Abstract
Introduction
Risk factors for disc degeneration depend on how the condition is defined, i.e. on the specific disc degeneration “phenotype”. We present evidence that there are two major and largely-distinct types of disc degeneration.
Methods
The relevant research literature was reviewed and re-interpreted.
Evidence
In the upper lumbar and thoracic spine, disc degeneration is closely associated with endplate defects and with inflammatory changes in the vertebral bodies. It has a relatively high heritability (i.e. a strong genetic influence), and its incidence does not increase markedly with age. In the lower lumbar spine, disc degeneration is closely associated with radial fissures and nucleus herniation. Here it has a relatively low heritability, and a correspondingly stronger association with mechanical loading, and its incidence increases steadily throughout life. Mechanical experiments on cadaveric spines show that endplate fracture and nucleus herniation can be caused by compressive loading, and by bending combined with compression, respectively. Both lesions cause an immediate decompression of the nucleus, so that it becomes difficult to create subsequently the other lesion in the same disc. This suggests distinct phenotypes.
Interpretation
The two types of disc degeneration are not entirely distinct, because disc herniation sometimes occurs at upper lumbar levels. Nevertheless, it may be useful to recognise two phenotypes when it comes to explaining and treating discogenic pain. Some other common disc changes (such as water loss and bulging) are attributable to ageing rather than degeneration, whereas disc narrowing probably represents a final common pathway for both types of disc degeneration.
Conflicts of Interest
None
Source of Funding
None
This abstract has not been previously published in whole or in part; nor has it been presented previously at a national meeting.
