While pressure ulcers can form in so many different places within the human body, some of the more common locations are areas with relatively little soft tissue between bones and the skin they’re pressing against. Examples include the back of the head, the elbows, the backs of the heels as they rest against a mattress while the patient is lying down.

Because of those examples, it’s easy to think pressure ulcer risk overall could be higher for wheelchair users who are underweight. And that overweight patients, who have extra soft tissue between bones and skin, could be at a relative advantage. But is that true?

And overall, what sorts of pressure considerations should seating & wheeled mobility professionals keep in mind when working with these two client populations?

Does a Lower Body Weight Raise Pressure Ulcer Risk?

For a multitude of reasons ranging from difficulties in getting adequate nutrition to high muscle tone and extraneous movements that possibly burn more calories than would otherwise be the case (see sidebar), some wheelchair users are significantly underweight. Are they automatically at increased risk for pressure ulcers on their buttocks, for example, because there is little soft tissue between their ischial tuberosities (IT) and their skin?

Jean Sayre, MSOT, COTA/L, ATP, CEAC, is the senior director of R&D clinical development at Quantum Rehab.

“One might think that a thinner person would be more susceptible to a pressure ulcer,” she says, “and it is sometimes true. If the person was to gain weight, it may help them in the preventive process — i.e., increasing their nutritional intake in return would assist in increasing the hemoglobin to decrease hypoxia. But there are too many other factors that contribute to pressure ulcer management to say that just being thinner would put a person at higher risk.”

“One can always make the case for clients that are underweight or undernourished to gain weight,” says Patrick Meeker, MS, PT, senior director of global sales at ROHO Inc. “However, gaining weight will not necessarily result in additional tissue on the buttocks, specifically in individuals with severe muscle atrophy. In fact, adding weight may increase pressure ulcer risk since the additional weight will result in the ITs having more applied force — increasing the deformation damage to the gluteus muscle and subcutaneous fat layers adjacent to the ITs.”

Meeker additionally points out that simply having more tissue between the ischial tuberosities and the skin doesn’t necessarily mean lowered risk of skin breakdown. “Spinal cord injury patients have shown significant fat infiltration occurring in their gluteal muscles,” he notes. “Changes in the tissues creates even more opportunity for damage, as fat is much softer, and the tissues are more susceptible to deformation injury.”

Does a Higher Body Weight Reduce Pressure Ulcer Risk?

On the other side of the scale, does a higher body weight — even to the point of clinical obesity — lower the risk of developing pressure ulcers?

“This could be true for some individuals who have more soft tissue under the ITs, for example,” Meeker says. “However, in the last few years we have learned the most severe pressure ulcers start deep within the body as a deep tissue injury caused by soft tissue deformation. The increased body mass that loads the pelvis escalates the susceptibility to a deep tissue injury. This is especially true when the load is not supported with a pressure redistributing surface designed to handle the heavier-than-average body weights of an obese or bariatric client.”

Sayre says that perhaps contrary to initial perceptions, clients who are overweight could be at greater risk of pressure ulcer formation, even though they have extra tissue between bone and skin.

“Studies that are available have shown that the person that has a Body Mass Index (BMI) over 40 is at higher risk for pressure ulcer development,” she explains. “Pressure ulcers are formed when a mechanical load is applied over a bony prominence for a period of time, hence causing breakdown of the sub-dermal tissues. The ‘tip-of-the-iceberg’ effect is known when the ischemia results from deformed tissue due to hypoxia that will begin in the sub-dermal layer of the skin, which will then present itself to the outer dermal skin layer ‘inside to outside.’”

According to the National Institutes of Health’s National Heart, Blood & Lung Institute, a BMI of less than 18.5 is considered underweight, while a normal-weight BMI is from 18.5 to 24.9. A BMI of 25 to 29.9 is considered overweight, and a BMI of 30 or more is defined as obese.

Sayre reports that in doing research on the subject, she has found “that there is a lower risk to develop pressure ulcers — approximately a 12.5-percent chance — for people that have a BMI lower than 40, and an approximately 26-percent chance for those that have a BMI 40 or greater.”

Adding Wheelchairs to the Equation

If body weight is a balancing act — too little weight and too much weight both possibly raising skin breakdown risk — then adding the wheelchair element can further complicate a pressure ulcer risk assessment.

“Wheelchair users in general are at risk for development of pressure ulcers,” Sayre says.

But that’s still only part of the picture.

“Body weight is a factor for many functional activities involving the use of a wheelchair,” Meeker says. “It becomes very important when it comes to maintaining tissue integrity. Pressure must also be combined with moisture management, friction, shear and heat. Therefore, pressure can be calculated by the equation: Pressure = Force/Area. Pressure is the mass of the body times gravity (weight or force) divided by the surface area over which it is distributed.”

Higher pressures aren’t the only factor that can raise pressure ulcer risk, of course.

“According to National Pressure Ulcer Advisory Panel, the mere fact of taking the interface between the body and supporting surface is not reliable for pressure ulcer development,” Sayre says. “The pathophysiology of pressure ulcers is formed by extrinsic and intrinsic factors. The extrinsic factors which would be considered primary factors include pressure, shear, friction, moisture and microclimate. The intrinsic or secondary factors are nutrition/metabolism, mobility, edema, cognition, sensation, incontinence, body type/weight, age, sleep, smoking, medication(s), neurological [conditions], etc., which are equally important as the primary factors for preventing/developing pressure ulcers.”

Understanding the Unique Factors for Wheelchair Users

Body weight and the pressure it causes for bones, tissues and skin seem to be straightforward factors that should create a straightforward math equation. But as with so many things in seating & positioning, there are nuances.

“The classic question,” Meeker says, “is who falls through the ice first when walking on a frozen pond? Is it the woman in stiletto heels or the woman in snow shoes? If their weight is the same (100 lbs.), but the surface area is 10 times smaller with the heels rather than the snowshoes, then the pressure will be 10 times greater. The concentration of pressure is very similar to a spinal cord injury client with 150 lbs. of weight with a very bony pelvis and very little soft tissue to protect it. This client requires a cushion that allows for maximum immersion, depth and envelopment to maximize surface area. The greater the atrophy, the more complicated the seating environment must become to successfully redistribute the pressure and protect the soft tissue.”

Another nuance can be the actual definitions of body weight, whether a client is considered at his/her ideal weight, over it or under it.

“Wheelchair users may find that using ‘ideal body weight’ tables is misleading regarding what their ‘ideal’ body weight should be,” Meeker says. “These tables are taken from ambulatory, able-bodied individuals. Typically, wheelchair users tend to redistribute weight from the lower extremities to the upper extremities and torso. These changes occur over time and begin rapidly after injury and/or with the use of a wheelchair for primary mobility. Body weight change is simply the difference between caloric intake and caloric expenditure combined with the catabolism that occurs with muscle atrophy.

“Additionally, these are some important extrinsic risk factors to include for consideration in tissue viability: pressure redistribution around bony prominences (ITs, greater trochanters and heels), nutritional health of the individual, minimization of shear strains and stresses on susceptible tissues, control of heat and moisture, and finally, control of trauma-induced damage to sensitive areas.”

Interventions to Minimize Pressure Ulcer Risk

Given all the pressure ulcer variables stacked against wheelchair users, it’s crucial to try to minimize pressure ulcer risks wherever possible.

“The most important objective for people who are wheelchair users to prevent pressure ulcers is to keep moving or repositioning by shifting weight through the use of manual or power mobility,” Sayre says, “whether it is leaning side to side or forward, or using the tilt and recline features in their manual or power wheelchair to assist in making it an hourly regimen. This doesn’t stop with their mobility device, but extends to their other support surfaces such as beds, cars, etc. The type of support surface also plays another important factor in pressure ulcer prevention, along with the other extrinsic and intrinsic factors.

“Skin checks should be part of a daily activity for the user and/or caregiver, and documenting skin conditions is helpful,” Sayre adds. “Some other preventive considerations would be to minimize skin exposure to moisture; to cleanse skin immediately post soiling; to avoid excessively dry skin; and to minimize pressure, friction and shearing in how the person transfers to variable support surfaces.”

Meeker says, “The first rule of thumb is to always perform at least one skin check per day using a mirror or by a caregiver. This is especially important for clients with impaired or absent sensation. Second, clients must avoid clothing that tends to collect or bunch up underneath the pelvic bony prominences. The use of thin, breathable and synthetic underwear with minimal to no seams is highly encouraged. To this end, cushion manufacturers spend a great deal of time developing covers and materials so that the cushion’s properties are not impeded by the cover material.”

Being aware of ambient temperatures is also important, Meeker adds.

“Third, clients must limit heat and moisture. This can also have a negative impact on tissue integrity. The choice of cushion materials in warm or humid climates may have a significant impact on safe sitting time. Materials with high thermal mass or insulation (R-factors) may store heat for a longer period of time. This effect will cause sudor, or sweating, based on the individual’s own personal thermoregulatory mechanisms. The increase in heat buildup can be mitigated through cushion material choices, breathable cover materials, auxiliary fans and relief areas.”

Wheelchair users can be taught to reduce heat buildup themselves.

“A very effective method for reducing increased temperature and humidity in the seat surface is a simple press-up,” Meeker notes. “Performing a press-up clears the seating surface. The cooler air has an ‘air wash’ effect, decreasing the surface temperature by several degrees. This is shown to have a significant effect based on the cushion’s ability to release heat through convection (air wash effect) and its lower thermal mass (ability to store heat). The results include lower cushion interface temperatures and decreased press-up holding times.”

And finally, wheelchair users of all shapes and sizes will be most functional using equipment that fits properly.

“One of the most critical best practices is to use a cushion that is adjustable to the anatomy, ensuring good immersion and envelopment regardless of size, weight or muscle atrophy,” Meeker says. “This reduces the risk of pressure ulcers and deep tissue injury. A cushion should also be adaptable to move and yield with the body as the person moves throughout their day and changes over time, ensuring ongoing protection.”