Intervertebral disc degeneration: evidence for two distinct phenotypes

Abstract

We review the evidence that there are two types of disc degeneration. 'Endplate-driven' disc degeneration involves endplate defects and inwards collapse of the annulus, has a high heritability, mostly affects discs in the upper lumbar and thoracic spine, often starts to develop before age 30 years, usually leads to moderate back pain, and is associated with compressive injuries such as a fall on the buttocks. 'Annulus-driven' disc degeneration involves a radial fissure and/or a disc prolapse, has a low heritability, mostly affects discs in the lower lumbar spine, develops progressively after age 30 years, usually leads to severe back pain and sciatica, and is associated with repetitive bending and lifting. The structural defects which initiate the two processes both act to decompress the disc nucleus, making it less likely that the other defect could occur subsequently, and in this sense the two disc degeneration phenotypes can be viewed as distinct.

Fig. 1

Intervertebral discs, shown here in blue, lie between the vertebral bodies of the spine. Compressive (C) and shear (S) forces on the spine act perpendicular to, and parallel to, the mid-plane of each disc A bending moment (BM) causes the spine to bend, and an axial torque (AT) causes it to rotate about its long axis. Adapted with permission from Adams et al. (2012).

Fig. 2

Schematic suggesting how structural damage to the extracellular matrix of an intervertebral disc leads to progressive degeneration. See text for details.

Fig. 3

Profiles of vertically-acting compressive stress measured along the sagittal midline of a cadaveric intervertebral disc which was loaded in compression. A, P: anterior, posterior. Measurements were repeated after a vertebral endplate was fractured by compressive overload. Endplate damage decompresses the central and anterior regions of the disc and generates a high concentration of compressive stress in the posterior annulus (arrow). Endplate damage also transfers some compressive load-bearing to the neural arch (Luo et al. 2007).

Fig. 4

Images of endplate-driven disc degeneration. (A) Microradiograph of a 5 mm-thick section of a lumbar vertebral body showing a Schmorl's node (arrow) in its lower endplate. (B) Mid-sagittal section through a cadaveric spine showing a vertical herniation of nucleus pulposus through the disc's superior endplate. Herniation was caused by compressive loading. Note that the inner lamellae of the annulus have collapsed into the nucleus cavity (arrow).

Fig. 5

Images of annulus-driven disc degeneration: two mid-sagittal sections through lower lumbar cadaveric intervertebral discs (posterior on the right). (A) There is a complete radial fissure in the posterior annulus, with blood in its peripheral margins. (B) Nucleus pulposus tissue (*) has herniated through this radial fissure in response to severe loading in bending and compression (Adams et al. 2000).