Metal treatment – Process of modifying or maintaining internal physical... – Carburizing or nitriding using externally supplied carbon or...
Reexamination Certificate
1999-05-05
2001-05-29
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Carburizing or nitriding using externally supplied carbon or...
C148S422000, C420S426000, C428S698000, C433S003000, C433S008000, C606S076000
Reexamination Certificate
active
06238491
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to niobium alloys suitable for use in the construction of dental and other medical implants, and devices. More specifically, this invention relates to a bio-compatible, corrosion-resistant niobium-based NbTiZrMo alloy for use in medical device applications which can be cold-worked to a high level of strength and flexibility for improving performance of orthodontic, endodontic, dental, orthopaedic, cardiovascular, and other medical implants and devices.
BACKGROUND OF THE INVENTION
Niobium is a corrosion-resistant material which can be expected to be biocompatible in the living body (A. Yamamoto, et. al., J. Biomed. Mater. Res., 1998). Niobium (Nb), in fact has been added as an alloy constituent inntitanium alloys currently used in medical implant and device applications, such as Ti13Nb13Zr and Ti7Nb6Al (protasul 100™). But niobium in the unalloyed condition is a relatively low-strength metal with a tensile strength ranging as high as 94 ksi and as low as 13 ksi (
Materials Engineering
, Materials Selector, 1989, Pentum Pub., Inc. Cleveland, Ohio, December 1988). Typically the strength of niobium lies between 48 and 85 ksi, thus, although a biocompatible metal, niobium has not been recognized as an implant metal.
A niobium alloy is an alloy with niobium and at least one other element, and where the niobium represents an equal or majority amount, by weight in the alloy. Niobium, when alloyed with titanium or zirconium can produce, in the proper concentration, a superconducting metal, and is thus used primarily in non-medical applications. The presence of zirconium (Zr) in niobium results in higher mechanical properties. The addition of titanium (Ti) to niobium reduces the melting temperature making it easier to process. The presence of titanium in niobium can also improve corrosion resistance, particularly in lower pH environments (Prourbaix, 1984). Niobium with moderate levels of titanium tends to maintain a more ductile, readily cold-workable, and easy to process alloy. This combination tends to stabilize a desired beta phase at room temperature. Titanium and zirconium also reduce the melting temperature of the Nb alloy, and improves the ability of molybdenum to mix during melting. Because Nb and Zr are closer in density to Mo, the Mo will have a reduced tendency to segregate during melting compared to Mo in titanium alloys. Further the addition of zirconium in sufficient quantity helps to further stabilize the desired beta phase, as well as improving strength. The zirconium also produces a more stable passive surface oxide to improve corrosion resistance and also allows for conversion oxidation or nitridization of the surface. Such conversion processes can produce a hard, abrasion-resistant, inert, oxide or nitride ceramic surface layer. The amount of Nb preferred in the present invention is between about 29 to 70 weight percent, and with Zr between about 10 and 46 weight percent. Finally, the presence of molybdenum in titanium can improve corrosion resistance in chloride-containing environments, when present at levels above about 3 weight percent and less than about 15 weight percent. Because the invention niobium alloy contains titanium, an amount of Mo is also included in the invention NbTiZrMo alloy system. Further, Mo is a strong stabilizer of the beta phase and its presence further assures the strong tendency for a stable, desirable beta phase to form at 500° C. or below.
Considering the metallurgical factors, the present invention describes a niobium alloy containing titanium, zirconium, and molybdenum for use in medical implants and devices. The composition specifically avoids the use of known allergens, toxins, or carcinogens, such as Ni, Cr, Co and V, and avoids aluminum which has been associated with an adverse interference of neurological processes. Other less bio-compatible elements such as tin, iron, and copper are also excluded from the composition of the present invention Nb alloy to optimize corrosion resistance and bio-compatibility.
The usefulness of the invention NbTiZrMo alloy is to develop a stable corrosion-resistant, beta structure which is tough, and which can be strengthened from cold-working, and will additionally result in a reduction of elastic modulus from the cold-working process. Further, the presence of a uniform beta phase (versus alpha plus beta) at about 500° C. improves the uniformity and effectiveness of conversion oxide surface layers found at that temperature. Thus the resultant NbTiZrMo alloy can be processed to be tough and fracture resistant or high-strength and flexible (low modulus) for medical device applications requiring strong, resilient materials such as orthodontic arch wire and other orthodontic devices, endodontic dental files for root canals, trauma and spinal plating and screws, dental implants and posts, pacing leads, vascular stents, and other medical devices. Higher strength and greater flexibility improve the resistance of such devices to breakage, and can improve load transfer to adjacent tissue.
Specifically, the invention niobium alloy is a NbTiZrMo alloy with no other alloy constituent exceeding the amount weight percent of niobium, and comprising a combination between about 10 and 46 percent of Zr, and about 3 to 15 weight percent Mo, and the balance titanium. Contaminants, each, less than about 1 percent, can be present, not exceeding a total of about 3 weight percent, and include Si, P, Cu, Fe, Ta, Hv Sn, or Pd.
The author is not aware of any specific niobium-molybdenum alloys, particularly NbTiZrMo alloys currently used in or described for medical device applications. However there are a variety of titanium and zirconium alloys currently in use or that have been proposed, and which may include Nb or Mo in the composition. These alloys are described in the following sections:
Examples of titanium alloy used for medical devices, include a low-modulus, room temperature beta titanium alloy for orthodontic arch wire as described in U.S. Pat. No. 4,197,643. This patent describes the use of Mo, Nb, Ta additionally, the use of Mn, Fe, Cr, Co, Ni, Cu, Al and Zr. However this is a titanium-based alloy material and not a niobium alloy. Alloy strength is achieved by aging to precipitate the alpha phase or cold working. The preferred composition is Ti-11.5Mo-6Zi-4.5Sn, commonly called Beta III a (or TMA) and which does not even contain niobium. U.S. Pat. No. 5,312,247 describes a shape-memory or super elastic alloy having a predetermined activation temperature for use in orthopedic applications. This patent further describes the use of nickel-titanium based and titanium-molybdenum based alloys but as in the previous example, is not a niobium alloy. The use of nickel-containing metals is undesirable, not only in orthodontics, but in most medical device applicant, and even jewelry, due to the common occurrence of nickel sensitivity of patients. The applicants are unaware of any niobium-based alloys with shape memory properties, at least at temperatures useful in the human body. Nitinol is a commonly used Ti—Ni alloy with shape memory behavior that is used in many types of medical device applications. However, this highly elastic alloy is less than optimum with respect to other alternative available titanium alloys or the invention niobium alloy because the high concentrations of nickel interfere with the corrosion resistance properties of the alloys and the presence of the nickel can induce a sensitivity problem. Additionally nickel can interfere with magnetic resonance imaging quality. U.S. Pat. No. 5,232,361 and reissue Re35,863 are directed to an orthodontic bracket formulated of at least one of a group of alloys based on Ti, Zr, Si, B, Be, Cr, Nb and Co in a composition in which at least one of these elements exists in a range of between 40 weight percent and greater than 99 weight percent. Mo is not included in the preferred composition as it is in the present invention, and a Ti-based orthodontic bracket containing at least 45 weight percent titanium is given as an e
Davidson James A.
Tuneberg Lee H.
Combs-Morillo Janelle
DaviTech, Inc.
Fulbright & Jaworski LLP
Wyszomierski George
LandOfFree
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