University of Southern California Mrs. T.H. Chan Division of Occupational Science and Occupational Therapy
Smaller text Bigger text Print this page
Home USC Rancho USC/Rancho Pressure Ulcer Prevention Project USC/Rancho Pressure Ulcer Prevention Project
About the Project People Articles
Home | Articles in depth |
Read the basics about this subject

Pressure and Shearing

Although there are many elements that can contribute to creating pressure ulcers (which means the same thing as "pressure sores" or "bedsores"), both inside a person (such as smoking, poor nutrition, aging skin or a condition called heterotopic ossification) and outside a person (such as exposure to urine or bowel movements for too long, an injury or friction), it should be no surprise what the biggest cause of pressure ulcers is: pressure! In fact, physicians in a recent research study even defined a pressure ulcer as "any lesion caused by unrelieved pressure that damages underlying tissue." 1 Either sitting or lying in bed puts a lot of pressure on the skin over what are called bony prominences, that is, areas where bones or joints "stick out" because there is very little tissue (that is, flesh) over them; these areas include the tailbone, hip bones, spine, heels, back of the head, backs of the ears, shoulders, elbows, and inside the knees where they rub together. As an example, the amount of pressure on the heels when lying down can be two or three times greater than when there is no pressure on them; the pressure on the bottom or tailbone when sitting can be 10 to 15 times higher than when there's no pressure! The problem with all this pressure is that it forces blood out of the area 1; since blood carries oxygen to all the body's cells, the area that is under pressure can experience ischemia, that is, tissue damage due to oxygen loss, and ischemia is the start of a pressure ulcer 2. And that is how pressure leads to a pressure ulcer; in fact, the forces of pressure and shearing (which will be explained below) are the two main causes of pressure ulcers 3.

But it's not that pressure alone creates a pressure ulcer. The other ingredient that leads to trouble is time. Spending too long in the same position without relieving pressure doesn't give the body a chance to let blood flow back into an area, bringing back the precious oxygen that can prevent ischemia (and so, prevent a pressure ulcer) 2. Add to this the fact that studies have shown that people with spinal cord injuries tend to have a reduced flow of blood and a reduced supply of blood below the level of their injury in the first place 3, and it's easy to see why it's so crucial to allow blood to circulate as freely as possible throughout the body. That's why people with spinal cord injuries are taught, usually in the hospital or a rehabilitation facility, to do regular pressure reliefs, such as changing position, or use equipment that will help to relieve pressure, such as "tilt-in-space" wheelchairs, customized cushions and special mattresses or mattress pads. By doing a pressure relief, you give the blood, and the oxygen that's in it, a chance to rush back into the area that was pressured and not getting enough oxygen; in fact, the body pours extra blood into an area that was cut off for an extra long period of time, helping the area heal 2. When that happens, people whose sense of touch is good sometimes say it feels like "pins and needles," especially right after an arm or other body part has "fallen asleep."

One reason it is so important for people with spinal cord injuries to think about pressure is that they are probably not feeling pressure when it occurs. When a person's sense of feeling, or sensation, is working well, they quickly become aware when there is too much pressure on a part of their body, because there is a feeling of discomfort or feelings people sometimes describe as "pins and needles," or "my arm (or hand or leg or foot) fell asleep." Other examples are when sunglasses start to press too hard on the nose or ears and a person needs to move them, or when jewelry that fits tightly, such as clip earrings, a ring or an arm cuff, starts to pinch or "dig in" to the skin and a person needs to adjust the jewelry or take it off. That uncomfortable or tingling feeling causes a person to move around into another position, which releases the pressure and restores a good flow of blood. Unfortunately, after a spinal cord injury, sensation is limited or even missing from the parts of a person's body below the location of their injury. The result is that there is no feeling of discomfort, and so people do not know when there is too much pressure on an area of skin or a bony prominence. Without sensation, a person might not move their body, and the pressure will continue, and a pressure ulcer is a likely result.

It's very easy for pressure to become too much pressure for too long; 13 out of our 20 participants developed pressure ulcers whose chief cause was pressure. Sometimes, it was due to circumstances beyond their control and not at all their fault, such as when Brenda waited on a steel exam table for an MRI for eight hours, Robert was stuck in his wheelchair for 19 hours when he was at the airport and his flight was delayed, and four of our study participants got pressure ulcers during their initial hospitalization due to not being moved enough, which was sometimes due to urgent health considerations, such as Tom's case, who had a bullet still lodged in his spine. Sometimes pressure goes on for too long because a person chooses to put a higher priority on participating in an important activity over taking health precautions, a gamble that might be understandable when a person has a job that is very important to them or another activity that gives their life meaning, but can be risky if it leads to a pressure ulcer. Some of our participants who lost this gamble included Judy, who developed a pressure ulcer that needed surgery following a very busy time at work when she didn't take her doctor's advice to take a few weeks of bedrest because of her work obligations; Dave, who was getting up at 4:00 in the morning and working till late at night for weeks when his company was experiencing financial setbacks, which resulted in a pressure ulcer that needed surgery; and Ken, a school counselor who continued spending long hours up in his wheelchair hoping to complete the semester with his students, which also led to a pressure ulcer that required surgery.

Another cause of pressure ulcers is the type of damage to your skin caused by shearing. Shearing is like a mixture of pressure and friction (which is yet another way that pressure ulcers can be caused). Shearing is caused when two surfaces have opposing forces, which might best be illustrated by an example: When a person slides over a surface, like a bed or chair, loose skin folds over itself. (You can see what this looks like if you slowly rub your hand down your arm from your elbow to your wrist, or down your leg from your thigh to your knees or knees to ankles - but, of course, don't do this too hard, or more than once, so you don't cause shearing! Or, maybe a friend or your care attendant will demonstrate on their arm if you tell them how.) The trouble is, if your skin is still folded over itself when you stop moving, the blood supply is cut off where the skin is folded, which can result in ischemia, and that, of course, can cause a pressure ulcer. Among our study participants, both Billy and Dave got pressure ulcers that were clearly caused by shearing.

Now that you have more information about exactly how pressure, and the related force of shearing, work, it might be easier to see exactly how pressure reliefs work to prevent pressure ulcers. Although a lot of books or instructors advise doing pressure reliefs every 15 to 30 minutes, it is up to you to discover how often you need to do them to stay healthy. Some of our participants said that their bodies "told them" when it was time to do pressure reliefs, and they listened to this signal that they sensed. As one of our study participants, Mitch, explained:

When you are in the hospital, when you first get injured, they give you all these instructions, you know. "You should relieve pressure every 15 minutes and you should turn and do this, and you should exercise in range of motion." And, I mean, you can't do all this stuff. I mean, you do it in the hospital, but in real life, you can't do all that stuff. So you do what you are able to do and what's comfortable and what's normal for you that fits into your daily life.... Find ways that fit you and the way you live. Not every situation's gonna fit a person. You have to personalize every routine or class or whatever to fit that particular person.... People who are newly hurt, they have to train themselves to do certain things. And that's where that comes in, where you personalize a particular routine. But once you already trained yourself to do a certain thing and it works, then it's pretty natural after that.... I had personalized my own routine to a point where I don't even think about it now, and I relieve pressure without even thinking about it.

There are other pages in this publication with more information about some of these subjects; reading them might give you some ideas about the best way for you, in your personal lifestyle, to avoid pressure and shearing. A health care professional, like your doctor, occupational therapist, physical therapist or care attendant, might also have suggestions for you about making pressure relief a habit. Creating the right pressure relief plan for yourself can really help keep you healthy, and living free of pressure ulcers!

If you would like to look on the Internet for more information about how pressure and shearing can cause pressure ulcers, New Mobility magazine online has two articles, 21st Century Seating, from 2004, and Save Your Butt: Custom Orthotic Seating Can Do What Off-the-Shelf Cushions Sometimes Can't, from 1997 (but which was published in 2000), about how orthotic, that is, customized and corrective, seating can help reduce pressure and shearing. (Note: although orthotic seating is often good at preventing pressure ulcers, it is very expensive, and usually insurance does not reimburse the entire cost, as both articles mention.)

General websites about pressure ulcers, which include information about pressure and shearing, include Bedsores (Decubitus Ulcers), on the Aetna insurance company's "InteliHealth" website. Information on their website is provided by Harvard Medical School. A good overview of Pressure ulcers can be found on the excellent, ad-free website maintained by the University Health Care System, in Augusta, Georgia. The information on that website is provided by Healthwise, which also supplies information to WebMD, a website run by the publicly-traded company WebMD Corporation, that displays ads along with health information. The Northwest Regional Spinal Cord Injury System of the University of Washington has a page called Taking Care of Pressure Sores, which discusses pressure and shearing, and shows black-and-white drawings of the stages of a pressure ulcer. Skin Management After Spinal Cord Injury is a one-page fact sheet about pressure, shearing and other pressure ulcer subjects, on the website of the Spinal Cord Injury Association of Illinois; it was written by Dr. David Chen.

If you are interested in information that is in the same style you might see in a hospital or rehabilitation facility, an excellent resource is Pressure Ulcers: What You Should Know, created by the Consortium for Spinal Cord Medicine, which is supported financially by the Paralyzed Veterans of America (PVA). This 45-page guide is easy to read and to use. You can also download a free copy in PDF format. Úlceras por Decúbito: Lo Que Usted Debe Saber Una Guía para Personas con Lesiones Medulares ha sido producida en Español de parte del PVA por el Consorcio de Medicina de la Médula Espinal. Usted puede llamar a 1-888-860-7244.

You can also try this Spinal Cord Injury Manual, which was prepared for consumers by the Regional Spinal Cord Injury Center of the Delaware Valley, part of Thomas Jefferson University Hospital in Philadelphia, PA. Also, try Prevention of Pressure Sores through Skin Care at the Spinal Cord Injury Model System, maintained by the University of Alabama at Birmingham.

1 Dharmarajan, T. S., & Ahmed, S. (2003). The growing problem of pressure ulcers. Postgraduate Medicine, 113, (5), 77-90.

2 Noble, M., Voegli, D., & Clough, G. F. (2003). A comparison of cutaneous vascular responses to transient pressure loading in smokers and nonsmokers. Journal of Rehabilitation Research and Development, 40, (3), 283-288.

3 Consortium for Spinal Cord Medicine. (2000). Pressure ulcer prevention and treatment following spinal cord injury: A clinical practice guideline for health-care professionals. Washington, DC: Paralyzed Veterans of America.