Chih-Han Chan, Liang-Chiu Hsieh, and Tim D. Yang
ABSTRACT
Pressure ulcer development is a common complication in people with spinal cord injury (SCI). Although not the sole cause, prolonged seating pressure is known to be a contributing factor to ulceration. Seating technologies use principles of immersion (how far the buttocks sink below the interface) and envelopment (how well the interface conforms to the buttock) to reduce interface pressure by increasing contact area; however, these properties can be difficult to achieve due to individual differences in body type. Effective support surfaces must accommodate a wide range of pathophysiological and body weight changes caused by SCI. The purpose of this project was to design and develop a contoured cushion that was both cost-effective and could be conveniently (re)configured per individual. We fulfilled these design requirements by using the rapid prototyping technique.
KEYWORDS
pressure ulcer; rapid prototyping; seat cushion; spinal cord injury; wheelchair
BACKGROUND
Wheelchair users with spinal cord injury (SCI) commonly develop pressure ulcers at the bony prominences of the buttocks (1, 2). As a result of decreased weight shifting from impaired mobility and sensation, prolonged mechanical loading induces tissue ischemia and eventually tissue necrosis. Seating technologies aim to reduce pressure ulcer risk by redistributing seating loads via immersion and envelopment (3). Nevertheless, pressure ulcer prevalence rates and treatment costs remain high, with the former ranging from one-quarter to one-third of people with SCI and the latter accounting for 11 billion US dollars (USD) annually (4-6).
PROBLEM STATEMENT
Seat cushion development is guided by principles of immersion and envelopment. Immersion refers to how far the buttocks sink below the interface, and envelopment refers to how well the interface conforms to the buttocks (3). Combined, these cushion properties can reduce interface pressure by increasing contact area; however, immersion and envelopment can be difficult to achieve due to individual differences in body type. People with SCI undergo pathophysiological changes in the buttock, such as decreasing of the cortical bone and flattening of the tips of the ischial tuberosities (7-10). Furthermore, obesity has been identified as a major risk factor for deep tissue injury among people with SCI (11). As a result, effective seating support surfaces must accommodate a wide range of body types.
DESIGN AND DEVELOPMENT
Configurability of the seating interface would allow clinicians to enhance immersion and envelopment by adapting the surface to each individual. While custom-contoured solutions exist, our goal with the CO3 cushion was to develop one that was cost-effective and could be conveniently (re)configured per individual. To facilitate the development process, we used the rapid prototyping technique. Medola, Fortulan, Purquerio Bde, and Elui (12) demonstrated the use of the rapid prototyping technique for assistive technology development. They manufactured wheelchair pushrim prototypes in polyurethane and designed them with anthropometric features and ergonomic concepts. For our CO3 cushion, we used the Pro/ENGINEER 3D product design suite (PTC, Needham, MA) to create a virtual model of the wheelchair seat cushion (Figure 1).
Figure 1. Virtual 3D model of the customizable cushion.
The wheelchair spring component was rapidly prototyped with polyoxymethylene (TJ-MAX 3D Print, Tronjen Technology, Taichung, Taiwan). We prototyped the cushion surface and base from a cork material and acrylonitrile butadiene styrene injection molding, respectively (Figure 2). Finally, triangular units were created by fastening the spring to the surface and base components using ethylene vinyl acetate. We designed these triangular cushion units to be modular, such that each unit could be removed and customized as needed.
Figure 2. Component view of each triangular module: (A) cork surface, (B) ethylene vinyl acetate, (C) polyoxymethylene spring, and (D) acrylonitrile butadiene styrene injection molding.
EVALUATION
We assessed our CO3 cushion for safety, durability, and configurability. Mentoring faculty and fellow classmates within the department helped with the cushion evaluation process. In the interests of safety, foam padding was placed atop the cork surface both to provide a conventional cushion padding material as well as to prevent pinching of the skin between the triangular modules. The cushion components, including the springs, were durable enough to withstand individuals weighing between 139 and 209 pounds while performing wheelchair pushups. Triangular modules were disassembled and reassembled seamlessly throughout the evaluation process.
DISCUSSION AND CONCLUSION
Our contoured cushion fulfilled our two original goals: cost effectiveness and configurability. In terms of expense, each spring component cost only 0.05 USD and required 35 minutes to produce. In terms of configurability, the triangular modules satisfied the original design goal of allowing the clinician or wheelchair user to reassemble modules as needed. The low cost and fast production time of each spring will allow clinicians to customize spring components based on individual needs.
For example, spring properties could be customized according to each individual’s body weight by customizing the spring constant k. In terms of the physics of springs, a higher k constant corresponds to a stiffer spring. People with obesity will require spring components with a higher k constant to support their increased weight (Figure 3). Furthermore, our design allows clinicians to contour the seating interface. Due to pathophysiological changes, balanced seating and balanced pressure distribution are major concerns for people with SCI (13). Previous studies have demonstrated the importance of the support surface on spinal alignment and lumbar support in people with SCI (14).
Figure 3. The stiffness of each spring is controlled by its spring constant. The cushion’s spring component can be customized for people with different body types.
Figure 4 presents a conceptual illustration of how our CO3 cushion could be used to redistribute interface pressures and improve postural support. Figure 4A illustrates balanced posture and pressure distribution in a person without impairment. In Figure 4B, pathophysiological changes have caused imbalanced posture and pressure distribution in a person with SCI. The individual’s weight is heavily redistributed toward the right ischial tuberosity, causing severe spinal misalignment. In Figure 4C, seating pressures have been redistributed and the spine has been realigned by appropriately adjusting the spring constants of the triangular modules under each ischial tuberosity.
Figure 4. Conceptual illustration of our contoured cushion’s configurability: (A) balanced posture and pressure distribution, (B) imbalanced posture and pressure distribution due to pathophysiological changes, and (C) realigned posture and redistributed pressure due to spring configuration.
While our design has not been tested in a research setting among people with SCI, we look forward to conducting research with our customizable cushion. The low cost and high configurability of our CO3 cushion will allow clinicians to improve immersion and envelopment of seating interfaces for people with SCI.
ACKNOWLEDGMENTS
We would like to thank the professors and classmates from Asia University who contributed to the cushion evaluation process. Furthermore, the project would not have been possible without the guidance and support of Chi-Wen Lung, PhD (Department of Creative Product Design, Asia University) and Yih-Kuen Jan, PT, PhD (Department of Kinesiology and Community Health, University of Illinois at Urbana-Champaign).
REFERENCES
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2. Jan, Y. K., Crane, B., Liao, F., Woods, J. A., & Ennis, W. J. (2013). Comparison of muscle and skin perfusion over the ischial tuberosities in response to wheelchair tilt-in-space and recline angles in people with spinal cord injury. Archives of Physical Medicine and Rehabilitation, 94(10), 1990-1996.
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14. Mao, H. F., Huang, S. L., Lu, T. W., Lin, Y. S., Liu, H. M., Wang, Y. H., & Wang, T. M. (2006). Effects of lateral trunk support on scoliotic spinal alignment in persons with spinal cord injury: a radiographic study. Archives of Physical Medicine and Rehabilitation, 87(6), 764-771. doi: 10.1016/j.apmr.2006.02.029
Contact: Tim Yang, tyang15@illinois.edu