Considerations on Friction & Shear in Human Soft Tissue

A pressure sore or pressure ulcer is mostly caused by three combined extrinsic factors: Pressure, shear and a non-physiologic microclimate (National Pressure Ulcer Advisory Panel, npuap.org). We generally understand that pressure is an important causative parameter for pressure ulcers, but we might not know that the other two factors are commonly present as well — it’s not necessarily that they’re less critical, but they have not as well been researched yet. Therefore, a pressure ulcer should probably be called a pressure/shear/micro-climate ulcer, or a “PSM” trauma.

We’ll concentrate here on the shear component of PSM trauma, because it is often misunderstood or mixed in with “friction” or “chafing” or excessive “rubbing” or “abrasion.” All of these can be involved in the application of shear as a root cause or as an effect, but they are not shear itself.

Then we’ll talk about how a particular technology can address this cause of PSM trauma and provide effective intervention.

Understanding Pressure, Friction & Shear

Pressure that occurs for too long can lead to blood vessels being occluded, and can result in vertical distortion of tissues between a support surface and a non-compressible structure, such as bone. Since the damaging effects are not immediate, regular changes of position can stop localized pressure build-up (Figure 1b).

Applied shear force or shear stress, on the other hand, produces immediate and potentially more harmful shear strain – i.e., distortions — to both blood vessels and cell tissues. Shear strain that happens around our skin where it meets the support surface when we sit or lie down will distort the cells of the skin. Hence, it is important to select the support surface carefully, because the degree to which the support surface can absorb these distortions will reduce the distortions to the skin’s cells (Figure 1c).

Static friction resists when two objects attempt to slide on top of each other. Friction acts as a force transfer mechanism when a shear force is applied.

Dynamic friction occurs when the shear forces reach the point that the friction between two surfaces cannot hold them together any longer. The surfaces then start to slide on each other. Even when it is somewhat lower, dynamic friction can still initiate damage to the cells of the skin. The materials of the support surface will have a major influence on the potential damage caused to the skin when movement occurs across the surface.

decubitusulcervictims.com/pressure-sore-statistics).

Therefore, the need to reduce or avoid pressure and shear as much as possible is clear and justified.

Figure 4: High-friction pairing.

Here’s an example of high friction: A hand, palm-down, that’s pushed down on a highly polished surface (e.g., a nice table, a glass plate) and pushed forward at the same time significantly resists sliding. Why is that so? We can set a heavier mug on that same table and push it around more easily! Why does the hand “stick” to the table and the mug not so much?

It is because there is a higher friction between skin and the table than between the mug and the table.

A tangential force applied to skin, which is already weight bearing (think of a seated person) will therefore build and create stress and strain — at least in the skin surface, and potentially in the tissue below the skin. This is because all the biological tissue below the skin is connected to the skin on one side and to a bone on the other side.

So If All This Is Natural, What Can We Do About It?

Why don’t we find a way to eliminate friction? What would happen if we took away most or all of friction?

We certainly would not have a static situation anymore, because friction, the counter force, is much smaller than the applied shear force. We’d create a kinetic situation (sliding), and the stress and strain on the tissue (now not adhering to the support surface any more) might be significantly reduced or possibly eliminated.

Shear stress and strain basically could not build in this case, because friction, the necessary force transfer mechanism, would not be available.

Here’s another way of looking at it.

A child dressed in shorts — and therefore, with bare legs — tries to slide down a slippery metal slide on the playground. For some reason, this does not work. The child sits on the sloped slide and cannot manage to slide down because her bare skin sticks to the slide surface due to high friction. The result is a lot of pulling (shear stress and strain), squeaking, heat and related pain on the posterior skin and muscle tissue of the child’s legs.

Now in a similar scenario, imagine the child is dressed in a pair of long pants. The child easily slides down the slide, because the pants covering the skin have considerably reduced the friction (and the shear) in the contacting area. So this is a kinetic situation under basically identical circumstances. The only thing that changed was the friction, and that happened by introducing a friction reducer.