Intervertebral disc

During development and at birth, vertebral discs have some vascular supply to the cartilage endplates and the anulus fibrosus. These quickly deteriorate leaving almost no direct blood supply in healthy adults.[5]

The intervertebral disc space is typically defined on an X-ray photograph as the space between adjacent vertebrae. In healthy patients, this corresponds to the size of the intervertebral disc. The size of the space can be altered in pathological conditions such as discitis (infection of the intervertebral disc).

Stages of spinal disc herniation

A spinal disc herniation, commonly referred to as a slipped disc, can happen when unbalanced mechanical pressures substantially deform the anulus fibrosus, allowing part of the nucleus to obtrude. These events can occur during peak physical performance, during traumas, or as a result of chronic deterioration (typically accompanied with poor posture), and has been associated with a Propionbacterium acnes infection.[8] Both the deformed anulus and the gel-like material of the nucleus pulposus can be forced laterally or posteriorly, distorting local muscle function and putting pressure on the nearby nerve. This can give symptoms typical of nerve root entrapment, which can vary between paresthesia, numbness, chronic and/or acute pain, either locally or along the dermatome served by the entrapped nerve, loss of muscle tone and decreased homeostatic performance. The disc is not physically slipped; it bulges, usually in just one direction.

Another kind of herniation, of the nucleus pulposus, can happen as a result of the formation of Schmorl's nodes on the intervertebral disc.[9] This is referred to as vertical disc herniation.

Micrograph showing a fragment of an intervertebral disc demonstrating degenerative fibrocartilage with loss of nuclei and nests of chondrocytes, as seen in degenerative disc disease. HPS stain.

Before age 40, approximately 25% of people show evidence of disc degeneration at one or more levels. Beyond age 40, more than 60% of people show evidence of disc degeneration at one or more levels on magnetic resonance imaging (MRI).[10] These degenerative changes are a normal part of the ageing process and do not correlate to pain.[11]

3D medical animation still shot of scoliosis

One effect of aging and disc degeneration is that the nucleus pulposus begins to dehydrate and the concentration of proteoglycans in the matrix decreases, thus limiting the ability of the disc to absorb shock. This general shrinking of disc size is partially responsible for the common decrease in height as humans age. The anulus fibrosus also becomes weaker with age and has an increased risk of tearing. In addition, the cartilage endplates begin thinning, fissures begin to form, and there is sclerosis of the subchondral bone.[5] Since the fissures are formed in the anulus fibrosus due to osteo-arthritic bones or degeneration in general, the inner nucleus pulposus can seep out and put pressure on any number of vertebral nerves.[12] A herniated disc can cause mild to severe pain such as sciatica and treatment for herniated discs range from physical therapy to surgery.[13](see also: Intervertebral disc arthroplasty) Other degeneration of the vertebral column includes diffuse idiopathic skeletal hyperostosis (DISH)[14] which is the calcification or ossification of the ligaments surrounding the vertebrae. This degeneration causes stiffness and sometimes even curvature in the lumbar and thoraco-lumbar spinal region. Burgeoning evidence suggests that long-term running may mitigate age-related degeneration within lumbar intervertebral discs [15]

While this may not cause pain in some people, in others it may cause chronic pain. Other spinal disorders can affect the morphology of intervertebral discs. For example, patients with scoliosis commonly have calcium deposits (ectopic calcification) in the cartilage endplate and sometimes in the disc itself.[16] Herniated discs are also found to have a higher degree of cellular senescence than non-herniated discs.[5] In addition to scoliosis, which is the lateral 'S' curvature of the spine, the fused vertebrae can also experience other abnormalities such as kyphosis[12] (hunchback) which shows in old age, or lordosis[12] (swayback), which is often present in pregnancy and obesity.