What is the appropriate management for a patient presenting with knee swelling and joint line tenderness, considering potential diagnoses such as osteoarthritis or infection?

Management of Knee Swelling and Joint Line Tenderness

The immediate priority is to rule out septic arthritis through joint aspiration, as bacterial infection can cause irreversible cartilage damage within hours to days, while simultaneously obtaining plain radiographs and inflammatory markers (ESR, CRP) to guide further management. 1

Initial Diagnostic Algorithm

Immediate Clinical Assessment

• Document fever >101.3°F, inability to bear weight (have patient take at least 4 steps), and assess for acute monoarticular presentation 1, 2
• Measure range of motion specifically to 90 degrees of flexion 2
• Palpate for joint effusion, which may indicate inflammatory processes, structural damage, or infection 1, 2
• Assess for erythema, warmth, and severe tenderness that would elevate concern for septic arthritis 1

Risk Stratification for Septic Arthritis

If the patient meets ≥3 of the following criteria, septic arthritis likelihood approaches 100% and requires emergent joint aspiration: 1

• Fever >101.3°F
• ESR ≥40 mm/hour
• WBC ≥12,000 cells/mm³
• Inability to bear weight
• CRP >2.0 mg/dL

Initial Imaging and Laboratory Studies

• Order plain radiographs (AP, lateral, Merchant/sunrise, and tunnel views) immediately to exclude fracture, tumor, assess for degenerative changes, and identify indirect signs of infection (joint effusion, soft tissue swelling, periosteal reaction, gas in soft tissues) 3, 1, 4
• Order ESR and CRP immediately, as these are more sensitive than radiographs for detecting infection; when both are negative, infection is unlikely 4
• Radiographs may be completely normal in early acute infection (<14 days), showing only mild soft tissue swelling 1, 4

Definitive Diagnostic Procedure

Joint Aspiration Protocol

Perform joint aspiration immediately if inflammatory markers are elevated or clinical suspicion for septic arthritis remains high, as this is the definitive diagnostic procedure: 1

• For knee joints, aspiration can be performed at bedside without imaging guidance 1
• Send synovial fluid for: cell count with differential, Gram stain, culture, and crystal analysis 1
• Synovial fluid WBC ≥50,000 cells/mm³ is suggestive of septic arthritis 1
• Critical pitfall: Negative synovial fluid culture does not exclude infection; culture is positive in only ~80% of non-gonococcal cases 1
• Always perform crystal analysis even if infection is suspected, as gout and septic arthritis can coexist in 73% of reported cases 1

Advanced Imaging Indications

Order MRI with and without IV contrast if: 1, 2

• Clinical suspicion for septic arthritis remains high despite negative joint aspiration
• Concern for concurrent osteomyelitis exists (particularly in diabetic patients or if symptoms persist >7 days)
• Need to assess for soft tissue abscess or pyomyositis
• Initial radiographs are normal but symptoms persist after 6 weeks of conservative treatment
• Evaluating for meniscal pathology, cartilage lesions, or bone marrow edema when surgery is being considered

MRI demonstrates 82-100% sensitivity and 75-96% specificity for septic arthritis, and a negative MRI effectively excludes infection 1

Management Based on Diagnosis

If Septic Arthritis is Confirmed

Initiate immediate surgical drainage combined with empiric IV antibiotics: 1

• Start IV vancomycin 15 mg/kg every 6 hours immediately after cultures for MRSA coverage 1
• Surgical drainage is mandatory; repeated needle aspiration alone fails in 46% of cases 1
• Transition to oral antibiotics after 2-4 days if patient is clinically improving, afebrile, and tolerating oral intake (oral antibiotics are not inferior to IV therapy) 1
• Total antibiotic duration: 3-4 weeks for uncomplicated bacterial arthritis 1
• Monitor CRP and ESR to assess treatment response 1

If Osteoarthritis is Diagnosed (Radiographic Evidence of Joint Space Narrowing, Osteophytes)

Initiate combination therapy with acetaminophen, exercise, and patient education: 2, 5, 6

• Start acetaminophen up to 4g/day as first-line oral analgesic 2, 5
• Prescribe regular, progressive exercise therapy with load progression individualized based on patient tolerance 2, 6
• Recommend weight reduction if patient is obese 2, 6
• If acetaminophen is ineffective, add topical NSAIDs or capsaicin as second-line 2
• Consider oral NSAIDs with gastroprotection if gastrointestinal risk factors exist 2, 5
• Perform intra-articular corticosteroid injection specifically for pain flares accompanied by effusion 2, 7

If Meniscal Tear is Suspected (Joint Line Tenderness, Positive McMurray Test)

Initiate conservative management with exercise therapy for 4-6 weeks: 6

• McMurray test has 61% sensitivity and 84% specificity; joint line tenderness has 83% sensitivity and 83% specificity 6
• Exercise therapy is first-line treatment even in the presence of mechanical symptoms (locking, catching) 6
• Surgery is indicated only for severe traumatic bucket-handle tears with displaced meniscal tissue 6
• Degenerative meniscal tears do not require surgery even with mechanical symptoms 6

Critical Pitfalls to Avoid

• Never rely solely on radiographs to rule out infection, as radiographic changes appear late (7-10 days) and require >30% osseous matrix destruction 1, 4
• Do not delay joint aspiration if septic arthritis is suspected, as cartilage damage can occur within hours 1
• Do not skip crystal analysis even when infection is suspected, as coexistent gout and septic arthritis occur in 73% of reported cases 1
• Do not use MRI as the initial imaging study; the 2022 ACR Appropriateness Criteria explicitly state insufficient evidence for MRI first-line 1
• Reassess at 6 weeks if no favorable outcomes are observed with conservative treatment 2

Surgical Referral Indications

Refer for orthopedic consultation when: 2, 6

• Radiographic evidence of end-stage OA (no or minimal joint space) with refractory pain and disability despite 6+ weeks of adequate conservative treatment
• Severe traumatic meniscal tears with displaced tissue
• Persistent symptoms despite adequate conservative treatment with concerning MRI findings