Seated Surfaces

The seat cushion in a wheelchair has many roles depending on the individual’s unique needs. Primarily, the cushion’s role is, to contribute to a functional and balanced posture and redistributing pressure away from the critical areas of the IT and the sacrum and redistributing 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 IT or sacrum and are therefore more likely to have been initiated while seated. Provision of a wheelchair cushion that redistributes pressure is an important prevention recommendation. Cushions should be evaluated based on postural support and stability provided, pressure redistribution capabilities, comfort, function temperature effects level of SCI, pressure redistribution abilities, transfer technique, and lifestyle (Garber 1985; Makhsous et al. 2007a; Fisher et al. 1978; Seymour & Lacefield 1985; Sprigle et al. 1990). Many of the studies reviewed for this section compare different cushions in an attempt to identify the “best” cushion. One study explored how the intensity of the load when sitting on a cushion influences blood flow, which is thought to influence pressure injury risk.

Summarized Level 5 Evidence Studies

The following level 5 evidence studies have been reviewed, and the overarching findings from the studies are highlighted in this section. As noted at the start of this chapter, these types of studies are not included in the discussion or in the conclusions.

Wu et al. (2016) provided participants with alternating pressure air cushions six times a week for two weeks, every three months for a total of 18 months. A high percentage of users were very satisfied with comfort and performance of these cushions. However, there were no measures of pressure or pressure ulcer incidences in relation to this trial therefore the full benefits of this type of cushion is not known.

Kovindha et al. (2015) surveyed chronic SCI wheelchair users in Thailand about their pressure ulcer prevalence, quality of life and health status. McClure et al. (2014) similarly surveyed a group of chronic SCI wheelchair users about their pressure ulcer prevalence and wheelchair cushion use. In both studies over half of the population had a pressure ulcer at some point. Common sites for current pressure ulcers were the IT, while that of healed pressure ulcers was the sacrococcygeal area. Kovindha et al. (2015), found those with current pressure ulcers were more depressed than those without current pressure ulcers. There was however no difference in health status between those with and without pressure ulcers. McClure et al. (2014) found that more than half of the participants used their wheelchair cushions when travelling in motor vehicles or airplanes.

Meaume et al. (2017) completed two observational studies exploring pressure ulcer incidence in recently spinal cord injured people who were at high risk for developing pressure injuries following a 35 day period of using an air-filled cushion; one study used a single compartment air-filled cushion (n=78) and the second using a multi-compartment air-filled cushion (n=74). They found an incidence of 2.6% for developing a pressure injury in the single compartment air-filled cushion group versus a 4% rate in the other group. The authors indicate that this rate is low, and therefore recommended the use of these types of cushions however, they do not reference support for this being a low incidence rate. They also did not account for any other variables that may have supported good pressure management strategies to reduce the risk of developing pressure injuries given the participants were newly admitted and in a very supportive environment. Additionally, the authors also declare an affiliation and funding support with the manufacturer of the air-filled cushions.

Sprigle and Delaune (2014), and Sprigle (2010) investigated the properties of cushions used by SCI wheelchair users at an adult inpatient rehabilitation center. Cushion type varied from air, foam and fluid cushions. The average cushion age was approximately 30 months, and the average cushion usage per day was 12 hours. The proportion of cushion damage from deformation, granulation, or stiffness to cushions was greater as cushions aged. Sumiya et al. 1997 reported similar findings with regards to frequency of replacement of cushions and types of cushions used.

Brienza et al. (2018) explored the effects ot tissue compositions (fat and muscle) and deformation under the ischial tuberosities of 6 participants (4 with SCI, 2 without), on 6 different seat cushions using Magnetic Resonance Imaging (MRI) and Interface Pressure Mapping (IPM). They found that no one cushion performed best for all participants. They also found a difference in tissue composition between SCI and non-SCI participants. Participants with SCI having higher tissue volume reductions when loaded (sitting). Higher IPM Peak pressure indexes were also associated with lower overall tissue thicknesses in the ischial tuberosity areas. These findings reinforce that cushion selection must be individualized and the need for a comprehensive assessment to support the prescription of individualized seating equipment. Individual anatomy composition and cushion type will affect deformation response (and therefore assumed pressure injury response). The authors identify limitations in this study such as the small number of participants and that findings are observational. However, the findings are similar to other findings in this section and throughout the chapter as well as the Pressure Ulcer chapter.

Discussion

Sonenblum et al. (2018b) identified clinical factors for consideration that influence buttock tissue response to loading. The study found that people with higher BMI experienced greater magnitude of deformation of the ischial tuberosity tissue and slightly increased blood flow at lower loads. They also found that buttock tissue reached maximum deformation (“bottomed out”) at a lower load for people who smoked compared to non-smokers. In regard to superficial blood flow, there was great variability across all participants, at both high (200+mmHg) and low loads (40-60mmHg). However, for people with a history of pressure injuries, there was a blood flow decrease even at low loads. These findings suggest that there are clinically related factors to consider during the process of determining the optimal seated surface for pressure management.

Crane et al. (2016) sought to measure the interface pressure characteristics of an offloading cushion (Ride Java in 3 configuration – full offloading, and addition of 2 well inserts) compared to an air inflation cushion (single valve ROHO). Their findings suggest that the offloading cushion provided improved pressure management than the air inflation, however since their isn’t a universally accepted interface pressure parameter directly linked with pressure ulcer risk or development it is not known if these differences are enough to impact pressure ulcer incidence. Generalizability is also challenging due to limited information on participants, about their posture on the cushions and the small sample size.

Sonenblum et al. (2018a) also compared the Java in its offloading configuration with the 4” roho (single valve) as well as with the MatrixVi cushions. Their goal was to determine differences in tissue deformation using an MRI and Peak pressure index via IPM. The participants they chose had signficant atrophied sitting surface tissue as this was felt to be one of the most challenging individuals to seat safely. Their findings suggest that there is a relationship between the tissue thickness under the iscial tuberosities and interface pressure, where the thinner the tissue the higher the pressures. However, they also found that all cushions deformed tissue in some location, and that tissue responds individually to load in different locations, supporting the need for indivudalized assessment for identifying the optimal seat cushion for each person.

Vilchis-Aranguren et al. (2015) provided a wheelchair cushion personally customized to each participant’s ischiogluteal area. After using these custom cushions for two months, pressure distributions around the ischiastic tuberosity zone decreased and participants reported increased satisfaction in performing activities of daily living compared to their regular cushions. These findings support the need for consideration of the sitting surface anatomy during the individualized assessment for seating.

The following studies evaluated different cushions using different interface pressure mapping systems and different pressure mapping outcomes. Typically, the studies used very small numbers of participants and did not evaluate a range of contributing factors such as posture on the cushions evaluated. Since there isn’t an absolute pressure threshold identified related to pressure injury incidence, the findings from these studies provide data for consideration in clinical practice but should be used with clinical judgment for determining the optimal cushion in conjunction with the other seating components and configuration of the wheelchair frame. Trewartha and Stiller (2011) used pressure mapping to evaluate the Roho Quadtro and the Vicair Academy among three people with SCI. Findings 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.

In the cushion comparison study by Gil-Agudo et al. (2009) 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). This study compared only four cushions, and based findings 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), compared the contact sitting surface areas in two different conditions; one where the ischial support was partially removed for 10 mintues periods and the other where push up were performed every 20 minutes. The investigators found that the anterior portion of the seat cushion had a larger contact area among those with tetraplegia with higher pressure in the anterior and middle portion of the cushion for the partially removed ischial support condition.The authosr suggest that the reducing the contact area at the posterior sitting surface can be achieved with increased contact at the middle and anterior areas, thereby reducing the pressure over the sittign surface bony prominences.

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 participants 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), three 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 IT, 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 , with the dry flotation cushion providing more pressure redistribution than the gel cushion during upright sitting. Mean pressure with the IT-bladder-inflated cushion was greater than upright pressures for either the dry flotation or gel cushions.

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, polyurethane 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 eight 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 seven 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 versus paraplegia (55% versus 45%).

These studies demonstrate that there are individual variations in cushions needs inherent in those with SCI (e.g., paraplegia versus tetraplegia). 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. 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.

Conclusion

There is level 2 evidence (three randomized controlled trials: Gil-Agudo et al. 2009; Crane et al. 2016, Sonenblum et al. 2018a, and one pre-post study: Vilchis-Aranguren et al. 2015) suggesting that cushions that envelope specific to the individual’s shape may have lower sitting surface pressures may have higher patient satisfaction than cushions that envelope less.

There is level 2 evidence (from one prospective controlled trial: Burns & Betz 1999, one randomized control study: Sonenblum et al. 2018a, and one cohort study: Makhsous et al. 2007) that cushions that reduce the pressure (e.g., dynamic versus static) or offload pressure in the ischial tuberosity region may be associated with potentially beneficial reduction in seating interface pressure and/or pressure injury risk factors.

There is level 4 evidence (from one pre-post test study: Sonenblum et al. 2018b) to suggest that the factors of body mass index, smoking status, and pressure injury history affect tissue response to different loads when seated on a cushion.