Abstract
Introduction Infection is a risk following surgical procedures that violate the disc. Bacteria, often from normal skin flora, can be introduced into the disc space causing inflammation with destruction of the adjacent vertebrae. The incidence of iatrogenic discitis is thought to be reduced with the use of prophylactic antibiotics. Although there is substantial evidence from animal studies that prophylactic antibiotics reduce the incidence of discitis, evidence in humans is not strong. The aim of this study was to determine if levels of cephazolin in human disc and serum reached the stated minimum inhibitory concentration (MIC > 1 mg/L) against Staphylococcus aureus following intravenous administration.
Methods Thirty patients (15 female and 15 male) with a mean age 42 years, (range 21–63 years) received 1g cephazolin prior to one or two level lumbar spinal fusion surgery. Venous blood was collected prior to intravenous administration of cephazolin and again at the time of disc removal to measure its concentration in serum and disc tissue.
Results The interval between cephazolin administration and tissue sampling ranged from 7 to 137 minutes. Cephazolin concentration in the serum (31.1 – 148 mg/L) was greater than in the disc (0 – 9.5 mg/L). The concentration of cephazolin peaked in the serum at 7 minutes and in the disc between 37 and 53 minutes. Over 70% of the disc samples had detectable levels of cephazolin at the time the disc was removed, although only half had cephazolin levels above 1 mg/L.
Discussion For a given time period the antibiotic concentration in the disc varies between patients. Only one measurement was taken from the disc using the method described when in fact if multiple measurements were taken over time there may be an antibiotic concentration peak in the disc followed by a steady decline. Variability between peak antibiotic concentration in the disc may be attributed to patient size, weight, gender, co-morbidities (diabetes, rheumatoid arthritis) and degree of disc degeneration. However the most critical variable for all patients is timing of administration of antibiotic to achieve optimal concentration in the disc when it is at greatest risk of inadvertent inoculation.
The abstracts were prepared by Professor Bruce McPhee. Correspondence should be addressed to him at Orthopaedics Division, The University of Queensland, Clinical Sciences Building, Royal Brisbane & Women’s Hospital, Herston, Qld, Australia
