Choosing the Vertebral Level — L4-L5 vs L5-S1
These two lowest lumbar levels bear the most mechanical load.
- L4-L5, L5-S1: Most common levels (over 90% of herniations)
- L5: L4-L5 root affected (exits below L5 pedicle)
- S1: L5-S1 root affected (exits below S1 pedicle)
- Highest: Load at L5-S1 (peak axial + shear stress)
Level anatomy
Each disc sits between two vertebral bodies, cushioning motion.
Why these levels
Lowest levels carry the most flexion and torsion load.
Root numbering
A herniated disc typically compresses the traversing lower root.
L4-L5 disc usually compresses the L5 root, not L4.
Annular Tear and Early Disc Bulge
Radial fissures form in the annulus, letting the disc bulge.
- 15-25: Annulus layers (concentric collagen lamellae)
- Posterolateral: Tear location (thinnest annulus region)
- Contained: Bulge vs herniation (annulus still intact)
- Flexion + load: Common trigger (lifting, twisting)
Annulus fatigue
Repeated loading fatigues and cracks the outer fibers.
Radial fissure
A tear opens a path toward the disc surface.
Contained bulge
Disc contour bulges but nucleus stays inside.
A contained bulge can still be asymptomatic at this point.
Nucleus Pulposus Prolapse Through the Tear
Gel-like nucleus material extrudes through the annular defect.
- Protrusion→Extrusion: Herniation type (progressive stages)
- Proteoglycan gel: Nucleus composition (high water content)
- Sequestrated: Extruded fragment (if fully detached)
- Posterolateral: Direction (toward nerve root)
Protrusion
Nucleus pushes outward but base stays broader than apex.
Extrusion
Material breaks through, apex now wider than base.
Sequestration
A free fragment can migrate within the canal.
Extruded fragments can resorb over months without surgery.
Direct Mechanical Compression of the Nerve Root
Herniated material physically pinches the exiting nerve root.
- Lateral recess: Compression site (or neural foramen)
- Flattening: Root deformation (axon + myelin distortion)
- Slowed: Conduction effect (nerve signal transmission)
- Congestion: Venous effect (impaired root microcirculation)
Direct pinch
Disc material presses the root against bone.
Microvascular effect
Compression slows venous return around the root.
Conduction block
Larger fibers fail first, weakness and numbness follow.
Mechanical pressure alone often causes mild, positional pain.
Phospholipase A2 and Chemical Radiculitis
Leaking nucleus material chemically inflames the nerve root.
- Phospholipase A2: Key enzyme (from nucleus pulposus)
- TNF-α, IL-6: Cytokines released (sensitize nerve fibers)
- Chemical + mechanical: Pain mechanism (combined radiculopathy)
- Dermatomal: Pain pattern (radiates below the knee)
PLA2 release
Nucleus material releases PLA2 into the epidural space.
Cytokine cascade
TNF-α and interleukins sensitize the nerve root.
Radicular pain
Sensitized fibers fire, sending pain down the sciatic path.
Chemical inflammation can hurt even without heavy compression.