A patient with a year-long history of knee osteoarthritis presents with a recent flare, sudden worsening pain one month ago without trauma, brief relief after an intra‑articular steroid injection 10 days ago, and now persistent severe pain; what is the most likely diagnosis and what investigations and management steps are indicated?

Diagnosis: Osteoarthritis Flare with Failed Steroid Response – Consider Septic Arthritis or Inflammatory Arthropathy

Given the failed steroid injection response (pain relief only lasting as long as the local anesthetic) and recurrent severe pain, you must immediately rule out septic arthritis before proceeding with any further management. The pattern described—initial brief relief followed by persistent severe pain despite corticosteroid injection 10 days ago—is a red flag that demands urgent investigation 1, 2.

Immediate Next Steps: Investigation

Urgent Joint Aspiration (Within 24 Hours)

• Perform arthrocentesis before any additional treatment
• Send synovial fluid for:
  • Cell count with differential (septic arthritis typically >50,000 WBC/mm³ with >90% neutrophils)
  • Gram stain and culture (aerobic and anaerobic)
  • Crystal analysis (to exclude pseudogout/gout mimicking OA flare)
• Check inflammatory markers: ESR, CRP, WBC count
• Blood cultures if fever or systemic symptoms present

The failure of corticosteroid injection is particularly concerning. While EULAR guidelines confirm that intra-articular corticosteroids provide short-term benefit (1-4 weeks) for OA flares 1, complete lack of sustained relief beyond the local anesthetic effect suggests either:

1. Septic arthritis (a known complication of intra-articular injections) 3, 4
2. Inflammatory arthropathy misdiagnosed as OA
3. Severe structural disease unresponsive to conservative measures

Additional Imaging

• Repeat knee radiographs to assess for:
  • Rapid joint destruction (concern with repeated corticosteroid exposure) 5, 6
  • Subchondral insufficiency fracture
  • Osteonecrosis
• Consider MRI if radiographs inconclusive and infection ruled out, to evaluate:
  • Bone marrow edema
  • Meniscal pathology
  • Occult fracture
  • Soft tissue abnormalities

Management Algorithm

If Septic Arthritis Confirmed:

• Immediate hospital admission
• Urgent orthopedic consultation for joint washout/arthroscopic debridement
• IV antibiotics (empiric coverage until culture results available)
• This is a medical emergency requiring aggressive intervention 3, 4

If Infection Ruled Out – Refractory OA Flare:

Step 1: Optimize Oral Analgesia

• NSAIDs (oral or topical) if not contraindicated—more effective than acetaminophen for inflammatory flares 2
• Consider glucosamine sulfate (1A evidence) and chondroitin sulfate (1A evidence) for symptomatic benefit 1

Step 2: Consider Hyaluronic Acid (HA) Injection

• HA provides longer-lasting pain relief (several months vs. several weeks with steroids) 1
• Evidence shows HA may reduce future flares and steroid injection requirements 1
• Better outcomes in patients with less severe disease and without effusion 1
• Recent 2025 data suggests HA associated with decreased OA progression compared to corticosteroids at 2-year follow-up 6
• Requires 3-5 weekly injections (logistical consideration) 1

Step 3: Orthopedic Referral for Surgical Evaluation

• Joint replacement must be considered given radiographic OA evidence and refractory pain/disability 1
• EULAR guidelines state: "Joint replacement has to be considered in patients with radiographic evidence of knee OA who have refractory pain and disability" 1
• Total knee replacement shows 89% good-to-excellent outcomes for pain and function at 5 years 1

Critical Pitfalls to Avoid

1. Do NOT repeat corticosteroid injection without ruling out infection first—this could be catastrophic if septic arthritis present 3, 4

2. Do NOT assume this is simple OA progression—the sudden worsening without trauma and failed steroid response pattern is atypical and warrants investigation 7

3. Avoid repeated corticosteroid injections—recent evidence shows association with accelerated OA progression, subchondral insufficiency fractures, and rapid joint destruction 5, 6, 8

4. Do NOT delay surgical referral in patients with severe symptoms unresponsive to conservative measures—prolonged disability significantly impacts quality of life 1

5. Recognize obesity as risk factor for injection complications and consider imaging-guided injection if repeat injection contemplated 4

Why This Patient Failed Steroid Injection

The evidence shows corticosteroids provide only short-term benefit (1-4 weeks maximum) 1, 9, but this patient had no benefit beyond local anesthetic duration. This suggests:

• Possible infection (must exclude)
• Severe structural disease beyond what corticosteroids can address
• Possible inflammatory arthropathy rather than pure OA
• Patient may be in the subset that doesn't respond to corticosteroids (predictors of response remain unclear per guidelines) 1

The priority is infection exclusion, followed by either HA trial or surgical evaluation depending on disease severity and patient factors 1, 7.