Effect of fluoride substitution on the biocompatibility of hydroxyapatite

Hydroxyapatite (HA) and 0.2 and 0.3 wt% fluoride-substituted apatite (F-HA) were studied using in vitro testing by both immersion in simulated body-fluid and direct cell culture on ceramic substrates. X-ray diffraction and scanning electron microscopy revealed the formation of a bonelike apatite layer on F-HA discs after only 3 days immersion in simulated body-fluid, compared to 14 days on HA discs. The increased activity of the substituted apatite was confirmed by in vitro assessment using a primary human osteoblast-like (HOB) cell line. HOB cells cultured on the substituted apatites demonstrated elevated levels of tritiated thymidine incorporation compared to pure HA. However, alkaline phosphatase expression after 7 days in vitro was found to be independent of substrate composition, indicating that fluoride substitution did not affect the phenotypic expression of the HOB cells. Scanning electron microscopy of the cultures after 1 day in vitro demonstrated formation of a bone-like apatite layer on the surface of the 0.3wt% F-HA, and a protein-like layer was identified on both 0.2 and 0.3 wt% F-HA discs after 3 days in vitro.