Intervertebral disc

Intervertebral discs are cartilaginous pads between the vertebral bodies, constituting 25% of the total length of the spine. There are 23 discs in the spine (between C2 and the sacrum). They provide mobility of the spine and shock absorption during loads.

Building

The disk consists of two components:

Nucleus pulposus (nucleus pulposus) is a gelatinous mass in the center of the disc, containing up to 80% water in young people. The core works like a hydraulic shock absorber: when loaded, water is displaced, and when unloaded, it is absorbed back. This is why a person’s height in the morning is 1–2 cm higher than in the evening.

Ring of fibrosus (annulus fibrosus) - consists of 15–25 concentric layers of collagen fibers located at an angle of 30° to the horizontal, and the direction of the fibers in adjacent layers alternates. This design, similar to a car tire, provides strength under all types of loads.

Disk power

The intervertebral disc is the largest avascular structure in the body. Nutrition is carried out by diffusion from the vertebral bodies through the endplates. This mechanism requires regular change of load: when compressed, fluid with metabolic products leaves the disk, when unloaded, fresh nutrients are absorbed.

This is why prolonged sitting is harmful to the discs - with static load, the metabolism slows down.

Degeneration and hernias

With age, the water content in the nucleus decreases (from 80% to 70%), the height of the disc decreases, and cracks appear in the annulus fibrosus. Stages:

1. Protrusion - the nucleus is displaced, but the fibrous ring does not rupture. The disc bulges, but the contents do not come out.
2. Extrusion (hernia) - the nucleus breaks through the fibrous ring and exits into the spinal canal
3. Sequestration - a fragment of the nucleus separates and lies freely in the canal

The most common localization of hernias is L4–L5 and L5–S1 (about 90% of cases), because the lumbar region bears the greatest load.

Symptoms and diagnosis

A disc herniation can compress:

• Spinal nerve - pain radiating to the leg (sciatica) or arm
• Spinal cord (in the cervical region) - weakness in the limbs
• Cauda equina (L1–L2) – dysfunction of the pelvic organs (emergency surgery!)

The gold standard for diagnosis is MRI, which allows visualization of the soft tissue of the disc and nerve structures.

You can see how the nucleus pulposus is displaced during a hernia 3D atlas Humio - interactive models of the spine allow you to examine each disc and nerve root.