When the spine is straight, such as in standing or lying down, internal pressure is equalized on all parts of the discs. While sitting or bending to lift, internal pressure on a disc can move from (lying down) to over (lifting with a rounded back). Herniation of the contents of the disc into the spinal canal often occurs when the anterior side (stomach side) of the disc is compressed while sitting or bending forward, and the contents (nucleus pulposus) get pressed against the tightly stretched and thinned membrane (annulus fibrosus) on the posterior side (back side) of the disc. The combination of membrane-thinning from stretching and increased internal pressure () can result in the rupture of the confining membrane. The jelly-like contents of the disc then move into the spinal canal, pressing against the spinal nerves, which may produce intense and potentially disabling pain and other symptoms. Some authors favour degeneration of the intervertebral disc as the major cause of spinal disc herniation and cite trauma as a minor cause. Disc degeneration occurs both in degenerative disc disease and aging. With degeneration, the disc components – the nucleus pulposus and annulus fibrosus – become exposed to altered loads. Specifically, the nucleus becomes fibrous and stiff and less able to bear load. Excess load is transferred to the annulus, which may then develop fissures as a result. If the fissures reach the periphery of the annulus, the nuclear material can pass through as a disc herniation. Mutations in several genes have been implicated in intervertebral disc degeneration. Probable candidate genes include type I collagen (sp1 site), type IX collagen, vitamin D receptor, aggrecan, asporin, MMP3, interleukin-1, and interleukin-6 polymorphisms. Mutation in genes – such as MMP2 and THBS2 – that encode for proteins and enzymes involved in the regulation of the extracellular matrix has been shown to contribute to lumbar disc herniation. Disc herniations can result from general wear and tear, such as weightlifting training, constant sitting or squatting, driving, or a sedentary lifestyle. Herniations can also result from the lifting of heavy loads. Professional athletes, especially those playing contact sports, such as American football, Rugby, ice hockey, and wrestling, are known to be prone to disc herniations as well as some limited contact sports that require repetitive flexion and compression such as soccer, baseball, basketball, and volleyball. Within athletic contexts, herniation is often the result of sudden blunt impacts against, or abrupt bending or torsional movements of, the lower back.
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