Abstract
Introduction: Although prophylactic antibiotic administration is common in spinal surgery, the choice of drug, dose, and timing of administration often varies. Little is known about the activity of antibiotics in the spine and indeed if they are distributed throughout the disc and if the time intervals are optimal. Because infections that produce iatrogenic discitis generally arise within the disc, the antibiotic concentration of the disc is more relevant than serum concentrations. The aims of the study were to determine if a 2g dose of cephazolin was effective at preventing discitis over a four-hour period in immature ovine discs that were both non-degenerate and degenerate; and also to determine the concentration of cephazolin in serum and disc tissue.
Methods: In 10 Merino wethers aged 12 weeks, three lumbar discs were “degenerated” by incising the posterolateral annulus with a scalpel blade and using ronguers, removing the bulk of the nucleus pulposus. After 12 weeks nine animals were anaesthetised and given a 2g dose of cephazolin (David Bull Laboratories, Australia) at predetermined time intervals over a four-hour period. The antibiotic was chosen for effectiveness against Staphylococcus aureus a common discitis-causing organism. One sheep (control) did not receive any antibiotics to follow the natural progression of infection. All animals had discography with radiographic contrast that contained S. aureus at two incised levels and at two non-incised levels. Lateral radiographs of the lumbar spine were taken at two, six and 12 weeks to monitor the bony changes. At 12 weeks all sheep were given a 2g intravenous dose of cephazolin at time intervals before being killed. 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: The control sheep that did not receive any antibiotics developed discitis at four levels. Histology at 12 weeks confirmed discitis in 10/36 “prophylactic discs”. Of these “prophylactic discs” 7/10 had previously been “degenerated”. Discitis only developed in immature discs that were administered cephazolin two hours prior to inoculation. When antibiotic was administered after inoculation discitis was prevented. Biochemistry results confirmed that antibiotic diffused throughout the disc but was concentrated in the annulus more than the nucleus. Antibiotic levels in the disc peaked at 15 minutes (annulus mean concentration 15.5 mg/L, nucleus mean concentration 3.2 mg/L). Serum levels at 15 minutes were up to 50 times greater at this time (serum mean concentration 178 mg/L).
Discussion: The discs that were “degenerate” had a higher incidence of discitis compared to “non-degenerate” discs. However the concentration of antibiotic in degenerate discs was not significantly different than in non-degenerate discs. A 2 gram dose of cephazolin is reasonably effective (approx 70% success rate) at preventing discitis over a four-hour period.
The abstracts were prepared by I. B. McPhee. Correspondence should be addressed to the Spine Society of Australia Secretariat, The Adelaide Centre for Spinal Research, Institute of Medical and Veterinary Science, PO Box 14, Rundle Mall, Adelaide SA 5000, Australia.
