The posterior lumbar ramus: CT-anatomic correlation and propositions of new sites of infiltration

Abstract

Background and purpose: Lumbar zypapophyseal joints are innervated by the medial branch of the posterior lumbar ramus. The aim of this work was to describe the precise course of the medial ramus on axial CT scans and to define a precise location for its selective infiltration.

Methods: Lumbar spines of two cadavers were first dissected to assess the route of the L1-L5 posterior ramus. Thirty lumbar spinal nerves of three cadavers were injected in the epineural space with a mixture of iodine contrast and stain to perform a correlation between anatomic gross sections and CT sections in the axial plane. A histologic study was also performed to ensure the neurologic nature of the structure identified.

Results: The fibroosseous canal located between the mamillary and the accessory processes was a constant pathway for the medial branch of the L1-L4 posterior ramus. This former was always located closer to the accessory process. The L5 posterior ramus and its divisions could also be identified into a groove bounded laterally by the ala of the sacrum and medially by the base of the superior articular process of S1.

Conclusion: The accessory process and the groove bounded laterally by the ala of the sacrum and medially by the base of the superior articular process of S1 can be easily depicted on CT images and may allow a precise and selective infiltration of the medial branch of the posterior lumbar ramus.

Fig 1.

Photograph (A) and schematic drawing (B) of a right posterolateral view of a dissection of a posterior lumbar ramus and its divisions at L2–L3. The medial branch of a L2 posterior ramus (arrowheads) runs against the lateral surface of the caudal edge of the superior articular process (black curved arrow) and then passes under a ligament (long black arrow) connecting the accessory process (a) and the mamillary process (m). Note the lateral branch of L2 posterior ramus (long white arrow) and vessels (white curved arrow) as well as the duplicity of the medial branch in the fibroosseous canal (twin medial branch).

Fig 2.

Axial CT section (A) and corresponding gross anatomic section (B) show the L1 posterior ramus dividing into a medial branch (black arrow) and a lateral branch (white arrow) at the back of the transverse process. The medial branch is demonstrated (black arrow) on the corresponding histologic section (C).

Fig 3.

Anatomic axial CT scan (A) after nerve injection and corresponding gross anatomic section (B) showing the L2 posterior ramus (long arrow) located into the groove bounded by the accessory process (a) and the mamillary process (m). Note the vascular pedicle (short arrows) located at the medial aspect of the medial branch. The histologic section (C) performed at this level demonstrates the medial branch (long arrow), the vascular pedicle (short arrows), the accessory process (a), and the mamillary process (m).

Fig 4.

Right posterior view of a dissection at L2–L3 intervertebral level showing the division of the medial branch into a branch (white arrow) innervating the adjacent zypapophyseal joint and into another branch (black arrow) innervating the subadjacent zygapophyseal joint.

Fig 5.

Drawing showing on a left posterolateral view the course of a posterior lumbar ramus (long arrow) and its division into a lateral branch (short arrow) running at the back of the transverse process and into a medial branch (arrowhead) running dorsally and caudally against the anterolateral edge of the superior articular process and then passing between the accessory and mamillary process.

Fig 6.

Posterior view of the left part of the first sacral vertebrae (S1) showing the groove bounded by the base of the superior articular process of S1 (black arrow) and the ala of the sacrum. The course of the medial branch of the posterior ramus is materialized by a yellow plastic wire (white arrows).

Fig 7.

Axial CT scan (A) and corresponding gross anatomic section (B) showing the L5 posterior ramus (white arrow) located into a groove bounded medially by the superior articular process of S1 (black arrow) and laterally by the ala of the sacrum (as). The gross anatomic section was dissected to better show the whole course of the posterior ramus into the bony groove. S1, first sacral vertebrae; SG, spinal ganglia.