Counterpoint: Hydroxyapatite crystal deposition is not intimately involved in the pathogenesis and progression of human osteoarthritis.

The association of hydroxyapatite deposition with osteoarthritis pathogenesis and progression remains controversial, even after decades of study. Hydroxyapatite crystals are found in osteoarthritis in advanced disease only. Even then, hydroxyapatite crystals are found in such small amounts that special analytical techniques are required to detect the crystals. Further, the osteoarthritic joint fluid appears noninflammatory, suggesting that such hydroxyapatite crystals have very small effect on the pathologic process. Formal histopathologic evidence is scant, but hydroxyapatite crystal deposition within osteoarthritic synovium or cartilage is a rare event. Hydroxyapatite crystals may be present at end-stage osteoarthritis, but in insufficient amounts to contribute significantly to osteoarthritis pathogenesis or progression. This review critically examines the evidence from osteoarthritic synovial fluids, imaging, and histopathology to determine whether the well-characterized in vitro cellular reactions to hydroxyapatite apply to the pathogenesis of human osteoarthritis.