Abstract
INTRODUCTION: Infection can occur after any spinal procedure that involves entry into the disc and although it is not common, the potential consequences are serious. Treatment usually requires identification of the bacteria followed by a course of antibiotics. This treatment remains controversial since it is not clear whether antibiotics actually penetrate the disc and if so, whether they are effective, or even if the outcome would be the same without antibiotics.
For an antibiotic to be effective against the infecting organism it must diffuse through the disc matrix. Blood vessels that surround the disc facilitate the diffusion process, but with age this vascularity decreases and may impede diffusion.
The aims of the pilot study were to assess the effectiveness of antibiotic in treating infection in both normal and degenerate sheep discs and to measure the concentration of antibiotic in non-operated discs at varying ages.
METHODS: In each of six Merino wethers aged 12 weeks (n=3) and 24 months (n=3), two lumbar discs were “degenerated” by incising the posterolateral annulus with a scalpel blade. After four weeks all animals had discography with radiographic contrast that contained Staphylococcus aureus at the incised levels and at two non-incised levels. Seven days after infection four animals began IV antibiotic treatment with cephazolin sodium (David Bull Laboratories, Australia) for 21 days at a dose of 50 mg/kg/day. The antibiotic was chosen for effectiveness against S. aureus. One control animal from each age group did not receive any antibiotics, to follow the natural progression of infection. Lateral radiographs of the lumbar spine were taken at two, six and 12 weeks. At 12 weeks all sheep were given a single intravenous dose of cephazolin sodium as either a 1, 2 or 3 g dose. The sheep were then killed after 30 minutes. The spines were removed and prepared for light microscopy to assess pathology of the discs and for biochemical analysis of antibiotic concentration. Success of treatment was judged using histologic and radiographic features.
RESULTS: Discitis was evident radiologically as early as two weeks after inoculation in all animals. Histology at 12 weeks confirmed discitis in all discs regardless of treatment. Biochemistry results confirmed that antibiotic diffused throughout the disc but was more concentrated in the annulus than the nucleus. At all doses disc concentration of antibiotic was higher in lambs than sheep.
DISCUSSION: Treatment with cephazolin sodium at a dose of 50 mg/kg/day for 21 days administered from seven days after inoculation, did not prevent discitis. This does not appear to be due to inability of antibiotic diffusion into the disc.
These abstracts were prepared by Dr Robert J. Moore. Correspondence should be addressed to him at Spine Society of Australia, Institute of Medical and Veterinary Science, The Adelaide Centre for Spinal Research, Frome Road, Adelaide, South Australia 5000.
