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Milwaukee Shoulder BCP — SCE Rheumatology MCQ

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HardCrystal ArthropathyMilwaukee Shoulder BCPSCE Rheumatology

A 78-year-old woman has gradually progressive loss of movement of her right shoulder with relatively little pain. Examination shows a large cool effusion and marked glenohumeral instability. Aspiration yields blood-stained fluid with a leucocyte count of 900 × 10^6/L; Gram stain and bacterial cultures are negative. Compensated polarised microscopy shows occasional calcium pyrophosphate crystals, and specialist analysis confirms abundant basic calcium phosphate crystal aggregates. Imaging demonstrates a massive rotator-cuff tear, superior migration of the humeral head and rapidly progressive glenohumeral cartilage and subchondral bone destruction. What is the most likely diagnosis?

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Correct answer: B — Milwaukee shoulder syndrome (BCP-associated destructive arthropathy)

This is Milwaukee shoulder syndrome, a rapidly destructive arthropathy associated with intra-articular basic calcium phosphate, usually hydroxyapatite, deposition. The discriminating combination is an older woman with limited pain, a large cool haemorrhagic low-cell-count effusion, massive rotator-cuff failure and rapid glenohumeral destruction. CPP crystals may coexist but do not make CPP crystal arthritis the primary diagnosis. Non-crystal rotator-cuff tear arthropathy can produce superior humeral migration but does not explain the characteristic BCP-rich haemorrhagic effusion. Rheumatoid arthritis would usually have inflammatory synovitis and an appropriate systemic or polyarticular context. Osteonecrosis causes humeral-head collapse without this crystal-associated effusion and cuff phenotype. Septic arthritis is generally painful and inflammatory, with a substantially raised synovial leucocyte count; cultures here are negative.

Reference: Rosenthal AK. Basic calcium phosphate crystal-associated musculoskeletal syndromes: an update. Current Opinion in Rheumatology. 2018;30(2):168–172. https://pubmed.ncbi.nlm.nih.gov/29227355/