4.4 Skin, Soft Tissue, Bone & Joint Infections

1. Skin & Soft Tissue Infections (SSTIs)

Classification & Empirical Management

SSTI Classification:
  ├── Purulent (Abscess, Furuncle, Carbuncle) -> S. aureus (MSSA/MRSA)
  │     └── Primary Therapy: Incision and Drainage (I&D)
  │     └── Add Oral MRSA Antibiotics if systemic signs, high risk, or extremes of age
  └── Non-Purulent (Erysipelas, Cellulitis) -> Group A Strep >> MSSA
        ├── Erysipelas: Superficial, fiery red, raised, sharp distinct margins
        ├── Cellulitis: Deeper subcutaneous, diffuse non-demarcated borders
        └── Primary Therapy: Anti-streptococcal Beta-lactams (Cephalexin, Cefazolin)

Necrotizing Soft Tissue Infections (Necrotizing Fasciitis)

• Microbiology:
  • Type I (Polymicrobial ~70–80%): Mixture of aerobes (E. coli, Klebsiella, Enterococcus) and anaerobes (Bacteroides, Peptostreptococcus, Clostridium spp.). Occurs in diabetics, peripheral vascular disease, post-surgical wounds, and perianal regions (Fournier Gangrene).
  • Type II (Monomicrobial ~20–30%): Caused by Group A Streptococcus (Streptococcus pyogenes) +/- S. aureus. Occurs in healthy individuals, blunt trauma, lacerations, or injection drug users.
• Clinical Clues for Necrotizing Infection:
  1. Severe, excruciating pain out of proportion to visible skin findings.
  2. Rapid margin progression despite standard antibiotic therapy.
  3. Violaceous bullae, cutaneous necrosis/gangrene, and wooden-hard subcutaneous induration.
  4. Cutaneous anesthesia (destruction of cutaneous nerves traveling through the fascial plane).
  5. Subcutaneous crepitus (palpable gas bubbles or soft tissue gas on radiography/CT).
  6. Sepsis, hemodynamic instability, or refractory septic shock.
• LRINEC Score (Laboratory Risk Indicator for Necrotizing Fasciitis): Evaluates CRP (>=150 mg/L = 4 pts), WBC count (<15k = 0, 15–25k = 1, >25k = 2 pts), Hemoglobin (>13.5 = 0, 11–13.5 = 1, <11 = 2 pts), Sodium (<135 mEq/L = 2 pts), Creatinine (>1.6 mg/dL = 2 pts), and Glucose (>180 mg/dL = 1 pt). A score >=6 raises high suspicion, and >=8 indicates strong risk (though normal lab values do not rule out necrotizing fasciitis in high clinical suspicion).
• Emergency Management:
  1. Immediate Emergent Surgical Consultation for Operative Exploration and Extensive Debridement: This is both diagnostic ("dishwater pus", non-bleeding fascia, lack of fascial resistance to finger dissection) and life-saving. Repeat surgical exploration ("second-look") is required within 24 hours.
  2. Empiric Broad-Spectrum IV Antimicrobial Regimen:
     • Vancomycin (15–20 mg/kg IV q8–12h) OR Daptomycin (6–8 mg/kg IV daily) PLUS
     • Meropenem (1 g IV q8h) OR Piperacillin-tazobactam (4.5 g IV q6h) PLUS
     • Clindamycin (600–900 mg IV q8h).
     • Why Clindamycin is Mandatory: Clindamycin is a ribosomal 50S protein synthesis inhibitor that suppresses bacterial production of Streptococcal Pyrogenic Exotoxins (SPE-A, SPE-B, SPE-C) and overcomes the Eagle Effect (where beta-lactams lose bactericidal activity against high-inoculum, stationary-phase streptococci).

2. Bone & Joint Infections

Acute Septic Arthritis

• Microbiology: Staphylococcus aureus (most common overall across all age groups), Streptococcus spp., aerobic Gram-negative bacilli (elderly, IVDA), and Neisseria gonorrhoeae (sexually active young adults).
• Diagnostic Arthrocentesis: Synovial fluid aspiration is mandatory prior to antibiotic therapy. Synovial fluid WBC count >50,000/mcL with >75–90% PMNs confirms septic arthritis (often >100,000/mcL in non-gonococcal disease).
• Disseminated Gonococcal Infection (DGI):
  • Occurs in sexually active young adults. Manifests in two classic presentations:
    1. Tenosynovitis-Dermatitis-Polyarthralgias Syndrome: Migratory polyarthralgias, wrist/ankle tenosynovitis, and painless pustular or hemorrhagic necrotic skin lesions on extremities (synovial fluid cultures are frequently negative, but mucosal NAAT is positive).
    2. Purulent Monoarthritis: Typically knee, wrist, or ankle with positive synovial fluid Gram stain/culture.
  • Treatment: IV Ceftriaxone 1 g IV every 24 hours for at least 7 days PLUS oral Doxycycline 100 mg PO BID for 7 days if chlamydial infection has not been ruled out.
• Non-Gonococcal Septic Arthritis Treatment: Emergency joint drainage (serial needle aspiration, arthroscopy, or open arthrotomy) plus IV antibiotics (Vancomycin + Ceftriaxone empiric) for 3–4 weeks.

Osteomyelitis

• Mechanisms: Contiguous focus (diabetic foot ulcers, peripheral vascular disease), Hematogenous (vertebral osteomyelitis, pediatric long bones), or Direct inoculation (open fractures, orthopedic hardware).
• Diabetic Foot Osteomyelitis:
  • Probe-to-Bone Test: Palpating hard, gritty bone with a sterile metallic blunt probe at the base of an open ulcer has a positive predictive value >85–90% in high-prevalence settings.
  • Imaging: Plain radiographs have low sensitivity in early disease (bone demineralization and periosteal elevation require 10–14 days and >30–50% bone loss). Magnetic Resonance Imaging (MRI) is the gold standard imaging modality (sensitivity and specificity >90%).
  • Microbiological Confirmation: Bone biopsy (percutaneous or intraoperative) for histopathology and quantitative microbiology is required prior to antimicrobial therapy in hemodynamically stable patients. Superficial wound or sinus tract swabs correlate poorly with deep bone pathogens and must not guide targeted antibiotic regimens.