nucleus pulposus

**Structure and embryology** - The intervertebral disc has two parts: the outer annulus fibrosus (concentric lamellae of type I collagen) and the inner nucleus pulposus. - The nucleus pulposus is gelatinous and made of type II collagen, proteoglycans (aggrecan), and about 70-90% water. Its high water content comes from the hydrophilic glycosaminoglycans (chondroitin sulfate, keratan sulfate). - It is a remnant of the notochord. Notochord remnants can give rise to chordoma, a tumor of the clivus or sacrum with physaliferous (bubbly) cells that stain S100 and cytokeratin positive. - The nucleus pulposus is avascular and receives nutrients by diffusion from the vertebral endplates.

**Function** - It distributes axial load evenly and acts as a hydraulic shock absorber. - It allows spinal flexibility.

**Pathology: herniated nucleus pulposus (HNP, "slipped disc")** - Mechanism: the nucleus pulposus herniates through a weakened or torn annulus fibrosus. It usually goes posterolaterally because the posterior longitudinal ligament is thinnest laterally and reinforces the midline. - Common sites are L4-L5 and L5-S1, because the lumbar spine bears the most load and motion. The cervical spine (C5-C6, C6-C7) is the next most common. - Risk factors: age (disc dehydration and degeneration), heavy lifting, repetitive flexion, obesity, smoking. - Nerve root rule: a posterolateral herniation usually compresses the nerve root exiting at the level below. For example, an L4-L5 herniation compresses L5, and an L5-S1 herniation compresses S1. - Clinical features: low back pain radiating down the leg (sciatica), worse with sitting, coughing, or Valsalva. Straight-leg raise and crossed straight-leg raise are positive. Findings depend on the root involved: - L4: weak knee extension, decreased patellar reflex. - L5: foot drop and weak dorsiflexion or great toe extension, sensory loss over the dorsum of the foot. - S1: weak plantarflexion, decreased Achilles reflex, lateral foot numbness. - Central herniation can cause **cauda equina syndrome**, a surgical emergency. It presents with saddle anesthesia, bilateral leg weakness, urinary retention or overflow incontinence, fecal incontinence, and decreased anal tone. - Diagnosis: MRI is the best test. Imaging is reserved for red flags or symptoms lasting more than 6 weeks. - Management: most cases resolve with conservative care (NSAIDs, activity as tolerated, physical therapy). Epidural steroid injections can be used for persistent pain. Surgery (discectomy) is indicated for cauda equina syndrome, progressive motor deficit, or refractory pain.

**Aging** - With age the nucleus pulposus loses water and proteoglycan content, so the disc becomes less elastic and more prone to herniation. Herniation is therefore less likely in the elderly, whose nucleus is more fibrotic, and most common in ages 30-50.

**High-yield associations** - Notochord remnant, so chordoma. - Type II collagen in the nucleus and type I in the annulus. - Posterolateral herniation affects the nerve root exiting below. A far-lateral herniation affects the root at the same level. - Ankylosing spondylitis involves the annulus fibrosus (syndesmophytes, "bamboo spine"), not the nucleus pulposus.