Section 4.2: Infectious & Inflammatory MSK Disorders (Osteomyelitis, Septic Arthritis)

Infectious & Inflammatory MSK Disorders

PANCE High-Yield Focus: Infectious bone and joint disorders are orthopedic emergencies that carry a high risk of permanent disability or systemic sepsis. The PANCE focuses on identifying key causative pathogens in specific patient populations (e.g., Salmonella in sickle cell patients, Pseudomonas in sneaker puncture wounds), selecting the appropriate imaging modality (MRI for early osteomyelitis), interpreting synovial fluid analysis to confirm septic arthritis, and initiating prompt empiric antibiotic regimens and joint drainage.

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Pathophysiology and Pathogens

Osteomyelitis

Osteomyelitis is an infection of the bone and bone marrow. It can occur via three primary mechanisms:

1. Hematogenous Spread: Most common in children, typically affecting the highly vascular metaphysis of long bones (such as the femur or tibia).
2. Direct Inoculation: Occurs from open fractures, orthopedic surgery, or penetrating trauma (e.g., stepping on a nail).
3. Contiguous Spread: Occurs from adjacent soft tissue infections, highly prevalent in patients with diabetic foot ulcers or peripheral vascular disease.

Septic Arthritis

Septic arthritis is a bacterial infection of the joint space. It is a medical emergency because the inflammatory response leads to the release of chondrolytic enzymes from polymorphonuclear leukocytes (PMNs) and chondrocytes, which can destroy the articular cartilage within 24 to 48 hours, leading to permanent joint damage. The infection typically seeds the joint hematogenously, but can also occur via direct inoculation or contiguous spread.

Causative Pathogens by Clinical Context

Identifying the patient's risk profile directs initial diagnostic suspicion and empiric therapy:

• Staphylococcus aureus: The overall most common pathogen in all age groups, including patients with rheumatoid arthritis, diabetes, and joint prostheses. Includes methicillin-resistant S. aureus (MRSA).
• Salmonella species: Highly associated with patients who have Sickle Cell Disease or other hemoglobinopathies. Note that S. aureus is still a frequent cause in these patients, but Salmonella is highly specific and tested on exams.
• Pseudomonas aeruginosa: Classically associated with osteomyelitis resulting from a puncture wound through a sneaker (rubber soles support the growth of Pseudomonas in warm, moist environments) or in intravenous drug users.
• Neisseria gonorrhoeae: Most common cause in sexually active young adults. Presents either as a localized purulent arthritis or as disseminated gonococcal infection (characterized by the triad of tenosynovitis, dermatitis [purpuric macules/pustules], and migratory polyarthralgias).
• Staphylococcus epidermidis: The most common cause in patients with prosthetic joint infections (due to its ability to produce biofilms on foreign materials).
• Group B Streptococcus (Streptococcus agalactiae) and Escherichia coli: Most common causes in neonates (< 3 months old).

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Clinical Presentation and Diagnostic Evaluation

Osteomyelitis

• Clinical Signs: Presents with localized bone pain, tenderness, warmth, erythema, and swelling over the affected bone, often accompanied by systemic symptoms such as fever, chills, and malaise. In adults with chronic contiguous osteomyelitis (e.g., diabetic foot ulcers), systemic signs are often absent, and the primary sign is a non-healing ulcer. A clinical pearl is the 'probe-to-bone' test: if a sterile metal probe can easily reach the bone when inserted through a diabetic ulcer, osteomyelitis is likely.
• Diagnostics:
  • Laboratory Findings: Elevated white blood cell (WBC) count, Erythrocyte Sedimentation Rate (ESR), and C-Reactive Protein (CRP). Blood cultures are positive in 50% of hematogenous cases.
  • Plain Radiography (X-ray): The initial imaging modality. However, X-ray changes (including periosteal elevation, cortical bone destruction, and focal osteopenia) lag behind symptoms by 10 to 14 days. Up to 50% of bone mineral density must be lost before it is visible on an X-ray.
  • Magnetic Resonance Imaging (MRI): The most sensitive and specific imaging modality for early osteomyelitis. It can detect bone marrow edema and soft tissue involvement within 1 to 2 days of symptom onset.
  • Gold Standard: Bone biopsy and culture.

Septic Arthritis

• Clinical Signs: Presents with the acute onset of severe, excruciating monoarticular joint pain, swelling, warmth, and erythema, accompanied by systemic fever and chills. On physical examination, the joint is held in a position of maximum comfort (often slight flexion), and there is severe restriction of both active and passive range of motion (the patient will resist any movement due to severe pain). The knee is the most commonly affected joint (50% of cases), followed by the hip, shoulder, and ankle.
• Diagnostics:
  • Arthrocentesis (Joint Aspiration): The single most critical diagnostic step and must be performed immediately prior to starting empiric antibiotics. Synovial fluid is analyzed for appearance, cell count with differential, Gram stain, cultures, and crystals (to rule out gout/pseudogout).
  • Synovial Fluid Findings in Septic Arthritis:
    • Appearance: Purulent, turbid, yellow-green.
    • WBC Count: Typically > 50,000 cells/mcL (often > 100,000 cells/mcL).
    • Differential: > 75% polymorphonuclear leukocytes (PMNs / neutrophils).
    • Gram Stain: Positive in 50-70% of Gram-positive infections but often negative in Gram-negative or gonococcal infections.
    • Glucose: Significantly decreased (< 50% of serum glucose levels).
    • Lactate: Elevated.

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Management Guidelines

Septic Arthritis Management

1. Joint Drainage (Emergent): Drainage of the purulent joint fluid and inflammatory enzymes is mandatory. This is achieved via serial needle aspirations, arthroscopy, or open arthrotomy (particularly for hip septic arthritis, which is difficult to aspirate and requires urgent surgical decompression to prevent avascular necrosis of the femoral head).
2. Empiric Intravenous Antibiotics: Initiated immediately after arthrocentesis has been performed.
   • Gram-Positive Cover (MRSA): Vancomycin is first-line.
   • Gram-Negative Cover (including N. gonorrhoeae): Add Ceftriaxone (or a third-generation cephalosporin like Ceftazidime if Pseudomonas is suspected, such as in IV drug users).
   • Duration: Typically 3-4 weeks of IV therapy.

Osteomyelitis Management

1. Surgical Debridement: Indicated for chronic osteomyelitis, abscess formation, joint involvement, or failure to respond to antibiotic therapy. Debridement of necrotic bone (sequestrum) is essential.
2. Long-Term Intravenous Antibiotics: Typically administered for 4 to 6 weeks through a peripherally inserted central catheter (PICC). Empiric regimens include:
   • Vancomycin (for MRSA cover) plus Cefepime or Ceftazidime (to cover Gram-negatives, including Pseudomonas).
   • Sickle Cell Patients (Salmonella cover): Fluoroquinolones (such as Ciprofloxacin) or third-generation cephalosporins (such as Ceftriaxone).

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Synovial Fluid Analysis Classification