hydroxyapatite
/ hy-drox-ee-AP-uh-tite /
Hydroxyapatite is the calcium-phosphate ceramic that your own body is already made of: it is the hard mineral, about two-thirds of bone by weight and the bulk of tooth enamel, that stiffens your skeleton and lets you bite. Its formula is Ca10(PO4)6(OH)2 — a calcium phosphate crystal with a hydroxyl group. Because it is chemically almost identical to bone mineral, synthetic hydroxyapatite is the go-to bioceramic for coating implants and filling bone defects: living bone recognises it as one of its own and bonds directly to it rather than walling it off as foreign.
This bonding to living tissue is what makes hydroxyapatite bioactive, in contrast to a bioinert ceramic like pure alumina that the body merely tolerates behind a scar-like capsule. When hydroxyapatite is implanted, its surface chemistry is so close to natural bone mineral that bone-forming cells attach, lay down new bone directly on it, and the new bone becomes continuous with the implant — a true chemical bond, not just a mechanical grip. That is why a titanium hip stem or dental implant is often sprayed with a thin hydroxyapatite coating: the metal gives the strength, and the hydroxyapatite skin coaxes the surrounding bone to grow fast onto the implant and lock it in. In porous block form, hydroxyapatite also serves as a bone-graft substitute and scaffold, its interconnected pores acting as a trellis that new bone and blood vessels grow into.
The honest limits keep it in a supporting role. Dense hydroxyapatite is a ceramic — brittle, weak in tension, and far too fragile to carry the loads a hip or a jaw sees, so it is used as a coating or a filler, not as a load-bearing implant on its own. As a coating it must be thin and well bonded, or it can crack or peel. And bioactivity cuts both ways: the very solubility that lets bone integrate means some calcium-phosphate phases slowly dissolve or resorb in the body, so formulations are tuned — sometimes toward more soluble tricalcium phosphate to encourage resorption and replacement by natural bone, sometimes toward stable hydroxyapatite to stay put. It is the material that lets metal and bone become friends, but it does not replace the metal's job of bearing load.
A titanium dental implant is plasma-sprayed with a thin hydroxyapatite coating before it is screwed into the jaw: the metal root bears the chewing load, while the HA skin, chemically matched to bone mineral, invites the surrounding bone to grow directly onto it and fuse the implant in place over a few months.
Hydroxyapatite is bone's own mineral — bioactive, so bone bonds straight to it; used as an implant coating and bone-graft filler, but too brittle to bear load alone.
Hydroxyapatite bonds beautifully to bone but is a brittle ceramic — it cannot carry structural load on its own. It works as a thin coating on a strong metal implant or as a porous scaffold, letting the body do the integrating while the metal does the load-bearing.