Anatomy of the Epidural Space
The epidural space is a potential space containing fat, nerves and blood vessels. It begins at the formamen magnum and extends to the sacral hiatus.
The epidural space is deep to the highly resistant ligamentum flavum. The depth from the skin is highly variable but averages 4-5cm. The depth depends on the patient's body habitus, anatomy and the angle of your needle.
Surface Anatomy
The following landmarks are useful when inserting your epidural
- The line between the iliac crests (intercristal line) corresponds to the L3/L4
- Inferior angle of the scapula corresponds to T7
- Vertebra Prominens (C7). (The lowest cervical vertebra protrudes the most.)
Differences between lumbar and thoracic epidural insertions
Lumbar
The spinal cord finishes at L1 in adults so is safe for injections below L1
The spinal processes are fairly perpendicular to the spine so the needle can be inserted at 90 degrees.
Midline is best: It has less vessels and the ligamentum flavum is thickest.
Thoracic
The spinal cord is susceptible to injury
The spinous processes angle downwards
The interspace is narrower
The ligamentum flavum is not a thick as in the lumbar region.