During a midline spinal block at the L3-L4 level, which structure would the needle NOT pass through?

During a midline spinal block at the L3-L4 level, which structure would the needle NOT pass through?

• A. Supraspinous ligament
• B. Interspinous ligament
• C. Ligamentum flavum
• D. Posterior longitudinal ligament

Answer: (D)

In a midline lumbar spinal block, the needle tracks from the back through the posterior ligaments before reaching the subarachnoid space. You pass through the supraspinous ligament, then the interspinous ligament, and then the ligamentum flavum. After that, you go into the epidural space and finally through the dura and arachnoid to access CSF for a spinal injection. The posterior longitudinal ligament sits on the anterior surface of the vertebral canal, in front of the dural sac, so it is not encountered along a standard posterior, midline approach. Therefore, the structure not crossed is the posterior longitudinal ligament.

In a midline lumbar spinal block, the needle tracks from the back through the posterior ligaments before reaching the subarachnoid space. You pass through the supraspinous ligament, then the interspinous ligament, and then the ligamentum flavum. After that, you go into the epidural space and finally through the dura and arachnoid to access CSF for a spinal injection. The posterior longitudinal ligament sits on the anterior surface of the vertebral canal, in front of the dural sac, so it is not encountered along a standard posterior, midline approach. Therefore, the structure not crossed is the posterior longitudinal ligament.