Normal Disc vs. Disc Degeneration

The human intervertebral disc (IVD) is a complex organ composed of fibrous and cartilaginous connective tissues, and it serves as a boundary between 2 adjacent vertebrae. It provides a limited range of motion in the torso as well as stability during axial compression, rotation, and bending. Adult IVDs have poor innate healing potential due to low vascularity and cellularity.

Comparison of a healthy and degenerated IVD disc. The intervertebral disc (IVD) is comprised of three primary constituents: the inner nucleus pulposus (NP), the outer annulus fibrosus (AF), and the cartilaginous endplates (CEPs), which serve to secure the disc to the surrounding vertebrae. In a healthy state, pressure within the NP is uniformly distributed across the adjacent CEPs. However, in cases of degeneration, the disc’s capacity to retain water under compression diminishes, resulting in a significant reduction in disc height. Conversely, the AF in its healthy state exhibits a well-organized fibrous structure, characterized by concentric layers of angled collagen fibers and dispersed proteoglycan. In a degenerated IVD, the AF undergoes severe deformation and disorganization. In degenerating IVDs, the CEP experiences damage. Overall, degenerated IVDs distinguish themselves from their healthy counterparts through the depletion of extracellular matrix, dehydration of the fibrous nucleus pulposus, significant structural alterations in the collagen fibers of the AF, extensive damage to the CEP, and the occurrence of subchondral osteosclerosis. The reduction in disc height, as observed in spinal radiographs, serves as a common finding and an indicator of IVD degeneration

Degenerative disc disease (DDD) generally arises from the disruption of the homeostasis maintained by the structures of the IVD, and genetic and environmental factors can accelerate the progression of the disease.