Lumbar vertebrae have massive vertebral bodies (kidney-shaped) without costal facets. The spinous process is thick and directed sagittally. The superior articular processes are concave and inferior are convex. Both have articular surfaces directed sagittally. The typical lumbar vertebrae have two additional, small processes called the mamillary process and accessory process.
For lumbar puncture the patient should be positioned in the lateral recumbent position with the neck and back fully flexed. Usually the puncture site is found at the line joining the superior part of both iliac crests (Tuffier’s line) which cross the midline between the L3 and L4 or L4 and L5 verterbrae. In this position, the interspace between the adjacent spinous processes of lower spine is increased, and a thin needle can be threaded into the spinal canal.
A local anaesthetic is used to numb or “freeze the area” of the puncture site.
A needle is inserted into the lower back along the superior aspect of the spinous process, directed toward the umbilicus. The needle goes into a space below where the spinal nerve ends. The lumbar puncture needle pierces the following structures:
- Subcutaneous tissue
- Supraspinous ligament
- Interspinous ligament
- Ligamentum flavum
- Epidural space
- Internal vertebral venous plexus
- Dura mater
- Arachnoid mater
- Subarachnoid space
During penetration of the needle through the ligamentum flavum, a small passive resistance is felt by clinicians and a second one after passing the dura mater entering the subarachnoid space.
During epidural anesthesia the needle enters the epidural space and during lumbar puncture the needle enters the subarachnoid space.
The most common complication after lumbar puncture is headache, caused by irritation of the dura mater.