RESNA Annual Conference - 2019

How Does It Shape Up? Buttocks Shape Across Wheelchair Cushions.

Sharon Sonenblum, PhD1, Stephen Sprigle, PhD, PT1, Mary Shea, MA, OTR, ATP2, Kelly Waugh, PT, MAPT, ATP3, Trevor Dyson-Hudson, MD4

1Georgia Institute of Technology, 2Kessler Institute for Rehabilitation, 3University of Colorado Denver, 4Kessler Foundation


While many factors are considered when selecting a wheelchair cushion for an individual, the ability of the cushion to prevent pressure ulcers by limiting tissue deformation is an important factor. Shape Compliance describes the ability of a cushion to support the buttocks with minimal buttocks deformation. Research has shown that when a cushion matches the shape of a measured contour, it results in an improved loading profile at the buttocks [1-3]. While bench tests to measure shape compliance are still under development, initial data collected on both humans and compliant models allow us to compare the contours of different types of wheelchair cushions when loaded by different buttocks.

In addition to differences across cushions, differences across individuals may also be explored in terms of the contour at the buttocks-cushion interface. Individuals who experience more tissue deformation and a sharper curvature when seated are considered to have a higher Biomechanical Risk for pressure ulcer development. In distinction, deformation resistance is defined as “the intrinsic characteristic of an individual's soft tissues to withstand extrinsic applied forces.” [4, 5]. Therefore, we would expect to see differences in the shape of the buttocks of high and low risk individuals at the buttocks-cushion interface.

The objective of this paper was to describe the average contours of human buttocks and of a compliant cushion indentor across different wheelchair cushions and across individuals of different levels of pressure ulcer risk.


On the left, a generic ischium is plotted with 5 contours representing the sagittal view, and on the right is the coronal view. Four of the contours, representing Jay, HR45, Roho and Matrx are very similar near the ischium with some differences as you move anterior, posterior and lateral. The Java contour is more inferior to the ischium than the others.
Figure 1. 3rd order polynomial best fit to all subjects’ buttocks in each 5 groups of cushions under the peak of the ischium.
The buttocks of 36 individuals (7 able-bodied individuals, 29 individuals who use a wheelchair as their primary mobility device) were scanned sitting in a FONAR Upright MRI. T1-weighted Fast Spin Echo scans were collected with the individuals seated on flat HR45 followed by a subset of wheelchair cushions that could have included: Roho HP (n=15), J2 Deep Contour (n=9), Jay Fusion (n=3), Jay Active (n=1), Matrx Vi (n=13), and/or Ride Java (n=15) in randomized order. For the sake of this analysis, the three different Jay products were combined.

A compliant cushion loading indentor (CCLI) was used to evaluate wheelchair cushions in a more standardized fashion. The CCLI contains an internal substructure with medial and lateral protuberances to mimic the load-bearing ischial tuberosities and trochanters and an elastomeric shell to mimic soft tissue [6]. Ultrasound sensors at 7 locations were used to measure deflection of the elastomer and allowed for the calculation of deformed CCLI contour. The model was loaded with 61 kgf, representing 95 kg person on a Jay3, Roho HP, and Matrx Vi.


The generic ischium is plotted along with 3 contours representing the human buttocks and 3 representing the phantom or compliant cushion indentor when loaded on the Jay, Roho and Matrx. Contours are very similar across each cushion, and the compliant indentor is similar to the human contour.
Figure 2. 3rd order polynomial best fit to all subjects seated on each surface, compared with trigonometric compliant cushion loading indenter results.
Best fit buttocks contours based on all participants seated on each surface are presented in Figure 1 in the sagittal and coronal planes. All of the cushions tested in this study (HR45 notwithstanding) are designed for high-risk individuals. With the exception of the Java, which uses a different strategy (an orthotic offloading design), they all take an immersion and envelopment approach. The Roho is an example of an air cell design, in which air redistributes between a matrix of connected cells in response to buttocks loading, resulting in envelopment of the buttocks. HR45 is a flat foam while Matrx Vi is a contoured foam with multiple layers of different foam stiffness including a viscoelastic layer. Both seek to envelop the buttocks and distribute loading based on compression of the foam layers. Jay cushions take a combination of approaches, creating immersion using a contoured foam base, and then providing a fluid layer on top. Across these surfaces that manage body weight using immersion and envelopment, average contours look fairly similar near the ischium, with the biggest differences emerging as you get farther from the ischium. It is likely that the differences in HOW the cushions manage the load – using air, fluid, foam, etc. is important. Further characterization of the material construction of the cushions will be important to understand how the tissue will respond in dynamic situations as opposed to in static scenarios like that studied here.

Three plots, from left to right include able-bodied, wheelchair users with no pressure ulcer history, and wheelchair users with a pressure ulcer history. Each has a generic ischium and the best fit contour and confidence interval. On the able-bodied plot, contours for able-bodied users fall below the confidence intervals. On the middle plot, contours fall mostly within the confidence intervals, and on the wheelchair users with a pressure ulcer history plot many contours wrap even closer to the ischium than the best fit contour.
Figure 3. Coronal contours on HR45 and best fit 3rd order polynomial across the entire population demonstrates how each risk group compares to the average of the overall population.
The CCLI, when loaded on the Jay3, Roho HP, and Matrx Vi, also produced similar contours across cushions as seen in the human data (Figure 2). A slight difference in elastomeric thickness under the ischium was seen, also similar to human data. This provides some validation for the CCLI and the test method under development.

Buttocks contours for the able-bodied and PrU history groups look distinctly different, with the able-bodied contours typically falling one confidence interval away from the mean polynomial calculated across all 36 participants, and the high risk group (pressure ulcer history) falling much closer to the best fit contour, and wrapping much more tightly around the ischium (Figure 3). Participants in the middle group (wheelchair users with no pressure ulcer history) tend to be at very high risk for pressure ulcers, relative to the general population. In terms of their contours, however, they present as a mix of the other two groups.


Tissue contours in the loaded buttocks present an interesting way to investigate cushion shape compliance and to compare biomechanical risk of individuals.


1.         Sprigle, S., K.C. Chung, and C.E. Brubaker, Reduction of sitting pressures with custom contoured cushions. J Rehabil Res Dev, 1990. 27(2): p. 135-40.

2.         Sprigle, S. and J.Z. Schuch, Using seat contour measurements during seating evaluations of individuals with SCI. Assist Technol, 1993. 5(1): p. 24-35.

3.         Brienza, D.M. and P.E. Karg, Seat cushion optimization: a comparison of interface pressure and tissue stiffness characteristics for spinal cord injured and elderly patients. Arch Phys Med Rehabil, 1998. 79(4): p. 388-94.

4.         Sonenblum, S.E., et al., 3D anatomy and deformation of the seated buttocks. J Tissue Viability, 2015. 24(2): p. 51-61.

5.         Sonenblum, S.E., et al., 3-dimensional buttocks response to sitting: A case report. J Tissue Viability, 2012.

6.         Kumar, N., S. Sprigle, and J.S. Martin, Measurement of Load Redistribution Properties of Wheelchair Cushions Using a Compliant Cushion Loading Indenter. Assist Technol, 2015. 27(3): p. 129-35.


Funding for this project was provided by NIDILRR through a field initiation grant (90IF0120). The authors thank Dr. John Greenhalgh for his assistance with collecting MRI scans, and gratefully acknowledge materials support and a donation from Ride Designs, which supplemented the cost of MRI scanner time.