ATP Series

Friction and Shear

Understanding what shear is and how to lower client risk is an evolving progress.

friction and shear


The challenge in understanding shear, its risk factors and implications is that those concepts are moving targets, no pun intended. What the healthcare industry knows about shear and the damage it can cause is evolving rapidly. Consider: Just a decade ago, there was little enough understanding of deep tissue injuries (DTI) that the National Pressure Ulcer Advisory Panel (NPUAP) referred to them as “Suspected Deep Tissue Injuries” (see sidebar). In 2016, the terminology changed as a result of an improved understanding of what DTI are and how they can be caused. Shear, of course, is one of the possible causes of DTI. And while shear is better understood now than just a few years ago, there is still considerably more to learn.

Definitions: Shear vs. Friction

Curt Prewitt, MS, PT, ATP, is the Director of Education for Ki Mobility.

“It’s still an evolving thought process in terms of what’s really going on,” he said of shear. “It seems the jury’s still out sometimes in terms of some of the mechanisms and what is indeed happening, but there are some common schools of thought.”

A good place to start, Prewitt suggested, is differentiating between shear and friction.

“Shear is a deformation of the tissues, which could be superficial and visible, but most commonly in terms of how it affects us in this industry, it’s probably deeper and not as visible and not as immediate in its effect,” he said.

“That’s opposed to friction. Think of tripping in the hallway: We put our hands out, catch ourselves, slide a little on the floor and get a bit of a friction burn. We see what at the surface appears to be the same type of redness we might see from a pressure injury due to problems with bloodflow. But [the friction burn] is actually a very different type of injury. It’s very superficial, and it’s not necessarily related to long-term exposure issues that you would see from lack of weight shifts and so forth.”

Compare that injury with the deeper involvement of shear.

“Shearing is basically layers of tissues sliding laterally, one in respect to the other,” Prewitt said. “If you cut a cube of Jell-O and sit it on a plate, and if you push on the top corner of one side of that piece of Jell-O, the bottom of it, where it’s stuck to the plate, would stay on the plate. But the top would lean over; it would be deformed. Instead of a cube, it would be more of a rhombohedral shape. It would tilt.

“If you think of that Jell-O as having multiple layers — fat, muscle, other tissue types — and each layer slides on itself, each layer has a connective tissue that joins one to the next, and every cell joins to other cells nearby. When we have shear going on, the thing to really be aware of is shear strain. Shear is the physical action that’s happening. Shear strain is the result of that physical action; it’s the damage that’s created. It is the deforming of the normal alignment or normal cell structure.”

Shear & Pressure Injuries

Historically, pressure injuries have been blamed on lack of bloodflow. The typical pressure injury example might be a person with compromised mobility who sits or lies in the same position long enough that the compressed tissues are starved of oxygen due to lack of bloodflow.

But Prewitt said that compression isn’t the only result.

“Think of your ITs [ischial tuberosities] or any of your bony prominences,” he said. “In unloaded buttocks in the lying down position, the gluteal tissues are a certain profile. And then when they’re weighted, by lying on a mattress for example, that bony prominence pushes on that tissue. Some of the pressure in traditional pressure ulcers that we’ve seen is producing shear strain effects as well. Because when [the IT] squishes the muscle tissue, it’s not just cutting off the blood flow. It’s also literally pulling on those different tissue layers and creating some of that shear strain.

“How much of an effect is actually going on is a little undetermined, I think. But shear strain has become a bigger topic. Some of what we thought was just lack of bloodflow to an area, for example, is not just that. It’s in combination with what has happened to individual cells in the tissue layers because of shear strain that has caused those layers to get damaged.”

Think back to that Jell-O analogy.

“If that was actually tissue and it was being pulled sideways because someone was sitting on their seat cushion and sliding,” Prewitt said, “one side of the tissue layer is anchored to the seat, and the other is being pulled on by its chain link connection to the ITs. Things get stretched, and cell walls don’t maintain their normal integrity. So it’s not just because there’s no longer blood to an area. It’s because it’s been mechanically pulled apart.”

Movement & Micro-Movement

It might be a new idea, that injuries long believed to be caused solely by compression are actually also caused by shear. But experienced clinicians and ATPs know that movement abounds, even when clients are thought to be immobile.

Prewitt referenced recently pressure mapping a wheelchair user who was supposedly sitting still.

“We highlighted their center of mass sitting still on the chair,” he said, “and it’s moving. Very minute movements, but even when they’re sitting still, they’re not truly sitting still.

“There are slight changes because of external forces or because of internal balancing forces, muscular reactions and so forth. There are always minor movements happening. The chair they’re sitting in is moving around and causing vibrations, which they have to stabilize and counteract. Gravity would cause us to fall backwards if our trunk muscles weren’t holding us up. So even in someone with a spinal cord injury, who doesn’t have any movement below a certain level, there’s still movement going on because of what’s going on above, where there is voluntary muscular control.”

In a very literal way, everything is connected.

“It’s impossible to completely isolate what happens in one area of the body from affecting some of the other things biomechanically down the kinetic chain,” Prewitt said. “You move an arm, and your butt’s going to do something to counteract or stabilize the moving of that arm.”

An additional danger of DTI is that they’re typically more difficult to detect than pressure injuries that begin with the top layers of skin.

“At the surface, they don’t look much beyond a Stage 1 pressure ulcer,” Prewitt said. “The skin’s kind of reddened. But when it progresses, it suddenly opens up and it’s full-blown — what would otherwise be known as a Stage 4. All the damage has been going on under the surface without anybody necessarily being aware of what’s going on in the deeper tissue layers. It’s suddenly a fatal risk for some people.

“My suspicion is that a lot of deep tissue injuries are a result of shear strain. The damage is not happening at the skin and moving inward, but it’s starting — and using an IT as an example — it’s starting at the IT and working towards the surface.”

Shear Risk Factors

Risk factors for shear, especially at intensities that can damage tissues, include impaired sensation as well as cognitive impairment in wheelchair users, Prewitt said: “Functionally, it becomes the same thing. They don’t have a way to recognize that there’s an issue going on. Because they’re dependent for care, whether they cognitively don’t know to weight shift or are physically incapable of weight shifting or offloading. Research has shown that with tilt-in-space chairs, whether someone is dependent for it or they do their own tilting schedule, they don’t do as much as recommended in terms of angle, nor do they spend as much time there as is recommended.”

Prewitt described a continuum of shear risk, starting with the fact that complex rehab wheelchair users spend a lot of time sitting. Poor nutrition, poor circulation, loss or atrophy of muscle, heat and moisture can all raise the chances of shear-related injuries. Transfers done with poor mechanics — dragging instead of lifting — can also raise injury risk, though active wheelchair users who transfer a number of times per day might counteract some of that risk simply by being more active.

“They certainly are at risk, as is anybody in a chair,” Prewitt said. “But if you put it on a continuum, the more active user is probably less likely to have some of those issues purely because of their activity levels. The more dependent a person is, for their mobility, for their transfers, etc., probably the higher at risk they are. The more active they are, the less risk.”

Technology & Shear Risk

Referring to the role that complex rehab technology (CRT) can play in reducing shear risk and related injuries, Prewitt said, “Proper-fitting equipment, properly supportive equipment to facilitate a stable posture, is probably one of the biggest things that can be done. We talk about movement that occurs, and that everybody, even sitting still, is always moving. But if you’ve got someone who’s at risk, the more you can do to have them not move around any more than necessary, the better their chances are. So, proper offloading of the bony prominences or directing of the load for proper fit to facilitate a stable posture. And whether we’re talking about offloading by tilting a chair for pressure purposes or simply for better mechanics in self propulsion, you can change seat angle to make someone more stable in their chair. It’s not just because of pressure. If you put them at a slightly different seat angle, they have suddenly become more stable against gravity, and there’s less movement of their ITs underneath them.”

That said, every CRT client is different. Combine that with the still-evolving nature of understanding shear, and Prewitt admitted, “It’s so complicated. Every individual is just that — an individual. It becomes very difficult to start making generalizations because the sample size is still relatively small. There’s a lot of traditional pressure ulcer history out there, but understanding the deep tissue injury and some of the effects of shear strain is relatively new. And by relatively new, we mean in the past 10 or 15 years, starting to say, ‘Wait a minute: This wound isn’t the same as that wound in terms of how it progressed. Why?’

“There are a lot of whys being asked without a lot of answers to the whys yet.”

This article originally appeared in the August 2018 issue of Mobility Management.

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