Lumbar Puncture: Site, Needle Direction, and Structures Pierced

Lumbar puncture (LP), also known as a spinal tap, is a medical procedure used to collect cerebrospinal fluid (CSF) for diagnostic purposes or to administer medications. It involves advancing a needle through specific anatomical structures to access the subarachnoid space.

Procedure Site

The lumbar puncture is performed at the L3-L4 or L4-L5 intervertebral space to avoid damage to the spinal cord, which typically terminates at L1-L2 in adults.

Needle Direction

The needle is inserted in a slightly cephalad (upward) direction with a bevel facing lateral to minimize nerve fiber damage.

Sequential Structures Pierced

• Skin – The first layer penetrated after sterilization.
• Subcutaneous Tissue – Fat and connective tissue beneath the skin.
• Supraspinous Ligament – Connects adjacent spinous processes.
• Interspinous Ligament – Located between the spinous processes.
• Ligamentum Flavum – Offers resistance; piercing may produce a ‘pop’ sensation.
• Epidural Space – Contains venous plexuses and fat.
• Dura Mater – Tough, protective outer meningeal layer.
• Arachnoid Mater – The second meningeal layer.
• Subarachnoid Space – The target site containing CSF for collection.

Clinical Considerations

Careful needle advancement prevents complications such as post-lumbar puncture headache, infection, and spinal nerve damage.