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Rapidly Progressive OA — SCE Rheumatology MCQ

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HardOsteoarthritisRapidly Progressive OASCE Rheumatology

A 68-year-old woman has radiographic medial-compartment knee osteoarthritis that had been clinically stable for 5 years. She develops sudden, atraumatic pain localised over the medial femoral condyle, particularly on weight-bearing. CRP is 3 mg/L. Synovial fluid contains 0.4 × 10^9 leucocytes/L, with no crystals, and culture is negative. Compared with radiographs taken 4 months earlier, there is rapid medial joint-space loss and slight flattening of the medial femoral condyle. MRI shows extensive bone-marrow-oedema-like signal in the condyle and a thin subchondral hypointense line parallel to the articular surface, with oedema extending to the subchondral plate. There is no geographic lesion with a serpiginous rim. Which diagnosis best explains the accelerated structural deterioration?

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Correct answer: E — Subchondral insufficiency fracture of the knee

The diagnosis is a subchondral insufficiency fracture of the knee (SIFK). The discriminating features are sudden atraumatic focal pain in an older adult and a subchondral hypointense line with extensive surrounding marrow-oedema-like signal on MRI. SIFK can progress to osteochondral collapse and rapid secondary joint-space loss. Lesions formerly labelled spontaneous osteonecrosis of the knee are now generally regarded as advanced SIFK or SIFK with osteonecrosis. Uncomplicated primary OA does not adequately explain the abrupt symptoms and fracture-line pattern. CPP arthritis and septic arthritis are undermined by the non-inflammatory, crystal-negative, culture-negative aspirate. Systemic-type osteonecrosis is less likely because it typically produces a geographic marrow lesion with a serpiginous rim rather than this subchondral fracture pattern.

Reference: Ochi J, Nozaki T, Nimura A, et al. Subchondral insufficiency fracture of the knee: review of current concepts and radiological differential diagnoses. Japanese Journal of Radiology. 2022;40:443–457. https://pubmed.ncbi.nlm.nih.gov/34843043/