This patient has Milwaukee shoulder syndrome, caused by basic calcium phosphate deposition. Milwaukee shoulder syndrome is characterized by symptoms of pain, stiffness, and swelling that tend to occur gradually over time, often with a preceding trauma or history of overuse on the affected side; this entity is classically described in women over the age of 70 years. This patient has typical features of the syndrome, including a large effusion on examination and synovial fluid that is blood tinged with a low leukocyte count. The crystals are often not visible by routine light microscopy due to their small size, and they are not birefringent (hence, not appreciated with a polarizing microscope); however, they may be revealed with alizarin red staining (with crystals visualized as red, globular clumps). This patient's shoulder radiograph reveals narrowing of the glenohumeral joint, calcification of the periarticular cartilage, and erosive changes of the humeral head, all typical findings in this syndrome. Upward subluxation of the humeral head due to rotator cuff destruction and bony cysts are also common.

Acute calcium pyrophosphate crystal arthritis (also known as pseudogout) can lead to significant joint swelling; however, the absence of inflammatory synovial fluid makes this diagnosis unlikely.

This patient has a history of gout; however, her current symptoms are atypical for a gout attack, particularly the gradual onset of these symptoms. The synovial fluid is also very uncharacteristic for gout, which typically causes an inflammatory effusion. The absence of crystals in the synovial fluid further speaks against gout, although monosodium urate crystals are sometimes missed on synovial fluid analysis.

Monoarticular joint swelling must always raise suspicion for an infected joint; however, in this case, the slow onset of symptoms, lack of fever, and normal serum leukocyte count go against this diagnosis. Synovial fluid with low leukocyte count and negative Gram stain also point away from a diagnosis of an infected joint.