Hydroxyapatite: A major component and an essential ingredient of normal bone and teeth. Hydroxyapatite makes up bone mineral and the matrix of teeth. It is hydroxyapatite that gives bones and teeth their rigidity.

Hydroxyapatite molecules can group together (crystalize) to form microscopic clumps. If these tiny crystals of hydroxyapatite are deposited by mistake in or around joints, they may cause inflammation of the joints and nearby tissues, such as tendons and ligaments, particularly causing rotator cuff problems in the shoulder. The inflammation caused by hydroxyapatite crystals has been referred to as hydroxyapatite crystal disease.