Seeing the Difference

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November 2003

By Jillian M. Swaine, BSc, OT

Figure 1. IPM of a man, 5’11”, 180 pounds, with no disability. The highest interface pressures (red) correspond to his ischial tuberosities while sitting on a firm surface.

Assessing a client for a new wheelchair cushion is a daunting task given the myriad of available cushions and their physical properties. The cushions must meet seating goals, of which there are typically three: to provide comfort, to enhance and correct postural alignment, and to prevent sitting-acquired pressure ulcers (SAPUs). There are many commercially available wheelchair cushions, but no one cushion has been found to provide the optimal pressure distribution for all clients. Therefore, individual assessment is required to best match a client’s needs and abilities to a cushion’s features.

SAPUs can be prevented by using a systematic assessment framework that includes interface pressure mapping (IPM). Pressure is determined by dividing an applied force by the area perpendicular to it.1 In sitting, average pressure is the weight of the client’s upper body, buttocks, and thighs divided by the contact area. The goal is to provide maximum contact area in order to distribute the pressure at the seat-buttock interface. Higher interface pressures have been associated with a higher incidence of SAPUs in the elderly.2

Clinicians describe SAPUs as pressure-induced ulcers on the ischial tuberosities. There is debate about whether an ulcer at the coccyx and/or sacrum would be considered sitting-acquired. Many clinicians believe that ulcers in these locations are more likely due to lying or semireclining in bed, unless the client sits with a severe posterior pelvic tilt. Ulcers located on other areas of the buttocks, such as the cheeks, are generally considered to be friction-related opposed to being due to pressure. Therefore, the clinician must determine the anatomical location and etiology of the ulcer during the seating assessment. Too often an expensive wheelchair cushion is blamed for an ulcer that was actually caused by another factor, for example, friction from a too-wide wheelchair.

Figure 2. IPM of a client with a right, above-knee amputation and a T10 spinal cord injury with muscle flap surgeries on both sides.

Because SAPUs are significant health issues for individuals using wheelchairs,3 a method such as IPM is used in wheelchair seating clinics and home care settings, despite a lack of protocol in the interpretation of IPM data. This lack of consensus on interpretation led to the development of an international listserv for exchanging information on interpreting IPM data, research, seating, and wound care.4

Cushion properties

There are nine different materials typically used to make cushions: foam or flexible polyurethane, rubber latex, honeycomb plastic, water, viscous fluid, gel, air, combination, and dynamic computerized or custom molded. For each cushion, there are five distinct properties that clinicians need to consider when using their assessment framework to match client needs, skills, and abilities to a wheelchair cushion: density, stiffness, resilience, dampening, and envelopment.5 Of these properties, stiffness, dampening, and envelopment are pertinent to IPM.

Stiffness is the relative measure of depth to which a client will sink into a wheelchair cushion.6 IPM will provide the clinician with peak interface pressure measurements thereby indicating whether the client is bottoming out on a cushion that is not stiff enough or sitting on a cushion that is too stiff. Dampening is the cushion’s ability to dampen or soften impacts similar to that of a shock absorber, and it affects tissue loading during activity. Remote IPM—using wireless technology—will assist the clinician to map during an activity, thereby evaluating the effect of the cushion’s dampening property.7 Envelopment is the cushion’s ability to surround or contain the buttock. IPM will assist the clinician to evaluate the contact area of the envelopment and its consequent effect on the peak pressure.

Getting Results from IPM

Widely used in wheelchair seating clinics and home care settings to help prevent SAPUs, IPM technology assists in comparing cushions so clinicians can help a client select the one that is most appropriate. In the past, the only method to ensure that a wheelchair cushion provided sufficient support and pressure reduction was to use clinical observation skills: monitor skin redness, blanching, or bruising, or place one’s hands underneath the buttocks to detect “bottoming out.” These clinical methods were not always reliable and were difficult to teach clinicians new to the field. Therefore, IPM augments these clinical observations by providing objective outcome measurements. The technology may also be helpful in the future for predicting those individuals who are at low and high risk for developing SAPUs.

IPM has been useful for providing objective data to clients reluctant to alter their wheelchair cushion or follow a pressure-relief protocol, such as doing push-ups. For example, clients with spinal cord injuries cannot feel the pressure under their buttocks, and it is often difficult for them to change from a cushion that they have been using for years. Visual depiction of their pressure distribution compensates for their lack of sensation and assists in biofeedback.

There are three major manufacturers of IPM technology in North America, all of whom use common configurations. They are computer-based systems that include proprietary software, pressure pads in a variety of sizes, an interface box, and a computer that is commonly a laptop. It is important to understand that the pressure pads differ in their technology (capacitive vs resistive).

In addition, each IPM system has standard measurement errors (hysteresis and creep) that the clinician must also understand. Creep is the increase in pressure over time, and hysteresis is the energy that is lost as something is loaded, then unloaded. It is recommended that the clinician unweight the cushion and IPM entirely between readings to minimize these effects. The manufacturers’ software has also addressed these issues. Frequent calibration is critical for some systems and should be done every 3 months in order to maintain accuracy of the sensors.

Adopting a protocol for IPM

Despite limited research, IPM is being used in clinical settings and there is a need to adopt a protocol for IPM—for collecting the data and its interpretation. Since IPM is essentially a comparison between clients sitting on a variety of wheelchair cushions, clinicians need to develop skills for comparing and ranking the various IPMs from best to worst.

The IPM protocol8 includes pressure mapping the client on their existing wheelchair cushion while documenting their posture, cushion, and wheelchair. Then, the client is pressure mapped on a firm flat surface and then on a new cushion(s). This protocol provides the clinician with a baseline in which to compare IPMs. A cushion may not provide more pressure reduction than a firm flat surface.

The IPM protocol in detail is as follows:

Place the pressure pad on top of the firm flat surface or wheelchair cushion. It is recommended that the clinician place the pad in the same orientation each time, with the cord in the front right-hand corner.
The clinician sits briefly on the firm surface to complete a clinical check of the IPM’s calibration and to check orientation of the buttocks on the computer screen.
Transfer the client onto the pressure pad. Ensure that the entire buttocks are on the pressure pad.
Check with the client to see if the sensor pad’s orientation on the computer screen makes sense to them, since the client participates in the interpretation of their IPMs. Orient the client on how to interpret their IPMs.
Ensure that the client sits on the cushion for 8 to 10 minutes.
Choose representative frames of IPM to record, or record for several minutes.
Palpate bony prominences to correlate with peak pressures seen on IPM (eg, ischial tuberosities, greater trochanters). Do not assume that the high pressure observed on the computer screen matches a typical bony prominence, especially if the client has had surgery on the buttocks, such as surgery of the muscle flap with shaving of the ischial tuberosity.
Make notes in the IPM software: clinician name, date, client name, client ID number, age, weight, height, diagnosis, presenting problem, wheelchair model and size, angle of inclination of the seat, cushion, backrest, backrest angle, and related measurements.
Take a digital photograph and insert it into the IPM posture software (front and side views).

Figure 3. IPMs of one client with a spinal cord injury sitting on three different air-filled cushions.

Interpretation of IPM for Sitting

The clinician chooses as many representative frames from the IPM of each cushion to interpret as possible. For example, it is common to use the averaged data from 100 representative frames for each cushion so long as the client remains in the same posture. The pressure distribution is represented by a rainbow of colors on the computer screen. High pressures correspond to red, pressures in the mid range are yellow, while pressures at the low end of the range are depicted as blue (see Figure 1, page 26).

Interpreting IPMs is a combination of art and science not unlike interpreting a histology slide in a pathology department. There is an element of pattern recognition to interpreting IPMs. Clients who have had surgery on their buttocks (ie, muscle flap surgery for a SAPU) display IPMs that are unique. In addition, clients with orthopedic issues such as amputations also show unique IPMs (see Figure 2, page 28).

There has been much debate in the literature and within standards committees about what parameters are valid for interpreting IPM. Since IPM compares sitting surfaces and their relative pressure distributions, three domains are commonly used:

Peak pressure index, the average of four of the highest sensor cells surrounding a bony prominence (ie, ischial tuberosity). Clinicians aim to find a cushion that provides a peak pressure index of 100 mmHg or below under their client’s ischial tuberosities. Note: many clients have peak pressure indices greater than or equal to 150 mmHg inferior to their ischial tuberosities and are wound free, but they reduce their risk with additional strategies such as leaning forward every 15 minutes or lying down during the day.
Area of the client’s buttocks making contact with the wheelchair cushion surface (in2 or cm2).
Pressure distribution asymmetries between left and right sides of the IPM, especially in the ischial tuberosities and greater trochanters or an excessive loading in the coccyx/sacrum area as compared with the ischial tuberosities. A client who has a pelvic obliquity will demonstrate higher peak pressures on one side as compared with the other side of their IPM.

See Figure 3, page 30, for a comparison of IPMs for one client sitting on three different air cushions. The IPMs have been ranked from worst to best. This client sustained a T10 spinal cord injury. He used to sit on air cushion (a). He developed a right ischial tuberosity SAPU. He now sits on air cushion (c) and has been wound-free for 2 years.

Pressure gradient at the bony prominences has been discussed as a salient domain to use when clinically interpreting IPMs; however, a definition of a “good” or “bad” pressure gradient has not been consensually defined. Pressure gradient is the spatial rate of change of pressure or how close high pressures are to neighboring low pressures, similar to looking at mountains on a topographical map.

An additional domain being tested is the dispersion index.9 This is the ratio of the area of the ischial tuberosities and the coccyx/sacrum to the rest of the contact area of the seat on the IPM.

Future for IPM for sitting

While there is a movement toward adopting an international protocol for administering and interpreting IPM, research is required to validate each IPM domain and its relationship to the development of SAPUs.

Jillian M. Swaine, BSc, OT, is an occupational therapist in private practice at Swaine and Associates in Calgary, Alberta, Canada. She is preparing for a PhD in seating and sitting-acquired pressure ulcer development.

References

Ferguson-Pell M, Parry E. Pressure mapping-uses and abuses. Seventeenth International Seating Symposium; February 22-24, 2001; Orlando, Fla.
Geyer MJ, Brienza DM, Karg P, Trefler E, Kelsey S. A randomized control trial to evaluate pressure-reducing seat cushions for elderly wheelchair users. Adv Skin Wound Care. 2001;14(3):120-9.
Tavakoli K, Rutkowski S, Cope C, et al. Recurrence rates of ischial sores in para- and tetraplegics treated with hamstring flaps: an 8 year study. Br J Plast Surg. 1999;52(6):476-479.
Swaine J. Pressuremapping@coolist.com. Rehabilitation Engineering and Assistive Technology Society of North America (RESNA) 26th International Conference on Technology and Diversity; June 19-23, 2003; Atlanta.
Sprigle S. The matching game. TeamRehab Report. May 1992:20-21.
Shaw G. Retention of supportive properties by eggcrate and foam wheelchair cushions. J Rehab Res Dev. 1998;35:396-404.
Andreoni G, Pedott A. Pressure distribution on wheelchair cushions in static sitting and during manual propulsion. J Mech Biol. 2001;1:33-44.
Taylor V. Pressure mapping clinical protocol. Canadian Seating and Mobility Conference; September 22-24, 1999; Toronto.
Sprigle S. Interface pressure measurement: applying research findings to clinical use. RESNA 26th International Conference on Technology and Diversity; June 19-23, 2003; Atlanta.

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November 2003