INTRODUCTION: Discogenic back pain may be related to the ingrowth of nerves and blood vessels, although this is controversial. We hypothesise that ingrowth is greater in painful discs, and that it is facilitated by proteoglycan loss from the anulus.
METHODS: We compared tissue removed at surgery from 22 patients with discogenic back pain and/or sciatica, and from 16 young "control" patients with scoliosis. Wax‐embedded specimens were sectioned at 7μm. Nerves and blood vessels were identified using histological stains, and antibodies to PGP 9.5 and CD31 respectively.
RESULTS: Blood vessels were identified in 77% of "painful" discs compared to 44% of scoliotic discs (p=0.016), and they were more common in the anterior anulus compared to the posterior (p=0.026). Maximum penetration of blood vessels from the peripheral anulus was 4.7 mm ("painful" discs) and 2.0 mm (control discs), and penetration increased with histological grade of disc degeneration in the "painful" discs (p=0.002). In 16/17 painful discs, blood vessels were within 1 mm of an anulus fissure, or the disc periphery. Nerves were found in 36% of ‘painful’ discs (all with blood vessels) and 25% of control discs. Nerve ingrowth was always less than or equal to blood vessel ingrowth, with a maximum observed penetration of 1.5 mm from the periphery. Loss of proteoglycans (saffranin O staining) correlated with macroscopic grade of disc degeneration (r= 0.57, p<0.001).
DISCUSSION: In degenerated and painful discs, the ingrowth of nerves appears to follow that of blood vessels, although no nerves were seen >2mm from the periphery. Ingrowth of both structures appears to favour anulus fissures, possibly because focal loss of proteoglycans reveals a collagenous scaffold that is conducive to cell migration. Results support the view that fissures in the outer anulus can give rise to discogenic back pain.