Anatomy of the Epidural Space at the Level of the Fourth Lumbar Vertebra
The epidural space at the L4 level is a potential space located between the dura mater and the vertebral canal, containing primarily epidural fat, venous plexuses, spinal nerves, and connective tissue elements with no fibrous tissue. 1
Boundaries and Location
- Superior boundary: Foramen magnum at the base of the skull
- Inferior boundary: Sacrococcygeal membrane at the sacral hiatus
- Anterior boundary: Posterior longitudinal ligament covering the vertebral bodies and intervertebral discs
- Posterior boundary: Ligamentum flavum and laminae
- Lateral boundaries: Pedicles and intervertebral foramina
Contents of the Epidural Space at L4 Level
Adipose Tissue
- Composed of uniform cells enclosed in a fine membrane 1
- Dorsal fat is attached to the canal wall only in the dorsal midline
- Often tenuously attached to the dura mater
- Semi-fluid consistency allowing movement of the dura relative to the canal wall
- May act as a natural barrier to the spread of injected solutions in the midline 1
Vascular Structures
- Extensive venous plexuses, predominantly in the ventral epidural space 1
- Internal vertebral venous plexus forms a network that:
- Communicates with basivertebral veins
- Drains primarily into the lumbar and azygous veins 2
- The anterior epidural cavity contains:
- Two medial spaces enclosing anterior and medial venous plexuses
- Two lateral spaces receiving anterior longitudinal veins 3
- Communication between medial and lateral cavities
Neural Elements
- Spinal nerve roots traversing the space
- Multiple fascicles that disperse as they approach the dorsal root ganglion 1
- Envelope of arachnoid enclosing the roots near the exit site from the dura
- Recurrent meningeal nerves (sinuvertebral nerves)
Connective Tissue Elements
- Posterior longitudinal ligament forms a cross-shaped structure at L4 including:
- Central septum
- Superficial part extending into intervertebral foramina
- Anterior ligaments of the dura mater 3
- System of connective tissue trabeculae in lateral vaults 4
- No fibrous tissue in the epidural space itself 1
Dimensions and Special Features at L4 Level
- Width of epidural space:
- Anterior: 3-5 mm
- Posterior: 5-7 mm (widest at this lumbar level)
- Lateral: 2-3 mm
- Dura is joined to the canal wall only ventrally at the discs 1
- Lateral vaults connect with the intervertebral canal and dorsally into the posterior epidural space 3
Clinical Significance
For Epidural Anesthesia
- L4 level provides optimal access due to wider posterior epidural space
- Lack of substantial attachments between dura and canal wall facilitates spread of injected solutions 1
- Midline fat may impede solution spread, potentially explaining asymmetric epidural anesthesia 1
- Accurate identification of L4 level is crucial - studies show tendency for practitioners to perform punctures more cranially than intended 5
For Pathological Conditions
- Epidural space can be affected by various pathologies:
- Degenerative conditions
- Infections (epidural abscess)
- Neoplastic lesions (benign or malignant)
- Vascular abnormalities 6
- MRI is the modality of choice for evaluating epidural pathologies due to excellent soft tissue contrast 2
- Epidural abscess is a feared complication with incidence of 2.5-3 per 10,000 hospital admissions 2
For Vertebral Augmentation Procedures
- Venous channels in the epidural space represent potential pathways for extraosseous cement leakage during vertebroplasty 2
- Cement leakage into epidural space can cause serious complications requiring emergency decompression 2
Embryological Development
- The epidural space forms during primary and secondary neurulation
- By the end of the fourth week of embryogenesis, primary neurulation is complete
- The conus medullaris ascends during development, reaching the L1-L2 disc space by 2 months after birth 2
- A conus medullaris ending below the middle third of L2 is radiographically tethered 2
Understanding the detailed anatomy of the epidural space at L4 is essential for safe and effective performance of epidural procedures and for accurate interpretation of pathological processes affecting this region.