The present invention relates to low modulus biocompatible titanium alloys suitable for use as a material for a medical prosthetic implant, and in particular to a biocompatible titanium alloy comprising xcex1xe2x80x3 phase as a major phase and the process of preparing the same.
Concern has been raised from time to time about the stress shielding phenomenon, i.e., insufficient loading of bone due to the large difference in modulus between implant device and its surrounding bone. This phenomenon, more often observed in cementless hip and knee prostheses (Sumner et al., 1992), can potentially lead to bone resorption (Engh et al., 1988) and eventual failure of the arthroplasty (Sumner et al., 1992).
Both strain gauge analysis (Lewis et al., 1984) and finite element analysis (Koeneman et al., 1991) have demonstrated that lower modulus (more flexible) hips produce stresses and strains that are closer to those of the intact femur, and a lower modulus hip prosthesis may better simulate the natural femur in distributing stress to the adjacent bone tissue (Cheal et al., 1992; Prendergast et al, 1990). Canine and sheep implantation studies have shown significantly reduced bone resorption in animals with low modulus hips (Bobyn et al., 1992), and the bone remodeling commonly experienced by hip prosthesis patients may be reduced by a prosthesis having lower modulus (Bobyn et al., 1990 and 1992).
Titanium and titanium alloys have become one of the most attractive implant materials due to their light weight, high biocorrosion resistance, biocompatibility and mechanical properties, including low modulus. For example, the most widely used titanium alloy, Ti-6Al-4V, according to Pilliar (Pilliar, 1990), has an elastic modulus (108 GPa) only about half that of 316L stainless steel (200 GPa) or Coxe2x80x94Crxe2x80x94Mo alloy (210 GPa) that is still popularly used today.
Although alpha-beta type Ti-6Al-4V alloy is widely used as an implant material, studies have reported that the release of Al and V ions from the alloy might cause some long term health problems (Rao, 1996, Yumoto, 1992, Walker 1989, McLachlan et al., 1983). Moreover, the low wear resistance of Ti-6Al-4V could accelerate the release of such harmful ions (Wang et al., 1996, McKellop, 1990, Rieu, 1992).
Recently much research effort was devoted to the study of more biocompatible, lower modulus, better processability beta or near-beta Ti alloys, such as Ti-13Nb-13Zr (Mishra 1996), Ti-11.5Mo-6Zr-2Fe (Wang 1996) and Ti-15Mo (Zardiackas et al. 1996). The near-xcex2 Ti-13Nb-13Zr alloy issued to Davidson et al (U.S. Pat. No. 5169597, 1992), was reported to consist of hexagonal martensite phase under water-quenched condition. With subsequent aging, the bcc xcex2 phase was precipitated. This aged Ti-13Nb-13Zr alloy had a lower (by 30-40%) modulus than mill-annealed Ti-6Al-4V alloy (Mishra et al., 1996).
The xcex2 phase Ti-15Mo alloy is being evaluated for orthopaedic implant applications by Synthes USA. The rapidly quenched Ti-15Mo alloy was reported to have a fine-grained bcc structure with a lower modulus (77.7 GPa) than those of 316L stainless steel, Grade IV Ti, Ti-6Al-4V and Ti-6Al-7Nb (Zardiackas et al., 1996).
It is therefore an object of the invention to provide a biocompatible titanium alloy having even lower modulus, equivalent strength and appropriate hardness, suitable for use in a wide range of medical implant applications.
It has been found by the inventors that titanium alloy comprising from about 5 to about 10 wt % of molybdenum if subject to fast cooling can induce a significant amount of xcex1xe2x80x3 phase. The titanium alloy having a martensite structure xcex1xe2x80x3 phase exhibits a desirable combination of properties, i.e. low modulus of elasticity, equivalent bending strength and appropriate hardness.
It has also been found by the inventors that addition of alloying elements, i.e. niobium and zirconium into the Tixe2x80x94Mo system can increase the bending strength while maintaining the low modulus of elasticity.
Specifically, the biocompatible titanium alloy of the invention includes from about 5 to about 10 percent by weight of molybdenum, from 0-3 percent by weight of an alloying element and the balance titanium. The alloying element is niobium (Nb) or zirconium (Zr) or the mixture of the two elements.
In case small size or complicated prosthetic implants are fabricated, the alloy is first melted at a temperature greater than 1750xc2x0 C. The molten titanium alloy is then directly cast into a mold of desired shape in a vacuum or inert atmosphere, with a cooling rate greater than 10xc2x0 C./second.
In case simple shape prosthetic implants are fabricated, the alloy is first subjected to cold or hot working, including rolling, drawing, extrusion or forging, followed by annealing at a temperature of 600-1200xc2x0 C. and fast cooling at a cooling rate greater than 10xc2x0 C./second to obtain the xcex1xe2x80x3 phase.
According to an aspect of the invention, the titanium alloy preferably comprises 6-9 wt % of molybdenum. The Ti-7.5 wt % Mo alloy exhibits very low modulus of elasticity, very high springback capability, equivalent bending strength and appropriate micorhardness. Specifically, the Ti-7.5 wt % Mo alloy shows a bending modulus of 55 GPa which is closer to the modulus of human long bone, 20 GPa.
According to another aspect of the invention, when 1 wt % of Nb or Zr is added, the acicular martensitic structure of xcex1xe2x80x3 phase of the titanium alloy remains, microhardness increases 25-29%, bending strength increases 13-21% while the modulus and springback capability only slightly change.