Assessment of geometric and mechanical parameters in wheelchair seating: a variability study

A measurement method has been developed to quantify the posture of able-bodied subjects seated in their wheelchair. Fourteen geometric parameters were measured in order to represent the pelvis, trunk and lower limbs orientations. They were defined by digitizing the three-dimensional (3-D) position of 23 anatomical landmarks using a mechanical articulated arm (Microscribe3D, Immersion Corporation). Mechanical parameters were used to measure the maximum pressure, mean pressure and peak pressure gradient on the seat and the back of the wheelchair using a force sensing array (Vista Medical, Inc.). A third set of parameters combining mechanical and geometric measurements were defined to represent pelvic tilt and ischial pressure orientations. However, different types of errors are associated to the measurement of these geometric and mechanical parameters. The purpose of this study was to evaluate these errors and their impact on the precision of the various parameters on a sample group of five able-bodied subjects. Results showed that variability of most of the geometric parameters is below 2 degrees with the sagittal rotation of the pelvis presenting the highest variability (3.8 degrees) and the thigh angle the lowest one (0.5 degrees). The variability of the mechanical parameters were respectively equal to 4.9% for the mean pressure, 9.3% for the peak pressure gradient and 16.9% for the maximum pressure under ischial tuberosities. It is suggested that the method proposed in this paper could be used as an accurate procedure to characterize the posture of subjects sitting in a wheelchair.