Hydroxyapatite grafting promotes new bone formation and osseointegration of smooth titanium implants

Titanium is the ideal metal for intra-osseous dental implants. It permits the natural formation of an oxide layer on its surface and thereby it prevents the release of potentially toxic molecules. New formation of bone around implants, partially placed into the bone marrow cavity, is a gradual process that runs from the endosteum to the surface of the implant. Deposition of hydroxyapatite crystals on collagen type I fibrils is initiated by acidic proteins and leads to bone mineralization. This study analyzed the effects of hydroxyapatite upon peri-implant bone formation after insertion of smooth titanium implants. Screw-shaped smooth titanium implants of 3.75 mm thickness and 8.5 mm length were inserted into the metaphysis of rabbit tibia, either together with bovine hydroxyapatite into the right tibia or in controls without hydroxyapatite into the left tibia. Polyfluorochrome tracers (alizarin complex, calcein, tetracycline) were injected subcutaneously at different time intervals after implantation to evaluate the time frame of bone new formation over a period of 8 weeks. All samples were processed for histology and analyzed by fluorescence and polarizing microscopy. Our results showed a higher quantity of mature type I collagen fibers around implants and an acceleration of bone formation in the presence of hydroxyapatite. Mainly immature organic matrix was formed at the surface of implants in controls. The presence of hydroxyapatite seems to promote the maturation of collagen fibers surrounding the titanium implants and to support osteoconduction. Moreover, new formation of bone was faster in all samples where implants were inserted together with hydroxyapatite.