Concept and Fabrication of Beta-Type Titanium Alloy Rod with Parts Possessing Different Young’s Moduli for Spinal Fixation

This paper proposes the concept and fabrication process of titanium alloy rods for spinal fixation. A part of rod for fixing the lower side of the lumbar vertebra is strengthened, while the other part for fixing the upper side has low stiffness. The results obtained by finite element analysis reveal that a rod with partially lowered Young’s modulus has higher flexibility and fixity compared with a rod possessing high Young’s modulus throughout. Using Ti-29Nb-13Ta-4.6Zr alloys with oxygen contents of 0.2 and 0.4％ as the model alloys, rods with partially different Young’s moduli were fabricated by aging treatment at 723 K, followed by partial heating up to above the β-transus temperature and quenching by high-frequency induction heating (IH-treatment). A single β-phase, which has low Young’s modulus, is obtained by IH-treatment and has lower strength. With regard to the as-aged parts, the precipitated condition of the α-phase can be changed by varying the aging time. The obtained Young’s modulus and strength reflect this change. Near the boundary between the as-aged and IH-treated parts, the hardness is gradually changed, and it is possible to gradually soften the material from the as-aged part to the IH-treated part.