Electromagnetic and mechanical wave energy treatments of UHMWPE

Details Electromagnetic and mechanical wave energy treatments of UHMWPE Electromagnetic and mechanical wave energy treatments of UHMWPE

2000-02-21

2002-05-28

Seidleck, James J. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Compositions to be polymerized by wave energy wherein said...

C522S157000, C522S150000, C522S003000, C204S157150

Reexamination Certificate

active

06395799

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for reducing or quenching free radical concentration in irradiated ultra high molecular weight polyethylene (UHMWPE). More particularly, the present invention relates to rapid quenching of free radicals to avoid long term oxidative degradation in irradiated cross-linked ultra high molecular weight polyethylene used to construct orthopedic implants.
BACKGROUND OF THE INVENTION
The replacement of destroyed or damaged human joints is one of the greatest achievements of Twentieth Century orthopedic surgery. However, total joint prosthesis, composed of various combinations of metal, ceramic, and polymeric components, continue to suffer from limited service lives.
UHMWPE is commonly used as an articulating, load-bearing surface in total joint arthoplasty. In the last decade, however, it has become apparent that wear debris from UHMWPE components may be a primary contributor to osteolysis, loosening and eventual failure of prosthetic joints. With steady increases in human life expectancy, there is a driving need to significantly increase the effective lifetime of a single implant. A desire to use prosthetic implants in younger patients is another strong incentive for improving the wear resistance of UHMWPE. As indicated below, the present invention discloses a process to improve long term wear characteristics of prosthetic implants made with UHMWPE.
When a human joint is destroyed or damaged by disease or injury, surgical replacement (arthoplasty) is normally required. A total joint replacement includes components that simulate a natural human joint, typically:
(a) a more-or-less spherical ceramic or metal ball, often made of cobalt chromium alloy;
(b) attachment of a “stem”, which is generally implanted into the core of the adjacent long bone; and
(c) a hemispherical socket which takes the place of the acetabular cup and retains the spherical ball. This hemispherical joint typically is a metal cup affixed into the joint socket by mechanical attachments and “lie” with UHMWPE. In this way, the ball can rotate within the socket, and the stem, via the ball, can pivot and articulate.
One of the difficulties in constructing any device for implantation into the human body is the need to avoid adverse immune responses. The probability of a severe immune response is reduced when certain synthetic materials are used. For example, synthetic UHMWPE implants have minimal immunogenicity problems. However, the wear and breakdown of the UHMWPE components are known in the art to cause immunology-related problems.
Histologic studies have demonstrated that wear of UHMWPE from orthopedic inserts leads to several problems. First, tissue surrounding implants constructed with UHMWPE has been shown to contain extremely small particles of UHMWPE which range from sub-micron to a few microns in size. While large particles of UHMWPE appear to be tolerated by the body, as is the intact solid wall of the UHMWPE implant, the body apparently does not tolerate smaller particles of UHMWPE. In fact, the small particles of UHMWPE can cause powerful histiocytic reactions by which the body unsuccessfully attempts to eliminate the foreign material. Agents released during this process attack neighboring bone to cause “wear debris” induced osteolysis. This in turn leads to loss of fixation and loosening of the prosthesis due to “remodeling” of the bone. The breakdown of UHMWPE during wear which leads to these adverse biological problems, can be due, in part, to degradation of polymer chains resulting from chemical oxidation of free radicals.
Numerous techniques have been proposed to improve wear resistance of UHMWPE in orthopedic implants. In these instances, however, many of the new versions of articulating polymers have generally failed to demonstrate significant reduction in wear and often prove to be inferior to conventional polyethylene. Recent attempts at improving wear properties of UHMWPE use special pressure/temperature processing techniques, surface treatments, formation of composites with high modulus fibers, and cross-linking via ionizing irradiation or chemical agents. Some of these attempts are summarized below.
1. Temperature/Pressure Treatments
Special thermal and pressure treatments have been used to increase physical performance and wear resistance of UHMWPE (e.g., U.S. Pat. Nos. 5,037,928 and 5,037,938). For example, “Hipping” (Hot Isostatic Pressing), produces material alleged to comprise fewer fusion defects, increased crystallinity, density, stiffness, hardness, yield strength and resistance to creep, oxidation, and fatigue. Clinical studies, however, indicate that “Hipping” treated UHMWPE may possess inferior wear resistance in comparison to conventional UHMWPE. The inferior wear resistance being due to increased stiffness which leads to increased contact stresses during articulation (Livingston et al., Trans. ORS, 22, 141-24, 1997).
Post-consolidation temperature and pressure treatment, such as solid phase compression molding (Zachariades, U.S. Pat. No. 5,030,402), have also been attempted. Zachariades utilized solid state processing to further consolidate and orient UHMWPE chains. Resistance to wear in orthopedic implants, however, was not improved.
2. Surface Treatments
Focusing upon the surface of UHMWPE components, attempts have been made to decrease wear by increasing smoothness and/or lubricity of the UHMWPE components surface. A group from Howmedica used a heat pressing technique to melt the articulating surface and remove machine marks from the surface of UHMWPE components such that the “wearing in” of rough machine marks could be avoided. This modification, however, resulted in delimination and high wear due to the fact that high articulation-induced stresses were located in regions where there was a sharp transition in crystalline morphology (Bloebaum et al., Clin. Orthop. 269, 120-127, 1991).
Andrade et al. (U.S. Pat. No. 4,508,606) suggested oxidizing the surface of a wet hydrophobic polymer surface to reduce sliding friction. The preferred means included applying a radio frequency glow discharge to the surface. With this technique, surface chemistries were altered by changing the time of gas plasma exposure and by altering the gas composition. The invention was proposed for the treatment of catheters to decrease surface friction properties while in a wet state. Similarly, Farrar (World Patent Application No. WO 95 212212) proposed using gas plasma treatments to cross-link the surface of UHMWPE and, thereby, increase its wear resistance. None of the plasma treatments, however, were practical because any perceived benefit would most likely wear away with articulation.
3. Composites
Because creep may be a contributor to UHMWPE wear, investigators have also included high modulus fibers in polyethylene matrices to reduce plastic deformation. (U.S. Pat. No. 4,055,862.) developed a “poly-to-carbon polyethylene composite” which failed significantly via delimination. Recently, Howmedica reported that a PET/carbon fiber composite exhibited 99% less hip simulated wear than conventional polyethylene over ten million cycles. (Polineni, V. K. et al., J. 44
th
Annual ORS, 49, 1998.)
4. Cross-Linking
A. Ionizing Radiation Induced Cross-Linking
In the absence of oxygen, the predominant effect of ionizing radiation on UHMWPE is cross-linking. Cross-linking of UHMWPE forms covalent bonds between polymer chains which inhibit cold flow (creep) of individual polymer chains. Free radicals formed during irradiation, however, can exist indefinitely if termination by cross-linking or other forms of recombination do not occur. Furthermore, reacted intermediates are continuously formed and decayed. Exposure of these free radical species at any time (e.g., during irradiation, shelf-aging, or in vivo aging) to molecular oxygen or any other reactive oxidizing agent can result in their oxidation. Extensive oxidation leads to a reduction in molecular weight, and subsequent changes in physical properties, including wear resistance.
To reduce oxidation a

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