DEGENERATIVE DISC DISEASE

The intervertebral disc is the ‘cushion’ that separates the vertebral bodies in the spinal column. The disc acts as both a shock absorber and a universal joint. It allows for controlled movement in multiple planes. A disc and the vertebrae immediately adjacent to it make a motion segment.

 
The disc is surrounded by a thick fibrous band, called the annulus fibrosus. The annulus attaches the disc to the adjacent vertebra. Inside the fibrous ring is a gelatinous core, known as the nucleosus pulposus. This substance acts as a shock absorber and ball-bearing. The upper and lower surfaces of the disc are lined with cartilage. This cartilage endplate attaches the disc to the flat surfaces of the adjacent vertebrae. The disc has no direct blood supply and nutrition has to diffuse through the cartilage layer from the adjacent bone. A healthy disc has a nerve supply limited to the outer aspect of the annulus.
(Fig.1 MRI scan showing normal intervertebral discs)
 
 
Disc degeneration may be initiated by an injury to the annulus. This may trigger an inflammatory reaction and lead to the growth of abnormal nerve fibres into the disc. This injury may cause acute or chronic back pain, also known as discogenic back pain. A tear of the annulus can be seen on an MRI scan or on a discogram. Not all annulus tears seen on scans are painful.
(Fig.2 Annulus tear seen on MRI scan)
 
 
 
The soft nucleosus may extrude through the annulus tear. This is referred to as a prolapsed disc or a ‘slipped disc’. The prolapsed disc may cause compression or inflammation of an adjacent nerve root, resulting in sciatica. Prolapsed discs are usually diagnosed on an MRI scan.
(Fig. 4 A discogram showing leakage of contrast material from a disc annulus tear)
 
 
 
Disc degeneration may also result from inadequate disc nutrition. When the blood supply from the adjacent vertebrae is impaired, disc degeneration may be accelerated. Cigarette smoke is a significant risk factor for accelerated disc degeneration. The soft, pulpy nucleus may become dehydrated and its collagenous fibres undergo degradation. The degenerate discs may be a source of chronic back pain.
(Fig. 5 An end-stage dehydrated disc showing loss of disc height and ligament thickening)
 
 
 
The dehydrated discs can be seen on an MRI scan. They are also known as ‘black discs’ because of the colour change on MRI. It is important to know that not all black discs are painful. Confirming that a particular degenerate disc is the source of back pain may be very difficult. Other tests, such as discography may be helpful in diagnosing the cause of back pain.
(Fig. 3 A ‘black disc’ associated with a torn annulus)
 
 
 
Ultimately, the disc may lose its shock absorbing ability. The disc space will become narrow and movement at that level will be abnormal. This places excessive strain on the adjacent weight-bearing structures in the spine, such as the facet joints. Excessive strain on the facet joints and surrounding ligaments may cause them to hypertrophy (enlarge). Joint hypertrophy and ligament thickening reduces the space available for the adjacent nerve roots. This may cause nerve root compression, a painful condition called spinal stenosis.
(Fig. 6 An x-ray showing a chronic degenerate disc)
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