Bioactivity of a sol-gel-derived hydroxyapatite coating on titanium implants in vitro and in vivo

Background: Hydroxyapatite is widely used as a coating on metallic implants to promote bioactivity. The coating is typically produced using a high temperature, resulting in phase heterogeneity and coating delamination, which may lead to failure of the coating clinically. Development of a simple and low-temperature hydroxyapatite coating technique may improve the bone bonding ability of implants. Objectives: To investigate responses to hydroxyapatite-coated titanium produced by a newly developed sol-gel by osteoblasts in vitro and bone in vivo. Methods: Osteoblast proliferation was characterized using a methyl thiazolyl tetrazolium assay and cell calcification with an Alizarin red S assay, and the results were compared with those of uncoated titanium. Uncoated and coated screws were inserted into the trabecular bone of New Zealand white rabbit legs. These implants were evaluated mechanically and histologically after 7, 12, and 24 weeks. Results: Hydroxyapatite-coated titanium showed a significantly greater cell proliferation and mineralization than uncoated titanium. Extraction torques for the coated screws increased with time of implantation and were significantly greater than those of uncoated screws. We observed bone fragments attached to the surface of all coated screws after removal, but none on uncoated screws. Hematoxylin and eosin-stained bone showed no active inflammatory responses to implantation at any time examined. lone surrounding either uncoated or coated screws followed typical remodeling stages, but maturation of bone healing was faster with coated screws. Conclusions: The sol-gel-derived hydroxyapatite coating showed bioactivity, indicating its potential application as an alternative coating technique to improve the bone bonding ability of implants.