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

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, 70 kg).

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 15 Hz.

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 2 cm 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 0.8 N/cm(2).

Conclusions: Cushion use reduced the maximum subcutaneous stress inferior to the ischial tuberosity. However, increasing the cushion thickness beyond 8 cm 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.