On the tissue response to hafnium, titanium and thin calcium phosphate coatings

Abstract

Background: Titanium (Ti) is the gold standard material for load-bearing implants. Although high success rates have been reported for Ti implants, failures do occur. Alternative materials/surface modifications might further improve the performances of Ti implants, particularly in situations with non-optimal local tissue conditions. Hafnium (Hf) is a metal closely related to titanium, but harder and its biocompatibility has not been evaluated. Calcium phosphate (CaP) coatings have been suggested to speed up bone healing. Despite that various types of CaP coatings have been used, evidence of their long-term behaviour and clinical superiority is still lacking. Aims: To evaluate the biological performance of Hf and thin magnetron sputtered CaP coatings with different properties in comparison with commercially pure (cp) Ti implant. Materials and Methods: Forty-two plugs and 36 screw-shaped implants of Hf and Ti were inserted in the abdominal wall of 21 rats for 8 days, 6 and 12 weeks, and in the cortical bone of 18 rabbits for 6, 12 and 24 weeks. In addition, 800 implants of noncoated titanium and magnetron sputtered thin CaP coatings with different thicknesses (100 nm and 2.0 µm), and crystallinity, were inserted in the cortical and trabecular bone of 100 rabbits for 1, 3, 6 weeks and 9 months. The implant surfaces were characterized using SEM, stylus profilometry, TopScan 3D, XRD, ICP-OES, and FTIR-ATR. The biological evaluation (fibrous capsule thickness and presence of inflammatory cells (in rats), and bone contact and bone area (in rabbits) was performed using light microscopy. Results: Hf showed to have a smoother (Sa=0.43-0.57 µm) surface than Ti (Sa=0.75 µm). The surface topography (Sa) of noncoated and CaP coatings varied in the range of 0.3-1.2 µm. The heat treatment of the coatings increased the degree of crystallinity of the coatings. The Ca/P ratio was 1.665 and the coatings showed absorption bands characteristic for hydroxyapatite. No significant differences were observed between Hf and Ti in neither implantation sites. After 6 weeks and 9 months, both crystalline calcium phosphate coatings had significantly higher bone contact than noncoated and amorphous CaP in both bone types. Noncoated Ti and both amorphous CaP coatings showed significantly higher bone area in their threads in both implantation sites. Summary and Conclusions: Hf showed a biocompatibility comparable to Ti. Hf might be an interesting metal as load-bearing implant, when a harder metal than Ti is desired. The 100 nm thin crystalline CaP coating induced an improved early bone apposition/bone contact that was maintained over a long time without inducing adverse inflammatory bone reaction. Submicron crystalline CaP coating may be an interesting surface modification since it adds bioactive property to the bioinert Ti. Clinical trials are needed to assess the clinical efficacy of such coatings.