Seat-interface pressures on various thicknesses of foam wheelchair cushions: A finite modeling approach

Ragan R, Kernozek TW, Bidar M, Matheson JW. Seat-interface pressures on various thicknesses of foam wheelchair cushions: a finite modeling approach. Arch Phys Med Rehabil 2002;83:872-5. Objective: To investigate the effect of cushion thickness on subcutaneous pressures during seating by using a finite element modeling approach. Design: Seat-interface pressure measurements were used in a computational model. Setting: Biomechanics laboratory. Participant: A single healthy man (weight, 70kg). Interventions: Subject sat upright either with or without cushions of various heights. Seat-interface pressures measured by using a sensor mat interfaced to a personal computer sampling at 15Hz. Main Outcome Measures: Peak seat-interface pressure; finite-element software was used to model the buttock, ischial tuberosity, and seat cushion. Subcutaneous stresses were calculated from the model. Results: The region of highest subcutaneous stress in the soft tissue was concentrated within 1 or 2cm of the ischial tuberosity, with the maximum compressive stress inferior to the bottom surface of the ischial tuberosity. The maximum subcutaneous stress, maximum seat-interface pressure, and maximum subcutaneous shear stress each changed with cushion thickness. Subcutaneous pressures decreased with thicker cushions, but almost all of the reduction was obtained with an 8-cm cushion. The amount of subcutaneous shear stress increased slightly for thicker cushions. The maximum subcutaneous stress was greater than the maximum interface pressure but not by a constant factor. Instead, the former was consistently larger by 0.7 to 0.8N/cm2. Conclusions: Cushion use reduced the maximum subcutaneous stress inferior to the ischial tuberosity. However, increasing the cushion thickness beyond 8cm was ineffective in further reducing subcutaneous stress. It was also found that seat-interface pressures were a good indicator of the subcutaneous stress reduction in seating. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation