Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2001-07-24
2004-04-27
Lucchesi, Nicholas D. (Department: 3624)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S022150, C623S023500, C623S023550
Reexamination Certificate
active
06726725
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to metallic orthopedic implants with load bearing surfaces coated with a thin, dense, low friction, highly wear-resistant coating of oxidized zirconium. The devices of the present invention possess two or more of oxidized zirconium surfaces in which the oxidized zirconium surfaces contact one another in an articulating (i.e., load bearing) or non-articulating (non-load-bearing) manner. Because these high hardness coatings are resistant to galling and fretting, they are particularly useful on the articulating portions of these prostheses. Typically, such prostheses with articulating surfaces were constructed of materials of different hardness. By having one “yielding” surface, such prior art devices eventually form an optimal fit, i.e., a tight tolerance, whereby galling, fretting, and other erosive phenomena are minimized, resulting in longer-lasting prosthetic devices. An example of these early-generation devices is the femoral head of a hip-stem prosthesis which engages a counter-bearing surface in an acetabular cup which is often made of a softer material such as ultra-high molecular weight polyethylene. However, use of contacting surfaces of varying hardness has disadvantages avoided by the unique properties of the present invention. The softer surface is, by nature sacrificial; it will eventually fail, the trade-off being realized in an overall increase in the useful life of the prostheses. Additionally, fretting of the softer surface results in debris that may have deleterious effects on health on the patient. The invention described herein is a particular type of ceramic-on-ceramic prosthesis; its unique compositional properties affords the traditional advantages of ceramic-on ceramic systems while avoiding their major disadvantage.
While efforts have been made to fabricate prosthetic devices having contacting surfaces of the same material, these efforts have had limited success in overcoming the deficiencies of the prior art. By taking advantage of the unique nature of the blue-black or black oxidized zirconium, the instant invention teaches a prosthetic device having contacting surfaces (either articulating or non-articulating) of the same material that dispenses with the need for a yielding or sacrificial surface, yet keeps wear to a minimum. The basic technology upon which the improvement described herein is based, is described in U.S. Pat. No. 5,037,438 to Davidson and pending application 09/381,217, filed Nov. 24, 1999 of Hunter, et al., both of which are fully incorporated by reference herein.
The invention also relates to medical implants having contacting oxidized zirconium surfaces and also having oxidized zirconium coatings on the non-articulating surfaces of an orthopedic implant where the latter oxidized zirconium surfaces provides a barrier between the metallic prosthesis and body tissue thereby preventing the release of metal ions and corrosion of the implant. Additionally, this oxidation process and the associated increase in surface oxygen content and hardness increases the strength of the metal substrate and improves the fatigue properties of the implant.
The longevity of medical implant devices is of prime importance as it is desirable that the implant should function for the complete lifetime of a patient. This is particularly true if the patient is young and the number of surgical revisions is to be kept to a minimum and preferably zero. To this end, orthopedic implant materials should preferably combine high strength, corrosion resistance and tissue compatibility. One of the variables affecting the longevity of load-bearing implants such as hip-joint implants is the rate of wear of the articulating surfaces and long-term effects of metal ion release. A typical hip-joint prosthesis includes a stem, a femoral head and an acetabular cup against which the femoral head articulates. Wear of either or both of the articulating surfaces results in an increasing level of wear particulates and “play” between the femoral head and the cup against which it articulates. Wear debris can contribute to adverse tissue reaction leading to bone resorption, and ultimately the joint must be replaced.
The rate of wear of the acetabular cup and the femoral head surfaces of artificial hips is dependent upon a number of factors which include the relative hardness and surface finish of the materials which constitute the femoral head and the acetabular cup, the frictional coefficient between the materials of the cup and head, the load applied and the stresses generated at the articulating surfaces. The most common material combinations currently used in the fabrication of hip-joint implants include femoral heads of cobalt, titanium, or zirconium alloys articulating against acetabular cups lined with organic polymers or composites of such polymers including, for instance, ultra-high molecular weight polyethylene (UHMWPE) and femoral heads of polished alumina in combination with acetabular cups lined with an organic polymer or composite or made of polished alumina.
Of the factors which influence the rate of wear of conventional hip-joint implants, the most significant are patient weight and activity level. Additionally, heat generated by friction in the normal use of the implant has been shown to cause accelerated creep and wear of the polyethylene cup. Furthermore, there is a correlation between the frictional moment which transfers torque loading to the cup and the frictional coefficient between the femoral head and the surface of the acetabular cup against which the head articulates. Cup torque has been associated with cup loosening. Thus, in general, the higher the coefficient of friction for a given load, the higher the level of torque generated. Ceramic bearing surfaces have been shown to produce significantly lower levels of frictional torque. It is also noteworthy that two of the three commonly used hip-joint systems as indicated above include a metallic femoral head articulating against a UHMWPE liner inside the acetabular cup. UHMWPE, being a polymeric material, is more susceptible to creep when heated than the commonly used metal alloys or ceramics due to its relatively lower melting point and is consequently more susceptible to wear than the alloys or ceramics.
It has also been found that metal prostheses are not completely inert in the body. Body fluids act upon the metals causing them to slowly corrode by an ionization process thereby releasing metal ions into the body. Metal ion release from the prosthesis is also related to the articulation and rate of wear of load bearing surfaces because, as may be expected, when a metallic femoral head, for instance, is articulated against UHMWPE, the passive oxide film which forms on the femoral head is constantly removed. The repassivation process constantly releases metal ions during this process. Furthermore, the presence of third-body wear (cement or bone debris) accelerates this process and micro fretted metal particles can increase friction. Consequently, the UHMWPE liner inside the acetabular cup, against which the femoral head articulates, is subjected to accelerated levels of creep, wear, and torque.
A number of attempts to correct these problems were the subject of much of the early work in this area. U.S. Pat. No. 4,145,764 to Suzuki taught a metal prosthesis plasma sprayed with a bonding agent which is in turn covered with a porous ceramic coating which would allow the in-growth of bone spicules into the pores. However, the Suzuki patent did not address the issue of friction or wear of orthopedic implant bearing surfaces but confined itself to the single issue of the biocompatibility of metal prostheses and did not address the issue of dimensional changes that occur when applying such a coating. U.S. Pat. No. 3,677,795 to Bokros is directed to the application of a carbide coating over a metallic prosthetic device. The method is said to produce a prosthetic device which has “excellent compatibility with body tissue and is non-thrombogenic”. However, Bokros does no
Hunter Gordon
Mishra Ajit
Fulbright & Jaworski L.L.P.
Hamilton Lalita M
Lucchesi Nicholas D.
Smith & Nephew Inc.
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