Musculoskeletal Infections and Microbiology

Musculoskeletal Infections and Microbiology

Prepared by Dr. Alper DUNKI

Spot Knowledge

• Staphylococcus aureus is the leading cause of MSK infections.
• MRSA strains (community vs hospital) differ in virulence.
• Children 6 mo–4 yrs: Kingella kingae is most common.
• Sickle cell disease: Salmonella is typical pathogen.
• Implant infections: Biofilm formation → requires debridement.
• MRI is nearly 100% sensitive in early osteomyelitis.

Epidemiology & Microbiology

• Main pathogens: S. aureus, S. epidermidis, coagulase-negative staphylococci.
• Gram-negative: E. coli, Proteus, Klebsiella, Enterobacter.
• IV drug users: Pseudomonas, Serratia, fungi.
• Gonococcal arthritis: Neisseria gonorrhoeae in young adults.
• Post-shoulder surgery: Propionibacterium acnes.

Pathogenesis

• Synovium lacks basement membrane → easy microbial entry.
• S. aureus virulence factors: Protein A, polysaccharide capsule, biofilm, PVL toxin.
• Biofilms protect bacteria in prosthetic joint infection → need surgery + antibiotics .

Clinical Findings

• Septic arthritis: monoarticular, knee most common.
• Kocher criteria (peds): fever, non-weight bearing, ESR >40, WBC >12,000.
• Osteomyelitis (peds, MRSA risk): fever >38°C, Hct <34%, WBC >12,000, CRP >13.

Diagnosis

• Radiology: joint space narrowing, periosteal reaction, Codman’s triangle.
• MRI: gold standard, early detection.
• Lab: CRP, ESR monitoring.
• Synovial fluid: WBC >50,000, >90% PMN highly suggestive.

Treatment

• Osteomyelitis: 4–6 wks (≥6 for MRSA).
• Septic arthritis: 3–4 wks.
• Adults empiric: Vancomycin + Ceftriaxone.
• Children (MRSA): IV Vancomycin (15 mg/kg q6h).
• Implant infection: add Rifampin (synergy vs biofilm).
• C. difficile must be considered in prolonged antibiotic use .

Antibiotic Prophylaxis in Orthopaedics

• Not routine in elective surgery without implants.
• Give ≤1 h before incision (Vanco: 2 h prior).
• 1st line: cephalosporins.
• Clinda/Vanco for β-lactam allergy.
• Duration: ≤24 h.

Prevention of Surgical Site Infection

• Risk factors: DM, obesity, malnutrition, smoking, RA, MRSA colonization.
• Measures: chlorhexidine prep, double gloving, monofilament sutures, drains <24h, normothermia, glycemic control.

Periprosthetic Joint Infection

• Knee arthroplasty: Synovial WBC >2,500/mm³ or >90% PMN → chronic infection.
• Gram stain not useful.

Atypical & Rare Infections

• Necrotizing fasciitis: S. pyogenes, CA-MRSA; urgent surgery.
• Gas gangrene: Clostridium spp., surgery + high-dose PCN/Clinda.
• TB: spine most common, 4-drug ≥6 months.
• NTM: M. marinum (hand infections post-water exposure).
• Vibrio vulnificus: severe necrotic infection after seawater.
• Candida albicans: rare prosthetic infection.
• Lyme (Borrelia): late monoarthritis.
• HIV/AIDS: optimize immunity pre-surgery .

References

• Masters EA, et al. Nat Rev Microbiol. 2022.
• Touaitia R, et al. Antibiotics. 2025.
• Sanpera I, et al. Current Concepts in Septic Arthritis. 2024 .