Chapter 13

Wheelchair seating

Appropriate wheelchair seating is an integral aspect of the overall management of people with spinal cord injury. It not only determines patients’ mobility but also has implications for skin, posture, pain and contracture management.

Over recent years a highly commercialized industry has evolved around mobility and seating equipment for the disabled. Consequently, there are hundreds of different types of cushions, wheelchairs, backrests and accessories, making selection of appropriate equipment increasingly complex. In specialized spinal units, wheelchair seating and prescription is predominantly done by seating teams comprising engineers, technicians, physiotherapists and occupational therapists. These teams are solely devoted and specifically trained for wheelchair prescription, and have an in-depth knowledge of locally available products and pressure management. Typically, commercial products are used but then individually modified to suit patients’ specific needs and to minimize the deleterious effects of pressure.

This chapter outlines some of the key features of wheelchairs and cushions which need to be considered when this equipment is selected and adjusted for patients. The first section provides an overview of wheelchair cushions with particular emphasis on the effects of upright sitting on pressure distribution. The second section summarizes different types of wheelchairs and the effects of wheelchair set-up on mobility, stability and pressure. Those who require more information are well advised to refer to the excellent books solely devoted to this topic.1–3

Wheelchair cushions

It is important that patients sit on appropriate cushions to prevent pressure ulcers. A poorly fitted, maintained or prescribed cushion or a cushion placed upside down or around the wrong way can cause debilitating pressure ulcers necessitating months of bedrest. The soft tissues overlying the ischial tuberosities are most vulnerable to damage from sitting and cushions are primarily designed to protect these areas (see Chapter 1 for discussion on causes and management of pressure ulcers).

Most of the commercially-available cushions are air-, foam- or gel-based (see Figure 13.1a–c).⁴ A recent Cochrane systematic review found insufficient evidence to recommend one type of cushion over another, suggesting that decisions about appropriate cushions for patients need to be based on rationale and clinical reasoning and cannot yet be based on good quality evidence.⁵ Often a cushion which provides adequate pressure relief for one patient will be inappropriate for another. This is partly because the pressure-relieving features of cushions are influenced by many factors, including the wheelchair and its set-up, and patients’ mobility, skin integrity, nutrition and weight. Cushions need to be prescribed on a case-by-case basis after examining their effects on pressure distribution.

Figure 13.1 Most cushions are air- (a), foam- (b) orgel-based (c).

The pressure-relieving qualities of cushions need to be assessed every time a new cushion is trialled. This can be done using simple or sophisticated equipment to measure skin-interface pressures.⁴^,^6–8 These pressures are measured with patients sitting on their cushions in their wheelchairs. However, there is not one critical pressure below which patients will be safe from skin damage and above which they will not. The appropriate pressure is determined by patients’ susceptibility to pressure ulcers and their ability to relieve pressure.² However, as a general rule, peak pressures over vulnerable sites should be kept well below 60 mm Hg.⁴^,⁷^,⁹^,¹⁰

The pressure-relieving qualities of cushions should also be assessed by examining skin integrity immediately after patients return to bed following a period of sitting in their wheelchairs. When a new cushion is trialled, patients should only sit for between 30 minutes and 1 hour. The length of time spent sitting can be gradually increased but the skin should continue to be checked after patients return to bed and always checked at least once a day. If the skin looks red and does not blanch with localized pressure, the cushion is not providing adequate protection.⁵ Either the cushion needs to be modified or changed, or the length of time spent sitting needs to be reduced. Alternatively, pressure needs to be more effectively or frequently relieved when sitting, or the set-up of the wheelchair needs to be changed.

Air-based cushions

Air-based cushions relieve pressure by distributing air from pockets of high pressure to pockets of low pressure. In this way, they mould to the shape of patients and distribute pressure over a larger surface area.⁵ The ischial tuberosities should submerge into the cushion but should not press hard up against the seat of the wheelchair. The effectiveness of air-based cushions is dependent on appropriate inflation. An under-inflated cushion provides little or no protection because the ischial tuberosities bury through the cushion onto the hard seat of the wheelchair. An over-inflated cushion prevents submersion and mimics the effects of sitting on a hard seat. Therapists can use their fingers to crudely check the inflation of air-based cushions by ensuring there is enough room to slide two fingers between the ischial tuberosities and seat. Insufficient space for the fingers indicates that the cushion is under-inflated.

Some air-based cushions are power operated, cycling air between different compartments. They constantly vary pressure, avoiding long periods of high pressure in any one spot.⁵ These types of cushions are primarily used for patients in power wheelchairs with ongoing pressure problems.

Gel-based cushions

Gel-based cushions work on a similar principle to air-based cushions. They dissipate pressure by allowing gel to move from areas of high pressure to areas of low pressure. Most have a contoured foam base upon which the gel sits.⁴ The foam base has a specially-designed hollow or ‘well’ for the ischial tuberosities (see Figure 13.2). This helps ensure that most pressure is borne by the soft tissues over the lateral aspect of the thighs, leaving the ischial tuberosities free to submerge within the gel-filled well. Needless to say, if the well is too wide both the lateral thighs and ischial tuberosities fall into it with a high risk of the ischial tuberosities burying through the gel, pressing up hard against the base of the cushion or wheelchair.

Figure 13.2 The ischial tuberosities sit in a well filled with gel. The lateral thigh bears weight through the firmer outer rim of the cushion.

It is a common mistake to prescribe obese patients cushions with inappropriately wide wells. It is wrongly assumed that all obese patients have broad bony pelvises. These patients need to be prescribed cushions according to the width of their pelvises, not according to the width of their hips or the size of their wheelchairs. Often these patients require cushions with narrow wells individually modified to accommodate excessive adipose tissue around the hips.

Foam-based cushions

Foam-based cushions also redistribute pressure. Their effectiveness is dependent on the compressibility of the foam and the cut of the cushion. Some cushions use two or more types of foam, typically with firmer foam under the lateral aspect of the thighs and more compressible foam under the ischial tuberosities. This encourages more weight to be borne through the thighs and less weight through the vulnerable ischial tuberosities.

Foam-based cushions can be cut and contoured to meet the individual needs of patients but this is best done by trained seating specialists. Technology is also available to cut and shape foam-based cushions from plaster moulds of patients. This technology provides individualized and sophisticated seating solutions but often requires a commitment to expense without an opportunity to trial the cushion first. If the cushion is not effective, money is wasted. For this reason foam-based cushions cut and shaped from plaster moulds of patients are primarily used for particularly difficult seating and postural problems.

Other considerations

Ease of maintenance

The choice of an appropriate cushion is not only dictated by its pressure-relieving qualities but also by its ease of maintenance and its durability. For example, air-based cushions need to be regularly checked for correct inflation. Air-based cushions are also vulnerable to puncture, rendering them immediately useless until repaired. Air-based cushions are therefore not generally recommended for non-compliant patients, those with little hand function or carer support, or those in situations where punctures are a substantial risk. In contrast, gel- and foam-based cushions require little maintenance. It is, however, important that users of gel-based cushions ensure that the gel is evenly distributed prior to sitting on the cushion. Those living in cold climates also need to ensure that gel-based cushions are not stored in subzero temperatures.

All cushions require replacing. For example, foam-based cushions can require replacing every year because the foam compresses with time, decreasing its pressure-relieving qualities. Gel- and air-based cushions generally last longer, sometimes for several years.

Effect on seating stability, mobility and posture

The choice of an appropriate cushion is also determined by its effect on stability, mobility and posture.¹¹ Some patients feel unstable on air-based cushions and prefer the rigidity provided by foam- or gel-based cushions. More rigid cushions are also easier to transfer from because the cushion does not compress under the hands and patients do not lose height on the vertical lift of the transfer. Transferring from cushions with deep wells can be difficult if patients struggle to get their buttocks up and out of the well.

Cushions also influence seating posture.¹¹ For example, foam can be strategically placed on cushions to prevent legs falling into abduction or sweeping to one side. Similarly, foam can be used to lift one side of the pelvis for patients with a tendency to sit asymmetrically. However, it can be difficult to attain optimal seating posture while also ensuring sufficient pressure protection, especially in patients with deformities and complex seating and skin problems. To improve seating posture it is often necessary to increase pressure over vulnerable bony prominences. The solution is the best possible seating posture which provides adequate pressure protection. It is advisable to compromise on posture before compromising on pressure protection. Foam- and gel-based cushions generally provide greater potential to correct posture but air-based cushions provide greater skin protection.

Weight

Air- and foam-based cushions are lighter than gel-based cushions. This can be a consideration for patients doing a lot of wheelchair pushing or needing to regularly lift their cushions in and out of cars.

Cost considerations

The cost of cushions is variable but foam-based cushions are usually the cheapest. The cost can be prohibitive, particularly for those in developing countries and those with limited financial resources. In third world countries, cushions can be cheaply made with a sharp knife, an appropriate piece of foam and some initial training.²^,¹²^,¹³ Alternatively, bicycle inner-tubes can be bound together to create an air-based cushion.¹ Cushions made in this way are not ideal but they provide some skin protection and are a better option than sitting directly on the hard base of a wheelchair.

Manual wheelchairs

Like cushions, there are hundreds of different types of wheelchairs. Large international companies supply wheelchairs to the majority of countries with ongoing customer support. There are also local manufacturers of wheelchairs in most countries. All wheelchairs come with an array of different features and accessories which need to be considered. Some features are critical and determine safety, comfort, pressure distribution and manoeuvrability, while others are less important and may reflect personal preference.

Wheelchair prescription not only involves finding the appropriate product but also ensuring it is appropriately fitted and set up for the patient. For example, a poorly fitted wheelchair which is too narrow for a patient can cause skin breakdown, and an excessively ‘tippy’ wheelchair can cause a backward fall (see Chapter 4).Most wheelchairs have substantial adjustability, although highly specialized sports wheelchairs do not.

Ideally, the set-up of a wheelchair should enable patients to sit comfortably with weight borne through the buttocks and thighs. Sitting posture should be as ‘normal’ as possible. The wheelchair set-up should provide sufficient upright stability to enable patients to sit without needing to grasp the wheelchair or rest the elbows on armrests to prop themselves upright. Those with upper limb function should also be able to raise their arms without toppling forwards and propel themselves up a slope without tipping the wheelchair backwards. If patients are unable to sit or move in these ways, it is usually indicative that their wheelchairs are inappropriately set up for them.

Inevitably wheelchair set-up is a compromise between providing optimal mobility, stability, skin protection and posture. Therapists and patients need to trial different set-ups until the best solution is reached. Sometimes appropriate seating cannot be achieved with the adjustability provided in commercial products. This is particularly common in patients with complex seating needs and spinal deformities. Often these patients require sophisticated custom-made seating systems, a service which can only be provided with appropriate technical and engineering support.

The optimal set-up of a wheelchair often changes over the first year following injury as patients’ function and mobility changes. For example, with time and better wheelchair control it may be appropriate to move the back wheels forwards, increase the tilt of the seat or position the wheels higher on the frame (the effects of all these changes are discussed below). For this reason it is often advisable for patients’ first wheelchairs to be highly adjustable. Alternatively, the prescription of first wheelchairs can be delayed until mobility and function have stabilized and patients have a better understanding of what they want and need. If there are no financial constraints then a first wheelchair can be prescribed or provided on loan soon after injury and a second and better suited wheelchair can be provided 6–12 months later.

Below is an overview of some of the key issues which need to be considered for fitting, setting up and choosing a manual wheelchair. Several generic issues are equally relevant to power wheelchairs and will be briefly discussed at the end of the chapter.1–3^,¹⁴^,¹⁵

Type of frame

There are two types of wheelchair frames, rigid (see Figure 13.3a) or folding (see Figure 13.3b). Rigid frames are primarily prescribed for active patients. They are generally lighter, sturdier, more adjustable and easier to push. Folding fames are better suited to ambulating patients because the footrests can be lifted when standing up. Folding frames are also used by patients who rely on car hoists to stow their wheelchairs on the roofs of cars. However, folding frames are more likely to break and do not always provide a comfortable ride. Some wheelchairs are fitted with suspension to provide a smoother ride; however, suspension is expensive and increases the weight of the wheelchair.

Figure 13.3 A rigid (a) and folding (b) framed wheelchair.

Seat

The seat of a wheelchair can be either flexible (sling) or rigid. Most manual wheelchairs have sling seats because they are lighter and enable the wheelchair to be readily collapsed. However, sling seats often sag with time and, depending on the rigidity of the cushion, can create skin and postural problems. This problem can be overcome by placing a rigid but removable base on a sling seat. Alternatively, the tension of some sling seats can be adjusted, with similar mechanisms used to change the tension of sling backrests (see Figure 13.7).

Seat-to-floor height

The seat-to-floor height determines the overall height of the wheelchair (see Figure 13.4). The back of the seat is usually lower than the front of the seat; consequently, the seat-to-floor height at the rear of the wheelchair is usually less than the seat-to-floor height at the front of the wheelchair. Seat-to-floor height is varied primarily to accommodate heel-to-knee length and to ensure adequate footplate clearance. Taller patients generally require higher seats. However, if the seat is too high, patients are unable to get their knees under tables. They may also have problems with head clearance when sitting in wheelchair-accessible vans. A high seat is also less stable than a low seat, increasing the risk of tipping. In contrast, if patients are short and the seat is low, they cannot comfortably rest and use their arms on the top of a table. A seat which is inappropriately low for a patient raises the knees, concentrating pressure under the pelvis. Patients propelling wheelchairs with their feet require a low seat to enable the feet to touch the ground.