Advances in Development of Titanium Alloys for Spinal Fixation Applications -Titanium Alloys with High Fatigue Strength and Low Springback for Spinal Fixation Applications-

The fatigue strength of the newly developed beta-type titanium alloy Ti-29Nb-13Ta-4.6Zr (referred to as TNTZ) is important for its applications in spinal fixation rods. Enhancements in the fatigue strength of TNTZ upon solution treatmnet and thermo-mechanical processing (aging after severe cold-caliber rolling or sever cold-swaging) have been investigated. Aging the rods at 723 K for 259.2 ks after cold-swaging is shown to be the optimal thermo-mechanical processing method for the TNTZ rod, yeilding a high 0.2% proof stress of about 1200 MPa, high elongation of 18%, and high fatigue strength of 950 MPa. High springback is undesirable for application of TNTZ spinal-fixation rods. To reduce springback, a High Young's modulus is required; on the other hand, a low Young's modulus is beneficial to reduce bone absorption and obtain good bone remodeling. To achieve a balance between these two factors, TNTZ has been modified by adding Cr to obtain an alloy with a high Young's modulus at the deformed region and a low Young's modulus throughout the undeformed region. TNTZ-8Ti-2Cr and TNTZ-16Ti-4Cr are novel alloys whose beta phase is more stable than that of TNTZ. Only the beta phase is present in these alloys, which exhibit relatively low Young's moduli of <65 GPa after solution treatment, and higher Young's moduli after cold rolling, owing to a deformation-induced omega-phase transformation. These modified TNTZ alloys show significantly less springback than the original TNTZ, based on tensile and loading unloading bending tests.