Apatite, hydroxyapatite, Ca₁₀(PO₄)₆OH, basic calcium phosphate, octacalcium phosphate, tricalcium phosphate, dicalcium phosphate dihydrate, carbonate-substituted apatite.

Basic calcium phosphates, in particular hydroxyapatite, are among the most common crystals seen in articular fluid and periarticular tissues. Unlike calcium pyrophosphate dihydrate and monosodium urate crystals, which cause specific clinical syndromes characterized primarily by destructive, inflammatory arthritis, hydroxyapatite crystal deposition can complicate a variety of disorders ranging from scleroderma to the shoulder arthritis of syringomyelia, or may itself cause erosive arthritis, as seen in Milwaukee shoulder or Philadelphia finger.

Hydroxyapatite crystals are generally small (50–500 nm) and rod-shaped. Due to their size (found typically as 1–5 μm clumps of crystals) and chemical properties, they are not visible with polarizing microscopy. Hydroxyapatite clumps stain red with...