Editor’s Note: Ride Designs has graciously donated their blog post space to Sharon Sonenblum, a rehabilitation engineer and researcher at Georgia Tech, to share her thoughts and research findings on some very important topics: sitting mechanics, pressure ulcers, wheelchair cushions, and butts. Sharon is Senior Research Scientist at Georgia Tech, and she has been studying wheelchair use, pressure ulcer prevention, and the response of buttocks to loading for over a decade. This is the first blog post of two, so stay tuned.
On a related note, see the interview with Tom Hetzel, CEO of Ride Designs, with a manufacturer's perspective on the need for evidence-based seating systems.
Sitting-acquired pressure ulcers are a critical problem for people who use wheelchairs for mobility. More than 50% of people with SCI develop a pressure ulcer during their lifetimes, with over 20% requiring surgery. Pressure ulcers reduce activity and participation, affect quality of life, and increase the risk of premature death. And once you get one, you are at increased risk for future pressure ulcers.
These ulcers most often occur at the ischial tuberosities (the sitting bones that bear most weight), and sometimes at the greater trochanters. Our research has shown that individuals using power wheelchairs spend about 12 hours per day in their chairs, while manual users spend about 10.5 hours. That’s a long time.
You might ask, “Surely they are doing weight shifts every 30 minutes, just like they were taught?” Not exactly. In our studies, we have found that people do move around a lot, performing small tilts 3 times per hour in power wheelchairs and weight shifts (or partial unloading of the buttocks) 2.4 times per hour in manual wheelchairs. But in terms of bigger movements that unload more of the buttocks, people using power tilt-in-space only did large tilts past 30⁰ every few hours. People in manual chairs only fully unloaded their buttocks (with a depression lift or full front lean) once every few hours. Most individuals we studied in manual wheelchairs had at least one 2-hour stretch during the day without any weight shifts at all.
Now here is where things get interesting. Nearly half of individuals do NOT experience tissue break down, even though they have stretches of time without weight shifts, and even though they don’t do full pressure reliefs very often. Some individuals seem to have much more tolerance for loading than others, who seem to get away with anything without developing a pressure ulcer. I like to call this difference a person’s “Biomechanical Risk.”
What it means is that we all have different buttocks, and the same amount of load applied causes a different amount of deformation on different buttocks. For those who have palpated many rears, you might have noticed a variety of differences that I will describe here in completely non-technical terms based on my non-clinical “expertise:”
- No butt: very skinny, often a young man with chronic paraplegia, and there is almost no tissue covering the pelvis.
- Saggy butt: This individual has more tissue covering the buttocks, but it is very loose and on palpation the ischial tuberosity can easily be palpated.
- Well-padded butt: This rear end may be larger, but that isn't what makes it stand out. More important is the fact that the tissue holds up well to palpatation. It doesn’t collapse, making the ischial tuberosities more difficult to locate.
- Surprising butt: This rear end might appear well padded to the uninitiated, based on size and adipose content, but upon palpation, it is noted that the adipose does not have any ability to support load and the ischial tuberosity is easily identified and not well supported.
Genetics and demographics certainly play a role. Tissue is different in men versus women, lighter skin behaves differently than darker skin and skin compliance changes with age. Behavioral risk factors play a role, too. Smoking, dietary choices and hydration levels, for example, all can change tissue compliance. In addition, our recent MRI research at Georgia Tech showed that while some people sit on muscle, many do not – more people sit directly on connective tissue and fat.
Our research shows that if we could consistently identify someone’s Biomechanical Risk, we could prevent more pressure ulcers. MRI is a great research tool that we are using for visualizing the buttocks' anatomy and deformation during sitting, but it’s not going to help in the clinic.
So here’s what I want to know from you: what different buttocks do you see in your practice, and which ones seem to be at the greatest risk of breakdown and which one seem to have greater tolerance? Is there a clinical tool out there (besides some well-trained hands) than can be used to identify someone’s Biomechanical Risk?