annulus fibrosus

**Structure** - The intervertebral disc has two parts. The outer annulus fibrosus is fibrocartilage. The inner nucleus pulposus is a gelatinous remnant of the notochord. - The annulus is made of concentric lamellae. Collagen fibers run obliquely and alternate in direction between adjacent layers, which resists torsion and tension. - The outer annulus is mostly type I collagen (tensile strength). The inner annulus has more type II collagen and transitions into the nucleus. - The nucleus pulposus is rich in proteoglycans and water, with mostly type II collagen. It acts as a shock absorber and distributes load. - The annulus attaches firmly to the vertebral body rims and endplates. The anterior longitudinal ligament (ALL) reinforces the disc anteriorly. The posterior longitudinal ligament (PLL) reinforces it posteriorly, but it is thinner and weaker in the lateral part of the posterior disc. - The disc is avascular in adulthood, so healing is poor. Only the outer annulus has some innervation (sinuvertebral nerve, a recurrent meningeal branch), so annular tears can cause pain.

**Pathophysiology and clinical significance** - **Disc herniation (HNP):** Degeneration, trauma, or repetitive strain causes annular tears or fissures. The nucleus pulposus then protrudes through the weakened annulus. - Herniation is most common **posterolaterally**, where the PLL is thinnest. It is most common at **L4–L5 and L5–S1**, then in the cervical spine at C5–C6 and C6–C7. - **Nerve root involvement:** A posterolateral herniation typically compresses the nerve root that exits at the level *below* (the traversing root). For example, an L4–L5 herniation compresses L5. A far lateral herniation compresses the exiting root (L4 in this example). - L4 root: weak knee extension, decreased patellar reflex. - L5 root: foot drop and weak dorsiflexion or great toe extension. - S1 root: weak plantarflexion, decreased Achilles reflex. - **Central herniation:** Can cause **cauda equina syndrome**, a surgical emergency. Findings are saddle anesthesia, bilateral leg weakness, and urinary retention or overflow incontinence with fecal incontinence. - **Symptoms and exam:** Radicular pain, often worse with sitting, coughing, or Valsalva. Straight leg raise (positive at 30–70°) suggests L5 or S1 radiculopathy. Crossed straight leg raise is more specific. Femoral stretch test suggests L2–L4. - **Imaging and management:** MRI is the best test. Most cases resolve with conservative care (NSAIDs, activity as tolerated, PT). Surgery is indicated for cauda equina syndrome, progressive neurologic deficit, or refractory pain. - **Aging:** The nucleus loses water and proteoglycans, so the disc height falls and the annulus bears more load. This predisposes to annular fissures and herniation, which is why herniation is most common in young to middle-aged adults (30–50 years). In older adults the nucleus is more fibrotic and herniation is less likely, but spondylosis and stenosis become more common. - **Embryology:** The nucleus pulposus comes from the notochord. The annulus fibrosus and vertebral bodies come from sclerotome (mesoderm).

**High-yield pearls** - Annulus = outer, type I collagen. Nucleus = inner, type II collagen, notochord remnant. - Posterolateral herniation is the classic pattern because the PLL is weaker laterally. - Always ask about cauda equina red flags (saddle anesthesia, bladder or bowel dysfunction).

**Related terms:** nucleus pulposus, intervertebral disc herniation, radiculopathy, cauda equina syndrome, posterior longitudinal ligament, notochord.