Annulus fibrosus

The annulus fibrosus surrounds the nucleus pulposus and together they form the intervertebral disk.

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 disorganised collagen bundles, and a greater proportion of vertical fibers, contributing to it being the weakest part, and accounting for the high rate of disk 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.