Accessing the influence of repositioning on the pelvis' 3-D orientation in wheelchair users.

This study aimed at evaluating the effects of mechanical repositioning, obtained by the increase in seat-to-back (STB) and system tilt angles, on the position of the pelvis with spinal-cord injured subjects seated in a wheelchair. The noninvasive method used combined magnetic resonance imaging (MRI) images of the whole pelvis obtained in a supine posture and ultrasound images of the pelvic iliac crests obtained in four seating positions. The matching of the two image data sets enabled the location of fourteen pelvic landmarks in the seated positions. From these landmarks, the pelvic tilt, obliquity, and transverse rotation, and the three-dimensional (3-D) motion of the pelvis were calculated. Results showed that the increase in STB angle is not equal to the calculated increase in pelvic tilt and that the pelvis rotated posteriorly, moved forward and downwards. An increase in the system tilt moved the pelvis rearwards and downwards, which counter-balanced the movement seen with the increase in STB. At the return to the first position, no significant changes were observed in the pelvis' position and orientation compared to the initial posture. Results also demonstrated the importance in calculating the total 3-D rotations and translations to characterize adequately the pelvic movement.

Differences between pelvic skin and bone landmark identification in different seated positions on spinal-cord injured subjects.

The purpose of this paper was to determine the differences between internal and external pelvic landmark locations in different seating positions. A computer tool developed for the registration of two series of images was used to obtain the internal geometry. First, images of the pelvis were acquired by magnetic resonance imaging (MRI) for each subject, in a supine position; internal landmarks were then identified on the images. Second, ultrasound images of the iliac crests were acquired in four seated positions. A registration algorithm was applied to obtain the transformation matrix between the two image reference systems. The MRI anatomical landmarks were, therefore, transferred into the ultrasound referential, to obtain their three-dimensional (3-D) location in the different seating positions. The external landmarks in those seated positions were identified with a 3-D digitizer. The results revealed that generally the internal and external coordinates of corresponding landmarks are statistically different. The differences are not only due to soft tissue thickness but also to different interpretations of the landmarks' locations between the supine and the seated postures. However, these differences generally did not affect significantly the accuracy with which orientation indexes can be estimated (pelvic tilt, obliquity, transverse rotation). Correlations were found between the internal and external coordinates, implying that linear regressions can be established.