Eitzen (2004) identified the role of the cushion as two fold. Firstly, to contribute to a functional and balanced posture and secondly, redistributing pressure away from the critical areas of the IT and the sacrum and re-distributing pressure over a larger contact area to reduce overall and peak pressures (Eitzen 2004).Bogie et al (1995) stated that 47% of pressure ulcers occur at the ischial tuberosities or sacrum and are therefore more likely to have been initiated while seated. Provision of a wheelchair cushion that relieves and redistributes pressure and reduces risk of pressure ulcer formation is an important prevention recommendation. It is important for clinicians to understand and evaluate the pressure redistribution capabilities of various seating cushions.
Cushion design has been based on the belief that sitting interface pressure should be distributed evenly to reduce areas of high pressure underneath bony prominences (Yuen & Garrett 2001). Cushion selection can be difficult as there are numerous cushions on the market each citing specific characteristics along with various amounts of pressure reduction and redistribution that make a cushion “superior.” When assessing an individual for a cushion, factors such as the degree of pressure reduction and redistribution (Garber 1985), temperature effects (Fisher et al. 1978; Seymour & Lacefield 1985); level of SCI, pressure relief abilities, transfer technique and lifestyle (Garber 1985; Makhsous et al. 2007a) also need to be considered. In addition to a reduction in pressure ulcer risk, cushions must also promote adequate posture and stability for the individual with SCI (Sprigle et al. 1990). Seat cushions can be made from a variety of materials, can be static or dynamic (Garber 1985; Makhsous et al. 2007a), and are incorporated into a variety of wheelchairs.
Numerous authors have investigated various wheelchair cushions and seating systems to try and determine which offer the most pressure or risk factor reduction to prevent occurrence of pressure ulcers in individuals with SCI.
Trewartha and Stiller (2011) used pressure mapping to evaluate two different cushions among three people with SCI. Findings from phase 1 indicated that the Roho Quadtro had significantly fewer cells in the greater than 100 mmHg range than the Vicair Academy but there was no significant difference in the 66-99mmHg range. The study did not examine the number of cells in the less than 65mmHg range. The location of the cells with greater than 100mmHg were not identified as being over bony prominences. Other pressure characteristics such as peak pressure gradient, area of distribution, or symmetry were not measured.
Gil-Agudo et al. (2009) aimed to characterize the clinical utility of interface pressure mapping for cushion comparisons. The dual compartment air cushion exhibited the best mechanical performance with regard to the distribution of pressures and contact surface interface compared to the other three cushions studied (low profile air, high profile air, and gel and firm foam cushions). Variances in the pressure mapping variables were noted between this study and others for the same cushion. This study compared only four cushions, based only on distribution of pressure and not any of the other factors that are required for cushion selection. The main finding was that using interface pressure mapping could augment cushion selection but is only part of the cushion selection process.
Makhsous et al. (2007b), in a case-control study, exposed subjects to two 1-hour protocols: alternate, where sitting posture was alternated dynamically every 10 minutes between normal (sitting upright with ischial support) and sitting upright with partially-removed ischial support and lumbar support (WO-BPS), and normal (normal posture plus push-ups performed every 20 minutes). These investigators found that the anterior portion of the seat cushion had a larger contact area among those with tetraplegia compared to those in the other groups. It also was determined that those with a SCI had a larger contact area in the mid portion of the seat cushion. There were significant differences between the groups when looking at the average pressure over the whole seat (p<0.001) and the total contact area on the seat cushion. With the WO-BPS posture, the average pressure for the tetraplegia group was higher than it was for the other groups (p<0.001). Most importantly, the total contact area on the posterior portion of the cushion was less for the WO-BPS posture group. As well, peak interface pressure was lower for all groups, with the greatest decrease from normal posture seen in the tetraplegia group. The average pressure increased on the anterior and middle portion of the cushion in all groups.
Hamanami et al. (2004) used a pressure mapping system to evaluate the pressures found on an air floatation cushion (high profile ROHO) with 36 subjects with SCI. The results indicated that the optimal reduction in interface pressure was just before bottoming out on the cushion. No reliable method was found for systematically determining the appropriate air pressure for a ROHO for subjects with SCI (Hamanami et al. 2004). Takechi and Tokuhiro (1998) also found that the air cushion had the lowest peak pressure and the highest area of pressure distribution followed by the silicone (gel) cushion.
In the study conducted by Burns and Betz (1999), 3 wheelchair cushions were tested: dry flotation (ROHO High Profile), gel (Jay 2), and dynamic (ErgoDynamic), the last consisting of two air-filled bladders (H-bladder, IT-bladder). These were compared to each other under high pressure conditions (upright sitting or IT-bladder inflated) and low pressure conditions (seat tilted back 45° or H-bladder inflated). When analyzing the pressure placed on the ischial tuberosities, it was found that the pressure was higher during upright sitting than in the tilted back position for both the dry flotation and the gel cushion (p<0.001), with the dry flotation cushion providing more pressure relief than the gel cushion during upright sitting (112 versus 128 mmHg, p=0.01). Mean pressure with the IT-bladder-inflated cushion (157 mmHg) was greater than upright pressures for either the dry flotation or gel cushions (111 and 128 mmHg, respectively p<0.01). Most importantly, ischial tuberosity pressure for the dynamic cushion during H-bladder inflation in an upright position was comparable to the pressure for the dry flotation cushion in a tilted back position (71 versus 74 mmHg, p=0.91) and significantly less than the pressure obtained with the gel cushion (71 versus 86 mmHg, p<0.05).
Takechi and Tokuhiro (1998) studied the seated buttock pressure distribution in six patients with paraplegia using computerized pressure mapping. Five wheelchair cushions were evaluated (air cushion, contour cushion, polyurethrane foam cushion, cubicushion, silicone gel cushion). Tests showed that if the area of contact was more widespread, the peak pressure was lower. The air cushion and the silicone cushion were found to have the lowest peak pressures.
Gilsdorf et al. (1991) studied subjects sitting on ROHO and Jay cushions. Normal force, shear force, centre of force, lateral weight shifts and amount of weight supported by armrests were studied under static and dynamic conditions. The ROHO cushion showed a tendency to carry a larger percentage of total body weight; have a more anterior centre of mass; and showed more forward shear force. There were more lateral weight shifts on the Jay cushion. Armrests supported a portion of body weight.
Seymour and Lacefield (1985) evaluated 8 cushions for pressure, temperature effects and subjective factors influencing cushion purchase. While data indicated a wide variability in pressure measurements in individual subjects, the air filled cushion (Bye Bye Decubiti) had the best pressure readings. The alternating pressure and foam cushions had consistently higher temperature readings across both groups.
Garber (1985) evaluated 7 cushions based on amount of pressure reduction. The author also looked at how frequently each cushion was prescribed to subjects with quadriplegia and paraplegia. The ROHO cushion produced the greatest pressure reduction in the majority of subjects (51%) but was prescribed more often for subjects with quadriplegia vs. paraplegia (55% vs. 45%).
These studies demonstrate that there are individual variations in cushions needs inherent in those with SCI (e.g. paraplegia vs. tetraplegia). As a result the need for additional measures such as pressure mapping is needed to assist with individualizing a wheelchair cushion prescription. Pressure mapping is a useful clinical tool to assist in determining pressure redistribution properties of cushions but pressure is not the only factor to consider in cushion selection (Gil-Agudo et al. 2009). This is an important consideration as most of the studies reviewed have identified air inflation cushions as providing the lowest pressures but have not examined any other suitability factors.
It is also important to note that not all types of cushions have been studied. While pressure mapping is a useful tool for cushion comparisons it is more useful in identifying cushions with inadequate pressure redistribution characteristics rather than identifying the best cushion among those with similar pressure redistribution characteristics (Jan, 2006). Objective findings together with the clinical knowledge of the prescriber, individual characteristics and the client’s subjective reports need to be considered when prescribing a wheelchair cushion to minimize pressure ulcer risk factors. None of these studies included direct evidence of pressure ulcer prevention associated with a particular cushion type.
There is level 2 evidence (from one prospective controlled trial and several supporting studies; Burns & Betz 1999) that various cushions or seating systems (e.g. dynamic versus static) are associated with potentially beneficial reduction in seating interface pressure or pressure ulcer risk factors such as skin temperature.
There is level 2 evidence (from one randomized controlled trial and several supporting studies; Gil-Agudo et al. 2009) to support the air cushion as producing low average ischial tuberosity pressures and a large area for pressure distribution. However, not all cushions have been studied and pressure performance is not the only parameter for consideration in cushion selection.
No one cushion is suitable for all individuals with SCI.
Cushion selection should be based on a combination of pressure mapping results, clinical knowledge of prescriber, individual characteristics and preference.
More research is needed to see if decreasing ischial pressures or decreasing risk factors such as skin temperature via the use of specialty cushions will help
prevent pressure ulcers post SCI.
Pressure mapping is a useful tool for comparing pressure redistribution characteristics of cushions for an individual but it needs to be a part of the full evaluation not the main part or only evaluation.