intervertebral disc

**Structure** - Nucleus pulposus: the gelatinous center, a remnant of the embryonic notochord. It is rich in water, proteoglycans (aggrecan), and type II collagen. It is the shock-absorbing component. - Annulus fibrosus: concentric lamellae of fibrocartilage. The outer layer is mostly type I collagen and the inner layer has more type II. It contains the nucleus and resists tension and torsion. - Vertebral endplates: hyaline cartilage layers that anchor the disc to the vertebral bodies. They supply nutrients to the avascular disc by diffusion. - Discs account for about 25% of spinal column length. There is no disc between C1 and C2 (atlas and axis). The sacrum and coccyx are fused and have no discs. - The disc is largely avascular and aneural. Only the outer annulus has sensory innervation, from the sinuvertebral (recurrent meningeal) nerves, so it heals poorly.

**Function** - Shock absorption, load distribution, and spinal mobility (flexion, extension, lateral bending, rotation). - Cervical and lumbar discs are relatively thicker, which gives these regions the most mobility.

**Ligamentous support** - The anterior longitudinal ligament is broad and strong and limits hyperextension. - The posterior longitudinal ligament is narrower and weaker, especially posterolaterally. This is why herniation occurs in that direction.

**Herniated disc (herniated nucleus pulposus)** - Pathophysiology: age-related desiccation and degeneration, or acute trauma or heavy lifting, produce annular tears. The nucleus then protrudes through the annulus. - Most common sites: L4-L5 and L5-S1 (lumbar, about 90–95%), followed by C5-C6 and C6-C7 (cervical). - Posterolateral herniation compresses the nerve root exiting at the level below. Because the nerve root exits above the disc in the lumbar spine, a herniation at L4-L5 compresses the traversing L5 root. The L4 root has already exited at that level. - Lumbar presentation: - L4-L5 herniation → L5 radiculopathy: weak dorsiflexion and great toe extension (foot drop), numbness over the dorsum of the foot. - L5-S1 herniation → S1 radiculopathy: weak plantar flexion, diminished ankle (Achilles) reflex, numbness over the lateral foot. - L3-L4 herniation → L4 radiculopathy: weak knee extension, diminished patellar reflex. - Cervical herniation: C6 or C7 radiculopathy with arm pain and paresthesias. Upper extremity reflexes may be diminished. - Central (posterior midline) herniation of a large lumbar disc can cause **cauda equina syndrome**. Findings are saddle anesthesia, bilateral leg weakness, urinary retention or overflow incontinence, fecal incontinence, and decreased rectal tone. This is a surgical emergency. - Exam: positive straight leg raise (Lasègue sign, reproduces pain at 30–70° elevation) is sensitive for L5/S1 radiculopathy. Crossed straight leg raise is more specific. - Diagnosis: MRI is the imaging test of choice. Use it when there are red flags, progressive neurologic deficit, or symptoms persisting beyond 6 weeks. - Management: - Most cases resolve with conservative care: NSAIDs, activity as tolerated, and physical therapy. - Epidural steroid injections may be used for persistent radicular pain. - Surgery (microdiscectomy) is indicated for cauda equina syndrome, progressive motor deficit, or refractory pain.

**Other disc-related high-yield points** - Degenerative disc disease and spondylosis: loss of disc height, osteophyte formation, and facet arthropathy. These can cause spinal stenosis with neurogenic claudication, which improves with flexion (for example, leaning on a shopping cart). - Ankylosing spondylitis: inflammation of the annulus fibrosus insertion (enthesitis) with syndesmophytes produces the "bamboo spine." Associated with HLA-B27. - Ehlers-Danlos syndrome and other connective tissue defects may predispose to annular weakness. - Schmorl nodes: herniation of the nucleus pulposus through the vertebral endplate into the vertebral body. They are usually incidental. - Discitis/vertebral osteomyelitis: usually from hematogenous spread (Staphylococcus aureus). Risk factors include IV drug use and diabetes. Presents with back pain and fever, with elevated ESR/CRP. Workup includes MRI and blood cultures. Consider Mycobacterium tuberculosis (Pott disease) in the right setting. - Embryology: the nucleus pulposus derives from the notochord, and the annulus fibrosus and vertebral bodies derive from sclerotome. Notochord remnants can give rise to chordoma. - With aging, the nucleus loses water and proteoglycan content. Discs lose height and become more prone to annular tears.