Pressure injury: definition and etiology

What is a pressure injury?

NPIAP defines a pressure injury as "localized damage to the skin and/or underlying tissue, as a result of pressure or pressure in combination with shear. Pressure injuries usually occur over a bony prominence, but may also be related to a medical device or other object. Tolerance may be affected by microclimate, perfusion, age, health status, co-morbidities, and conditions of the soft tissue."

How does pressure injury happen?

There are multiple forces acting on the body that change the shape of the skin and soft tissue between the support surface and the bony prominences.

Superficial skin and deeper tissues are distorted and damaged. Death at the cellular and tissue level occurs, resulting in a pressure injury.

Recent studies suggest that cellular deformation, rather than just oxygen deprivation, plays a significant role in pressure injuries, raising questions about the primary cause of cell death.

Intrinsic & Extrinsic Factors

Why are patients susceptible to pressure injuries in a seated posture?

The effects of intrinsic and extrinsic factors of a pressure injury.

What are intrinsic factors?

Factors stemming from within the body that make an individual more susceptible to a pressure injury include:

What are extrinsic factors?

Factors that stem from the outside environment and/or seating surface. Pressure, shear, and microclimate are the extrinsic factors of a pressure injury. These are also factors we have greater control of.

Pressure

What is pressure?

Pressure results when force is applied on or against an object by making contact. In seating, equipment such as the seat and/or back support surface are in contact with the body, creating pressure.

What is peak pressures?

Peak pressures are high pressure directly under or against the bony prominences.

Microclimate

What is microclimate?

Microclimate refers to the temperature, humidity, and airflow next to the skin surface. The CPG notes that an increasing body of evidence suggests that the microclimate between skin and the supporting surface play a role in the development of pressure injuries.

How is microclimate created?

Increased body temperature at the skin surface, increased tissue temperature, and/or increased moisture at the skin surface due to:
• sweat
• urinary or fecal incontinence
• drainage from wounds
• sitting on a non-breathable seat surface preventing ventilation
• contact with a non-breathable support surface

How does microclimate increase the effect of pressure and shear?

Raised skin temperature and excessive moisture weaken the outermost layer of skin, making the superficial and deeper layers, tissue, muscle, and bone more susceptible to pressure and shear forces.

Elevated body temperature:

• increases metabolic rate, in turn increasing the demand of O2 to be delivered to the tissues
• pressure and shear cut off O2 supply
• as demand increases but supply decreases, ischemia occurs more quickly than when the body temperature is normal

Excessive moisture:

• increases the skin’s coefficient of friction, making the skin and deeper tissues “stick” more easily to the seat surface when shear forces are applied
• weakens the collagen fiber connections between the dermis and the epidermis and increases the risk of maceration
• increases the risk of maceration, which:
1. weakens the collagen fiber connections between the dermis and epidermis, and increases the risk of maceration and damage from stress and strain
2. makes blood vessels more susceptible to tearing and damage from stress and strain

Global experts

There are global leaders that collaborate to provide state-of-the-art recommendations for best practice based upon literature review, research, and product test standards. 

What other evidence can be considered when making decisions about pressure injury management?

Standardized tests that quantify performance characteristics of support surfaces have been developed to assess support surface capabilities, which can be connected to user's needs.

Who develops these standardized tests, and what are they?

Authorities that develop standardized testing are:
• International Organization for Standardization (ISO)
• Rehabilitation Engineering and Assistive Technology Association of North America (RESNA)
• American Nation Standards Institute (ANSI)

These multi-disciplinary volunteer organizations have published standard test methods for quantifying characteristics for mattresses and cushions. Measurements include:
• immersion
• envelopment
• heat and water vapor dissipation
• horizontal force
• and more…

At risk clients

Who is at risk for pressure injury?

There are both intrinsic (stemming from within the body) and extrinsic (stemming from the environment) factors that increase a person's risk for a pressure injury.

The CPG lists a number of contributing or compounding intrinsic factors associated with pressure injuries:

Why are patients susceptible to pressure injuries in a seated posture?

While seated, our patients are more susceptible to the extrinsic factors of a pressure injury due to:
• 40-90% more stress going directly through the spine than when standing
• 75% of weight going through the pelvis

This makes it imperative that we position our patients with the appropriate seating system to prevent them from sliding into one of the five postural asymmetries shown below, when coupled with skin changes and exposure to the extrinsic factors, place them at even greater risk for a pressure injury.

These are the five most common postural asymmetries caused by inappropriate wheeled mobility systems:

Windswept deformity

Pelvic rotation with spinal rotation

Why is a wheelchair user’s skin more susceptible to a pressure injury?

Recall that the CPG states that the primary factor associated with the risk of a pressure injury is immobility, making a wheelchair user automatically at higher risk for the development of a pressure injury.

Other factors alter skin integrity, making a wheelchair user's risk even higher.

Where do pressure injuries most commonly occur?

Most patients who develop a pressure injury spend the majority of their time in bed or in a wheelchair, placing the skin and tissue under the bony prominences at risk. These “at risk” areas are almost identical in bed and in the wheelchair, making it imperative to simultaneously prevent and treat pressure injuries from the supine and seated postures.

What can I do to minimize the risk of or to treat pressure injuries in these areas?

The use of skin protection support surfaces is necessary as part of your pressure injury management program. Unlike a basic or general use product, skin protection support surfaces are constructed with properties to maximize pressure redistribution and stability.

Support surfaces

What are "support surfaces"?

Support surfaces are any surface supporting the body for optimal posture and function.

What are we missing in the treatment of a pressure injury?

As healthcare providers—especially physicians, nurses, and specialized therapists in wound care—we understand bedside treatment well. We search for the most appropriate support surfaces and stay up-to-date with the most innovative techniques to promote healing and prevention of new pressure injuries.

However, too often we forget a critical piece of the puzzle in the fight against pressure injuries: prevention and treatment at the wheelchair level!

What support surfaces should I be thinking about with a person who is at risk?

Every surface in daily use needs to be taken into consideration:
• mattress and mattress overlays
• wheelchair cushions and back supports
• commodes/toilets
• shower bench
• travel seats and recreational seating

This guide will focus primarily on considerations for mattresses, wheelchair cushions, and wheelchair back supports.

Will any support surface protect my at-risk client?

Skin protection support surfaces, unlike basic or general use products, are constructed with properties to maximize pressure redistribution and stability for optimal posture and function. These help to minimize PI risk when any of the above risk factors are identified.

Specialty support surfaces, as defined by the CPG are, “specialized devices for pressure redistribution designed for management of tissue loads, microclimate, and/or other therapeutic functions (i.e. a mattress, integrated bed system, mattress replacement, overlay, or seat cushion, or seat cushion overlay).

Support surfaces alone neither prevent nor heal pressure injuries, but support surfaces play a significant role in an individualized comprehensive management plan for pressure injury prevention and treatment.

What are the CPG recommendations for support surfaces?

Pressure injuries (PIs) pose a significant risk for people with restricted mobility, and choosing the correct support surface is essential for their prevention and management. The Clinical Practice Guidelines (CPG) offer specific recommendations for selecting suitable support surfaces to reduce tissue deformation and improve blood flow to vulnerable or damaged tissue.

Pressure distribution methods

How do I redistribute pressure away from high-risk areas?

The risk for pressure injury increases when force is concentrated over a small surface area creating high pressures (Figure 1). By using the pressure redistribution methods of immersion, envelopment, and/or offloading, we are able to spread that same force over a greater surface area, thereby reducing pressure and the risk of pressure injury development.

Cushion construction achieves pressure redistribution in one of two methods: immersion/envelopment, or redirection/off-loading (CPG – 10 Support Surfaces)

Force, when concentrated in a small surface area, creates high peak pressures (see Figure 1). When the force is spread over a greater surface area, the peak pressure is reduced (see Figure 2).

According to the CPG, when immobility is a key issue, the seat support surface should redistribute pressure away from high risk areas. The method we choose should be based on the individualized needs of the client.

Offloading

What is offloading?

The principle of taking pressure off a small surface area and loading it onto a greater surface area that can withstand more pressure, preventing unwanted skin breakdown.

Complete and partial offloading

Complete offloading is the principle in which pressure is completely removed from a small, vulnerable surface area and loaded onto a greater surface area of more tolerant tissue to provide pressure redistribution and reduction of peak pressures.

This relocates pressure to more tolerate areas.

With partial offloading, instead of completely removing contact from the small, vulnerable area, contact is still partially made between the body and support surface. Peak pressures are greatly reduced by being spread over a greater surface area. Highest pressure is still relocated to more tolerant areas of the body.

This relocates higher pressure and increases surface area.

How is the pressure loaded onto a greater surface area?

This is achieved through the built-in contours of a cushion that help align and stabilize the spine, pelvis, and LEs to allow the pressure and shear forces to be transferred to a greater surface area. Deep posterior pelvic well, medial thigh support, and lateral tapered thigh supports are types of contours needed for offloading pressure. 

Posterior pelvic well, straight or tapered adductors, and abductors can play a roll in pressure redistribution by affecting pelvic stability, weight distribution, and muscle engagement.

Using an offloading cushion for pressure redistribution comes with both benefits and considerations.

Immersion and Envelopment

Immersion

The principle in which a material allows the body to "sink" into it and provide some pressure redistribution.

Envelopment

The principle in which a material allows the body to "sink" into it  while the material conforms to the body's shape to create the greatest surface contact between the body and the support to reduce peak pressures.

There are some key benefits and considerations to keep in mind when using an immersion and envelopment cushions. 

Comparing support surfaces

There are standardized tests for wheelchair support surfaces as well as tests for mattresses, to measure immersion and envelopment, published by both ANSI/RESNA and ISO. 

Utilizing the seating system to prevent pressure injuries

Does the wheelchair cushion alone protect against wounds?

It is true that we must utilize appropriate skin protection cushions. However, the wheelchair back support, features, and set-up (or configuration), are just as critical to create even pressure distribution in order to minimize risk of pressure injury or aid in healing. Considerations for every surface in use should also be part of the intervention (toilet, mattress, shower chair, etc).

This makes it imperative that we position our patients with the appropriate seating system to prevent them from sliding into one of five abnormal postures shown below that, when coupled with skin changes and exposure to the extrinsic factors, place them at even greater risk for a pressure injury.

What is the role of the back support?

There are multiple benefits to using a specialty back support to optimize posture and minimize the risk of a pressure injury, including:

• provides adjustability of seat-to-back angle (STBA) to more accurately match the body angles found in the assessment
• capture the curvature whether normal or abnormal for optimal upright, midline posture
• allow for even pressure redistribution to alleviate peak pressures along the spinous processes

Understanding Seat-to-Back Angle (STBA) in Pressure Redistribution

STBA is often mistaken for tilt, but it's actually the adjustable angle between the seat and back support. Unlike tilt, STBA helps accommodate range of motion (ROM) limitations by increasing surface contact with support surfaces, improving pressure redistribution. Opening the STBA can enhance support and reduce pressure for patients with the same ROM limitations.

How does a specialty back support help reduce the risk for a pressure injury?

Proper spinal support is essential to prevent pressure injuries, especially for those with abnormal curvatures like kyphosis, lordosis, scoliosis, or pelvic rotation. Using a specialty back support that immerses and envelops the spine helps redistribute pressure evenly, stabilize posture, and reduce shear forces for optimal comfort and protection.

Four common myths about pressure injuries

Pressure injuries pose a major risk for those with restricted mobility. Sadly, numerous misunderstandings about their prevention and treatment can result in poor care practices. Here are four prevalent myths about pressure injuries. Click on each to discover how we dispel these myths and examine evidence-based methods to more effectively assist those at risk.

Myth #1: A person with a pressure injury should be on bed rest

Myth #2: A scar from a previous pressure injury reduces the risk for another PI

Myth #3 Bariatric individuals have lower risk for pressure injury

Myth #4: Daily cushion checks are not required for some cushions

What is the simple equation?

Now it is clear that it is the interaction between the appropriately configured wheelchair frame, back support and cushion that creates the optimal seating system to reduce the risk for developing a pressure injury! Click here to learn more about the optimal seating system equation.

The CPG makes these recommendations about support surfaces for an at-risk client:

Implementation

The most important part of being successful in providing equipment: understand the team!

Get to know the essential team members in your community who will be crucial in this process. Your team might include a physician, a clinician (like an Occupational Therapist or Physical Therapist), an equipment provider, an equipment technician, and a manufacturer representative. Each member offers distinct expertise to guarantee that you obtain the equipment most suited to your needs which can include the physician’s referral and assessment, the occupational therapy (OT) / physiotherapy (PT) evaluation and interventions, as well as the support provided by the equipment provider.

Identifying at-risk clients for specialty equipment is crucial for preventing pressure injuries. Clinical Practice Guidelines offer standardized screening processes, enabling healthcare teams to proactively address needs through risk screenings, assessments, and prevention plans, while using protective support surfaces.

What is my role in the equipment ordering process?

No matter if you are the doctor, nurse, therapist, or ATP, you play a critical role in the recognition, treatment, and prevention of a pressure injury. Take a look at the following flow chart to understand your role and how it fits in the big picture of providing every patient with the appropriate equipment.

What if my team is not up to date with the clinical practice guidelines?

What if they don't understand the importance of proper seating equipment to prevent or treat a pressure injury?

It is important to implement education and training for proper PI prevention and treatment strategies to help protect patients. The CPG has very clear recommendations, and Permobil has comprehensive education programs to bring your facility up to speed!

How do I find a team to assist with this process?

A therapist who specializes in wheelchair seating interventions is an important partner in the process of getting the right solutions.

Summary & References

When we as healthcare providers understand these key principles and address pressure injuries from all support surfaces, then do we have a chance to win the battle against pressure injuries!

• Appreciate the role of tissue deformation in the formation of pressure injuries.
• Embrace and incorporate the guidance of global experts and authorities on pressure injuries by consulting the Clinical Practice Guideline.
• Understand the intrinsic and extrinsic factors of a pressure injury in relation to the seated posture.
• Educate yourself in the skin changes that make a wheelchair user more susceptible to a pressure injury.
• Understand the critical role addressing the seated posture and all support surfaces play in the holistic approach to the prevention and treatment of a pressure injury.
• Learn the methods of pressure redistribution, the benefits and considerations of each method, and the importance they play in the construction of a cushion and back support to prevent and treat a pressure injury.
• Acknowledge all the components in the simple equation that will result in an optimal seating system.
• Recognize your role, no matter what discipline, in obtaining the appropriate skin protection equipment for the wheelchair system.

Download your free PDF copy of the Pressure Management Guide. 

References

European Pressure Ulcer Advisory Panel, National Pressure Injury Advisory Panel and Pan Pacific Pressure Injury Alliance. "Prevention and Treatment of Pressure Ulcers/Injuries: Clinical Practice Guideline. The International Guideline." (2019).

International Organization for Standardization (ISO). ISO 16840, Wheelchair seating – Parts 1-13. International Organization for Standardization. Geneva, Switzerland.

ANSI/RESNA WC-3:2018 American National Standard for Wheelchairs - Volume 3: Wheelchair Seating. Rehabilitation Engineering and Assistive Technology Society of North America. Arlington, VA.

ANSI/RESNA SS-1:2019 American National Standard for Support Surfaces – Volume 1: Requirements and Test Methods for Full Body Support Surfaces. Rehabilitation Engineering and Assistive Technology Society of North America. Arlington, VA.

Gefen, Amit, et al. "Our contemporary understanding of the aetiology of pressure ulcers/pressure injuries." International wound journal 19.3 (2022): 692-704.

Jan, Y. K., Jones, M. A., Rabadi, M. H., Foreman, R. D., & Thiessen, A. (2010). Effect of wheelchair tilt-in-space and recline angles on skin perfusion over the ischial tuberosity in people with spinal cord injury. Archives of physical medicine and rehabilitation, 91(11), 1758-1764.

Jones, D., Radar, J. (2015.) Seating and mobility for older adults living in nursing homes: What has changed clinically in the past 20 years? Topics of Geriatric Rehabilitation, 31, 10-18.

Lange, Michelle L., and Jean Minkel. (2018). Seating and Wheeled Mobility: a Clinical Resource Guide. Thorofare, NJ: Slack Incorporated.

RESNA Position on the Application of Wheelchair Standing Devices: 2013 Current State of the Literature

RESNA Position on the Application of Tilt, Recline, and Elevating Legrests for Wheelchairs: 2015 Current State of the Literature

Sprigle, S., Maurer, C., & Sorenblum, S. E. (2010). Load redistribution in variable position wheelchairs in people with spinal cord injury. The journal of spinal cord medicine, 33(1), 58-64.