Metal treatment – Stock – Cobalt base
Reexamination Certificate
2000-09-13
2004-08-10
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Cobalt base
C420S436000, C420S437000, C420S440000
Reexamination Certificate
active
06773520
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to biocompatible implants and to metal alloys and methods for constructing biocompatible implants. In preferred embodiments, the invention relates to biocompatible joint implants and to materials and methods for constructing these implants.
BACKGROUND OF THE INVENTION
The replacement of joints with man-made artificial joints, i.e., joint arthroplasty, has grown dramatically in the past several decades. Several different joint replacement systems are currently available. However, the worldwide standard remains a cobalt-base superalloy ball structure which fits into a socket formed of ultra-high molecular weight polyethylene (UHMWPE).
The cobalt-base superalloy used in joint implants, CoCrMo, is particularly desirable because of its biocompatibility, high yield strength, and high hardness. American Society for Testing and Materials (ASTM) Specification F 1537 94 defines the chemistry of this alloy as set forth below:
ASTM F 1537 94 Chemistry
Min.
Max.
C
—
0.35
Mo
5.0
7.0
Cr
26.0
30.0
Ni
—
1.0
Fe
—
0.75
Mn
—
1.0
Si
—
1.0
N
2
—
0.25
There is also an ISO specification covering CoCrMo, ISO 5832-12, which is identical in chemistry and very similar otherwise to ASTM F 1537 94.
This alloy is solid solution hardened by the presence of Mo and to some extent Cr, and precipitation hardened by chromium carbides. In addition, the wrought version of the CoCrMo alloy is strengthened by work hardening. This can be accomplished by cold working (e.g. drawing or rolling at room temperature) or by warm working (e.g. rolling at relatively low temperatures within the allowable working temperature range.)
The presence of carbon, in particular, is considered critical to achieve necessary strength properties via the precipitated carbides. Although carbide strengthening is more important for the cast alloy and particularly at elevated temperatures (see Sims, C. T., Stoloff, N. S. and Hagel, W. C.,
The Superalloys II
, pp 135-163 (John Wiley & Sons, 1987); nevertheless, even in the wrought alloy which has a relatively low carbon content (typically 0.05%), there are normally huge numbers of very small (<5 &mgr;m) carbide particles dispersed throughout the alloy matrix. It is also not unusual to find slightly larger particles of the intermetallic compound, sigma, present in the alloy. Sigma is a very hard and brittle compound of the general formula, CO
x
(Cr, Mo)
y
.
The CoCrMo ball structures used in joint implants are usually machined from wrought bar stock. The thermo-mechanical forging process reduces the size of the hard carbide particles (as compared to the carbides in cast balls) which increases both alloy strength and hardness, and provides a corresponding reduction in surface roughness of the ball; Streicher, R. M.,
Tribology of Artificial Joints
, Endoprosthetics, Morscher, E. W. (Ed.), Berlin:Springer, p. 38-48 (1995). More specifically, the wrought alloy has a fine grain size (ASTM 5 or finer), a high strength (120 Ksi min. Y. S.), and high hardness (Rc35 typical).
Polyethylene wear remains the limiting factor for the longevity of joint arthroplasty. Wear rates on the order of 0.09 mm to 0.3 mm per year have been reported. As a result of such wear, submicron particles are released from the joint at a rate of on the order of 40 billion particles per year.
Whether or not the debris cause an immediate clinical problem depends upon the body's response to the particulate wear debris. Nevertheless, the polyethylene wear particles can have long term effects of bone loss and loosening of the implant. In particular, the wear debris can overload the afferent transport system leading to accumulation of debris around the articulation. A soft tissue membrane forms as a result of the biological reaction to the debris producing soluble factors that stimulate bone resorption, causing osteolysis and loosening of the implant.
The effect of polyethylene wear debris as a primary cause of long term joint implant failure has been known for over two decades and has generated widespread efforts to develop new joint implant structures and materials to reduce wear debris. Substantial effort has been focused on improving the polymers used to form the cup or socket portion of the artificial joint to directly minimize polyethylene debris. Such proposals have included improved sterilization and polymer hardening techniques.
Proposals for improving the ball structure to reduce wear debris in artificial joint implants have focused on frictional properties of the ball surface, and on hardness of the ball alloy. Hardness properties are significant because of third body wear which occurs when particles become trapped between two articulating surfaces. The presence of bone, cement and/or metal debris are believed responsible for roughening of the ball surface in artificial joint implants, causing in turn, increased abrasive, two body wear as the roughened ball grates across the softer polyethylene. Indeed, the tendency of titanium alloy hip prostheses, which were used in the 1970's, to cause rapid polyethylene wear was due to titanium's susceptibility to oxidative and third body wear. This experience led to acceptance of the harder and stronger CoCrMo alloys as the “gold standard”, Cukier et al.,
Femoral Head Technologies To Reduce Polyethylene Wear In Total Hip Arthroplasty
, Clinical Orthopaedics and Related Research, No. 317, pp. 57-63 (1995).
Despite the high hardness values associated with CoCrMo alloys used in joint implants, scratched and pitted surfaces are also seen in studies of balls of these alloys recovered from patients after use. This has led to proposals for improving surface hardness of the CoCrMo alloy ball. For example, U.S. Pat. No. 5,308,412 to Shetty et al. proposes nitrogen ion implantation to enhance the hardness of the surface of a cobalt-chromium implant. Similarly, Streicher, (cited above), reported improved wear resulting from a TiN coating on cast CoCrMo femoral parts. Alternative ball constructions based on ceramics, particularly zirconia and alumina, have also been investigated because of the extremely high hardness values associated with these materials as disclosed in U.S. Pat. No. 5,180,394 to Davidson; U.S. Pat. No. 3,871,031 to Boutin; and Cooper et al.,
Ceramic Bearing Surfaces In Total Artificial Joints: Resistance to Third Body Wear Damage From Bone Cement Particles
, Journal of Medical Engineering and Technology, Vol. 15, No. 2, pp. 63-67 (1991).
Various efforts and proposals have also been made to improve frictional characteristics of the surfaces of artificial implants. As noted previously, the smaller size of hard carbide particles on the surface of the current standard wrought CoCrMo alloy balls provide reduced surface roughness. In particular, reduction in size of the carbides from a diameter of 20 &mgr;m (cast alloy) to 2-3 &mgr;m (wrought alloy) reduced polyethylene wear by 20%, (Streicher, cited above). Gold et al.,
Metal
-
On
-
Plastic Total Hip Joints
, Clinical Orthopaedics and Related Research, No. 100, pp. 270-278 (1974) suggest a surface roughness less than four micro-inch (0.1 &mgr;m) but greater than 2 micro-inch would be ideal in a metal ball/plastic cup joint implant. Nishimura et al.,
Modification Of The Frictional Surfaces Of Artificial Joints
, Journal of the American Society for Artificial Internal Organs, Vol. 39, No. 3, pp. M762-M766 (1993) propose patterned surfaces to improve the frictional characteristics of the articulating surfaces of artificial joints. The pebbled and dimpled patterns can provide reservoirs for lubrication fluids and also provide possible sites for trapping otherwise deleterious wear particles.
Despite numerous studies and research extending over many years, polyethylene wear debris generation in artificial joints remains a significant problem and a substantial cause of long term implant failure. Although numerous modifications and alternatives have been proposed for CoCrMo alloys, the current wrought alloys continue to be the material of choice, particularly for construction
Chaffin, III John Harvie
Fehring Thomas K.
Kennedy Richard Lee
Alston & Bird LLP
University of North Carolina at Charlotte
Wyszomierski George
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