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

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.