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The release of metal ions from the Ti-15Zr-4Nb-4Ta alloy in pseudo body fluids was compared with those from Ti-6Al-4V and vanadium-free Ti-6Al-7Nb alloys widely used as implantable titanium alloys throughout the world, in order to choose an optimum acceleration solution for immersion testing. Bone plates, artificial hip joints of the cementless type and artificial tooth implants were experimentally fabricated using the Ti-15Zr-4Nb-4Ta alloy. The quantities of titanium ions released from the titanium alloys into phosphate-buffered saline, α-medium and fetal bovine serum were very small, and much lower than those released into 1.2 mass% L-cysteine, 0.05 mass%HCl and 1 mass% lactic acid solutions with lower pH values than the phosphate-buffered saline and α-medium. It was suggested that 1 mass% lactic acid solution was promising as an acceleration solution for immersion test. The quantities of titanium ions released from the Ti-15Zr-4Nb-4Ta alloy into fetal bovine serum, 1.2 mass% L-cysteine, 0.05 mass%HCl and 1 mass% lactic acid solutions were approximately 30% of those of titanium ions released from the Ti-6Al-4V alloy. The total quantity of zirconium, niobium and tantalum ions released from the Ti-15Zr-4Nb-4Ta alloy was much smaller than that of elements released from the Ti-6Al-4V and Ti-6Al-7Nb alloys. Bone plates, artificial hip joints and artificial tooth implants were successfully fabricated with the Ti-15Zr-4Nb-4Ta alloy using conventional manufacturing processes. The Ti-15Zr-4Nb-4Ta alloy with its excellent corrosion resistance is expected to become the preferential titanium alloy for implant applications in the future.

To develop orthopedic implants that are optimized for each patient’s needs or skeletal structure (custom-made implants), evaluations of the bending strength, bending stiffness, and durability of various types of conventional osteosynthesis devices have become important. Four-point bending tests and compression bending tests of osteosynthesis devices (bone plates, intramedullary nail rods, spinal rods, compression hip screws (CHSs), short femoral nails, and metaphyseal plates) were carried out to measure their bending stiffness, bending strength, and durability. The bending stiffness of bone plates, intramedullary nails, spinal rods, CHSs, short femoral nails, and metaphyseal plates increased with increasing bending strength. The durability limit of various types of osteosynthesis devices linearly increased with increasing bending strength. The relationship (durability limit at 106 cycles) = 0.67 × (bending strength) (N·m) (R2 = 0.85) was obtained by regression. The relationship for the highly biocompatible Ti-15Zr-4Nb-4Ta alloy was also linear. The mechanical strength and ductility of specimens that were cut from various osteosynthesis devices were excellent and their microstructures consisted of fine structures, which were considered to be related to the excellent durability. These results are expected to be useful for the development of implants suitable for the skeletal structure of patients using three-dimensional (3D) layer manufacturing technologies.

Ti, Zr, Nb, and Ta are biocompatible elements. A Ti-15Zr-4Ta-4Nb alloy for medical implants is being developed. In terms of in vitro cytocompatibility, the new bone tissue response to the Ti-15Zr-4Ta-4Nb alloy determined through rat tibia implantation was equal to or better than that to the Ti-6Al-4V alloy. An acceptable level of biological response can be expected when this alloy is used appropriately because it consists of biocompatible elements and has excellent biocompatibility, corrosion resistance, microstructure, and mechanical and fatigue properties. Therefore, the Ti-15Zr-4Ta-4Nb alloy with a low metal release rate is considered advantageous for long-term surgical implants.