Characterization and Biological Application of Hydroxyapatite-Glass-Titanium (HA-G-Ti) Functional Gradient Composite Material

Osteolysis has emerged as a common cause of loosening in cementless total hip arthroplasty. For the purpose of resolving osteolysis, plasma-sprayed hydroxyapatite (HA)-coated Ti implants were developed. However, delamination of HA has often occurred. There is a serious problem of adhesive quality between the HA layer and the substrate, mainly caused by the large difference in the linear thermal expansion coefficient between the substrate and HA. Bioactive composites comprising HA containing glass (G)-coated titanium (Ti) or titanium alloy (Ti-6A1-4V), so-called HA-G-Ti functional gradient composite materials, providing both the bioactivity potential due to the HA and the mechanical strength of titanium or alloy, were subjected to experiments under in vitro and in vivo conditions. In the former, as a characterization of HA-G-Ti composites, the structural features and properties, chemical stability, and electrochemical corrosion and then the bioactivity, involving the precipitation of apatite under physiological conditions, are summarized. In the latter, the experiments of implantation of the HA-G-Ti composites into mandibular bone, femur, and tibia of dogs were performed. The bonding behavior at the interface between the implants and the surrounding bone tissue, as well as the bonding strength and histocompatibility, have been extensively investigated. It is concluded that the HA-G-Ti composite is an excellent biomaterial, applicable in prostheses such as cementless artificial joints (HAPG-Profile). Clinical trials of HAPG-Profile are now going on.