The annulus fibrosus is the tough circular exterior of the intervertebral disc that surrounds the soft inner core, the nucleus pulposus. This outer portion is composed of a ring of ligament fibers that encases the inner core of the disc and securely connects the spinal vertebrae above and below the disc.
The annular fibers prevent the nucleus pulposus, which is comprised of a gelatinous material, from herniating or leaking out of the disc by hydraulically sealing the nucleus and evenly distributing any pressure and force imposed on the intervertebral disc.