What is the appropriate evaluation and management for a patient presenting with knee pain, swelling, and an overlying wound?

Evaluation and Management of Knee Pain, Swelling, and Overlying Wound

Immediately obtain plain radiographs (AP, lateral, and additional views as needed) and perform arthrocentesis with synovial fluid analysis before any other intervention—septic arthritis must be ruled out urgently as it can cause permanent joint destruction and death if treatment is delayed. 1, 2

Initial Clinical Assessment

Critical Red Flags Requiring Immediate Action

• Suspect septic arthritis if the patient presents with acute monoarticular joint pain, swelling, warmth, and erythema, especially with fever (present in only 57% of cases), though constitutional symptoms are poorly sensitive. 3, 4
• High-risk patients include those >80 years old, with diabetes mellitus, rheumatoid arthritis, recent joint surgery, prosthetic joints, skin infections, or immunosuppressive medication use. 3, 4
• Never administer intra-articular corticosteroids before excluding infection—this can worsen septic arthritis and lead to irreversible joint destruction. 5, 6

Physical Examination Findings

• Joint pain occurs in 85% of septic arthritis cases, history of joint swelling in 78%, but sweats (27%) and rigors (19%) are uncommon. 3
• Inability to bear weight (4 steps) or flex the knee to 90° mandates immediate radiographic evaluation. 1, 2
• Examine for visible bone in the wound or ability to probe to bone with a sterile instrument—this strongly suggests underlying osteomyelitis. 1

Diagnostic Algorithm

Step 1: Plain Radiographs (Mandatory First Study)

• Obtain AP, lateral, and additional views (tunnel/Rosenberg, tangential patellar) to exclude fracture, osteomyelitis, foreign body, or other bony pathology. 1, 2
• Radiographs may show bone destruction in chronic osteomyelitis, though early disease is often radiographically occult. 1
• A lateral view with 25-30 degrees of flexion can demonstrate joint effusion or lipohemarthrosis suggesting intra-articular fracture. 1

Step 2: Arthrocentesis and Synovial Fluid Analysis (Before Antibiotics)

Perform arthrocentesis immediately if septic arthritis is suspected—synovial fluid studies are required to confirm the diagnosis and guide antibiotic selection. 4, 7

Synovial Fluid Interpretation:

• WBC >50,000/μL: Likelihood ratio (LR) 7.7; strongly suggests septic arthritis. 3, 7
• WBC >100,000/μL: LR 28.0; highly specific for septic arthritis. 3
• Polymorphonuclear cells ≥90%: LR 3.4 for septic arthritis. 3
• WBC <25,000/μL: LR 0.32; makes septic arthritis less likely but does not exclude it. 3
• Send fluid for Gram stain, culture (aerobic, anaerobic, fungal, mycobacterial), and crystal analysis—septic arthritis and crystal arthropathy can coexist. 7

Critical Pitfall: In prosthetic joint infections, the diagnostic threshold is much lower—WBC >1,100/μL with >64% neutrophils suggests infection. 7

Step 3: Wound Assessment and Tissue Culture

• Cleanse and debride the wound before obtaining specimens—superficial swabs yield contaminants and are unreliable. 1
• Obtain deep tissue specimens by curettage or biopsy from the debrided wound base for culture (aerobic, anaerobic, fungal). 1
• If bone is visible or palpable with a sterile probe, strongly suspect osteomyelitis and obtain bone biopsy if surgical debridement is performed. 1

Step 4: Advanced Imaging (After Radiographs)

For Suspected Osteomyelitis:

• MRI without and with IV contrast is the most appropriate next study (rating 9/9) for evaluating osteomyelitis extent and soft tissue involvement. 1
• MRI without contrast alone (rating 7/9) is acceptable if contrast is contraindicated. 1
• CT with IV contrast (rating 7/9) is an alternative if MRI is contraindicated or extensive metal artifact is present. 1

For Suspected Septic Arthritis:

• MRI without and with contrast (rating 9/9) complements arthrocentesis for evaluating extent of infection, abscess, or muscle involvement. 1
• Ultrasound can detect soft tissue abscesses or fluid collections but is not routinely used for initial evaluation. 1

For Suspected Foreign Body:

• Ultrasound (rating 8/9) is preferred for radiolucent foreign bodies (wood, plastic). 1
• CT without contrast (rating 7/9) is best for radiopaque foreign bodies. 1

Management Strategy

Empiric Antibiotic Therapy (Start After Cultures Obtained)

Initiate empiric antibiotics immediately after obtaining synovial fluid and tissue cultures if septic arthritis is strongly suspected clinically. 4, 7

Antibiotic Selection Based on Gram Stain:

• Gram-positive cocci: Vancomycin (covers MRSA and methicillin-sensitive Staphylococcus aureus, the most common pathogen). 4, 7
• Gram-negative cocci: Ceftriaxone (covers Neisseria gonorrhoeae). 7
• Gram-negative rods: Ceftazidime. 7
• Negative Gram stain with high clinical suspicion: Vancomycin plus ceftazidime or aminoglycoside. 7

Route and Duration:

• Oral antibiotics are not inferior to IV therapy in most cases and can be used for definitive treatment. 4
• Total duration: 2-6 weeks for septic arthritis, depending on organism and clinical response. 4
• For osteomyelitis: 2-5 days parenteral therapy initially, then switch to oral when possible; total duration varies by severity. 1

Surgical Intervention

• Arthroscopic or open drainage is necessary for septic arthritis to evacuate purulent material—antibiotics alone are insufficient. 7
• Surgical debridement is required for osteomyelitis, especially if there is bone destruction, abscess, or failure to respond to antibiotics. 1
• Re-evaluate the wound if infection persists beyond expected duration—consider undiagnosed deep abscess, osteomyelitis, antibiotic resistance, or severe ischemia. 1

Wound Care and Adjunctive Measures

• Ensure adequate off-loading and wound care compliance—wounds will not heal without mechanical pressure relief. 1
• Optimize glycemic control in diabetic patients—hyperglycemia impairs wound healing and immune function. 1
• Reassess vascular status—severe ischemia may require revascularization before healing can occur. 1

Common Pitfalls to Avoid

• Do not delay arthrocentesis in patients with risk factors or clinical suspicion for septic arthritis—permanent joint damage occurs within days. 3, 4
• Do not rely on fever or constitutional symptoms to diagnose septic arthritis—they are present in <60% of cases. 3, 4
• Do not assume polyarticular pain excludes septic arthritis in rheumatoid arthritis patients—SA can masquerade as an RA flare. 8
• Do not perform MRI before plain radiographs—radiographs are mandatory first-line imaging and may reveal the diagnosis. 1, 2
• Do not use superficial wound swabs for culture—they yield contaminants and miss deep pathogens. 1
• Do not stop antibiotics when the wound remains open—antibiotic duration is defined by infection biology, not wound closure. 1