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Normally the intervertebral disc, which is sandwiched between two vertebrae, contains a highly pressurized center called the nucleus pulposus, which is held in place by the ligamentous annulus fibrosis.
The posterior 1/3 of the annulus is filled with the pain-carrying nerve fibers of the sinu-vertebral nerve.
An annular disc tear occurs when the substance of the annulus fibrosus allows that highly pressurized nucleus pulposus to escape outward toward the periphery of the disc.
Nucleus pulposus material is irritating to the sensitive pain-carrying nerve fibers.
If the tear grows large enough to connect the nucleus pulposus to the posterior longitudinal ligament the disc herniation develops and may cause back and/or leg pain / weakness.
The usual cause of annular tearing is from degeneration and/or trauma.
The pain that arises from an annular tear is called discogenic pain.
Annular tears give rise to disc herniation, which in turn may compress and/or chemically irritate the adjacent sciatic nerve rootlets causing radiculopathy.
Three main types of annular tears that occur: The rim lesion a horizontal tearing of the very outer annular fibers of the disc, the concentric tear, which is a splitting apart of the lamellae of the annulus in a circumferential direction; and the radial tear, which is usually a horizontally orientated annular tear that courses from the inner nucleus pulposus to the very outer region of the disc.
Radial tears may allow the nucleus pulposus to herniate.
Annular tears cause pain by irritating the well innervated posterior 1/3 of the annulus.
Blood vessels and nerve fibers can grow into the inner annulus in chronic back pain patients and even into the nucleus aa well.
Peripheral annular tears lead to severe premature degeneration of the disc.
Annular tears are not always seen on MRI, and never seen on x-ray.
CT discograms are the best way to demonstrate annular disc tears.
A discogram is performed by injecting contract material into the center of the disc, and then watching to see if the dye leaks from that center along a radial tear.