Prophylactic cephazolin to prevent discitis in an ovine model.

STUDY
DESIGN: An ovine model of discitis was used to determine the efficacy of the antibiotic cephazolin for prophylactic use in spinal surgery.
OBJECTIVES: To determine in juvenile and adult sheep spines if 2-g cephazolin given at intervals over a 4-hour period would prevent iatrogenic discitis, and determine the concentration of cephazolin in nucleus and anulus tissue after administration of a bolus dose. SUMMARY OF BACKGROUND DATA: It is standard practice at our institutions to give patients undergoing spinal surgery a single prophylactic (1-2 g) dose of cephazolin with a second dose for prolonged (>4 hours) procedures. Although this regimen provides therapeutic serum levels, the levels of antibiotic in the intervertebral disc are significantly lower. Because cephazolin is a negatively charged molecule, it is thought to diffuse poorly into the disc, raising questions about its efficacy as a prophylactic antibiotic for spinal procedures.
METHODS: There were 18 animals, including 9 lambs and 9 sheep, that received a single 2-g dose of cephazolin intravenously at 30-minute intervals over a 4-hour period. Two control animals (1 sheep and 1 lamb) did not receive antibiotic. All animals had diskograms at 2 previously incised lumbar levels and 2 nonincised levels using contrast that was deliberately contaminated with bacteria. Lateral spine radiographs were taken at postoperative intervals. After 12 weeks, all animals received another 2-g dose of cephazolin intravenously at intervals before the spines were removed for pathologic and biochemical analysis.
RESULTS: Discitis was detected in all control animals. Of those animals given cephazolin, discitis developed in 1 sheep and 4 lambs. Discitis did not develop in any of the sheep that received cephazolin 30 minutes before inoculation. Cephazolin was detected throughout the disc but was more concentrated in the anulus fibrosus. Disc levels of cephazolin peaked at 15 minutes, at which time serum levels were up to 50 times higher. Cephazolin levels in nonoperated and incised discs were not significantly different.
CONCLUSIONS: A single prophylactic 2-g dose of cephazolin administered anytime over a 4-hour period prevented discitis in almost all animals but was not as effective in lambs. Although lambs have a higher vascular supply to deliver antibiotics to the disc, it is likely that some other physiologic factor may be responsible for the increased infection rate. This study supports that timing of antibiotic prophylaxis is critical to prevent iatrogenic disc infection.