Disc herniation involves a complex cascade of tissue injury. To understand the pathophysiological processes involved with disc herniation, it is first important to understand normal spinal conditions. There are several phases associated with disc herniation and each phase determines conservative management strategies, severity and frequency of signs and symptoms, and clinical outcomes.
Normal spinal disc conditions
During flexion (forward bending) and extension (backwards bending) in spinal discs, there is normal migration/movement of the central portion of the disc called the nucleus pulposus (inner gel-like matrix), which functions to tension the annulus fibrosis (outer disc fibres) resulting in greater tensile forces at the fibres nearest to the annulus fibrosis. The nucleus pulposus also contains small fibres suspended in the matrix, which provide shock absorption.
During different postures, the nucleus pulposus changes its shape, becoming longer and thinner, which is more pronounced in flexion of the low back. In sitting postures, there is increased compressive loading on the front part of discs, causing a decrease in height at the front part of the disc, resulting in some deformation of the disc. Therefore, extension is responsible for decreased tension on the disc.
Phases of disc degeneration
Acute tearing or chronic degeneration of spinal discs is responsible for disc herniation. A cascade of injury and degenerative changes can explain disc herniation:
Acute (initial and resolvable) phase
- Injury, for example a small tear in the annulus fibrosis from heavy lifting with poor technique, causes a change in biochemistry (cell interaction) of the nucleus pulposus leading to an autoimmune response in which the body fights itself sending cells to the site of injury that promote inflammation.
- The nucleus’ ability to hold water is then reduced, which hinders its ability to provide shock absorption and accelerates progressive degenerative changes. Surrounding muscles will often go into spasm acting as a natural brace or splint to reduce potentially harmful movements. This decreases disc movement, which also decreases the ability of nutrients to flush through the disc, which further perpetuates degeneration.
- With time, the body will repair the damaged tissues and in some circumstances, the patient may be left with some minor disc derangement and poor muscle function, which predisposes them to further injury.
Subacute (derangement and reoccurance) phase
- If conditions do not fully resolve, there is an increased chance of the disc favouring flexion and with repetitive loading of the annulus fibrosis at end range will lead to further micro-trauma and disc derangement causing a disc ‘bulge’.
- Nucleus pulposus fragments (suspended fibres) become ‘blocked’ and tears through the outer fibres of the disc can pursue. This further reduces extension movement and increases local inflammatory processes, leading to disc degeneration.
Chronic (degenerative) phase
- At this point, there is potential for the inner disc material to push through the outer fibres, causing a disc herniation, which irritates the adjacent spinal nerve root (a section of nerve that comes off the spinal nerve and provides motor and sensory information to the limbs). In this circumstance, signs and symptoms of nerve root involvement such as burning and travelling pain, muscle weakness and numbness can occur. This is referred to as ‘sciatica’ (radiculopathy). There is potential for this to develop into compression of the spinal nerve (spinal stenosis). There are varying degrees of disc herniation severity.
- Scans may be indicated during this phase if significant neurological deficit is present.
- With age, spinal discs loose their ability to retain water and there is further decline in mechanical ability of the nucleus pulposus and reduced mobility in joints and muscles, which degrades components of the spinal disc. This further perpetuates the degenerative cascade and risk of spinal disc herniation and re-injury.
To learn more about disc herniation, please follow the link to read an overview on herniated lumbar discs.