Multiple reports of bacterial isolates in human disc tissue have suggested a role of low-grade infection on intervertebral disc degeneration (IDD) and modic changes (MC) generation. Animal models have been extensively used to study IDD; however, until recently, no consideration had been given to eventual infectious processes. To reproduce the phenomena by inoculating an infecting agent would support the infectious hypothesis. Therefore, we studied the effect of Propionibacterium acnes (PA) inoculation on rat tails and determined whether it would produce MCs on the adjacent endplates.

Disc infection with PA would accelerate IDD compared with the standard model and would also lead to MCs on the adjacent endplates.

Twelve Sprague-Dawley rats were randomized to receive a needle puncture in a caudal tail disc with either saline (control) or an inoculum of 5×10⁷ CFU of Strain 1a PA. Twelve weeks later, the rats were euthanized, and the tails were analyzed. The main assessment criteria were obtained from the post-mortem MRI: T2 values of punctured discs and adjacent endplates, as well as disc volumes. A histological grading score for IDD was also used, measuring the morphology and cellularity of the nucleus and annulus, as well as endplate disruption.

The median T2 value and disc volume were smaller in PA-punctured discs [T2 value: 30ms (23–44) vs 61ms (38–132), respectively, P=0.01; 0.01mm³ (0.01–0.05) vs 0.5mm³ (0.01–5.35), respectively; P=0.049]. There was no change in the adjacent endplates. There was no significant difference in histological grading between the test and control [13 (10–14) vs 10.5 (6–13); P=0.05].

Inoculation of caudal discs with PA generated increased degeneration; however, no MCs were observed on the adjacent endplates. A better understanding of low-grade disc infections is still needed.

V (Animal study).