Significance of cartilage endplate within herniated disc tissue

Disc herniations sometimes contain hyaline cartilage fragments, but their origins and significance are uncertain.

Herniations were removed surgically from 21 patients (aged 35-74 years) whose main symptom was sciatica (10 patients) or back pain (11 patients). Frozen sections, 5 µm thick, were examined histologically, and antibodies were used to label the matrix-degrading enzyme MMP 1, pro-inflammatory mediator TNFα, and cell proliferation marker Ki-67. Proportions of each tissue type were quantified by image analysis. Cartilage and bone components of the endplate were examined in 7-µm frozen sections from 16 cadaveric spines, aged 61-98 years.

Cartilage fragments were found in 10/21 herniations. They averaged 5.0 mm in length, comprised 25 % of the herniation area, and two had some bone attached. Hyaline cartilage was more common in herniations from patients with sciatica (7/10) than with back pain (3/11, P = 0.050), and the area (%) of the herniation occupied by the cartilage was greater in sciatica patients (P < 0.05). Cartilage fragments showed little evidence of swelling, proteoglycan loss or inflammatory cell invasion, although cell clustering was common, and TNFα was sometimes expressed. Each cartilage fragment showed at least one straight edge, as if it had been peeled off the bony endplate, and this mechanism of failure was demonstrated in preliminary mechanical experiments.

Disc herniations often include hyaline cartilage pulled from the vertebral endplates. Cartilage fragments show little swelling or proteoglycan loss, and may be slow to resorb, increasing the risk of persisting sciatica. Loss of cartilage will increase endplate permeability, facilitating endplate inflammation and disc infection.