Hydroxylapatite

Hydroxylapatite, often spelled hydroxyapatite, is a calcium phosphate mineral of the apatite group with the idealized formula Ca10(PO4)6(OH)2, commonly written Ca5(PO4)3OH. It is the principal inorganic component of bone mineral and tooth enamel in vertebrates, where it forms a crystalline phase that provides stiffness and a scaffold for mineralization.

In its pure form it is a hexagonal crystal with channels along the c-axis that house hydroxide ions; natural material often contains carbonate and other substitutions that modify its properties.

Substitutions: The lattice readily accommodates carbonate for phosphate, fluoride for hydroxide, and trace ions such as chloride, sodium, and magnesium. Such substitutions occur in biological apatite and synthetic variants and influence solubility, crystallinity, and bioactivity.

Occurrence and production: Hydroxylapatite occurs naturally in bone and teeth and can be synthesized by precipitation from calcium and phosphate solutions, often at near-neutral pH. Common methods include wet chemical precipitation, hydrothermal synthesis, and sol-gel processing.

Applications: It is used as a bioceramic material for bone grafts and as coatings on metal implants due to osteoconductivity and biocompatibility. It is also used in dental products for remineralization and as a filler in composites.

Biocompatibility and safety: Hydroxylapatite is generally regarded as biocompatible and supports bone ingrowth; crystallinity, porosity, and substitutions can affect resorption rates.