What urgent evaluation and management are indicated for a 60‑year‑old woman who received intra‑articular corticosteroid injections in both knees two weeks ago and now presents with acute posterior right‑knee pain radiating up the posterior thigh?

Urgent Evaluation for Deep Vein Thrombosis and Septic Arthritis

This patient requires immediate evaluation for deep vein thrombosis (DVT) with urgent duplex ultrasound of the right lower extremity, followed by assessment for septic arthritis if DVT is excluded. The acute onset of posterior knee pain radiating up the posterior thigh two weeks after intra-articular corticosteroid injection raises critical concern for DVT, which can present with posterior leg pain and is a medical emergency requiring anticoagulation to prevent pulmonary embolism.

Immediate Diagnostic Workup

First Priority: Rule Out DVT

• Obtain urgent lower extremity venous duplex ultrasound within hours of presentation to evaluate for deep vein thrombosis, as the pain distribution (posterior knee extending up posterior thigh) follows the anatomic course of the popliteal and femoral veins 1
• Assess for clinical signs of DVT including unilateral leg swelling, warmth, erythema, palpable cord, and positive Homan's sign (though absence does not exclude DVT) 1
• Measure D-dimer if ultrasound is delayed, though a negative D-dimer has higher utility for exclusion than a positive result has for diagnosis 1

Second Priority: Rule Out Septic Arthritis

• Perform immediate arthrocentesis with synovial fluid analysis if DVT is excluded, as septic arthritis is an absolute contraindication to further corticosteroid administration and can cause devastating joint destruction 1
• Send synovial fluid for cell count with differential, Gram stain, and culture looking for white blood cell count >50,000 cells/μL with >90% neutrophils, which suggests septic arthritis 1
• Examine the knee for warmth, erythema, effusion, and restricted range of motion, as these findings increase suspicion for infection 1
• Check inflammatory markers including ESR and CRP, though these are nonspecific and can be elevated in both infection and inflammatory arthritis 1

Third Priority: Evaluate for Post-Injection Complications

• Consider osteonecrosis of the tibial plateau, a rare but documented complication of intra-articular corticosteroid injection that presents with sudden onset of severe acute knee pain 2
• Obtain plain radiographs of the right knee to assess for subchondral collapse, sclerosis, or other signs of osteonecrosis if infection and DVT are excluded 2
• Consider MRI if plain films are negative but clinical suspicion for osteonecrosis remains high, as MRI is more sensitive for early detection 2

Critical Management Principles

If DVT is Confirmed

• Initiate therapeutic anticoagulation immediately with low molecular weight heparin, unfractionated heparin, or direct oral anticoagulant per institutional protocol 1
• Admit for monitoring if extensive thrombosis, hemodynamic instability, or high bleeding risk 1

If Septic Arthritis is Confirmed

• Never administer corticosteroids in the presence of joint infection, as steroids are absolutely contraindicated and will worsen outcomes 1
• Initiate empiric intravenous antibiotics immediately after obtaining cultures, typically vancomycin plus a third-generation cephalosporin to cover Staphylococcus aureus and gram-negative organisms 1
• Consult orthopedic surgery urgently for arthroscopic or open irrigation and debridement 1

If Both DVT and Infection are Excluded

• Optimize oral and topical analgesics before considering repeat injection, given that only two weeks have elapsed since the last corticosteroid administration 1
• Initiate or escalate topical NSAIDs (e.g., diclofenac gel) applied to the affected knee four times daily, as topical agents have favorable safety profiles 3, 1
• Add oral NSAIDs (naproxen 500 mg twice daily or meloxicam 7.5-15 mg daily) if topical therapy provides inadequate relief after 2-4 weeks 3, 1
• Limit repeat corticosteroid injections to no more frequently than every 3 months, with a maximum of 3-4 injections per joint per year to minimize cartilage damage risk 1, 4

Common Pitfalls to Avoid

• Do not assume this is simply recurrent osteoarthritis pain without excluding life-threatening conditions (DVT, pulmonary embolism) and joint-threatening conditions (septic arthritis) 1
• Do not perform repeat corticosteroid injection without first ruling out infection, as injecting steroids into an infected joint leads to catastrophic outcomes 1
• Do not dismiss the two-week timing as "too soon" for complications—both osteonecrosis and septic arthritis can present within this timeframe after injection 2
• Do not rely solely on the presence or absence of knee effusion to guide your differential diagnosis, as DVT may present without obvious joint findings 1

Follow-Up Strategy

• Schedule reassessment at 2 weeks and 4 weeks if conservative management is initiated, to evaluate treatment response and guide further therapy 1
• Consider hyaluronic acid injections as an alternative if corticosteroids are contraindicated or have failed, though effect sizes are modest 1, 4
• Refer to orthopedic surgery for consideration of total knee arthroplasty if pain remains refractory to all conservative measures and radiographic severity is advanced 4