How should I evaluate and manage a patient with knee pain and swelling due to a skin breach over the knee joint?

Evaluation and Management of Knee Pain and Swelling from Broken Skin Integrity

When a patient presents with knee pain and swelling associated with a skin breach, you must immediately assume this is a potential septic arthritis or soft tissue infection until proven otherwise, and proceed urgently with diagnostic workup before initiating antibiotics. 1

Immediate Clinical Assessment

Critical Risk Factors to Document

• Age >60 years, diabetes mellitus, rheumatoid arthritis, recent joint surgery, prosthetic joint, skin infection, immunosuppressive medications, or recent corticosteroid injection all significantly increase the probability of septic arthritis 2, 3, 4
• Do not rely on the absence of fever, erythema, or warmth to exclude infection—chronic infections frequently present with pain alone without these classic signs 1, 5
• Joint pain (85% sensitive), history of joint swelling (78% sensitive), and fever (57% sensitive) are the most common presenting features, but their absence does not rule out infection 2

Wound Characteristics That Matter

• Puncture wounds require special consideration for foreign body evaluation and have higher risk of deep infection 6
• Animal or human bite wounds over the knee joint warrant coverage for Pasteurella multocida (animal bites) and polymicrobial organisms including anaerobes 6
• Pain disproportionate to the severity of injury located near a bone or joint should suggest periosteal penetration and possible osteomyelitis 6

Diagnostic Algorithm

Step 1: Obtain Plain Radiographs First

Plain radiographs of the knee (AP, lateral, and axial views) are the mandatory initial imaging study because they are safe, inexpensive, exclude fractures or tumors, detect joint effusion and soft tissue swelling, identify gas in soft tissues or foreign bodies, and establish baseline anatomic detail 1, 6

• Early radiographs may be normal in acute infection but remain essential to exclude other pathology 1
• Radiographs alone are neither sensitive nor specific for diagnosing infection 6

Step 2: Immediate Laboratory Workup (Before Antibiotics)

Obtain ESR, CRP, and blood cultures immediately before any antibiotic administration 1, 5

• The combination of ESR and CRP achieves 93% sensitivity, 100% specificity, and 97% accuracy when at least 2 of 3 tests are abnormal for septic arthritis 1, 5
• CRP >13.5 mg/L demonstrates 73-91% sensitivity and 81-86% specificity for prosthetic knee infection 5
• Adding serum interleukin-6 to CRP improves diagnostic accuracy and provides higher predictive value than most other serologic markers 1, 5
• Peripheral WBC counts are not elevated in most patients with infected joints, making a normal WBC count meaningless for excluding infection 1

Step 3: Urgent Image-Guided Joint Aspiration

Proceed immediately with image-guided knee joint aspiration for synovial fluid analysis if inflammatory markers are elevated or clinical suspicion remains high—this is the most valuable diagnostic tool and should not be delayed 1, 5

• Withhold antibiotics for at least 2 weeks prior to aspiration if clinically feasible, as preaspiration antibiotic treatment causes false-negative cultures 1, 5
• Image guidance for aspiration is ideal because it confirms proper needle placement and reduces vascular complications 1

Synovial Fluid Analysis Must Include:

• Total and differential cell counts, aerobic and anaerobic bacterial cultures, leukocyte esterase, alpha-defensin, synovial fluid CRP, and nucleic acid amplification testing 1, 5
• Synovial fluid WBC count >100,000/μL has a likelihood ratio of 28.0 for septic arthritis 2
• Synovial fluid WBC count >50,000/μL has a likelihood ratio of 7.7 2
• Polymorphonuclear cell count ≥90% suggests septic arthritis with a likelihood ratio of 3.4 2
• A "dry tap" does not exclude infection—weekly repeat aspirations are recommended if initial aspiration is negative and clinical suspicion remains high 5

Step 4: Foreign Body Evaluation (If Puncture Wound)

If there is a history of puncture wound with negative radiographs, ultrasound is the preferred next imaging study (rating 8/9) because it is favored for radiolucent foreign bodies like wood or plastic 6

• CT without IV contrast (rating 7/9) is an alternative to assess for radiopaque foreign bodies 6
• MRI without and with IV contrast (rating 7/9) is an alternative for assessing extent of infection, though CT is favored over MRI for foreign body identification 6

Step 5: Advanced Imaging for Suspected Osteomyelitis

If there is concern for osteomyelitis based on clinical presentation (pain disproportionate to injury near bone/joint), obtain MRI without and with IV contrast after initial radiographs (rating 9/9) 6

• MRI has 100% negative predictive value for excluding osteomyelitis 1
• Contrast is preferred to aid in soft-tissue evaluation and delineation of fluid collections and areas of necrosis 6

Empiric Antibiotic Management

Timing of Antibiotic Initiation

After synovial fluid has been obtained, empiric antibiotic therapy should be initiated immediately if there is clinical concern for septic arthritis 4

Empiric Coverage for Native Joint Septic Arthritis

The initial antibiotic regimen should cover methicillin-resistant Staphylococcus aureus (MRSA) and gram-negative and gram-positive organisms 3

• Staphylococcus aureus and coagulase-negative Staphylococcus are the most common causative organisms in septic arthritis 1, 5, 4
• Oral antibiotics can be given in most cases because they are not inferior to intravenous therapy 4

Special Considerations for Bite Wounds Over the Knee

For animal bite wounds, use amoxicillin-clavulanate as first-line oral therapy (covers Pasteurella multocida and anaerobes) 6

• Avoid first-generation cephalosporins (cephalexin), penicillinase-resistant penicillins (dicloxacillin), macrolides (erythromycin), and clindamycin because they have poor activity against P. multocida 6
• Alternative oral agents include doxycycline, or penicillin VK plus dicloxacillin 6
• Fluoroquinolones, trimethoprim-sulfamethoxazole, and cefuroxime may require an additional agent active against anaerobes such as metronidazole or clindamycin 6
• Intravenous options include ampicillin-sulbactam, piperacillin-tazobactam, second-generation cephalosporins (cefoxitin), and carbapenems 6

Duration of Therapy

Total duration of antibiotic therapy ranges from 2-6 weeks for uncomplicated septic arthritis 4

• 4 weeks for septic arthritis 6
• 4-6 weeks for osteomyelitis 6
• 3-4 weeks for synovitis 6

Surgical Management

Indications for Urgent Surgical Intervention

Most patients with septic arthritis respond within 72-96 hours of appropriate antibiotic therapy, and persistent symptoms beyond 4 days warrant reassessment for complications or surgical intervention 1

• Operative management involves either arthrotomy or arthroscopy of the knee with thorough irrigation and debridement of all infected tissue 3

Wound Management Principles

Wounds should be cleansed with sterile normal saline (no need for iodine- or antibiotic-containing solutions) and superficial debris removed 6

• Infected wounds should not be closed 6
• Suturing wounds early (<8 hours after injury) is controversial—approximation of margins by Steri-Strips and subsequent closure by delayed primary or secondary intent is prudent 6
• During the first few days after injury, elevation of the injured body part, especially if swollen, accelerates healing 6

Adjunctive Measures

Ensure tetanus prophylaxis status is current—if outdated or unknown, administer 0.5 mL tetanus toxoid intramuscularly 6

• Rabies prophylaxis should be considered for all feral and wild animal bites and in geographic areas with high rabies prevalence 6

Common Pitfalls to Avoid

• Never delay joint aspiration waiting for imaging beyond plain radiographs if septic arthritis is suspected 1
• Never start antibiotics before obtaining cultures unless the patient is hemodynamically unstable 1, 5
• Never assume a normal peripheral WBC count excludes infection 1
• Never close infected wounds primarily 6
• Never use first-generation cephalosporins or clindamycin alone for animal bite wounds over joints 6