Is periprosthetic infection the same as cellulitis in patients with a history of joint replacement surgery?

Periprosthetic Infection vs. Cellulitis: Critical Distinction

No, periprosthetic joint infection (PJI) is fundamentally different from cellulitis and must be distinguished immediately because PJI requires urgent surgical intervention and carries a fivefold increased mortality risk compared to aseptic complications. 1, 2

Key Distinguishing Features

Anatomic and Pathophysiologic Differences

• PJI involves deep infection of the joint space, prosthesis-bone interface, and periprosthetic tissues, often with biofilm formation on the implant surface that makes eradication extremely difficult 1
• Cellulitis is a superficial soft tissue infection limited to the skin and subcutaneous tissues without involvement of the joint space or prosthesis
• PJI requires definitive surgical management (debridement with prosthesis retention, one-stage exchange, or two-stage exchange), whereas cellulitis responds to antibiotics alone 1

Clinical Presentation Patterns

Acute PJI (within 3 months post-surgery):

• Pain, swelling, warmth, erythema, and fever are common—mimicking cellulitis superficially 1, 3
• However, night pain or pain at rest is characteristic of PJI, whereas cellulitis typically causes constant pain regardless of activity 1, 4
• Joint effusion and purulence surrounding the prosthesis distinguish PJI 1

Chronic/Low-grade PJI (>3 months post-surgery):

• May present with pain alone without fever, erythema, or warmth—easily mistaken for mechanical loosening rather than cellulitis 1, 4
• Pain persisting >6 months after joint replacement mandates infection exclusion 1

Diagnostic Algorithm to Differentiate

Step 1: Clinical Assessment

• Superficial erythema and warmth WITHOUT joint pain at rest suggests cellulitis
• Deep joint pain (especially at rest/night), joint effusion, or sinus tract indicates PJI 1, 4

Step 2: Laboratory Markers

• Obtain ESR, CRP, and interleukin-6 for all suspected cases 1, 4
• CRP >13.5 mg/L has 73-91% sensitivity for PJI (though can be elevated in cellulitis as well) 1, 4
• When ≥2 of 3 tests abnormal (CRP, ESR, fibrinogen), diagnostic accuracy reaches 93% sensitivity and 100% specificity for PJI 1, 4
• Peripheral WBC is NOT useful—normal in most PJI cases 1, 4

Step 3: Joint Aspiration (Critical Differentiator)

• Proceed immediately to joint aspiration if PJI suspected—this definitively distinguishes PJI from cellulitis 1, 4
• Withhold antibiotics for ≥2 weeks prior to aspiration if patient is stable to avoid false-negative cultures 1, 4
• Synovial fluid with >1700 cells/μL or >65% neutrophils indicates PJI (94-97% sensitivity) 1
• Alpha-defensin testing of synovial fluid has 97% sensitivity and 96% specificity for PJI 1
• Cellulitis does NOT produce purulent joint fluid or elevated synovial markers

Step 4: Imaging

• Radiographs first to assess for loosening, osteolysis, or periosteal reaction suggesting PJI 1, 4
• MRI can demonstrate deep inflammatory synovitis, bone marrow edema, and fluid collections characteristic of PJI but absent in superficial cellulitis 1

Critical Pitfalls to Avoid

• Never treat suspected PJI as cellulitis with antibiotics alone—this delays definitive surgical management and worsens outcomes 1
• Do not rely on absence of fever/erythema to exclude PJI—chronic infections frequently present with pain alone 1, 4
• Superficial wound erythema in a joint replacement patient requires joint aspiration to exclude PJI before assuming cellulitis 1, 4
• PJI increases one-year mortality by fivefold compared to aseptic complications—early recognition is life-saving 2

Microbiology Differences

• PJI organisms: Staphylococcus aureus and coagulase-negative staphylococci (including S. epidermidis) are most common, with biofilm formation 1, 4
• Cellulitis organisms: Typically Streptococcus species or S. aureus without biofilm
• Multiple tissue cultures (3-6 samples) are required for PJI diagnosis, whereas cellulitis is diagnosed clinically 1

Treatment Implications

• PJI requires surgical debridement and often prosthesis exchange in addition to prolonged antibiotics (typically 6 weeks minimum) 1
• Cellulitis requires antibiotics only (typically 1-2 weeks)
• Treating PJI as cellulitis leads to treatment failure, chronic infection, and increased mortality 2