The human spine performs a complex function of a supporting structure that provides significant mobility. These functional capabilities are determined by the segmental structure of the spine with a certain freedom of movement in its individual links. One of the most important elements of this structure are intervertebral discs that perform shock-absorbing functions and provide movement in the intervertebral joints.
Pathogenesis
Intervertebral discs consist of a fibrous ring and a nucleus pulposus located in the center of the disс. The discs largely determine the height of the vertebral column; their total length is 1/4 of the length of the spine. The disc is connected to the overlying and underlying vertebrae with the help of cartilaginous hyaline plates. The fibrous ring consists of concentrically arranged layers of strong connective tissue fibers. The nucleus pulposus is a round elastic structure consisting of connective tissue with high hydrophilic properties. In adults, nucleus pulposus contains up to 70% water. With age, the water content and elastic properties of the disc are significantly reduced.
Strong longitudinal connective tissue ligaments pass along the anterior and posterior surfaces of the spine, which makes the vertebral column even more durable. The posterior longitudinal ligament strengthens the surface of the fibrous ring of the intervertebral disc facing the spinal canal, preventing its displacement in this direction. At the same time, the lateral surfaces of the discs, especially in the lumbar region, where the posterior longitudinal ligament is narrowed, are less strong. This feature of the structure of the intervertebral discs and ligamentous system can explain the mechanisms of their degenerative changes. The strength and mobility of the overall structure of the spine are also provided by the joints of the vertebrae lying next to each other and the ligamentum flavum located between the vertebral arches. Intervertebral discs play a role of shock absorbers. Under vertical load, the nucleus pulposus is flattened and stretches the fibrous ring. As the load stops, it takes its normal position. When turning, bending, and extending the spine, the corresponding deformation of the nucleus pulposus and the fibrous ring also occurs.
Different parts of the spine experience different loads. The lumbar and cervical discs have a particularly significant load, since they are the most mobile. This largely explains the fact that the age-related degenerative changes in the vertebral column, discs and ligamentous system are most pronounced in the cervical and lumbar regions. Another feature of intervertebral discs should be noted — their blood supply. Small vessels that supply blood to the discs are completely obliterated by the age of 20–30, so metabolic processes in the discs occur by diffusion and osmosis.
All these factors (constant mechanical load, changes in metabolic processes and hydrophilicity) inevitably lead to degenerative changes in the discs, vertebrae themselves, facet joints and their ligamentous apparatus. Genetic predisposition, significant physical activity associated with the life and work, injuries affect the severity of structural changes in the intervertebral discs and determine the age, at which they occur. The fibrous ring loses its elasticity; over time, cracks are formed in it, into which the nucleus pulposus is embedded under load, more and more extensively, moving to the periphery of the disc. If the outer layers of the fibrous ring are still intact, then at the place of its greatest thinning, the disc protrudes, and so-called disc protrusion forms.
When the fibers of the fibrous ring are completely torn, the contents of the nucleus pulposus go beyond its borders — a herniated intervertebral disc occurs (Fig. 15.1, 15.2). The protruded fragment loses its connection with the nucleus pulposus and can be freely located in the lumen of the spinal canal (disc sequestration).
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Fig. 15.1. Median herniated disc
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Fig. 15.2. Lateral herniated disc causing compression of the spinal root
It is very important to determine, in which direction the disc protrudes or falls out. The posterior and anterior surfaces of the discs are additionally strengthened by the longitudinal ligaments of the spine, so, more often, there is a lateral displacement of the disc towards the intervertebral opening, through which the spinal root passes. A herniated intervertebral disc causes compression of the corresponding root with the development of pain syndrome (discogenic radiculopathy).
Less often, the disc may protrude into the lumen of the spinal canal along the middle line (median disc herniation). The neurological signs of median hernias depend on the spine, in which they occur (lumbar hernias can lead to compression of the roots of the cauda equina, and large cervical hernias can cause damage to the cervical spinal cord).
When the cartilage plates adjacent to the vertebrae are destroyed, the nucleus pulposus can penetrate into the vertebra located next to it, resulting in Schmorl’s nodes, which, as a rule, are asymptomatic, but can be detected radiologically. In some cases, spontaneous hernia lysis is possible due to the influence of proteolytic enzymes.
Due to the degeneration of the intervertebral discs, it become sclerotic. As result, the mobility of the vertebrae is impaired, the spaces between them decrease, and osteochondrosis is noted.
Spondyloarthritis is a complex of changes in the vertebrae, ligamentous apparatus, facet joints, which can cause pain syndromes. These changes include the formation of bone osteophytes that protrude into the spinal canal, the intervertebral foramen or the canal of the vertebral artery. These osteophytes are formed at the edges of the articular surfaces of the vertebrae, the area of the uncovertebral articulation. This is often accompanied by thickening and ossification of the longitudinal ligament, while the deformed edges of the vertebrae and ossified adjacent parts of the ligaments form “bony brackets” (Fig. 15.3). With age, signs of spondyloaritis are found in most people (90% of men over 50 years old and 90% of women over 60 years old). In most cases, it is asymptomatic or accompanied by moderate pain, which does not significantly affect activity and working capacity.