Which type of synovial fluid crystal is bipyrimidal (irregular) and positively birefringent, seen in hyperparathyroidism or myositis ossificans?

• A. Hydroxyapatite
• B. Monosodium urate
• C. Calcium pyrophosphate dihydrate
• D. Calcium oxalate

Answer: (D)

Recognizing synovial fluid crystals by their appearance under polarized light is how we distinguish crystal-induced arthritis. The crystals described as irregular rhomboid shapes that are positively birefringent fit calcium pyrophosphate dihydrate specifically. This combination—a rhomboid or bipyrimidal form and weak positive birefringence—is classic for CPPD crystals, which are associated with metabolic conditions such as hyperparathyroidism. The presence of these crystals helps explain a pseudogout picture rather than gout or other crystal arthropathies.

For comparison, monosodium urate crystals are needle-shaped and strongly negatively birefringent, which gives a blue color when aligned with the slow axis of the compensator. Hydroxyapatite crystals are typically too small to be reliably seen with standard polarized light microscopy and are not characterized by this positive birefringence pattern. Calcium oxalate crystals are not the usual presentation in synovial fluid crystal arthropathies and don’t fit the described rhomboid, positively birefringent pattern.