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Authors: Kowalski TJ Berbari EF Steckelberg JM Huddleston PM Osmon DR

Title: Microbiology of Spine Implant Infections in the Modern Era

Addresses: Mayo Clinic Rochester 200 1st St SW, Rochester, MN, 55905

Purpose: Spinal implant infections are not well characterized. This retrospective cohort study aims to characterize the microbiology of such infections in the modern era.

Methods: The records of all adult patients seen at our institution between 1/1994 and 12/2002 with spinal implant infections were retrospectively reviewed. Only patients' first episode was analyzed. Fisher's exact test was used for statistical comparisons.

Results: 103 infection episodes among 81 patients were reviewed. Among first episodes, 30 (37%) occurred early (< 30 days from implant). Staphylococcus aureus (SA), coagulase-negative staphylococcus (SCN) and gram-negative rods (GNR) were most commonly isolated ( 29 (36%), 21 (26%) and 17 (21%) isolates, respectively). Anaerobic bacteria, Enterococcus sp., and culture negative infections (15 (18%), 11 (14%) and 8 (11%), respectively) were less common. 19 (23%) infections were polymicrobial, 4 (5%) of which included Candida sp. Infections diagnosed after 1998 were more likely to include MRSA (10 (21%) vs 1 (3%) P = 0.02) and less likely to include SCN (8 (17%) vs 13 (39%) P = 0.04). Infections within 30 days of instrument implantation were more likely to include GNR (11 (37%) vs 6(12%) P = 0.01). By anatomic level of infection, the most common pathogens were as follows: cervical; polymicrobial 6 (50%), SA 2 (17%) thoracic; SCN 9 (26%), polymicrobial 8 (24%), SA 5 (14%), GNR 4 (12%) lumbosacral; SA 14 (40%), polymicrobial 5 (14%), non-staphylococcal gram positive cocci 4 (11%) and anaerobes 4 (11%). 88% of monomicrobial episodes involving SA had an elevated erythrocyte sedimentation rate at diagnosis vs 50% with SCN and 40% with Propionibacterium sp. By comparison, 82% of monomicrobial episodes involving SA had an elevated C-reactive protein at diagnosis vs. 83% with SCN and 50% with Propionibacterium sp. Blood cultures were positive in 15 of 50 (30%) episodes; 11 (44%) blood cultures were positive in SA cases vs. 4 (16%) in the others (P = 0.06). Vancomycin-resistant enterococcus (VRE) was isolated in two episodes, each after 1998.

Discussion: Staphylococcus aureus remains a primary pathogen in spinal implant infections, but polymicrobial infections (including anaerobes) contribute significantly. Nearly 10% of such infections are culture negative, highlighting the need for more sensitive culturing techniques. Resistant pathogens (MRSA, VRE) were more frequently isolated in recent years.

Significance: Knowledge of the microbiology of spinal implant infections is valuable to clinicians in choosing empiric antimicrobial therapy. Erythrocyte sedimentation rates and C-reactive protein levels vary by pathogen, implying their diagnostic value may vary by pathogen. Knowing the microbiology of such infections should prove useful when developing strategies to prevent spinal implant infections.

Musculoskeletal Infecton Society
Musculoskeletal Infecton Society
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Rochester, MN 55903