Hydroxyapatite precipitation on Na implanted titanium

Objectives: Titanium is known for its good biocompatibility as an implant material. But a structural integration of a foreign material into the body is known only for hydroxyapatite because of its osteoconductive properties. Surface coating of titanium with hydroxyapatite by means of plasma spraying is the current technique to improve the osteointegration of implants. Problems with this method are the high thickness and limited adhesion of the apatite coating on titanium. Further there are changes in crystallinity of the apatite due to the heating process. A surface preparation that spontaneously induces hydroxyapatite nucleation on its surface is desired.
Methods: By ion beam implantation of Na (3 x 1017 ions cm-1, 50 keV) in titanium a sodium titanate surface was created. Hydrolysis of this layer in deionidised water resulted in formation of TiOH. By incubation in simulated body fluid a hydroxyapatite layer was precipitated on the surface. The human osteosarcoma cell line was seeded out on samples after hydrolysis and after hydroxyapatite precipitation. Reference samples were untreated or activated by soaking in hot NaOH. After 3, 14, and 28 days samples ware investigated for cell growth, alkaline phosphatase activity and calcification.
Results: Cell growth on Na implanted hydrolysed samples was decreased compared with pure titanium at any time. After hydroxyapatite precipitation cell growth after two weeks and four weeks was significantly higher on Na implanted samples than on untreated or NaOH treated samples. Without previous hydroxyapatite deposition only on NaOH treated samples a production of hydroxyapatite could be detected, after hydroxyapatite precipitation there was also better calcification by the cells. Alkaline phosphatase did not differ significantly in the cells on the different substrates.
Conclusion: Na ion implantation in titanium can activate the surface for hydroxyapatite nucleation out of aqueous solutions. The surface without further treatment is too reactive for direct cell contact. After a calciumphosphate deposition on the surface the growth of bone forming cells is enhanced.