Lumbar intervertebral disk injury, herniation and degeneration

Abstract

This chapter reviews the mechanical function and failure of lumbar intervertebral discs. Disc herniation is essentially a bending injury to the annulus that can also involve the nucleus and vertebral endplate. Injuries to the annulus and vertebral body endplate create two distinct ‘phenotypes’ of disc degeneration: annulus driven and endplate driven. Either injury alters the mechanical environment of disc cells. Disturbed cell biology can lead to altered matrix composition, further structural disruption, revascularisation, reinnervation and pain. This defines the process of ‘disc degeneration’. A disc’s attempts to repair itself are frustrated by the very low cell density in the nucleus, which itself is attributable to poor metabolite transport. Disc injuries need not be traumatic, because intervertebral discs can be so weakened by genetic inheritance, ageing and loading history that injury occurs during the activities of everyday living. The situation is analogous to vertebral osteoporotic fracture. Medicolegal implications are considered.