Interface pressure mapping

Interface pressure mapping offers visual and statistical data about pressure distribution between the buttock and wheelchair seat cushion that can be used to optimize load distribution.

Pressure ulcers (PrUs) are a prevalent, costly, and potentially life-threatening secondary condition of spinal cord injury (SCI). Sitting acquired pressure ulcers (SAPrUs) are associated with chronic sitting among full-time wheelchair users and typically appear in soft tissues enveloping bony prominences compressed by the weight of the body against the wheelchair seat cushion. Current trends in the pressure ulcer literature point to two pathways for SAPrU development.

The first pathway leads to the formation of purely superficial skin damage with no underlying damage to deep tissues. There is evidence to support the idea that superficial PrUs may develop in response to an interplay between pressure, friction, shear, motion, and microclimate conditions. The microclimate refers to the local temperature, humidity, and moisture conditions experienced by the skin at the interface between weight-bearing bony prominences, clothing, and the wheelchair seat cushion. In order to successfully prevent superficial PrUs, clinicians need to solve the skin-clothing-cushion contact problem for individual patients.

The second pathway for SAPrU development leads to the formation of deep tissue injury (DTI) with intact superficial skin. There is evidence to support the hypothesis that muscle tissue is the most sensitive to load-induced compression stresses during sitting, making the interaction between the bony pelvis and the gluteal musculature of critical importance. In order to successfully prevent DTIs, clinicians need to solve the soft tissue deformation problem associated with the seated posture.


Pressure is a common variable in both SAPrU development pathways. A SCI patient’s seated posture directly impacts the pressure distribution between the buttock and the wheelchair seat cushion. A primary goal of any wheelchair seating system is to minimize pressures experienced by soft tissues compressed by the bony pelvis. Two clinical tools are available to assess posture and the impact of postural asymmetries on pressure distribution.

The first tool is the clinician’s hands. In order to effectively assess posture and define each patient’s optimal seated position, clinicians must conduct mat evaluations. The primary goal of a mat evaluation is to identify each patient’s available range of motion for optimal sitting. The optimal sitting for pressure relief can be understood as a neutral posture that optimizes weight-bearing and pressure distribution. The results of a hands-on mat evaluation in both the supine and seated positions are a critical first step that guides seating interventions that correct or accommodate postural asymmetry.

The second tool is interface pressure mapping (IPM), a tool used during the seating assessment process that provides visual and statistical data representing the pressure distribution between the buttock and the wheelchair seat cushion. This data, when utilized properly, can assist a clinician in determining how successfully a seating system optimizes load distribution. Consider, for example, the case of a paraplegic patient who presents with flexible postural deformities including posterior pelvic tilt and thoracic kyphosis. IPM can be combined with the seated portion of a mat evaluation to provide a visual representation of how load is distributed (IPM #1/#2, see page 14). In the case of IPM #1, the lack of posterior pelvic support results in uncontrolled posterior pelvic tilt and load distributed to the coccyx/sacrum. In the case of IPM #2, posterior pelvic support provided by the clinician during the seated assessment corrects the flexible deformity, leading to the redistribution of load away from the coccyx/sacrum. Using IPM to visually confirm the presence and flexibility of postural asymmetry is one of many ways IPM can be utilized as an adjunct to seating success.

Using interface pressure mapping

Using interface pressure mapping (IPM) to confirm the presence and flexibility of postural asymmetry is one way IPM can function as an adjunct to seating success.

Teaching SCI patients how to manage the skin-clothing-cushion contact problem is critical for superficial SAPrU prevention. First, patients should be educated that clothing choices directly affect the risk of developing superficial skin damage. Clothing made of natural textiles such as cotton generally have greater coefficients of friction, absorb moisture well, and have longer drying times. In contrast, clothing composed of synthetic textiles such as polyester generally have lower coefficients of friction, transport moisture well, and have shorter drying times. Practical recommendations for clothing selection include reducing the number of layers of clothing worn; avoidance of thick seams, pockets, or closures; and matching clothing material characteristics to the needs of the individual.

Second, SCI patients should be educated about appropriate moisture management strategies. Excessive moisture on the skin can lead to skin maceration, reduced shear strength, and increased friction. Practical recommendations for moisture management include implementing a consistent bowel and bladder routine, changing clothes regularly if wet from incontinence or perspiration, and maintaining responsible bathing habits that emphasize avoiding excessively dry or wet skin.

Third, SCI patients should be educated regarding effective movement strategies during scooting or transferring on and off the seat cushion. Current best practice movement strategies include emphasizing the importance of slow and controlled body movements. Emphasis should also be placed on moving the body weight anteriorly in order to take advantage of the pelvic anatomy by shifting weight off the curved ischial tuberosities.

Finally, clinicians must understand the properties of cushion materials and the impact they have on the microclimate. Cushions are composed of varied materials or combinations of materials that include but are not limited to foam, viscoelastic foam, solid gel, viscous fluid, and air. It is important to understand that each of these cushion materials presents with varied thermal and shear related characteristics that directly impact superficial skin integrity.

Thermal characteristics refer to the impact that each cushion material has on the contact temperature between the buttock and cushion. It has been suggested that as skin temperatures increase over time with prolonged sitting, the skin’s tolerance to superficial injury decreases. Foam cushions are poor conductors of heat, contributing to increases in skin temperature, whereas viscoelastic foam, solid gel, and viscous fluid cushions are better equipped to conduct heat away from the skin. Air cushions have limited heat conduction capabilities, however, their thermal properties can be improved with an appropriate cover. It should be noted that regardless of the cushion material utilized, the cushion cover plays a significant role in the transfer of heat away from the buttock. Therefore, clinicians should recommend cushion covers that stretch to promote proper fit and air exchange.

Shear characteristics refer to the cushion material’s ability to manage the stress that deforms compressed skin tissue during sitting and moving on the cushion surface. Superficial skin damage can result whenever the shear stress that is applied to the skin exceeds the shear strength of the skin. Foam and viscoelastic foam cushions tend to contribute to increased shear forces due to their propensity to grab onto bony prominences during scooting and transferring. Solid gel, viscous fluid, and air cushions may decrease shear forces experienced during scooting and transferring due to improved ability to move with the patient. Again, the cushion cover plays a significant role in managing shear stress, as a poor fitting cover that is made of high coefficient of friction materials will result in increased shear strain.

IPM view #1 IPM view #2

IPM view #1 (top) depicts lack of posterior pelvic support results in uncontrolled posterior pelvic tilt and load distributed to the coccyx/sacrum. IPM view #2 (bottom) depicts posterior pelvic support rovided by clinician during the seated assessment that corrects the flexible deformity, leading to the redistribution of load away from the coccyx/sacrum.


Numerous risk factors have been identified for the development of DTI in the SCI population. First, muscle atrophy has been linked to greater susceptibility to DTI. Unfortunately, muscle atrophy is a hallmark of the SCI diagnosis, as it has been documented that SCI patients experience up to 56% atrophy of muscle and 24% decrease in muscle cross-sectional area in as little as 6 months post-injury. In order to prevent such rapid reductions in muscle mass, it is recommended that patients participate in regular functional electric stimulation (FES) sessions that include the stimulation of the gluteal musculature. FES has been shown to prevent disuse muscle atrophy and may contribute to improved DTI resistance.

Second, continuous or unrelieved sitting leads to potentially damaging muscle tissue deformations that may contribute to DTI onset. SCI patients must be educated regarding effective pressure-relief strategies including type of pressure-relieving maneuver, duration of the maneuver, and time interval between maneuvers. Unfortunately, the current pressure relief recommendations are largely based on expert opinion rather than definitive data. To better educate patients, more research is needed to determine a critical time threshold for DTI onset. Current evidence regarding manual pressure relief strategies supports the use of the forward lean or anterior weight shift every 8 minutes to 15 minutes for 3 minutes to 5 minutes in duration.

Finally, recent literature suggests that selecting the right cushion can delay the onset of DTI. More than 200 cushions are currently on the market, and no single cushion has been identified as clinically more appropriate than others for use among the SCI population. IPM is a tool that can assist clinicians in identifying appropriate cushions that redistribute load away from soft tissue compressed by bony prominences. Specifically, IPM should be used during seat cushion trials in order to make relative comparisons between surfaces and to rule out cushions that fail to optimize load distribution. When comparing cushions, it is critical that clinicians implement a consistent IPM protocol built on defined application procedures and the use of reliable statistical measures for data interpretation.

Successful SAPrU prevention must address pathway-specific factors that begin with the wheelchair seating system but certainly do not end there. Specifically, clinicians must look inside and outside the wheelchair seating system along all stages of the continuum of spinal cord injury care in order to foster prevention success. It is of critical importance that clinicians empower patients to take responsibility for their individual prevention needs by teaching patients that skin care is a routine activity of daily living. Overall, SCI patients should be taught to minimize their lifelong SAPrU risk by recommending regular skin assessment and routine follow-up care with clinicians who specialize in SCI medicine and wheelchair seating.

Sean McCarthy, MS, OTR, ATP, is an occupational therapist at the Kessler Institute for Rehabilitation in West Orange, NJ. He is the clinical manager of Kessler’s Spinal Cord Injury Program. McCarthy became a RESNA certified Assistive Technology Professional (ATP) in 2003 and has practiced exclusively with the spinal cord injury population at Kessler throughout his 11-year career. For more information, contact .