Anatomy of the intervertebral disc
The intervertebral disc functions as an amphiarthrosis. Its structure can be compared to that of a true synovial joint:
- Nucleus pulposus (nucleus pulposus) — analogue of the joint cavity, containing fluid similar to synovial fluid.
- Annulus fibrosus (annulus fibrosus) — acts as the joint capsule.
- Hyaline cartilage endplates of the vertebral bodies — analogues of articular surfaces.
Each spinal motion segment is also reinforced by its own ligamentous apparatus (anterior and posterior longitudinal, ligamentum flavum, interspinous, and supraspinous ligaments).
Pathogenesis: how the disc degrades
The degenerative process begins at the molecular level:
- Biochemical changes. Depolymerization of glycosaminoglycans occurs in the nucleus pulposus along with collagen degradation mediated by enzymes (cathepsins, hyaluronidase).
- Dynamics of the nucleus. The nucleus pulposus initially swells and then dehydrates. The disc flattens.
- Damage to the annulus. The annulus fibrosus bulges, and fissures appear within its collagen fibers.
- Herniation. The nucleus pulposus herniates through the annular tears, forming a disc herniation capable of compressing the dural sac and nerve roots.
Subsequently, blood vessels ingrow through the tears into the disc; the nucleus becomes fibrotic and calcified. This leads to laxity of the facet joint capsules and abnormal motion — spinal motion segment instability.
Types of herniated intervertebral discs
Depending on the extent of disc disruption, the following are distinguished:
- Protrusion (protrusio).
- Extrusion (extrusio).
- Sequestration (sequestratio) — a condition where a herniated fragment is completely detached from the disc.
In relation to the posterior longitudinal ligament, herniations are classified into:
- Medial (anterior to the ligament).
- Paramedial (partially behind the ligament).
- Lateral (completely outside the ligament).
A distinct entity is Schmorl's node, in which disc material herniates not outward, but directly into the cancellous bone of the adjacent vertebral body.
Stages of osteochondrosis
Radiographically, the disease progresses through several stages:
- Stage 1: initial changes; the first sign is mutual displacement of the vertebrae and alteration of physiologic lordosis.
- Stages 2 and 3: the disc flattens, endplates become irregular and thickened, and anterior osteophytes develop.
- Stage 4: posterior osteophytes form, narrowing the spinal canal and potentially compressing the spinal cord or nerve roots. Normally, the sagittal diameter of the canal in the lumbar spine should be at least half the depth of the vertebral body.
Briefly: answers to topical questions
What is the difference between an intervertebral disc protrusion and extrusion?
Intervertebral disc protrusion and extrusion differ in the ratio of the geometric dimensions of the displaced disc material to its base in the axial plane.
| Parameter | Protrusion (protrusio) | Extrusion (extrusio) |
|---|---|---|
| Base width | Greater than the distance of displacement | Less than the distance of displacement |
| Ratio | The base of the displaced disc material is wider than the displacement itself | The herniated portion of the disc is narrower than its base |
In a protrusion, the width of the base of the herniation exceeds the distance of disc material displacement, whereas in an extrusion, the width of the base is less than the length of the displaced disc material.
What neurological syndromes develop with spinal nerve root compression at the L4-S1 levels?
Compression of the spinal nerve roots at the lumbosacral level leads to characteristic symptoms of radiculopathy, with pain and paresthesias radiating from the back into the buttock and down the lower extremity.
- L4-L5 level (L4-L5) — pain may wrap around the leg and radiate to the dorsum of the foot.
- L5-S1 level (L5-S1) — symptoms are more commonly localized to the plantar surface of the foot.
Class and exam questions
What is the difference between osteochondrosis, spondylosis, and spondylarthrosis?
In osteochondrosis, the primary site of degeneration is the nucleus pulposus of the disc. In spondylosis, the process begins in the annulus fibrosus and the anterior longitudinal ligament. Spondylarthrosis refers to degeneration of the facet (zygapophyseal) joints.
What factors trigger the development of osteochondrosis?
It is a multifactorial disease. The main causes include macro- and microtrauma, congenital spinal anomalies, rheumatoid and autoimmune disorders, as well as vascular disturbances that impair disc nutrition.
What is a disc sequestration?
It is a fragment of the nucleus pulposus that has extruded through a ruptured annulus fibrosus and has completely lost continuity with the parent disc.
What else to study on this topic
- Blood supply characteristics of the thoracic spinal cord
- Biomechanics and innervation of the thoracic spine
- Surgical management of scoliosis: indications and techniques
- Congenital spinal anomalies and their role in instability
- Autoimmune and biochemical cascade in disc degeneration
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