Spine | Anatomy | Common root sheath L5-S1: MRI

32-year-old patient with chronic low back pain.

Figure 1: Lumbar MRI in T2 weighted sagittal sequences: L5 root (red arrow), S1 root (orange arrow). On the left side (A), highlighting a common sheath, with joining (blue arrow) roots L5 and S1. Right (B), modal type anatomy.


Figure 2: MRI in T2 weighted cube axial sequence:

(A) early departure of the left S1 root (orange arrows) in a common sheath with L5 (red arrows), compared to the contralateral side.

(B) S1 root in contact with the spinal ganglion of L5.

(C) Spinal ganglion S1 left (blue star) high located, able to mimic a herniated disc.

(D) Right S1 spinal ganglion (green star) in normal position.

Anatomical variations of the root sheaths of the lumbar region are frequent and varied. Common sheaths of L4-L5 and L5-S1 roots are common, and should not be overlooked, as they can create images of pseudo hernia, especially at spinal ganglion-related areas. Associations between L5-S1 common sheath and controlateral S1-S2 joint sheath are possible. The implication of these variations in lomboradiculalgia remains controversial.

Visible signs in MRI are:
-visualization of four roots (2 motor and 2 sensory) within the same sheath facilitates diagnosis (Figure 2.A)
-an asymmetry of the anterior lateral dural recess
-a contiguous aspect of both nerve roots on sagittal sections



-Song S.J., Lee J.W., Choi J.Y., Hong S.H., Kim N.R., Kim K.J., and al. Imaging features suggestive of a conjoined nerve root on routine axial MRI Skeletal Radiol 2008 ;  37 : 133-138

-Pièges et astuces en imagerie neurologique : imagerie des espaces périmédullaires. J.-L. Dietemann, A. Bogorin, M. Abu Eid, R. Sanda, I. Mourao Soares, S. Draghici, and al. Journal de Radiologie Diagnostique et Interventionnelle, décembre 2012.