The annulus fibrosus surrounds the nucleus polposus and together they form the intervertebral disc. 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 fibres alternates from lamina to lamina. The inner portion of the annulus is made of fibrocartilage which gradually blends with the nucleus polposus. Posterolaterally, the annulus is not only thinner but also has more disorganised collagen bundles, and a greater proportion of vertical fibres, contributing to it being weakest part, and accounting for the high rate of disc herniation.
- RF Costello, DP Beall "Nomenclature and standard reporting terminology of intervertebral disc herniation" Magn Reson Imaging Clin N Am 15 (2007) 167-174
- Gray's Anatomy 39th Edition, Elsevier