Background and Objectives: To perform spinal anesthesia, patients are usually placed in the lateral decubitus position with the knees drawn up to the stomach, the legs fully flexed, and the neck flexed to curve the back outward. Because the nerve roots of the cauda equina have considerable mobility in the subarachnoid space, the position of the cauda equina in the lateral decubitus position may be different from that in the supine position. However, the anatomic position of the cauda equina in the lateral decubitus position with fully flexed legs has not been carefully studied. In the present study, we geometrically measured the movement of the cauda equina in the subarachnoid space by changing positions, using magnetic resonance imaging (MRI).

Methods: After obtaining the approval of the hospital ethics committee and written informed consent, 12 healthy volunteers (age, 34 years [SD, 10 years]; height, 169 cm [SD, 9 cm]; weight, 65 kg [SD, 10 kg]) were studied with MRI, and their positions were changed as follows: the supine position, lateral decubitus position without fully flexed legs, and lateral decubitus position with fully flexed legs. The movement of the central point of the spinal cord and cauda equina by changing position was evaluated.

Results: The spinal cord and cauda equina were observed in the gravity-dependent side of the subarachnoid space in each position. The movement to the gravity-dependent side by changing position from the supine to the lateral decubitus position was significant (mean [SD] in millimeters: T11/12, 0.7 [0.4]; T12/L1, 1.5 [0.9]; L1/L2, 3.0 [1.0]; L2/L3, 3.4 [1.0]; L3/L4, 2.7 [0.9]; L4/L5, 2.0 [0.9]; L5/S1, 1.2 [0.7]; S1/S2, 0.5 [0.2]). The most obvious shift was observed at the L2/3 intervertebral space. The fully flexed legs significantly moved the spinal cord and cauda equina to the ventral side of the subarachnoid space and created a free space in the dorsal side. The movement to the ventral side by the fully flexed legs was significant (mean [SD] in millimeters: T11/12, 2.2 [1.0]; T12/L1, 4.1 [1.5]; L1/L2, 6.1 [0.5]; L2/L3, 5.4 [0.9]; L3/L4, 5.1 [1.9]; L4/L5, 4.4 [1.1]; L5/S1, 5.0 [0.9]; and S1/S2, 2.6 [0.9]). The most obvious shift was observed at the L1/2 intervertebral space.

Conclusion: The present study using MRI showed dynamical movement of the spinal cord and cauda equina due to changing position. The most obvious movements by changing from supine to lateral decubitus position and fully flexed legs were observed at the L2/3 and L1/2 levels, respectively.