A 25-year-old man presents to the emergency depament complaining of a severe headache, stiff neck, and fever. While waiting for treatment, the patient has a seizure. A neurologist suspects meningitis, and performs a lumbar puncture on the patient. Immediately after the needle passes through the interlaminar space between the 4th and 5th lumbar veebrae, which space does it enter?

The interlaminar space is the space between the laminae of the lumbar veebrae. The ligamentum flavum crosses the interlaminar space. Since there is a gap between the right and left ligamentum flavum in the midline, this ligament is not penetrated if the needle is in the midline. Immediately within the bony veebra is the epidural space, which lies between the bone and the dura. The epidural space contains a considerable amount of fat and a venous plexus.Because the muscles of the back are located on either side of the midline, no muscle is penetrated when doing a lumbar puncture. The subarachnoid space, which is filled with cerebrospinal fluid, is reached after the needle passes through the epidural space and pierces the dura and arachnoid. The cauda equina, rather than the spinal cord, is located in the subarachnoid space of the low lumbar region where a lumbar puncture is performed. The subcutaneous space contains superficial fascia and fat and is found immediately beneath the skin. After passing through the subcutaneous fascia, the needle passes through the supraspinous ligament and the interspinous ligament. Ref: Moon D.A., Foreman K.B., Albeine K.H. (2011). Chapter 1. Back. In D.A. Moon, K.B. Foreman, K.H. Albeine (Eds), The Big Picture: Gross Anatomy.