Mechanical effects of leg position on vertebral structures examined by magnetic resonance imaging.

Background And Objectives: Leg manipulation has been postulated to affect spinal curvature and position of the cauda equina within the dural sac. However, no evidence of such mechanical effects has been shown in living subjects. We used magnetic resonance imaging to evaluate the mechanical effects of leg position on these 2 parameters.

Methods: Sagittal and axial magnetic resonance images of the lumbosacral vertebral canal were obtained in 5 healthy, female volunteers with the subject in the supine position with knees straight, knees slightly flexed, and knees fully flexed.

Results: In the straight leg position, physiologic lumbar lordosis was evident in all subjects on midline sagittal slices, whereas lumbar lordosis disappeared in the fully flexed leg position. On the axial slices the cauda equina moved ventrally within the dural sac in all subjects in the fully flexed leg position. In 1 of the 5 subjects the cauda equina moved ventrally and also separated completely into right and left parts.

Conclusions: Our findings indicate that 2 potential factors, flattening of the lumbar lordosis and some added tension on the lumbosacral nerve roots, may contribute to postoperative back and leg aching after spinal anesthesia in the lithotomy position.