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.