Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2001-07-18
2003-11-25
Lewis, Ralph A. (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S020140, C623S022170, C623S022240
Reexamination Certificate
active
06652586
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to metallic implants with load bearing surfaces coated with a thin, dense, low friction, highly wear-resistant, uniformly thick coating of oxidized zirconium. This invention also relates generally to metallic implants with load bearing, abrasion resistant surfaces. In the present invention, the load bearing oxidized zirconium surfaces or abrasion resistant surfaces contact counter bearing surfaces of cross-linked polyethylene (XLPE). XLPE has superior wear characteristics compared with other conventional polymer materials used in prostheses. Oxidized zirconium has thermal conductivity characteristics that are particularly advantageous when used in a prosthetic device in which it articulates against XLPE. The unique advantages of oxidized zirconium and abrasion resistant surfaces in combination with those of XLPE result in a synergy which allows one to accentuate the superior properties of XLPE as a counter bearing surface, resulting in a superior prosthetic device.
Historically prostheses of articulating surfaces were constructed of materials of differing hardness for the contacting surfaces. 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 different hardness is not a perfect solution. The softer surface is, by nature sacrificial; it will eventually fail, its main virtue is the realization of 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 the health on the patient.
The invention described herein is a particular type of ceramic-on-polymer prosthesis. Its unique compositional properties affords the traditional advantages of ceramic-on-polymer systems while avoiding their major disadvantage.
The invention overcomes the major disadvantage generally inherent in prosthetic devices having hard surfaces articulating against soft surfaces. The basic technology upon which the improvement described herein is based, is described in U.S. Pat. No. 5,037,438 to Davidson and to commonly assigned, copending application Ser. No. 09/381,217, filed Nov. 24, 1999 now U.S. Pat. No. 6,447,550 of Hunter, et al., both of which are fully incorporated by reference herein.
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 an ultra high molecular weight polyethylene (UHMWPE) liner inside the acetabular cup. UHMWPE, being a polymeric material, is more susceptible to creep at higher temperatures 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.
The original impetus for the inclusion of surfaces such as UHMWPE was that they would act sacrificially; they would fail slowly and fail before the harder surface, allowing for an overall extension of the useful life of the device. Additionally, polyethylene was thought to absorb shock much better than harder surfaces, thereby simulating the effect of real cartilage. While the advance in the art which was realized by the use of oxidized zirconium surfaces articulating against UHMWPE surfaces was a lessening of wear and cup loosening between the surface of the metallic component and the UHMWPE, the problem was not completely eliminated. Thus, the instant invention represents another advancement in the art, namely, a further improvement in wear and a simultaneous significant improvement of the creep problem associated with the prior art prostheses comprising polyethylene articulating against harder surfaces.
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 reduction in these deleterious effects will also improve the problem of metal ion release.
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
Hunter Gordon
Jani Shilesh C.
Fulbright & Jaworski L.L.P.
Lewis Ralph A.
Smith & Nephew Inc.
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