattern. These products can be non-powered devices, such as foam, air-filled and gel pads, mattresses and overlays, or powered units, such as low air-loss beds, mattresses and overlays, and fluidized air beds.

*Use of dynamic systems that alternately load and unload the pressure at different parts of the body. The alternating frequency is usually in minutes to avoid sustained compression on any one location.

Figure 1. Action of shear between skin and contact surface

For all such products, selection of the unit for a patient must:

*Avoid the doughnut effect. This is when the entire load from one site is transferred directly to the adjacent region, which can lead to ulceration in these adjacent regions. Sites of concern include the ischial tuberosities and heels.

*Avoid bottoming out. This can cause the patient to actually be lying or sitting on the harder base below the pad, mattress, or overlay.

*Be appropriate for the patient. The clinician should match the unit with the patient risk assessment, i.e., body weight, ulcer stage, and other physiological conditions.

Shear

Shear is defined as a mechanical load that is parallel to the skin and cover surfaces.² The shear force created by the sheet or cover causes the skin and subcutaneous tissues to stretch and deform. This traumatizes the cells and distorts the blood vessels. As long as no movement occurs between the cover and the skin the shear force will remain. When this shear is sustained over a long period of time it can lead to compromised circulation and eventually pressure ulcers. Figure 1 displays a crossectional view of the action of shear on the skin.

Most pressure-reducing pads and mattresses have covers that protect them from patient discharge and contamination. These covers can produce damaging shear forces, depending on their smoothness and softness. Examples of this shear effect include:

*The hammock effect. This is when the sheet or cover material becomes taut and supports much of the body independently of the mattress. Along the sides of the contact area the sheet will stretch the skin and subcutaneous tissues, resulting in shear.

*Raised head portion. Shear may occur whenever the head portion of the bed is raised and the upper part of the patient tends to move down due to gravity.

Friction

As long as there is no movement between it and the cover, the skin is subjected only to shear. When the force that tends to move the skin against the cover exceeds the force keeping them together, the skin will drag or slide against the cover. Now in addition to the shear force, the skin also is exposed to frictional drag. This results in abrasion of the upper layers of the skin, including the stratum corneum. Again, properties of the cover material, such as smoothness and softness, can influence the frictional drag.

Influence of Pressure and Moisture on Shear and Friction

Shear and friction increases proportionally with increasing pressure between the skin and the cover or sheet material with which it is in contact. Shear and friction will also increase significantly if the skin is moist. Therefore, the ability of the support system to prevent excess moisture on the skin is very important in reducing the effects of these forces.³

Summary

The underlying physical causes of pressure ulcers are pressure, shear, and friction. The common denominator for these physical factors is pressure. The support system must sufficiently reduce the pressure, lower the covering surface drag, and keep excess moisture from the skin to prevent or successfully treat pressure ulcers. The next article will discuss the measurement of the influences of support surfaces on these physical factors. ***

References

1. Allman RM. Pressure ulcers among the elderly. N Eng J Med 1989;320:850–3.

2. Makelbust J. Pressure ulcers. Etiology and prevention. Nurs Clin N Am 1987;22:359–77.