The annulus fibrosus (plural: annuli fibrosi) surrounds the nucleus pulposus and together they form the intervertebral disc.
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Gross anatomy
The annulus comprises 15 to 20 collagenous (type I) laminae which run obliquely from the edge of one vertebra down to the edge of the vertebra below. The direction of the fibers alternates from lamina to lamina. The inner portion of the annulus is made of fibrocartilage which gradually blends with the nucleus pulposus.
Posterolaterally, the annulus is not only thinner but also has more disorganized collagen bundles, and a greater proportion of vertical fibers, contributing to it being the weakest part and accounting for the high rate of disc herniation. Furthermore, the composition of particularly the annulus fibrosis varies by spinal level.
Within the posterior cervical discs, the annulus fibrosus is thinner and consists of a single set of longitudinally-oriented fibers, rather than the multilaminated structure seen in lumbar discs 3.
Functional anatomy
The peripheral fibers of the annulus fibrosus have the function of anchoring the intervertebral disc to the vertebra; they "merge", anteriorly and posteriorly, with the longitudinal ligaments 4. Unlike the nucleus pulposus, on which mainly compressive forces act, the fibers of the annulus fibrosus resist the tensile and the compressive stresses to which the intervertebral disc is subjected.
Radiographic features
MRI
Annulus fibrosus is of low signal on T2-weighted images 5.
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