Seating Systems as Extrinsic Enablers for Assistive Technologies

Center of Gravity

Center of Pressure

Compression

Dampening

Density

Envelopment

Equilibrium

Fixed Deformity

Flexible Deformity

Force

Frictional Forces

Fulcrum

Gravitational Line

Linear

Line of Application

Mobility

Pelvic Obliquity

Pelvic Rotation

Planar

Pressure

Pressure Ulcer

Recovery

Resilience

Rotational

Scoliosis

Shearing

Sliding Resistance

Stability

Stability Zone

Stiffness

Stress

Tension

Windswept Hip Deformity

Needs Identification

Figure 6-1 lists the desired outcomes of the identification of needs. It is important to determine exactly what an individual’s specific needs are regarding seating. From the identified needs, goals to be addressed by the seating intervention are developed. It is the ATP’s responsibility to facilitate the identification and prioritization of these goals. Design of a seating system sometimes involves compromising the various goals. For example, desired biomechanical alignment may not be possible for a person with severe postural deformities when the resulting properly aligned position is too uncomfortable. Any assessment with the goal of identifying seating needs and recommended technology starts with discussion of the occupations the user wants and needs to complete while using the seating system. A general measure such as the Canadian Occupational Performance Measure (Law et al, 1997) provides a systematic means of discussing key occupations in the area of self-care, productivity, and leisure. There are some measures that are specific to seating and wheeled mobility, including the Functioning Everyday in a Wheelchair measure (Mills et al, 2002). The Wheelchair Outcome Measure (Miller, Mortenson, and Garden, 2006) is a new measure that considers function in self-care, productivity, and leisure specifically from the view of an individual who uses seating and mobility devices.

The level of assistance an individual requires to use the seating system is an important consideration in the assessment. Consideration must be given to whether an individual can transfer to the system and fasten any straps independently when he or she expects to use the seating system independently. The complexity of the system and the ease of access influence the demands placed on an individual providing assistance with a transfer.

Functional skills, including transfers to and from different surfaces (e.g., bed to wheelchair, car to wheelchair), self-care skills (e.g., feeding, dressing), wheelchair mobility, written and verbal communication skills, and bowel and bladder care should be evaluated. Equipment the person will use while in the seating system needs to be taken into consideration. For example, respiratory equipment and augmentative communication devices are frequently mounted on the wheelchair and need to be in a position that is functional for the user.

It is important that the individual’s ability to perform functional activities be evaluated both in the existing system and in a simulation of the proposed system. By observing the consumer performing functional activities from his or her existing system, the ATP learns two things. First, the ATP can determine the consumer’s level of independence and areas where function is impeded. The ATP can also learn what strategies the individual currently uses to complete functional activities. By using the methods described below, the ATP can then simulate different positions with the consumer. The ATP can have the individual perform functional tasks while in these simulated positions. Changing the sitting position will affect the person’s ability to perform certain activities. It is important to select a system that maximizes the person’s function and does not interfere with the use of strategies that have proven to be beneficial. For example, a teenager who uses an abnormal asymmetrical tonic neck reflex to operate a single switch should not be prohibited from doing so unless another movement can be found that accomplishes this task. It will sometimes be necessary to trade an ideal seated posture for a posture that allows the individual to be more functional.

Human Factors

Sensory and Perceptual Skills

Vision and visual perception, as discussed in Chapter 3, contribute to a person’s balance and sitting posture, and deficits in these areas need to be considered during the evaluation. The configuration of the seat can affect the user’s line of vision. For example, an individual with poor postural control who is unable to maintain spinal extension with consequent neck flexion may not be able to maintain the head in an upright position if the seat to back angle is set at 90 degrees. The user’s line of vision will be downward in this seating configuration. A person’s awareness of body position (proprioception) in space also influences body posture.

Tactile sensation is another factor to consider. Some individuals may react defensively to the touch of certain textures or positioning components on the body. Other individuals lack tactile sensation, which can contribute to skin breakdown. The ATP should determine whether there is any known decrease in sensation, particularly in the buttock area, and whether there is a history of pressure ulcers. The condition of the person’s skin on weight-bearing surfaces (including areas on the trunk that are braced by lateral supports) should be checked for evidence of skin breakdown, circulation, color, smoothness, sensation, and moisture (Tredwell and Roxborough, 1991).

Cognitive Skills

Cognitive skills such as problem solving and motor planning are not as much of an issue in seating as in mobility. However, there are a few areas that require consideration. Individuals with poor safety judgment may not be aware of the need to keep a positioning belt fastened, and special considerations may be necessary. When the seating system is complex, understanding the client’s cognitive abilities will aid the decision to teach the client or the caregiver about the proper use of the system. Knowing the individual’s language and communication skills (see Chapter 11) will help determine how the ATP gathers information during the evaluation. For example, if a person relies on an augmentative communication device or on yes/no responses, these modes of communication should be used during the evaluation process. If it is known that the consumer is not reliable in his or her responses, then the ATP should seek assistance from a caregiver in interpreting the consumer’s responses to the seating system.

Psychosocial Factors

The meaning that technology holds for the individual is an important factor to explore with the user, although it is more significant for the mobility component of a seating and mobility system. Many clients prefer technology that does not draw attention to a disability. This preference will be a factor in the selection of a seating system. Esthetics is an important factor in acceptance and rejection of the technology (Pape, Kim, and Weiner, 2002). Behavioral problems, such as an agitated person who throws himself against the back of the chair, can also present a safety problem that needs to be addressed. Working together with the consumer and the caregiver to address these concerns is essential.

Environmental Considerations

Matching Device Characteristics to a Consumer’s Needs and Skills

The information that has been gathered regarding needs and skills provides a profile of the user. It can then be determined which of the three categories in Figure 6-1 matches the consumer’s profile, which allows identification of potential technologies and evaluation of their effectiveness in meeting the consumer’s needs.

The next step is to actually simulate with the consumer one or more of the alternatives. The ATP can observe the effects of changes in body position and materials by having the consumer try variations of the positioning system. Trial positioning is also helpful for assessing the person’s ability to use control interfaces such as the joystick of a power wheelchair. Changes in position can be made to see whether there are beneficial or adverse effects on the person’s ability to control a device or perform other functional skills. Simulation makes it easier to document the need for and effectiveness of a particular system so that funding can be obtained. If specific cushions or positioning components are being considered for a consumer, it helps to have him or her try the actual product and determine whether he or she likes it. In some instances it may be desirable for the consumer to take the system home for a trial period, which allows the person to use the system over a longer period and in his or her natural environment.

There are several critical questions that can help the ATP evaluate the effectiveness of the technologies that have been simulated and to select an appropriate seating system for the consumer. These questions, which summarize the needs evaluation, the skill assessment, and the simulation, are shown in Box 6-2. The primary concern is whether the simulated seating system meets the goals identified during the needs assessment. The ATP should consider the extent to which the system achieves desired goals with respect to positioning, support of function, and comfort. The caregiver’s ability to lift, carry, and maintain the seating system is a further factor to consider. A system that does not meet these goals to the satisfaction of the client of the caregiver will not be used.

Kinematics: Study of Motion

When seating systems are designed, the position of the consumer, the position of the seating system components, and their movements should be considered. The term kinematics describes movement. The term displacement is used to define the position of a body in space; a change in displacement results in a new position. For example, in a postural support system, one goal is to bring the trunk to a midline position. This action may require a displacement from the rest position to midline by application of an external lateral trunk support. The rate of change in displacement is called velocity. It is also important to know how fast the velocity is changing (increasing or decreasing); this change is called acceleration. One of the most common accelerations is that of gravity. The term gravity actually refers to the acceleration of an object toward the center of the earth. Acceleration of an object is directly related to the force generated by the object’s movement.

There are two fundamental types of displacement: linear and rotational. When all parts of a body move in the same direction, at the same time, and for the same distance, the movement is linear (Low and Reed, 1996). For example, a person generates translational movement when walking. Displacements caused by external positioning components can also be translational. If the direction, distance, and time of the movement occur simultaneously, but the movement is through an angle instead of in a straight line, the movement is called rotational. Rotational movements occur around an axis called the fulcrum. The majority of body movements are rotational, such as hip or elbow flexion and shoulder flexion or extension. Some positioning components cause rotational displacements (e.g., reclining the back of a wheelchair causes rotation at the pelvis and hip).

Kinetics: Forces

Force is a major element in biomechanics and seating for individuals with disabilities. Force is anything that acts on a body to change its rate of acceleration or alter its momentum (Low and Reed, 1996). It is described by both magnitude and direction (Sprigle, 2000). Forces always occur in equal and opposite action-reaction pairs between bodies, although it is often convenient to think of one body being in a force field. Forces can be applied to the body internally or externally. Internal forces are generated inside the body, such as muscle contractions that cause movement of the joints. Externally applied forces come from outside the body and act on it in some way, such as the forces applied by a support surface and components of a seating system such as lateral supports. The force resulting from the acceleration of gravity is another external and ever-present force that acts on the body and influences its posture and movement (Sprigle, 2000). This force on the body acts along a line called the gravitational line, and its effect is localized around a point in the body called the center of gravity. The force of the earth’s gravitational field tends to pull the body toward the center of the earth and must be accounted for in designing a seating system. The center of gravity changes as posture changes from standing to sitting and in different sitting positions.

Four properties of force, which ultimately determine its result, are magnitude, direction, line of application, and point of application.Magnitude is the amount or size of the force measured in newtons, pounds, or kilograms. Forces are applied in some direction, either pushing or pulling, and are applied along a particular line of application. The force acts at a particular point on the body, called the point of application (Low and Reed, 1996).

Sitting Posture and Center of Pressure

Stability and mobility are two related dimensions of seated postural control. Stability allows an individual to maintain an upright seated position while mobility allows movement that enables function; for example, mobility allows the individual to lean forward to reach to shake a friend’s hand. Seating interventions for postural control must achieve an optimal balance between stability and mobility.

Two constructs are important to consider when discussing postural control: center of gravity and center of pressure. The location of the center of gravity is fairly well defined in standing. Its location is described as passing through the mastoid processes of the jaw, a point just in front of the shoulder, a point just behind the center of the hip joints, a point just in front of the center of the knee joints, and approximately 5 to 6 cm in front of the ankle joints (Figure 6-3). In this posture the pelvis is in a neutral position and there is a natural lordosis of the lumbar spine (Zacharkow, 1988). The location in sitting is more difficult to determine, but it is usually considered to be lower, with the buttocks and thighs forming the base of support. The individual must maintain the center of gravity over the base of support to maintain an upright posture in either sitting or standing. Seating interventions for postural control assist the client to keep the center of posture within the limits of the base of support.

It is not practical to measure or monitor the center of gravity in the clinic. The center of gravity is defined by three-dimensional coordinates. The center of pressure is described only in the horizontal plane, which makes it a much more clinically useful outcome. Its location in the frontal and lateral planes can be identified and monitored in the clinic by using a pressure mapping system. These systems use various technologies to monitor the pressure between the individual and a support surface (i.e., between the client’s buttocks and thighs and the seat cushion). They are most commonly used to show pressure distribution when pressure-relief cushions are evaluated, so their function will be described in greater detail in that section.

As mentioned above, the aim of postural control in seating intervention is to provide the client with a functional upright position (i.e., provide enough support to enable him or her to retain a seated position but also to enable sufficient movement to promote function in sitting). Monitoring of the center of pressure during quiet and active sitting is one way to evaluate the outcome of specific seating interventions. Discussion of the center of pressure is a relatively recent occurrence in the literature. The ideal location of the center of pressure is midway between the ischial tuberosities. Dunk and Callaghan (2005) found that the location of the center of pressure in the frontal plane varied between men and women. They studied various sitting postural parameters of university students engaged in computer activities while sitting on different office chairs. They found that the center of pressure was behind the center of mass of the chair for men and ahead of it for women. This finding has interesting implications for seating intervention, although it has not been explored.

Parkinson, Chaffin, and Reed (2006) describe the stability zone or limit, which they define as the balance limits for a person in either sitting or standing. A seat back and laterals or armrests will affect the stability limits in sitting. The authors initially hypothesized that the stability was limited laterally by the ischial tuberosities and posteriorly by the coccyx in the absence of these system features. The thighs provide support when the individual is reaching forward. Age, strength, and range of motion were identified as additional factors that affected the stability zone. They quantified the center of pressure during a lateral reaching task with a sample that included both young and older individuals and subjects with a body mass index range from underweight to obese. The greater trochanter, rather than the ischial tuberosities, was found to be more indicative of the stability zone because subjects shifted their weight laterally as they reached. Stability during reach was also affected by age, reach direction (lateral and forward reach were greater than rearward), and hip breadth (Parkinson, Chaffin, and Reed, 2006).

The center of pressure is an interesting phenomenon that has been explored recently, primarily in a nonclinical population. The studies described above suggest that differences exist in parameters related to center of pressure between men and women (Dunk and Callaghan, 2005), body mass, and age (Parkinson et al, 2006). These studies did not include individuals with disabilities, so the implications of the findings to this group are not clear. Further study is needed to explore the relationship between center of pressure and function and the effect of various seating interventions on this relationship.

Guidelines for Postural Control

The most important principle related to postural control is that proximal stabilization, near the center of the body, facilitates movement and control of the head and the extremities (e.g., function). During normal development, the infant achieves stability in the proximal joints before using the distal limbs for manipulation. For example, before a baby can successfully reach out and grab a toy while sitting, he must have mastered the ability to maintain a balanced sitting posture (Bertenthal and Von Hofsten, 1998; Hadders-Algra, Brogen, and Forssberg, 1998; Hadders-Algra et al, 1999; Savelsbergh and Van der Kamp, 1994). Otherwise the hands must be used to maintain balance, which limits their use for manipulation. Seating for postural control provides external positioning components for the individual who does not have internal mechanisms to control body posture. Tredwell and Roxborough (1991) present a classification scheme (Box 6-3) that is useful in describing the amount of control a person exhibits in sitting. Each category is matched with a brief description of the recommended degree of support provided by the seating system.

When any type of external support is provided, care needs to be taken so that the individual is not excessively positioned. We need to keep in mind that sitting is a dynamic activity. We often associate sitting with relaxation and lack of activity and movement, when in fact many activities are performed while sitting, such as writing, driving, talking on the phone, and typing. Even during quiet sitting an individual frequently shifts weight to maintain comfort. It is not uncommon to see individuals “properly” positioned to the point that they are no longer able to use the motor movements they have used in the past to complete functional tasks. The fewest restraints necessary to optimize function should be used (Kangas, 2000).

In this section we present a set of general guidelines for proceeding with the development of a postural seating system for an individual.

Incidence and Costs of Pressure Ulcers

Individuals who remain in bed for prolonged periods of time or who use a wheelchair and have limited ability to reposition themselves are at risk for development of pressure ulcers. In particular, individuals with spinal cord injury are at a high risk because they lack sensation and have limited movement below the level of the lesion. It is estimated that approximately one third of individuals with spinal cord injury will encounter some type of tissue breakdown during their lifetimes (Krause et al, 2001) and that approximately 25% of the health care costs associated with the consequences of a spinal cord injury are related to a pressure ulcer (Krause et al, 2001). Other populations with a high incidence of pressure ulcers include individuals with hemiplegia caused by stroke, multiple sclerosis, cancer, the elderly, and individuals who have had a femoral fracture.

Chen, DeVivo, and Jackson (2005) examined pressure ulcer prevalence in persons with spinal cord injury who were followed up through the National Spinal Cord Injury Database over the past two decades. Their sample included 3361 community-dwelling individuals with spinal cord injury who were followed up by nine centers participating in the Model Spinal Cord Injury Systems project. These nine centers were chosen because they collected continuous data throughout the duration of the study. The authors explored the relationship of risk factors and prevalence of pressure ulcer over time following the injury. Thirty-three percent of the sample had a pressure ulcer on entry to the study. It was found that the risk of pressure ulcer was relatively stable in the first 10 years following the injury. There was also a significant prevalence of recent pressure ulcers, which was not fully explained by other factors. Older subjects (50 years and older) were more likely to have a pressure ulcer. Other significant risk factors included male sex, African-American race, single marital status, education less than high school, and presence of other comorbid medical conditions (Chen, DeVivo, and Jackson, 2005).

In addition to the costs for medical care, there are social costs, which have a greater effect (Krouskop et al, 1983). Krouskop et al (1983) identify these costs as including (1) time lost from work, which affects the person and his or her family, (2) time lost from school, (3) time away from family, which can affect the person’s social development, and (4) loss of personal independence and productivity, which results in decreased self-esteem and self-worth.

Origins of Pressure Ulcers

Many factors contribute to the development of pressure ulcers; these are shown in Figure 6-15. External forces applied to a localized area are considered to be the primary cause. With application of external pressure, the normal flow of blood and oxygen to tissue in that area is reduced. If this situation is sustained, changes occur in the tissue cells, and these changes eventually lead to death of the cells. Individuals who have limited movement and lie in bed or sit in a wheelchair for prolonged periods generate compression forces that reduce the blood supply to the tissues and make them prone to pressure ulcers. Pressure ulcers are most common over weight-bearing, bony prominences because the force at these sites is greater than at other locations covered by subcutaneous tissue.

The amount of external pressure sufficient to restrict the blood flow enough to cause tissue damage has been a point of discussion over the years. The average blood pressure in capillaries ranges from 12 mm Hg in the venous limb to 32 mm Hg in the arteriolar limb (Landis, 1930). External pressure on the weight-bearing surface that exceeds these pressures produces obstruction of the capillaries. When sitting pressures of subjects on various types of surfaces were measured, it was found that the pressure generated by each surface under the ischial tuberosities greatly exceeded capillary blood pressure (Kosiak et al, 1958). A contoured, alternating pressure chair was the only surface that provided intermittent reduction (in the down position) of pressure to levels in the range of capillary blood pressure. Because most of the seating surfaces in this study generated pressures that exceeded capillary pressure, investigators were led to question whether that is the primary cause of pressure ulcer formation or whether other factors are involved.

The duration of pressure is a significant variable in the development of pressure ulcers. Kosiak (1959) determined that there is an inverse relationship between the amount of pressure sustained and the time over which it is applied. In a study involving dogs, Kosiak found that a pressure of 600 mm Hg produced ulceration in approximately 1 hour and a pressure of 150 mm Hg produced ulceration in 12 hours. The results of this study are shown in Figure 6-16, A. Microscopic tissue changes were found after application of as little as 60 mm Hg of pressure over 1 hour. This finding is consistent with the theory that exceeding the capillary pressure deprives the cells of enough important nutrients to cause damage at some level.

Time as a variable in pressure ulcer development is taken into consideration with the broad guidelines developed by Reswick and Rogers (1976). These guidelines, based on years of clinical experience with individuals who have spinal cord injury, establish allowable amounts of pressure that tissue surrounding bony prominences can endure over certain periods. They recommend that pressures on the ischial tuberosities remain in the range of 30 to 60 mm Hg, as shown in Figure 6-16, B. Tissues that are not susceptible to the internal pressure exerted by bony prominences can tolerate higher skin surface pressures or lower pressures for longer periods.

Up to this point the effects of sustained perpendicular (compression) pressure forces on tissue have been discussed. Parallel (or shear) forces play a significant role in the formation of pressure ulcers as well. Shear forces are generated when two surfaces move across each other in opposite directions, for example, when an individual slides his hips forward in a wheelchair and assumes a sacral sitting posture. In this position the skin remains in contact with the seat surface and the superficial fascia is interlocked with the skin. The deeper portion of the superficial fascia, however, is mobile and slides forward. The blood vessels in this area are stretched and angulated, which causes occlusion. Resulting tissue damage is at a deeper level and typified by a large area of undermining around the base of the ulcer (Reichel, 1958). Bennett et al (1979) believe that it is the combination of pressure and shear that is so effective in occluding blood flow. They found that, when sufficient shear was present, only half as much pressure was needed to cause occlusion. Unfortunately, because of the difficulty in measuring shear, there is still uncertainty regarding the extent to which shear contributes to the development of pressure ulcers.

Friction, the force between two surfaces at rest or in motion, is another component of shear and the development of pressure ulcers. Friction leads to injury and ulceration of the surface of the skin. A typical friction injury to the skin occurs when it moves across a rough surface such as bedding. Dinsdale (1974) found that the skin’s susceptibility to pressure ulcer development is increased with friction. When pressure alone was applied, 290 mm Hg was required to produce ulceration. With the application of pressure and friction, ulcerations were produced with pressure levels as low as 45 mm Hg. Moisture, heat, or properties of materials such as clothing can increase frictional forces.

Other Factors That Contribute to Pressure Ulcer Development

Some individuals can be exposed to the mechanical forces of pressure and shear without pressure ulcers developing, whereas others have very little tolerance to these mechanisms. Although compression and shear forces are typically considered to be the chief causes of pressure ulcers, there are several other factors that contribute to skin breakdown and cause some individuals to be more susceptible than others.

Pressure Measurement

Pressure ulcers result from sustained compression of soft tissues, particularly under bony prominences. The predominant hypotheses concerning the pathogenesis of pressure ulcers include localized tissue ischemia, sustained deformation of the cells, impaired nutritional flow to the cells, reperfusion injury, and inadequate drainage of cellular waste products (Lander Ganz and Gefen, 2004; Stekelenburg et al., 2006). Lander Ganz and Gefen demonstrated that the pressure measured at the deep tissue level was significantly greater than that measured at the surface interface, although they could not predict a specific relationship. The prevalence of pressure ulcers and the resulting costs underscore the need to measure the forces applied to the muscle in an attempt to prevent prolonged exposure to high loads. Many sophisticated pressure measurement systems have been developed, including near infrared tissue spectrophotoscopy, Raman spectrography, and in-dwelling sensors. However, these systems are not feasible in the clinical situation.

In the clinic, pressure mapping systems are the primary means of quantifying pressure. These systems quantify pressure at the buttock/seat interface, allowing a comparison among various cushions. In measuring sitting pressure, it can also be determined whether the individual has any evidence of asymmetry in sitting and what influence changing the configuration of the seating system has on correcting the asymmetry. Many commercial pressure measurement systems are available for commercial and research use. These technologies are constantly improving, but issues remain with consistency of the measurement and lack of agreement around best practices for pressure measurement protocol. The three most common pressure mapping systems are the Force Sensing Array (Vista Medical;www.pressuremapping.com), F-Scan (Tekscan) (www.tekscan.com), and Xsensor (Xsensor Technology Corporation, www.xsensor.com). Each of these systems uses a flexible matrix of pressure sensors that provide a map of the distribution of pressure at the interface between the seat cushion or back and the client’s body. These sensors are arranged in a grid pattern on the pressure mat. The number of sensors and their sensitivity varies across the different systems and should be taken into consideration when considering the purchase of a system. They vary in the technology used to measure pressure. These technologies include capacitance sensors that measure the ability to store an electrical charge, piezo-resistive sensors that measure the change in resistance when force is applied, and electrically conductive sensors that measure the change in current flow. Two properties influence the reliability of the pressure measurements: creep and hysteresis. Creep refers to the stability of the pressure reading over time. Hysteresis refers to the change in pressure reading as the device is loaded and unloaded (e.g., as the client sits on the cushion). Each of these systems corrects for these two variables in their software, but these properties still influence the reliability of the measurement systems to varying degrees, which needs to be taken into consideration when these systems are used clinically.

The output from each of these systems is generally similar, although care must be taken when the results of research or measurements obtained by use of the different systems are compared. As will be seen below, these systems differ in their performance. All the systems provide a visual output (Figure 6-17) that allows a quick inspection of the pressure distribution. The visual output may show pressure distribution with a color display or as peaks and depressions. The actual pressure value for each cell can be displayed as well. Data are captured continuously at varying sampling rates. The system will provide data on peak and mean pressure, number of sensors activated, minimum and maximum pressure, and the location of the center of pressure. Some systems have the capacity for a split screen that displays the pressure map on one side and a video recording on the other. Another useful feature is the ability to define a particular area of the pressure map for which the system will generate pressure statistics. The breadth of information that these systems provide is both useful and a distraction. Although data showing peak and mean pressure seem easy to interpret, there is little consensus on what is desired pressure at the seat/buttock interface. Different ways of interpreting these data will be discussed below in the discussion of a pressure mapping protocol.

Ferguson-Pell and Cardi (1993) completed an evaluation of three computer-based pressure mapping systems. Although this work evaluated technology that is dated, it continues to influence thinking about pressure measurement systems as it applies a systematic protocol, involves both consumers and users, and identifies key parameters that must be considered in both the selection of a pressure mapping system and interpretation of the resulting data. The three systems that were evaluated included: the Force Sensing Array (Vista Medical), with a 15 × 15–cell array of force-sensing resistors, the Talley Pressure Monitor (TPM, Progressive Medical), consisting of an array of bladder-type sensors, and the Tekscan Seat (Tekscan Inc.), with an array of 2056 force sensors. With these systems, Ferguson-Pell and Cardi (1993) carried out “bench tests” to determine the properties of the sensors under controlled loads. The measurement variables of interest were accuracy, linearity and reproducibility, hysteresis, and stability (Ferguson-Pell and Cardi, 1993). In other words, they measured the relationship between the pressure measured by each system and a known applied pressure, creep, and hysteresis. These were examined with planar and contour loads under laboratory conditions and across four different pressure relieving cushions, with five wheelchair users as subjects. The four cushions used to evaluate the performance of each of the pressure mapping systems included (1) foam, (2) gel, (3) hybrid (Jay), and (4) air filled (ROHO). The actual results of this work are less critical today because of the changes that have occurred in both measurement systems and cushion technology. However, this study remains an important one because of its use of a systematic research protocol and identification of critical factors that influence the reliability of pressure mapping systems.

A recent study involved a similar comparison of more current pressure mapping technologies (Hadcock et al., 2002). Incremental loading, low threshold, and stability (creep) of the F-scan FSA and Xsensor systems were evaluated under static and dynamic conditions by using planar and curved surfaces. The curved surface was cylindrical; quite different from a contoured seating surface, so the results of this aspect of the study must be interpreted with caution for the purposes of seating intervention. This study involved bench testing and did not include measurement with any wheelchair users. The results on a flat surface indicated that the FSA system was the most accurate, followed by the Xsensor and the F-scan. Creep was similar for the Xsensor and F-scan (17.62% and 17.23%, respectively) systems, and both were better than the FSA system (19.54%) (Hadcock et al., 2003). The Xsensor was better at detecting pressures under light loading conditions.

The studies by Ferguson-Pell and Cardi (1993) and Hadcock et al. (2003) provide some evidence for the difference in performance across pressure mapping systems. In addition to the performance characteristics evaluated in these studies, the selection of a system will depend on familiarity with the technology, the specific purpose (e.g., clinical evaluation versus research), and the functionality of the systems as they meet the needs of the user.

A second major issue with respect to pressure mapping is the identification of an accepted protocol for clinical use. There is no compelling evidence in the literature to suggest a particular number above which a pressure ulcer will certainly develop. Consequently, pressure mapping in the clinical situation is used to make comparisons among various cushions so that the clinician can rank the cushions for their ability to distribute pressure as measured by the pressure mapping systems. Swaine (2003) has developed a protocol for both obtaining and interpreting pressure measurements that is becoming more prevalent internationally. Swaine describes a consistent setup of the equipment and cushions to be evaluated, an initial check of the equipment, length of recording time, palpation of bony prominences, and documentation. She also suggests that interpretation of the results is based on peak pressure, the area of the client’s buttocks that are in contact with the pressure mat, and any asymmetries of pressure distribution.

Swaine’s work provides a useful basis for clinicians using pressure mapping to determine the optimal cushion for their clients’ needs. Caution is still advised because questions remain about a protocol and the interpretation of the data. For example, Swaine (2003) suggests that peak pressure is determined by taking the average of the four highest sensor cells around a bony prominence, whereas Dunk and Callaghan (2005) take the average of all sensors within 10% of the cell, measuring the highest peak pressure. Swaine recommends that clients sit on a cushion for 8 to 10 minutes, whereas Stinson, Porter, and Eakin (2002) suggest that 6 minutes is sufficient. Pressure gradient rather than absolute pressure has been suggested as a better indicator of risk for development of a pressure ulcer, but there is no consensus on what constitutes an acceptable gradient. These concerns suggest that, although pressure mapping remains a very useful tool its augment clinical judgment, it does not replace it.

Behavioral strategies to reduce the risk of development of pressure ulcers were identified earlier. Two main technologies exist to manage pressure for persons who use wheelchairs: pressure relief cushions and tilt and recline components on wheelchairs. The latter will be discussed in Chapter 12. Numerous studies have measured characteristics and properties of a variety of pressure relief cushions. The majority of investigations have used tissue interface pressure as the basis for comparing these products. Some studies have also compared changes in transcutaneous oxygen tension and capillary blood flow. Although it has been shown that seating technologies play a significant role in pressure ulcer prevention by reducing the mechanical load on the tissue, there is no evidence that one type of pressure-reducing device works better than all others under all circumstances (Brienza et al, 2001; Conine et al, 1994; DeLateur et al, 1976; Ferguson-Pell et al, 1986; Garber, Krouskop, and Carter, 1978).

Design and Construction of Seating Systems

The design of the seating system refers to the degree of contouring present in the seat and back and the degree of adjustability that is present in the components. These technologies range from systems that are relatively flat, without any contouring to match the shape of the body segments they support, to custom-contoured systems that are constructed to match as closely as possible the body contours of the user. Prefabricated technologies are available so that the ATP no longer needs to construct the components in the seating system.

Standard Contoured Modules

Contoured technologies are useful for individuals with moderate seating and positioning needs for postural management or who are at low risk for pressure ulcer development. These technologies use curved surfaces that more closely match the shape of the human body. The amount of contact that the body has with the seating surface is increased by contouring the seating surface to the person’s body, providing increased support and control. Further, a generic contoured cushion will distribute pressure across the seating surface and will fit people within a certain size range. This approach is also generally less costly than a custom-contoured cushion. The Matrix is one example of a standard contoured cushion (Figure 6-20) (Invacare, www.invacare.ca).

Custom Contoured

The cushion that provides the greatest amount of body contact and therefore the most support is one that has been shaped, or custom contoured, to the individual’s body. A number of technologies are available to achieve a custom-contoured system. One example is shown in Figure 6-21. These types of systems differ primarily in terms of the fabrication techniques used and whether the fabrication is completed on site or in a central location. The disadvantages of custom-contoured support surfaces include the following: transfers to and from the system are more difficult; the system is static and has no dynamic properties, thus limiting the individual to one fixed position; and there is limited ability within the system to allow for growth of the individual.

Foam in place systems allows the ATP to fabricate a custom-contoured cushion on site. The client is placed in a frame that has a flexible covering over one side. This covering is matched to the shape of the person, and he or she is positioned in the most desirable seated posture. The foam materials are then added to the frame and allowed to expand to contour to the person’s shape. The person is removed from the frame, and the foam is allowed to harden for several hours. Once the foam hardens, the flexible covering is removed. Further shaping of the foam can be completed by hand at this time. The foam is encased in a fabric cover. The foam can then be attached to a solid backing (usually plywood or plastic) and mounted to the wheeled base. This approach can be used on site, and a cushion can be completed in about 12 hours.

A vacuum consolidation process seats the client on a latex bag filled with beads (Lemaire et al, 1996). This bag is placed on top of a wheelchair or fitting chair. The ATP assists the client to achieve an optimal position to support goals of function, pressure relief, or comfort. The latex bag is then manipulated to conform to this position and the vacuum is used to draw the air out of the bag, consolidating the enclosed beads. At this stage, the cushion can be fabricated either on site by a foaming procedure or sent to a central manufacturer for remote fabrication.

Seating simulators use technology that allows the ATP to map the client’s body contours, which are then digitized. Computer-aided manufacturing technology is then used to fabricate the cushion or back. The process of determining the optimal cushion or back shape is similar to that described for vacuum consolidation. A simulator chair is used that allows multiple adjustments to manipulate seat depth, seat to back angle, and other common wheelchair configurations. An initial baseline reading is taken without the client seated in the simulator. Once the client is comfortably positioned in the chair, mechanical plungers are positioned to simulate the optimal seated position of the client. This information, along with the baseline reading, is transferred to a pressure-sensitive data recording sheet. This sheet is then sent to a central fabrication facility, along with other recommendations related to foam density, special construction related to pressure relief (e.g., use of different foams), and required accessories such as laterals and strapping systems. The Shape Sensor system by Invacare (www.invacare.com) is an example of this technology.

Prefabricated adjustable Backs

Prefabricated adjustable backs have become available in recent years. These products provide a large degree of adjustability that can be accomplished in the clinical setting. The Infinity Back is an example of an adjustable back. The ATP can make adjustments on the basis of an optimal seated position determined by a mat assessment. These backs allow the clinician to adjust height, depth, width, back angle, and placement of the laterals. Some of these systems allow the ATP to create a biplanar back in which the upper and lower segments of the back are set at different angles. This configuration is often used to provide specific postural control. Although the pivot point can be placed at any spinal level, when placed at the level of the posterior superior iliac spine, it can assist with control of the pelvis. Studies have investigated the effect of pelvic stability on function (Miller Polgar et al, 2000; Rigby et al, 2001;), but there have been no clinical studies that have evaluated the effect of this particular back configuration on postural control and subsequently function.

Classification of Cushion Technologies

Sprigle, Press, and Davis (2001) describe uniform terminology for classification of the material used to construct wheelchair cushions. They described the following categories of cushions: (1) made from cellular materials, (2) containing fluid, and (3) other constructions.

Cushions Containing Fluid

Cushion Covers

Selection of a cover for a seat or back cushion can be as important as the determination of the material used to make the cushion because an improper cover can negate some of the benefits of that material. The cover selected should conform integrally to the cushion’s contours, particularly in nonplanar systems. It should not interfere with the envelopment properties of the cushion nor add to shearing and friction. A cover that is too tight will prevent the client from sinking into the contours of the cushion. One that is too large will wrinkle, creating additional pressure points.

The ATP should know how the fabric handles moisture, either as a result of incontinence or perspiration. Most cushions will be used in hot, humid conditions for at least part of the year, so perspiration is an issue even when incontinence is not a concern. Many technical fabrics, blending Lycra and polyester, wick moisture away from the body, which is an important consideration when prevention of pressure ulcers is a goal. The cushion cover should be easy to remove and clean.

Technologies to Enhance Sitting Comfort for Wheelchair Users

In a study that assessed the satisfaction of wheelchair users, comfort was rated as the most important variable for a wheelchair seating aid (Weiss-Lambrou et al, 1999). At the same time, comfort was rated as the least satisfying variable among these wheelchair users. The reasons stated for dissatisfaction related to comfort included seat cushions that caused pain and discomfort, fatigue, uncomfortable headrests and thoracic supports, sliding in wheelchair seat caused by discomfort, and poor posture as a result of unsuitable installation. Comfort is also related to the contact surface between the seating system and the person. For example, materials that provide good air exchange, maintain an even temperature, and control moisture are more likely to provide a comfortable sitting climate.

Wheelchair users who have discomfort and chronic pain need seating systems that allow them to relieve the discomfort and participate fully in activities of daily living. These needs are best addressed after a thorough mat assessment that identifies the user’s most comfortable seated position and the combination of technologies identified above that best addresses comfort needs. (Mobility technologies that provide users with the ability to adjust their positions in the wheelchair will be discussed in Chapter 12).

Technologies That Increase Ease of Sitting for the Elderly

People are living longer, which means that the number of well elderly and those in need of supervised care is growing considerably. As an individual ages, mobility may be reduced as a result of acute illnesses or trauma, such as stroke, hip fracture, or progressive conditions such as arthritis. Consequently, it is likely that the amount of time the individual spends sitting increases. The goal of seating in this category depends on the individual’s needs and skills, as it does for the other categories. Just as there is a range of needs for the elderly population, there is also a range of seating technologies. Seating technologies for the aging population can be matched to the level of functional mobility the individual has: (1) ambulatory (2) mobile, nonambulatory, and (3) dependent mobile (Fernie and Letts, 1991). Chairs are needed that promote comfort, safety, ease of ingress and egress, and propulsion if necessary.

Study Questions

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