The Healthy Lumbar Disc
A gel-like nucleus sits contained within tough annulus rings.
- 10–12mm: Disc height (L4-L5) (normal adult spacing)
- ~20: Annulus lamellae (concentric collagen rings)
- ~80%: Nucleus water content (young healthy adult)
- >2000N: Axial load capacity (everyday compressive force)
Annulus fibrosus structure
Concentric collagen lamellae wrap the nucleus in criss-cross layers.
Nucleus pulposus function
Hydrated gel core distributes compressive load evenly across the disc.
Disc nutrition
Avascular tissue relies on diffusion through the vertebral endplates.
Discs have no direct blood supply after early childhood.
Disc Bulge Formation
Weakened annulus fibers let the nucleus press outward slightly.
- <25%: Bulge extent (of disc circumference)
- None – Grade I: Annular tears (early fiber fatigue)
- Often none: Symptom presence (frequently asymptomatic)
- Common: Prevalence (by age 40 on MRI)
Annulus fiber fatigue
Repeated loading fatigues and thins the outer collagen rings.
Bulge vs herniation
A bulge stays broad and contained, unlike focal herniation.
Early symptoms
Bulges rarely compress nerves or cause radiating leg pain.
Most disc bulges are found incidentally and need no treatment.
Disc Herniation & Annular Tear
Nucleus material breaks through a full-thickness tear in the annulus.
- 3: Herniation types (protrusion, extrusion, sequestration)
- 30–50: Peak incidence age (working-age adults)
- L4-L5 / L5-S1: Common levels (lowest lumbar segments)
- 1–3%: Annual incidence (of adults)
Radial annular tear
A crack runs from nucleus to the disc's outer edge.
Extrusion vs protrusion
Protrusion bulges through fibers; extrusion breaks completely free.
Extruded fragments can migrate away from the disc space.
Inflammatory response
Extruded material triggers local inflammation and chemical nerve irritation.
Nerve Root Compression
Herniated tissue presses the nerve root against the bony canal.
- 3: Compression grades (mild, moderate, severe)
- L5 or S1: Common root involved (sciatic distribution)
- ~90%: Sciatica prevalence (of symptomatic herniations)
- 3: Deficit types (motor, sensory, reflex)
Mechanical compression
Direct pressure narrows the nerve root and blocks signals.
Chemical radiculitis
Inflammatory cytokines irritate the nerve beyond simple mechanical pressure.
Pain often outlasts the mechanical compression that started it.
Deficit presentation
Weakness, numbness, or lost reflexes mark worsening nerve damage.
Conservative Care vs Surgical Discectomy
Deficit severity, not imaging alone, drives the treatment choice.
- 80–90%: Conservative success (resolve within 6–12 weeks)
- Severe / progressive: Surgical candidates (deficit or failed therapy)
- Fast: Discectomy relief (often within days)
- 5–15%: Recurrence rate (after discectomy)
Conservative pathway
Physical therapy, NSAIDs, and time resolve most herniations.
Surgical indications
Severe or progressive deficit, or cauda equina, prompts surgery.
Cauda equina syndrome is a surgical emergency, not a choice.
Shared decision making
Patient goals and deficit trajectory guide the final choice.