Affiliations: Orthopaedic Research Department, Smith & Nephew Richards Inc., 1450 E. Brooks Road, Memphis, TN 38116, USA
Abstract: To optimize the performance of total hip replacement, scientists and clinicians are seeking new materials and noncemented, press-fit designs that cam improve load transfer to the bone and reduce the incidence of loosening and thigh pain. Currently used Co-Cr-Mo alloy has a relatively high elastic modulus (E = 227 GPa), which limits its ability to transfer load to the surrounding bone in the proximal calcar region. Thus to improve load transfer, designs are considered with less cross-sectional area to increase flexibility, but at the expense of fit and fill, and thus stability of the implant within the bone. Should stem loosening occur, the stem stresses may exceed the relatively low fatigue strength of the Co-Cr-Mo alloy and lead to stem breakage. To improve these conditions, lower modulus Ti-6Al-4V alloy (E = 115GPa) is being used. More recently, a new lower-modulus (E = 79 GPa) Ti-13Nb-13Zr alloy has been developed which does not contain any elemental constituents associated with adverse cell response (i.e., Co, Cr, Mo, Ni, Fe, AI, V), and which possesses comparable or superior strength and toughness to existing Ti-6Al-4V alloy. The carefully selected Nb and Zr constituents improve bone biocompatibility and corrosion resistance compared to that of currently used implant metals. Additionally, a unique diffusion hardening (DR) treatment can be conducted during the age-hardening process of this near-beta alloy to produce a hardened surface with abrasion resistance superior to that of Co-Cr-Mo alloy. This also provides an improvement in the micro-fretting tendencies that may occur within femoral head-neck taper regions and modular interfaces of other implant designs. The present study describes the metallurgy and mechanical properties of this unique low modulus Ti-13Nb-13Zr alloy, and the heat treatments used to obtain the high strength, corrosion resistance, and surface hardening that renders this biocompatible alloy well-suited for press fit hip replacement applications. Because of the relatively lower beta transus (735°C), this alloy is also much easier to net shape forge into more complex stem designs.
