KR100538915B1 - Ultra-high molecular weight polyethylene prosthesis treated with radiation and melt - Google Patents

Abstract

The present invention relates to the in vivo medical prosthesis of the present invention formed from radiation treated ultra high molecular weight polyethylene in which substantially free radicals are not detected. Preferred prostheses of the present invention reduce particle production from the prosthesis during wearing of the prosthesis and are substantially oxidation resistant. The present invention also relates to methods of making such devices and materials used in the devices.

Description

Radiation and melt treated ultra high molecular weight polyethylene prosthetic device

FIELD OF THE INVENTION The present invention relates to the field of orthopedics, to the provision of prostheses, such as buttocks and knee inserts, and to the materials used in methods and devices for making such devices.

The use of synthetic polymers, such as ultra high molecular weight polyethylene, together with metal alloys has revolutionized their use in the prosthetic implant field, such as total joint replacements for hip or knee joints. However, wear of synthetic polymers by articulated metals can cause severe adverse effects that appear prominently after many years. From various studies, it has been concluded that such wear can liberate ultrafine particles of polyethylene into peripheral prosthetic tissue. The study suggested that abrasion elongates the chain superimposed microcrystals to form anisotropic fiber structures at the joint surface. The elongated fibrils are then ruptured, producing particles of submicron size. As these polyethylene particles gradually enter between the prosthesis and the bone, macrophage induced resorption of peripheral prosthetic bone begins. Often macrophages unable to soak these polyethylene particles synthesize and release a number of cytokines and growth factors that can ultimately cause bone resorption by keel cells and protein cells. Such osteolysis can contribute to the mechanical relaxation of the prosthetic component, which often requires corrective surgery as a result of this problem.

Summary of the Invention

It is an object of the present invention to insert, at least in part, formed of irradiated ultra high molecular weight polyethylene (UHMWPE) in which free radicals are not detected, in order to reduce the formation of fine particles from the prosthesis during wearing of the prosthesis. It is to provide a prosthetic device.

Another object of the present invention is to reduce osteolytic and anti-inflammatory responses resulting from prosthetic inserts.

It is a further object of the present invention to provide a prosthesis that can remain in the body for a long time.

It is a further object of the present invention to provide an improved UHMWPE that can be used in the prosthesis and / or other assembled articles of the subject matter described above.

It is a further object of the present invention to provide an improved UHMWPE which has a high density of crosslinks and in which no free radicals are detected.

Another object of the present invention is to provide an improved UHMWPE having improved wear resistance.

According to the present invention there is provided an in vivo medical prosthesis formed of irradiated ultrahigh molecular weight polyethylene in which free radicals are virtually undetectable. The irradiation can be, for example, gamma or electron beam irradiation. UHMWPE has a crosslinked structure. Preferably, UHMWPE is not oxidized substantially, and is in fact oxidative resistant. What is different is that it includes, for example, UHMWPE having three melting peaks, two melting peaks or one melting peak. In certain embodiments, the UHMWPE has a polymeric structure with less than about 50% crystallinity, less than about 290 mm Lamellar thickness, and less than about 940 MPa tensile modulus, thereby reducing the formation of fine particles from the prosthesis during wear of the prosthesis. Let's do it. Some of the prostheses may be in the form of cup or tray shaped articles with a load bearing surface made of such UHMWPE, for example. This rod bearing surface may be in contact with the second portion of the prosthesis having an occlusal rod bearing surface of metallic or ceramic material.

Another aspect of the invention is irradiated UHMWPE in which no free radicals are detected in nature. Such UHMWPE has a crosslinked structure. Preferably, such UHMWPE is not substantially oxidized and is in fact oxidized. What is different is that it includes, for example, UHMWPE having three melting peaks, two melting peaks or one melting peak.

Another aspect of the invention is an assembly article made of said UHMWPE, for example an assembly article having a rod bearing surface and a wear resistant coating. One embodiment is in the form of bar stock, in which the assembled article can be molded into the article by conventional methods, for example by machining.

Yet another aspect of the present invention includes a method of making a crosslinked UHMWPE in which virtually no free radicals are detected. Conventional UHMWPEs with polymer chains are provided. Such UHMWPE is irradiated to crosslink the polymer chain. UHMWPE is heated to a temperature above the melting point of UHMWPE, so that virtually no free radicals are present in the UHMWPE. The UHMWPE is then cooled to room temperature. In certain embodiments, cooled UHMWPE is machined and / or sterilized.

One preferred embodiment of this method is called CIR-SM, ie cold irradiation and subsequent melting. The UHMWPE provided is at a temperature below room temperature.

Another preferred embodiment of the invention is called WIR-SM, ie warm irradiation and subsequent melting. The provided UHMWPE is preheated to a temperature below the melting point of the UHMWPE.

Another preferred embodiment of this method is called WIR-AM, ie warm irradiation and adiabatic melting. In such embodiments, the provided UHMWPE is preheated to a temperature below the melting point of the UHMWPE, preferably from about 100 ° C. to a temperature below the melting point of the UHMWPE. Preferably, the UHMWPE is kept in insulating material to reduce heat loss from the UHMWPE during processing. The preheated UHMWPE is then irradiated with a very sufficient total dosage and a sufficiently fast dosage to generate sufficient heat in the polymer to melt virtually all crystals in the material, for example virtually all detectable free radicals generated by the irradiation step. To ensure its removal. It is preferable that an irradiation step generate above-mentioned adiabatic heating using electron beam irradiation.

Another object of the present invention is to provide a product prepared according to the above-described method.

Yet another aspect of the present invention is to provide a method for producing a crosslinked UHMWPE called MIR, ie melt irradiation. Conventional UHMWPE is provided. Preferably, the UHMWPE is surrounded by an inert material which is virtually oxygen free. UHMWPE is heated to a temperature above the melting point of UHMWPE to completely melt all crystal phase structures. The heated UHMWPE is irradiated and the irradiated UHMWPE is cooled to about 25 ° C.

In an embodiment of the MIR, highly entangled and crosslinked UHMWPE is produced. Conventional UHMWPE is provided. Preferably, the UHMWPE is surrounded by an inert material which is virtually oxygen free. The UHMWPE is heated to a temperature above the melting point of the UHMWPE for a time sufficient to form a entangled polymer chain in the UHMWPE. The heated UHMWPE traps the polymer chain in an irradiated and entangled state, and the irradiated UHMWPE is cooled to about 25 ° C.

The present invention features a method of making a medical prosthesis that reduces particle production from the prosthesis during the wearing of the prosthesis from radiation treated UHMWPE where virtually no free radicals are detected. Radiated UHMWPE is provided in which no free radicals are detected. Medical prostheses are formed from these UHMWPE to reduce particle production from the prosthesis during wear of the prosthesis, which forms the rod bearing surface of the prosthesis. Prostheses can be produced by standard methods known to those skilled in the art, for example by machining.

Also provided herein are methods of treating a body in need of a medical prosthesis. Shaped prostheses formed of radiation treated UHMWPE are provided which are virtually free of free radicals detected. The prosthesis is applied to the body in need of the prosthesis. The prosthesis reduces the production of particles from the prosthesis during wearing of the prosthesis. In a preferred embodiment, the UHMWPE forms the rod bearing surface of the prosthesis.

The above and other objects, features and advantages of the present invention will be more readily understood from the following specification when read in conjunction with the accompanying drawings.

1 is a cross-sectional view through a central portion of a medical hip prosthesis according to a preferred embodiment of the present invention.

FIG. 2 is a side view illustrating the liner of the articulated cup as shown in FIG. 1.

FIG. 3 is a cross-sectional view showing the line 3-3 of FIG. 2.

4 is a graph showing the melting point and crystallinity of UHMWPE melt-irradiated at different dosages.

5 is an environmental scanning electron micrograph of an etched surface of a conventional UHMWPE showing a crystalline structure.

FIG. 6 is an environmental scanning electron micrograph of an etched surface of melt irradiated UHMWPE showing crystalline structure at approximately the same magnification as in FIG. 5.

7 is a graph showing the melting point and crystallinity at different depths of melt irradiated UHMWPE cups.

FIG. 8 is a graph showing DSC melt endotherm for Hoechst-Celanese GUR 4050 UHMWPE prepared using warm irradiation and partial adiabatic melting (WIR-AM) in the presence and absence of subsequent heating. to be.

FIG. 9 is a graph showing DSC melt endotherm for Hoskect-Celanies GUR 1050 UHMWPE prepared using warm irradiation and partial adiabatic melting (WIR-AM) in the presence and absence of subsequent heating.

10 is a graph showing adiabatic heating of UHMWPE treated with WIR-AM at a preheating temperature of 130 ° C.

FIG. 11 is a graph showing tensile strain behavior of unirradiated UHMWPE, CIR-SM treated UHMWPE and WIR-AM treated UHMWPE.

The present invention provides an in vivo medical prosthesis formed from radiation treated ultra high molecular weight polyethylene in which free radicals are virtually free of detection.

Medical prostheses in the form of hip prostheses are generally shown at 10 in FIG. 1. The prosthesis shown has a conventional ball head 14 connected by a neck to a stem 15 fixed to the femur 16 by a conventional binder 17. The ball head can be of conventional design and is formed of stainless steel or other alloys known in the art. The radius of the ball head is approximately equal to the inner cup radius of the joint cup 12, which can be fixed directly to the pelvis 11 with a bonding agent. In addition, a metallic articulated shell can be bonded to the pelvis 11 and the articulated cup 12 can form a coating or liner connected to the metallic articulated shell by means known in the art.

Certain forms of prostheses can vary widely as is known in the art. Many hip structures are known and other prostheses such as knee joints, shoulder joints, ankle joints, elbow joints and finger joints are known. All such conventional prostheses can benefit from forming at least one rod bearing surface of such a prosthesis made of a high molecular weight polyethylene material according to the present invention. Such rod bearing surfaces may be in the form of layers, linings or the actual overall device as shown in FIG. 1. In all cases, it is preferable that the rod bearing surface acts with the metallic or ceramic mating member of the prosthesis to form a sliding surface therebetween. Such sliding surfaces can break polyethylene as is known in the art. Such breakage can be greatly reduced by the use of the material of the present invention.

FIG. 2 shows an articulated cup 12 in the form of a semi hollow ball-shaped device, which is better seen in the cross section of FIG. 3. As noted above, the outer surface 20 of the articulation cup need not be circular or hemispherical, but it can be square or any shape that permits attachment to the pelvis directly or through a metallic shell as is known in the art. have. The radius of the articulated cup shown as 21 in FIG. 3 of a preferred embodiment is from about 20 mm to about 35 mm. The thickness of the joint cup from the generally hemispherical hollow part to the outer surface 20 is preferably about 8 mm. The outer diameter is preferably about 20 mm to about 35 mm.

It is desirable to make articulated cups or articulated cup liners made of UHMWPE to be one with metallic balls, but in some cases, the ball joint may be made of the UHMWPE of the present invention, and articulated cups may be made of metal. have. Specific methods of attaching the prosthetic component to the bones of the body can vary widely as is known in the art.

The medical prosthesis of the present invention is intended to include the entire prosthetic device or portions thereof, for example components, layers or linings. Medical prostheses include, for example, orthopedic joint and bone replacement portions, such as buttocks, knees, shoulders, elbows, ankles or finger substitutes. The prosthesis may be in the form of a cup or tray shaped article with, for example, a rod bearing surface. Other forms known to those skilled in the art are also included in the present invention. Medical prostheses include any wear surface of the prosthesis, for example a coating on the surface of the prosthesis where the prosthesis is made of a material other than the UHMWPE of the present invention.

The prosthesis of the invention is useful for contact with metals containing parts formed of, for example, cobalt chromium alloys, stainless steels, titanium alloys or nickel cobalt alloys, or ceramics containing parts. For example, a hip joint in which a cup-shaped article with an inner radius of 25 mm is contacted with a metal ball with a 25 mm outer radius is structured and combined with the cup-shaped article to become one. The rod bearing surface of the cup-shaped article of this example preferably has a thickness of at least about 1 mm, more preferably at least about 2 mm, even more preferably at least about ¼ inch, and most preferably at least about ⅓ inch. It is made of the UHMWPE of the present invention.

The prosthesis can be of any standard known shape, shape, or shape or of a conventional design, but has one or more rod bearing surfaces made from the UHMWPE of the present invention.

The prosthesis of the present invention is not harmful to humans. These prostheses are not degraded by normal body components, such as blood or interstitial fluids. These prostheses can be sterilized by standard methods or ethylene oxide, including for example by heating.

As used herein, the term “UHMWPE” means a linear unbranched ethylene having a molecular weight greater than about 500,000, preferably greater than about 1,000,000, and more preferably greater than about 2,000,000. Often, the molecular weight can be as high as at least about 8,000,000. As used herein, the expression “initial average molecular weight” means the average molecular weight of the UHMWPE starting material prior to any irradiation.

Conventional UHMWPE is formed by standard Ziegler-Natta catalysis, and polymer chains are generated from surface catalytic sites, where they crystallize and bind as chain overlapped crystals. Examples of known UHMWPE powders are Hifax Grade 1900 polyethylene (obtained from Montel, Wilmington, Delaware) which has a molecular weight of about 2,000,000 g / mol and does not contain any calcium stearate. ; GUR 4150 (obtained from Hooescht Celanese Corp.), also known as GUR 415 having a molecular weight of about 4,000,000-5,000,000 g / mol and containing 500 ppm calcium stearate; GUR 4050 (obtained from Houston Houston Escort Celanese Corporation) with a molecular weight of about 4,000,000-5,000,000 g / mol and no calcium stearate; molecular weight of about 2,000,000 g / mol and containing 500 ppm calcium stearate GUR 4120 [obtained from Houston Houston Escort Celanese Corporation]; GUR 4020 [obtained from Houston Houston Hoskect Celanese Corporation] with molecular weight of about 2,000,000 g / mol and containing no calcium stearate; GUR 1050 (obtained from Hoeskett Celanese Corporation, Germany) of about 4,000,000-5,000,000 g / mol and containing no calcium stearate; GUR having a molecular weight of about 4,000,000-5,000,000 g / mol and containing 500 ppm of calcium stearate 1150 [obtained from Hoeskeck Celanese Corporation, Germany]; molecular weight about 2,000,000 g / mol GUR 1020 [not obtained from German Hoescett Celanese Corporation], which also does not contain calcium tearate; and GUR 1120 [Germany Hoeskett Celanese Corp., containing about 2,000,000 g / mol and 500 ppm calcium stearate Available from GUR 4150, GUR 1050 and GUR 1020. Resin means powder.

UHMWPE powders can be solidified using a variety of different techniques, such as ram extrusion, compression molding or direct compression molding. In ram extrusion, the UHMWPE powder is pressurized through a heated barrel, obtained from rod stock, or bar stock (e.g., from Westlake Plastics, Rennie, Pennsylvania). ] Is solidified. In compression molding, the UHMWPE powder is solidified under high pressure (from Poly-Hi Solidur, Indiana Wayne, or from Perplas, Stanmore, UK). The shape of the mold can be a thick sheet, for example. Direct compression molding is preferably used to prepare reticulated products, such as articular components or tibial knee inserts (eg, from Zimmer, Inc., Washo, Indiana). Used. In this technique, the UHMWPE powder is compressed directly into the final shape. The "hockey puck", or puck, is generally machined from a ram extruded bar stock or a compression molded sheet.

Irradiated UHMWPE refers to UHMWPE that has been crosslinked between polymer chains of UHMWPE by irradiation, for example gamma radiation or electron beam irradiation.

In fact, no free radicals were detected, as described by Jahan et al., J. Biomedical Materials Research 25: 1005 (1991), and virtually no free radicals were measured by electron paramagnetic resonance. Means no. Free radicals include, for example, unsaturated trans-vinylene free radicals. UHMWPE irradiated at a temperature below the melting point with ionizing radiation contains crosslinked and persistent trapped free radicals. These free radicals react with oxygen over a long period of time and break down UHMWPE through oxidative degradation. An advantage of the UHMWPE and medical prosthesis of the present invention is that radiation treated UHMWPE is used in which no free radicals are detected. Free radicals can be removed by any method that gives this result, for example by heating the UHMWPE to a temperature above the melting point, leaving virtually no residual crystalline structure left. By removing the crystalline structure, free radicals can be removed by recombination.

UHMWPE used in the present invention has a crosslinked structure. The advantage of having a crosslinked structure is that this will reduce particle production from the prosthesis during wearing of the prosthesis.

It is preferred that UHMWPE is virtually not oxidized. It does in fact not oxidized, FTIR spectrum at 1740cm ^-1-carbonyl peak area for cross-linking the sample of the FTIR spectrum at 1460cm ^-1 peak area ratio of crosslinked to non-sample and approximately the same size of the prior of the following at the bottom of the It means.

UHMWPE is preferably virtually oxidation resistant. Persistent oxidation resistance means that it is virtually unoxidized for over 10 years. The UHMWPE preferably remains substantially unoxidized for at least about 20 years, more preferably at least about 30 years, even more preferably at least about 40 years, and most preferably while the patient is alive. .

In certain embodiments, the UHMWPE has three melting peaks. The first melt peak is preferably about 105 ° C. to about 120 ° C., more preferably about 110 ° C. to about 120 ° C., and most preferably about 118 ° C. The second melt peak is preferably about 125 ° C. to about 140 ° C., more preferably about 130 ° C. to about 140 ° C., even more preferably about 135 ° C., and most preferably about 137 ° C. The third melt peak is preferably about 140 ° C. to about 150 ° C., more preferably about 140 ° C. to about 145 ° C., and most preferably about 144 ° C. In certain embodiments, the UHMWPE has two melting peaks. The first melt peak is preferably about 105 ° C. to about 120 ° C., more preferably about 110 ° C. to about 120 ° C., and most preferably about 118 ° C. The second melt peak is preferably about 125 ° C. to about 140 ° C., more preferably about 130 ° C. to about 140 ° C., even more preferably about 135 ° C., and most preferably about 137 ° C. In certain embodiments, the UHMWPE has one melting peak. The melt peak is preferably about 125 ° C. to about 140 ° C., more preferably about 130 ° C. to about 140 ° C., even more preferably 135 ° C., and most preferably about 137 ° C. It is preferred that UHMWPE has two melting peaks. The number of melting peaks is measured with a differential scanning calorimeter (DSC) at a heating rate of 10 ° C./min.

The polymer structure of UHMWPE used in the prosthesis of the present invention reduces the production of UHMWPE particles from the prosthesis during the wearing of the prosthesis. Because a limited number of particles flow out of the body, the prosthesis has a longer insertion life. Preferably, the prosthesis may remain inserted in the body for at least about 10 years, more preferably at least about 20 years and most preferably the patient is alive.

The present invention also encompasses other assembled articles made from radiation treated UHMWPE, in which no free radicals are detected. Preferably, the UHMWPE used to make the assembled article has a crosslinked structure. Preferably, UHMWPE is substantially oxidation resistant. In certain embodiments, the UHMWPE has three melting peaks. In certain embodiments, the UHMWPE has two melting peaks. In certain embodiments, the UHMWPE has one melting peak. It is preferred that UHMWPE has two melting peaks. Assembly articles are for example machined objects such as cups, gears, nuts, sled runners, bolts, fasteners, cables, pipes and the like, and bar stocks, membranes, cylindrical bars, sheets, panels, and Including shaped fibers, and shaped articles and unshaped articles. Shaped articles can be produced, for example, by machining. The assembled article may be in the form of a bar stock, which may be shaped into a second article, for example, by machining. Assembly articles are particularly suitable for rod bearing applications, for example high wear resistance applications, and as metal replacement articles, for example as rod bearing surfaces (eg segment surfaces). Membranes or sheets made of the UHMWPE of the present invention can be attached onto the support surface using, for example, glues, and can be used as wear resistant rod bearing surfaces.

The present invention also encompasses radiation treated UHMWPE, in which no free radicals are detected. UHMWPE has a crosslinked structure. Preferably, UHMWPE is not oxidized substantially, and is in fact oxidative resistant. In certain embodiments, the UHMWPE has three melting peaks. In certain embodiments, the UHMWPE has two melting peaks. In certain embodiments, the UHMWPE has one melting peak. It is preferred that UHMWPE has two melting peaks. Depending on the particular process used to prepare the UHMWPE, certain impurities may be present in the UHMWPE of the present invention, including calcium stearate, mold release agents, extenders, antioxidants, and / or additives to other conventional polyethylene polymers. .

The present invention also provides a method for producing crosslinked UHMWPE in which free radicals are virtually free of detection. Preferably, such UHMWPE is for use as a high wear resistant rod bearing article. Conventional UHMWPEs with polymer chains are provided. Conventional UHMWPE can be, for example, in the form of bar stock, shaped bar stock (eg puck), coatings, or assembled articles for medical prostheses, such as cups or trays. By conventional UHMWPE is meant a commercially available high density (linear) polyethylene having a molecular weight greater than about 500,000. Preferably, the UHMWPE starting material has an average molecular weight greater than about 2,000,000. Initial mean molecular weight means the average molecular weight of the UHMWPE starting material before any irradiation. UHMWPE is irradiated to crosslink the polymer chain. Irradiation can be carried out in an inert or non-inert atmosphere. Preferably, the irradiation is carried out in a non-inert atmosphere, for example air. The irradiated UHMWPE is heated to a temperature above the melting point of the UHMWPE so that virtually no free radicals are detected in the UHMWPE. The heated UHMWPE is then cooled to room temperature. Preferably, cooling is performed at a cooling rate in excess of about 0.1 ° C./minute. Optionally, the cooled UHMWPE can be machined. For example, if any of the UHMWPE is oxidized during the irradiation step, it can be machined by any method known to those skilled in the art as needed. Also optionally, the cooled UHMWPE or machined UHMWPE can be sterilized by any method known to those skilled in the art.

One preferred embodiment of the invention is called CIR-SM, ie cold irradiation and subsequent melting. In this embodiment, the provided UHMWPE is at a temperature below room temperature. Preferably at a temperature of about 20 ° C. UHMWPE can be irradiated by gamma irradiation or electron beam irradiation, for example. In general, gamma-irradiation provides a higher depth of penetration but takes more time, allowing for deeper oxidation. In general, electron beam irradiation provides a more limited transmission depth but takes a shorter time, thus reducing the possibility of widespread oxidation. Irradiation is carried out to crosslink the polymer chains. The dosage can be varied to control the degree of crosslinking and crystallinity in the final UHMWPE product. Preferably, the total absorbed dose is about 0.5 to about 1,000 Mrad, more preferably about 1 to about 100 Mrad, even more preferably about 4 to about 30 Mrad, even more preferably about 20 Mrad, and most preferably About 15 Mrad. Preferably, irradiation rates that do not generate too much heat are used to melt the UHMWPE. When gamma-irradiation is used, the preferred irradiation rate is about 0.05 to about 0.2 Mrad / min. When electron beam irradiation is used, the preferred irradiation rate is about 0.05 to about 3,000 Mrad / min, more preferably about 0.05 to about 5 Mrad / min, and most preferably about 0.05 to about 0.2 Mrad / min. The irradiation rate in the electron beam irradiation is determined by parameters such as (i) the power of the accelerator (kW), (ii) the conveyor speed, (iii) the distance between the surface of the irradiated specimen and the scan horn, and (iv) the scan width. Determined by Emissivity in devices using electron beams is often measured in Mrads per pass under the rastering electron beam. Emissivity, expressed herein as Mrad / min, can be converted to Mrad / pass using the following equation:

Where D _{Mrad / min} is the emissivity expressed in Mrad / min, D _{Mrad / pass} is the emissivity expressed in Mrad / pass, V _c is the conveyor speed, and l is the sample traveling through the electron beam raster area. Is the length of. When electron beam irradiation is used, the transmission depth of the transfer line can be changed by changing the energy of electrons. Preferably, the energy of the electrons is from about 0.5 MeV to about 12 MeV, more preferably from about 5 MeV to about 12 MeV. Such operability is particularly useful when the irradiated object is an article of varying thickness or depth, for example a joint cup for a medical prosthesis.

The irradiated UHMWPE is heated to a temperature above the melting point of the UHMWPE so that no free radicals are present in the UHMWPE. Heating provides the molecule with sufficient fluidity to escape the confinement resulting from the crystals of UHMWPE, recombining virtually all residual free radicals. Preferably, the UHMWPE is about 137 ° C. to about 300 ° C., more preferably about 140 ° C. to about 300 ° C., even more preferably 140 ° C. to about 190 ° C., even more preferably about 145 ° C. to about 190 ° C. Even more preferably, at a temperature of about 146 ° C to about 190 ° C, and most preferably about 150 ° C. Preferably, the temperature of the heating step is maintained for about 0.5 minutes to about 24 hours, more preferably about 1 hour to about 3 hours, and most preferably about 2 hours. The heating is carried out, for example, in air, in an inert gas such as nitrogen, argon or helium, in a sensitive atmosphere such as acetylene, or vacuum. Preferably, the heating is carried out in an inert gas or vacuum for a longer time.

Another preferred embodiment of the process of the invention is one called WIR-SM , ie warm irradiation and subsequent melting. In this embodiment, the provided UHMWPE is preheated to a temperature below the melting point of the UHMWPE. Preheating can be carried out in an inert atmosphere or in an inert atmosphere. The preheating is preferably carried out in air. Preferably, the UHMWPE is preheated to a temperature of about 20 ° C. to about 135 ° C., more preferably greater than about 20 ° C. to about 135 ° C., and most preferably about 50 ° C. Other parameters are preferably about 0.05 to about 10 Mrad / min, and more preferably about 4 to about 5 Mrad / min for the irradiation step using electron beam irradiation; The irradiation rate for the irradiation step using gamma-irradiation is preferably as described above for the CIR-SM embodiment except that it is preferably about 0.05 to about 0.2 Mrad / min, more preferably about 0.2 Mrad / min.

Another preferred embodiment of the process of the invention is one called WIR-AM , ie warming irradiation and adiabatic melting. In this embodiment, the provided UHMWPE is preheated to a temperature below the melting point of the UHMWPE. Preheating can be carried out in an inert or non-inert atmosphere. This preheating is preferably carried out in air. Preheating can be carried out, for example, in an oven. Preheating is preferably carried out at a temperature below the melting point of about 100 ° C to UHMWPE. Preferably, the UHMWPE is preheated to a temperature of about 100 ° C. to about 135 ° C., more preferably to a temperature of about 130 ° C., and most preferably to a temperature of about 120 ° C. It is desirable to keep the UHMWPE in insulating material to reduce heat loss from the UHMWPE during processing. Heat includes, for example, preheating delivered prior to irradiation and heat generated during irradiation. By insulating material is meant all types of materials having insulating properties (eg, glass fiber pouches).

The preheated UHMWPE is then irradiated at a very sufficient total dose and at a rapid enough dose to generate sufficient heat in the polymer to melt virtually all crystals present in the material, for example generated by the irradiation step. Virtually all detectable free radicals are reliably removed. In the irradiation step, it is preferable to use electron beam irradiation to generate such adiabatic heating. Adiabatic heating means there is no heat loss to the surroundings during irradiation. Adiabatic heating causes adiabatic melting when the temperature is above the melting point. Adiabatic melting includes whole or partial melting. The minimum total dosage is that of the heat required to heat the polymer from its initial temperature (ie, the preheat temperature described above) to the melting point, and the heat required to melt all the crystals, and the heat required to heat the polymer to a predetermined temperature above its melting point. Determined by the amount. The following equation describes how to calculate the total dose:

_Wherein C _Ps (= 2 J / g / ° C.) and C _Pm (= 3 J / g / ° C.) are the heat capacities of UHMWPE in the solid and molten states, respectively, and ΔH _m (= 146 J / g) The heat of fusion of the unexamined Hoskect Celanese GUR 415 bar stock, T _i is the initial temperature and T _f is the final temperature. The final temperature should be higher than the melting point of UHMWPE.

The final temperature of the UHMWPE is preferably about 140 ° C to about 200 ° C, more preferably about 145 ° C to about 190 ° C, even more preferably about 146 ° C to about 190 ° C, and most preferably about 150 ° C. . At temperatures higher than 160 ° C., the polymer begins to form bubbles and cracks. Preferably, the irradiation rate during electron beam irradiation is about 2 to about 3,000 Mrad / min, more preferably about 7 to 25 Mrad / min, even more preferably about 20 Mrad / min, and most preferably about 7 Mrad / min. Minutes. Preferably, the total absorbed dose is about 1 to about 100 Mrad. Using the above equation, the absorbed dose for the initial temperature of 130 ° C. and the final temperature of 150 ° C. is about 22 Mrad.

In this embodiment, the heating step of the method results from the adiabatic heating described above.

In certain embodiments, adiabatic heating completely melts the UHMWPE. In certain embodiments, the adiabatic heating only partially melts the UHMWPE. Preferably, further heating of the irradiated UHMWPE is performed subsequent to irradiation induced adiabatic heating, so that after further heating the final temperature of the UHMWPE is higher than the melting point of the UHMWPE to ensure complete melting of the UHMWPE. The temperature of the further heated UHMWPE is preferably about 140 ° C. to about 200 ° C., more preferably about 145 ° C. to about 190 ° C., even more preferably about 146 ° C. to about 190 ° C., and most preferably about 150 ° C.

Yet another embodiment of the present invention is called CIR-AM , ie cooling irradiation and adiabatic heating. In this embodiment, the UHMWPE at a temperature below room temperature is melted by adiabatic heating in the presence or absence of subsequent additional heating as described above.

The present invention also includes a product prepared according to the above method.

Also provided herein is a method of making a medical prosthesis from UHMWPE where virtually no free radicals are detected, which reduces the production of particles from the prosthesis during wearing of the prosthesis. Radiated UHMWPE is provided in which no free radicals are detected. The medical prosthesis is formed from this UHMWPE, reducing the generation of entry from the prosthesis during the wearing of the prosthesis, the UHMWPE forming the rod bearing surface of the prosthesis. The formation of the prosthesis can be carried out by standard methods known to those skilled in the art, for example by machining.

Also provided is a method of treating a body in need of a medical prosthesis. Shaped prostheses formed of radiation treated UHMWPE are provided which are virtually free of free radicals detected. This prosthesis is applied to the body in need of the prosthesis. The prosthesis reduces the production of fine particles from the prosthesis during wearing of the prosthesis. In a preferred embodiment, the ultrahigh molecular weight polyethylene forms the rod bearing surface of the prosthesis.

In yet another embodiment of the present invention, a polymer structure having less than about 50% crystallinity, less than about 290 mm Lamellar thickness, and less than about 940 MPa tensile modulus to reduce the formation of fine particles from the prosthesis during wear of the prosthesis An in vivo medical prosthesis formed of ultra high molecular weight polyethylene (UHMWPE) having is provided.

The UHMWPE of this embodiment has a polymer structure with a crystallinity of less than about 50%, preferably less than about 40%. Crystallinity means the fraction of polymer that is crystalline. The crystallinity measures the weight of the sample (w (g)), the heat absorbed by the sample at the time of melting (E (cal)) and the calculated heat of fusion of polyethylene (ΔH ° = 69.2 cal / g) at 100% crystallinity In one state, it is calculated using the following formula:

The UHMWPE of this embodiment has a polymer structure with a lamellar thickness of less than about 290 mm 3, preferably less than about 200 mm 3, and most preferably less than about 100 mm 3. Lamellar thickness (l) means the calculated thickness of the lamella structure estimated in the polymer using the following formula:

Where σ _e is the terminal free surface energy of the polyethylene (2.22 × 10 ^-6 cal / cm ² ), ΔH ° is the calculated heat of fusion of the polyethylene in a 100% crystalline state (69.2 cal / g), and ρ is the crystalline phase The density of the region (1.005 g / cm ³ ), T _m ° is the melting point of the complete polyethylene crystal (418.15 K), and T _m is the melting point of the sample measured experimentally.

The UHMWPE of this embodiment has a tensile modulus of less than about 940 MPa, preferably less than about 600 MPa, more preferably less than about 400 MPa, and most preferably less than about 200 MPa. Tensile modulus refers to the ratio of nominal stress to corresponding strain for less than 0.5% strain as measured using the standard test ASTM 638 M III.

Preferably, the UHMWPE of this embodiment has a polymer structure with a crystallinity of about 40%, a lamellar thickness of about 100 GPa, and a tensile modulus of about 200 MPa.

The UHMWPE of this embodiment also has no trapped free radicals, for example unsaturated trans-vinylene free radicals. The UHMWPE of this embodiment preferably has a hardness of less than about 65, more preferably less than about 55, most preferably less than 50 on the Shore D scale. Hardness means immediate indenter station measured on the Shore D scale using a durometer as described in ASTM D2240. It is preferred that the UHMWPE of this embodiment is virtually not oxidized. The polymer structure is extensively crosslinked so that a substantial portion of the polymer structure does not dissolve in decalin. By substantial portion here is meant at least 50% of the dry weight of the polymer sample. Not soluble in decalin means not soluble in decalin at 150 ° C. for 24 hours. Preferably, the UHMWPE of this embodiment has a high density entangled structure, forming incomplete crystals and reducing crystallinity. The density of the entangled structure refers to the number of entangled points of the polymer chain in the unit volume; Higher density entangled structures are indicated by the inability of the polymer sample to crystallize to the same level as conventional UHMWPE to further reduce crystallinity.

The present invention also includes other assembled articles made from UHMWPE of this embodiment having a polymer structure having a crystallinity of less than about 50%, a lamellar thickness of less than about 290 GPa, and a tensile modulus of less than about 940 MPa. Such assembled articles include, for example, machining objects such as cups, gears, nuts, sled runners, bolts, fasteners, cables, pipes, etc., and bar stocks, membranes, cylindrical bars, sheets, panels and fibers. Shaped or unshaped articles are included. Shaped articles can be produced, for example, by machining. Assembly articles are particularly suitable as rod bearing applications, for example rod bearing surfaces, and metal replacement articles. Thin films or sheets made of melt-irradiated UHMWPE can be used as a transparent and wear resistant rod bearing surface, for example, attached onto a support surface using a binder.

The present invention also includes embodiments in which the UHMWPE has a unique polymer structure characterized by less than about 50% crystallinity, less than about 290 GPa lamellar thickness, and less than about 940 MPa tensile modulus. Depending on the particular process used to prepare the UHMWPE, certain impurities, including, for example, calcium stearate, mold release agents, extenders, antioxidants, and / or other conventional additives to polyethylene polymers, may be used in the preparation of the UHMWPE of the present invention. May exist in In certain embodiments, the UHMWPE has a high light transmittance, preferably greater than about 10% light transmittance in a sample of 517 nm to 1 mm thick, more preferably greater than about 30% light in a sample of 517 nm to 1 mm thick, and Most preferably it has a light transmittance of more than 40% in a sample of 517 nm to 1 mm thick. Such UHMWPEs are particularly useful as thin films or sheets that can be attached onto the support surfaces of various articles, which are transparent and wear resistant.

In another embodiment of the present invention, a method of preparing a crosslinked UHMWPE is provided. This method is called melt irradiation ( MIR ). Conventional UHMWPE is provided. Preferably, the UHMWPE is surrounded by an inert material which is virtually oxygen free. UHMWPE is heated to a temperature above the melting point of UHMWPE to completely melt all of the crystalline structure. The heated UHMWPE is irradiated and the irradiated UHMWPE is cooled to about 25 ° C.

Preferably, the UHMWPE prepared from this embodiment has a polymer structure having a crystallinity of less than about 50%, a lamellar thickness of less than about 290 GPa and a tensile modulus of less than about 940 MPa. Conventional UHMWPE, eg bar stocks, shaped bar stocks, coatings, or assembly articles are provided. By conventional UHMWPE is meant a commercially available high density (linear) polyethylene with a molecular weight greater than about 500,000. Preferably, the UHMWPE starting material has an average molecular weight greater than about 2,000,000. By initial average molecular weight is meant the average molecular weight of the UHMWPE starting material before any irradiation. Such UHMWPE is preferably surrounded by an inert material which is virtually oxygen free, for example nitrogen, argon or helium. In certain embodiments, an inert atmosphere can be used. UHMWPE is heated to a temperature above its melting point for a sufficient time to melt all the crystals. Preferably, the temperature is about 145 to 230 ° C, more preferably about 175 to about 200 ° C. Preferably, heating is continued such that the polymer is at the desired temperature for about 5 minutes to about 3 hours, more preferably about 30 minutes to about 2 hours. The UHMWPE is then irradiated with gamma radiation or electron beam irradiation. In general, gamma irradiation provides a high penetration depth, but takes a lot of time, allowing only some degree of oxidation. In general, electron beam irradiation provides a more limited penetration depth but takes less time, thus reducing the possibility of oxidation. The dosage can be varied to control the degree of crosslinking and crystallinity in the final UHMWPE product. Preferably, doses greater than about 1 Mrad, more preferably doses greater than about 20 Mrad, are used. When electron beam irradiation is used, the degree of crosslinking and crystallinity in the final UHMWPE product can be controlled by changing the energy of the electron beam to change the penetration depth of the electron beam. Preferably, the energy is about 0.5 MeV to about 12 MeV, more preferably about 1 MeV to about 10 MeV, most preferably about 10 MeV. This maneuverability is particularly useful when the irradiated subject is an article of varying thickness or depth, for example a joint cup for a prosthesis. The irradiated UHMWPE is then cooled to about 25 ° C. Preferably, the cooling rate is at least about 0.5 ° C./min, more preferably at least 20 ° C./min. In certain embodiments, the cooled UHMWPE can be machined. In one preferred embodiment, virtually no free radicals are detected in the cold irradiated UHMWPE. Examples 1, 3 and 6 describe certain preferred embodiments of the method. Examples 2, 4 and 5, and FIGS. 4-7 illustrate the specific properties of melt irradiated UHMWPE obtained from these preferred embodiments compared to conventional UHMWPE.

The present invention also encompasses articles made according to the methods described above.

In an embodiment of the MIR, highly entangled structures and crosslinked UHMWPE are produced. Conventional UHMWPE is provided. Preferably, the UHMWPE is surrounded by an inert material which is virtually oxygen free. The UHMWPE is heated to a temperature above the melting point of the UHMWPE for a time sufficient to form a polymer chain of entangled structure in the UHMWPE. The heated UHMWPE is irradiated to trap the polymer chain in an entangled state. The irradiated UHMWPE is cooled to about 25 ° C.

The invention also encompasses articles made according to the methods described above.

Also provided herein is a method of making a prosthesis from UHMWPE to reduce the production of fine particles from the prosthesis during wear of the prosthesis. UHMWPE is provided having a polymer structure having a crystallinity of less than about 50%, a lamellar thickness of less than about 290 GPa, and a tensile modulus of less than about 940 MPa. The prosthesis is formed from this UHMWPE, which forms the rod bearing surface of the prosthesis. Prostheses can be produced by standard methods known to those skilled in the art, for example by machining.

Also provided herein are methods of treating a body in need of a prosthesis. Shaped prostheses formed of ultra high molecular weight polyethylene having a polymer structure having a crystallinity of less than about 50%, a lamellar thickness of less than about 290 GPa, and a tensile modulus of less than about 940 MPa are provided. This prosthesis is applied to the body in need of the prosthesis. The prosthesis reduces the production of fine particles from the prosthesis during wearing of the prosthesis. In a preferred embodiment, the ultrahigh molecular weight polyethylene forms the rod bearing surface of the prosthesis.

The products and methods of the present invention also apply to other polymeric materials such as high density polyethylene, low density polyethylene, linear low density polyethylene and polypropylene.

The following non-limiting examples further illustrate the invention.

Example 1 Methods of Making Melt Irradiated (MIR) UHMWPE

This example describes electron beam irradiation of molten UHMWPE.

A 10 mm x 12 mm x 60 mm cubic specimen (puck) made from a conventional ram extruded UHMWPE bar stock (Hosk Cellanes GUR 415 Bar Stock, obtained from Westlake Plastics, Rennie, Pennsylvania) was placed in a chamber. Located. The atmosphere in the chamber was composed of low oxygen nitrogen gas (<0.5 ppm oxygen gas) (obtained from AIRCO, Murray Hill, NJ). The pressure in the chamber was brought to about 1 atmosphere. The temperature of the sample and irradiation chambers was controlled using heaters, variacs, and thermocouple readers (manual) or thermostats (automatic). The chamber was heated using a 270W heating mantle. The chamber was heated at a heating rate such that the steady state temperature of the sample was about 175 ° C. (controlled by bariac). This sample was held at steady state temperature for 30 minutes before irradiation.

Irradiation was performed using a van de Graaff generator with an electron energy of 2.5 MeV and an emissivity of 1.67 Mrad / min. A 20 Mrad dose of electron beam was provided to the sample to hit the 60 mm × 12 mm surface of the sample. The heater was turned off after irradiation and the sample was cooled to 25 ° C. at a cooling rate of about 0.5 ° C./min in an inert atmosphere, chamber under nitrogen gas. As a control, similar specimens were prepared using unheated, unirradiated bar stock made of conventional UHMWPE.

Example 2 Comparison of Properties of GUR 415 UHMWPE Bar Stocks with Melt Irradiated (MIR) GUR 415 UHMWPE Bar Stocks (20 Mrad)

This example illustrates various properties of the irradiated and unirradiated samples of UHMWPE bar stock (GUR 415) obtained from Example 1. The samples tested were as follows: The test samples were stocks which were examined after melting; The control was bar stock without any heating / melting and no irradiation.

(A) Differential Scanning Calorimetry (DSC)

Perkin-Elmer DSC 7 was used with an ice water heat sink and heating and cooling at 10 ° C./min with continuous nitrogen cleaning. The crystallinity of the sample obtained from Example 1 was calculated from the heat of fusion of polyethylene crystals (69.2 cal / g) and the weight of the sample. The temperature corresponding to the endothermic peak was taken as the melting point. Estimate the lamellae crystalline phase, the heat of fusion ΔH ° (69.2 cal / g) of the 100% crystalline polyethylene, the melting point of the complete crystal (418.15K), the density of the crystalline region (1.005 g / cm ³ ) and the terminal free surface energy of the polyethylene The lamella thickness was calculated by measuring (2.22 × 10 ⁻⁶ cal / cm ² ). The results are shown in Table 1 and FIG. 4.

TABLE 1

DSC (10 ° C / min)

These results indicate that the melt irradiated sample has a more entangled structure and a less crystalline polymer structure than the unirradiated sample, as evidenced by the low crystallinity, low lamella thickness and low melting point.

(B) expansion ratio

Samples were cut into cubes of size 2 mm x 2 mm x 2 mm, which were immersed in decalin at 150 ° C. for 24 hours. An antioxidant (1% N-phenyl-2-naphthylamine) was added to the decalin to prevent degradation of the sample. Before the experiment, the expansion ratio and percent extraction were calculated by inflating for 24 hours and vacuum drying the expanded sample after weighing the sample. The results are shown in Table 2 below.

TABLE 2

Swell in decalin with antioxidant at 150 ° C. for 24 hours

The results indicate that the melt irradiated UHMWPE sample is highly crosslinked and can dissolve the polymer chain in hot solvent even after 24 hours, whereas the unirradiated sample is completely dissolved in hot solvent in the same period as above. .

(C) tensile modulus

The ASTM 638 M III of the sample is as follows. The displacement rate was 1 mm / minute. Experiments were performed on an MTS machine. The results are shown in Table 3 below.

TABLE 3

Elastic test (ASTM 638 M III, 1 mm / min)

The results indicate that melt irradiated UHMWPE samples have significantly lower tensile modulus than unirradiated controls. The low stress at break of melt irradiated UHMWPE samples is more convincing evidence for crosslinking of the chains in these samples.

(D) hardness

The hardness of the sample was measured using a Durometer on the Shore D scale. The hardness for the immediate indenter station was recorded. The results are shown in Table 4 below.

TABLE 4

Hardness (Shore D)

The results indicate that the melt irradiated UHMWPE was softer than the unirradiated control.

(E) Light transmittance (transparency)

The transparency of the sample was measured as follows: The light transmittance was examined for a light wavelength of 517 nm passing through a sample of about 1 mm thickness placed between two glass slides. Samples were prepared by polishing the surface against 600 grit paper. After spraying silicone oil on the sample surface, the sample was placed between two slides. Silicone oil was used to reduce the spread of light scattering due to the surface roughness of the polymer sample. The standard material used for this purpose was two similar glass slides separated by a thin film of silicone oil. Transmittance was measured using a Perkin Elmer lambda 3B ultraviolet-visible spectrophotometer. The extinction coefficient and transmittance of exactly 1 mm thick samples were calculated using Lambert-Bee Law. The results are shown in Table 5 below.

TABLE 5

Light transmittance at 517nm

The results indicate that the melt irradiated UHMWPE sample transmits much more light through the sample than the control, making it much more transparent than the control.

(F) Environmental scanning electron speculum (ESSM)

ESEM (ElectroScan, Model 3) was performed at 10 kV (low voltage to reduce the radiation loss rate of the sample) with a very thin gold coating on the sample (about 20 kV to increase the quality of the picture). A study of the polymer surface under ESEM with or without gold coating proved that the thin gold coating did not produce any workpieces.

Samples were etched with a sulfuric acid to ortho-phosphate ratio of 1: 1 and potassium permanganate at a concentration of 0.7% (w / v) prior to observation under ESEM.

FIG. 5 shows the ESEM (magnification 10,000 ×) of the etched surface (GUR 415; not heat treated; not irradiated) of conventional UHMWPE. FIG. 6 shows the ESEM (magnification 10,500 ×) of the etched surface (GUR 415; molten; 20 Mrad) of melt irradiated UHMWPE. The ESEM showed that the size of the microcrystals was reduced and incomplete crystals occurred in the melt irradiated UHMWPE compared to conventional UHMWPE.

(G) Fourier Transform Infrared Spectroscopy (FTIR)

FTIR of the samples was performed using a microsampler on a sample washed with hexane to remove surface impurities. The peak observed at about 1740-1700 cm ⁻¹ was the band associated with the oxygen containing group. Thus, the degree of oxidation was measured using the ratio of the area under the carbonyl peak at 1740 cm ⁻¹ to the area under the methylene peak at 1460 cm ⁻¹ .

FTIR spectra show that the melt irradiated UHMWPE sample has a greater degree of oxidation than the conventional unirradiated UHMWPE control, but much less oxidation than the irradiated UHMWPE sample provided with the same irradiation as the melt irradiated sample at room temperature in air. Prove it was.

(H) electron paramagnetic resonance (EPR)

EPR on samples placed in a nitrogen atmosphere in an airtight quartz tube was performed at room temperature. The apparatus used was a Bruker ESP 300 EPR spectrometer, and the tube used was a Taperlock EPR sample tube obtained from Wilmad Glass Company, Buena, NJ.

Since irradiation is a method of generating free radicals in the polymer, the unirradiated sample does not have any free radicals therein. Upon irradiation, free radicals are produced that can last for years under suitable conditions.

The EPR results show that when the melt irradiated sample is studied using EPR immediately after irradiation, no free radicals appear, while the sample irradiated at room temperature under nitrogen atmosphere is trans-vinylene free even after storage at room temperature for 266 days. Indicates that a radical was formed. The absence of free radicals in the melt irradiated UHMWPE sample means that no further oxidation is possible.

(I) wear

The wear resistance of the samples was measured using a bi-axial pin-on-disk wear tester. The abrasion test involves the frictional action of UHMWPE pins (diameter = 9 mm; height = 13 mm) against Co-Cr alloy discs. These tests were performed in total 2,000,000 cycles. Unirradiated pins exhibited a wear rate of 8 mg / million-cycle, while irradiated pins exhibited a wear rate of 0.5 mg / million-cycle. The results indicate that melt irradiated UHMWPE had much better wear resistance than unirradiated controls.

Example 3 Methods for Making Conventional Joint Cups Made of Melt Irradiated (MIR) UHMWPE

This example illustrates electron beam irradiation of a conventional joint cup made of molten UHMWPE.

A conventional articulated cup (26 mm inner diameter manufactured from GUR 415 ram extruded bar stock (Adaptive, non-sterile UHMWPE cup obtained from Zimmer, Inc., Washo, IN) was controlled in a controlled atmosphere and Under temperature conditions were irradiated in an airtight chamber with a titanium cup holder at the bottom and a stainless steel foil (0.001 inch thick) at the top. The atmosphere in this chamber was composed of low oxygen nitrogen gas (<0.5 ppm oxygen gas) (obtained from AIRCO, Murray Hill, New Hampshire). The pressure in the chamber was about 1 atmosphere. The chamber was heated using a 270 W heating mantle at the bottom of the chamber controlled using a thermostat and variac. The chamber was heated so that the temperature at the top surface of the cup rose to about 1.5 ° C. to 2 ° C./min and finally reached a steady state temperature of about 175 ° C. Due to the special design of the apparatus used and the thickness of the sample cups, the steady state temperature of the cups varied between 200 ° C. at the bottom and 175 ° C. at the top. The cup was held at this temperature for 30 minutes before starting the irradiation.

Irradiation was carried out using a Vandegraph generator with an electron energy of 2.5 MeV and an irradiation rate of 1.67 MRad / min. The electron beam entered the chamber through the thin film foil at the top and hit the concave surface of the cup. The dose received by the cup was that a maximum dose of 20 Mrad was received about 5 mm below the surface of the cup hit by the electrons. After irradiation, heating was stopped and the cup was cooled to room temperature (about 25 ° C.) while continuously maintained in a chamber filled with nitrogen gas. The cooling rate was about 0.5 ° C./min. After the temperature of the chamber and the sample reached room temperature, the sample was removed from the chamber.

The irradiated cups that increase in volume (due to a decrease in density with a decrease in crystallinity after melt irradiation) can be machined back to suitable dimensions.

Example 4 Expansion Ratios and% Extraction at Different Depths for Melt Irradiated (MIR) UHMWPE Joint Cups

This example describes the expansion ratio and percent extraction at different depths of the melt irradiated articulated cup obtained from Example 3. Samples of 2 mm × 2 mm × 2 mm size were cut from the cup at various depths along the axis of the cup. These samples were then immersed in decalin at 150 ° C. for 24 hours. An antioxidant (1% N-phenyl-2-naphthylamine) was added to the decalin to prevent degradation of the sample. Prior to the experiment, the sample weight was measured after inflating for 24 hours and after the expanded sample was vacuum dried to calculate the expansion ratio and percent extraction. The results are shown in Table 6 below.

TABLE 6

Expansion ratio and percent extraction at different depths for melt irradiated UHMWPE joint cups

The results indicate that UHMWPE in the cup was crosslinked to a depth of 12 mm to the extent that no polymer chains were dissolved in hot decalin for 24 hours due to the melt irradiation method.

Example 5 Crystallinity and Melting Point at Different Depths for Melt Irradiated (MIR) UHMWPE Joint Cups

This example illustrates the crystallinity and melting point at different depths of the melt irradiated cups obtained from Example 3.

Samples were taken from the cups at different depths along the axis of the cup. Crystallinity is the fraction of crystalline polymer. Weight of sample (w (g)), heat absorbed by the sample upon melting (E (cal), measured experimentally using a differential scanning calorimeter at 10 ° C./min) and melting of polyethylene in 100% crystallization Heat (ΔH ° = 69.2 cal / g) was measured and crystallinity was calculated using the following formula:

The melting point is the temperature corresponding to the DSC endothermic peak. The results are shown in FIG.

The results indicate that the crystallinity and melting point of the melt irradiated UHMWPE in the joint cups obtained from Example 3, even up to a depth of 1 cm (thickness of the cup is 1.2 cm), were much lower than the values corresponding to conventional UHMWPE. .

Example 6 Second Method for Making Melt Irradiated (MIR) UHMWPE Articulated Cups

This example illustrates a method of making a joint cup made of melt irradiated UHMWPE.

A conventional ram extruded UHMWPE bar stock (GUR 415 bar stock obtained from Westlake Plastics, Lenny, Pennsylvania) was machined into a cylinder shape of 4 cm in height and 5.2 cm in diameter. One circular face of the cylinder was machined to contain an accurate hemispherical hole of 2.6 cm in diameter, coinciding the axis of the hole with the cylinder. This specimen was placed in an airtight chamber with a thin stainless steel foil (0.001 inch thick) on top. The cylindrical specimen was positioned so that the hemispherical hole faces the foil. The chamber was then flushed and filled with a low oxygen nitrogen gas (<0.5 ppm oxygen gas) atmosphere obtained from AIRCO, Murray Hill, NJ. After charging in this manner, a continuous nitrogen slow flow was maintained while maintaining the pressure in the chamber at about 1 atmosphere. At the bottom of the chamber controlled using a thermostat and variac, the chamber was heated using a 270 W heating mantle. The chamber was heated so that the temperature of the upper surface of the cylindrical specimen rose to a heating rate of about 1.5 ° C. to 2 ° C./min, eventually reaching a steady state temperature of about 175 ° C. The specimen was then kept at this temperature for 30 minutes prior to the start of irradiation.

Irradiation was performed using a Van de Graf generator with an electron energy of 2.5 MeV and an irradiation rate of 1.67 Mrad / min. The electron beam entered the chamber through the thin film foil at the top and hit the surface with the hemispherical hole. The dose received by the sample was that a maximum dose of 20 Mrad was received about 5 mm below the surface of the polymer hit by the electrons. After irradiation, heating was stopped while continuing to be maintained in a chamber filled with nitrogen gas and the specimen was cooled to room temperature (about 25 ° C.). The cooling rate was about 0.5 ° C./min. The sample was removed from the chamber after the temperature of the chamber and sample reached room temperature.

The cylindrical specimen was then machined into a joint cup with the dimensions of a joint cup made of highly adaptable UHMWPE with an inner diameter of 26 mm manufactured from Zimmer, Inc., Washo, Indiana, to provide the concave surface of the hemispherical hole. It was machined back into the joint surface. This method may allow for relatively large dimensional changes during melt irradiation.

Example 7 Irradiation of UHMWPE Puck

This example illustrates that electron beam irradiation of the UHMWPE puck provides a non-uniform absorbed dose profile.

A conventional UHMWPE ram extruded bar stock (Hosk Celanes GUR 415 bar stock, obtained from Westlake Plastics, Lenny, Pennsylvania) was used. The GUR 415 resin used for the bar stock had a molecular weight of 5,000,000 g / mol and contained 500 ppm calcium stearate. The bar stock was cut into cylinders shaped "hockey puck" (height 4 cm, diameter 8.5 cm).

The linear beam accelerator (obtained from AECL, Manitoba, Canada), operated at 10 MeV and 1 kW with a scanning width of 30 cm and a conveyor speed of 0.08 cm / sec, projects an electron beam into one of the circular bottoms of the puck to keep the puck at room temperature. Investigated. Due to the cascade effect, electron beam irradiation formed a non-uniform absorbed dose profile. Table 7 lists the absorbed doses calculated at various depths of polyethylene specimens irradiated with an electron beam of 10 MeV. The absorption dose was the value measured at the upper surface (surface on which the electron beam was incident).

TABLE 7

Change in absorbed dose expressed as a function of depth in polyethylene

Example 8 Method for Making UHMWPE Using Cold Irradiation and Subsequent Melting (CIR-SM)

This example provides a method for producing UHMWPE having a crosslinked structure and virtually no free radicals is detected by cooling and then melting the UHMWPE.

A conventional UHMWPE ram extruded bar stock (Hosk Celanes GUR 415 bar stock, obtained from Westlake Plastics, Lenny, Pennsylvania) was used. The GUR 415 resin used for the bar stock had a molecular weight of 5,000,000 g / mol and contained 500 ppm calcium stearate. The bar stock was cut into a "hockey puck" shaped cylinder (height 4 cm, diameter 8.5 cm).

Total absorption dose of 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 30, and 50 Mrad at an irradiation rate of 2.5 Mrad per bar pass, measured at room temperature, on the top surface (surface on which the electron beam was incident) at room temperature (Using an electron beam accelerator obtained from AECL, Manitoba, Canada). Irradiated in the air without packaging the puck. After irradiation, the puck was heated to 150 ° C. under vacuum for 2 hours to melt the polymer, thereby recombining free radicals such that virtually no free radicals were detected. The pucks were then cooled to room temperature at a cooling rate of 5 ° C./min.

Free radicals remaining by electron paramagnetic resonance were measured as described in Jahan et al., J. Biomedical Materials Research 25: 1005 (1991).

Example 9 Method for Making UHMWPE Using Warm Irradiation and Subsequent Melting (WIR-SM)

This example illustrates a method of preparing a UHMWPE having a crosslinked structure and virtually free of free radicals, by irradiating UHMWPE heated to a temperature below the melting point and subsequently melting the UHMWPE.

A conventional UHMWPE ram extruded bar stock (Hosk Celanes GUR 415 bar stock, obtained from Westlake Plastics, Lenny, Pennsylvania) was used. The GUR 415 resin used for the bar stock had a molecular weight of 5,000,000 g / mol and contained 500 ppm calcium stearate. Bar stock was cut into cylinders of "hockey puck" shape (height 4 cm, diameter 8.5 cm).

The puck was heated to 100 ° C. in air in an oven. The heated puck was then irradiated with an electron beam (E-beam service, Cranberry, NJ) at a total dose of 20 Mrad at an irradiation rate of 2.5 Mrad per pass with a scan width of 30 cm and a conveyor speed of 0.08 cm / sec. After irradiation, the puck was heated to 150 ° C. under vacuum for 2 hours to recombine free radicals such that virtually no free radicals were detected. The pucks were then cooled to room temperature at a cooling rate of 5 ° C./min.

Example 10 Method for Producing UHMWPE Using Warm Irradiation and Adiabatic Melting (WIR-AM)

This example illustrates a method of producing a UHMWPE having a crosslinked structure and virtually free of free radicals, by irradiating UHMWPE heated to a temperature below the melting point to generate adiabatic melting of the UHMWPE.

A conventional UHMWPE ram extruded bar stock (Hosk Celanes GUR 415 bar stock, obtained from Westlake Plastics, Lenny, Pennsylvania) was used. The GUR 415 resin used for the bar stock had a molecular weight of 5,000,000 g / mol and contained 500 ppm calcium stearate. Bar stock was cut into cylinders of "hockey puck" shape (height 4 cm, diameter 8.5 cm).

Two pucks were packed in fiberglass bags (obtained from Fisher Scientific Co., Pittsburgh, Pennsylvania) to minimize heat loss in subsequent processing steps. First, the packed puck was heated overnight in an air convection oven maintained at 120 ° C. As soon as the puck was taken out of the oven, it was placed under an electron beam incident on one of the circular bottoms of the puck from a linear electron accelerator operating at 10 MeV and 1 kW and immediately irradiated with a total dose of 21 and 22.5 Mrads, respectively. The irradiation rate was 2.7 Mrad / min. Therefore, 21 Mrad was irradiated for 7.8 minutes and 22.5 Mrad was irradiated for 8.3 minutes. After irradiation, the pucks were cooled to room temperature at a cooling rate of 5 ° C./min at which temperature the fiberglass bags were removed and the samples analyzed.

Example 11 Comparison of Properties of GUR 415 UHMWPE Bar Stock Puck with CIR-SM and WIR-AM Treated Bar Stock Puck

This example illustrates various properties of the irradiated and unirradiated samples of UHMWPE bar stock GUR 415 obtained from Examples 8 and 10. The tested samples were (i) tested at room temperature, then heated to about 150 ° C. to completely melt the polyethylene crystals, and then cooled to room temperature (CIR-SM) as test samples from the stock (puck), (ii ) A test sample (puck) from bar stock heat-melted polyethylene crystals (WIR-AM) by heating to 120 ° C. in a fiber glass bag to minimize heat loss from the puck and then irradiating immediately, and (iii) (any Control bar stock without heating / melting and irradiation).

A. Fourier Transform Infrared Spectroscopy (FTIR)

Infrared (IR) spectroscopy of the samples was performed using a BioRad UMA 500 infrared microscope on the thin portions of the samples obtained from Examples 8 and 10. The slender portion (50 μm) was prepared using a sledge microtome. IR spectra were collected at 20 μm, 100 μm, and 3 mm below the irradiation surface of the puck with a pore size of 10 × 50 μm ² , respectively. The peak observed at about 1740-1700 cm ⁻¹ was associated with the oxygen containing group. Thus, the after subtracting the corresponding baselines, carbonyl area ratio is a measure of the degree of oxidation of the peak area under the methylene peak at 1460cm ^-1 under at 1740cm ^-1. Tables 8 and 9 below summarize the degrees of oxidation for the samples described in Examples 8 and 10.

These data indicate that some oxidation occurred in the thin layer about 100 μm thick after crosslinking. In the case of machining this layer, the final product will have the same oxidation level as the unirradiated control.

TABLE 8

Oxidation degree of specimen from Example 8 (CIR-SM) (melting after irradiation in vacuum)

TABLE 9

Oxidation degree of specimen from Example 10 (WIR-AM)

B. Differential Scanning Calorimetry (DSC)

Perkin-Elmer DSC 7 was used with an ice water heat sink and at a heating rate and cooling rate of 10 ° C./min with continuous nitrogen wash. The crystallinity of the samples obtained from Examples 8 and 10 was calculated from the heat of fusion and sample weight of the polyethylene crystals measured during the first heating cycle. Crystallinity (%) is obtained from the following formula:

Where E and w are the heat of fusion (J or cal) and the weight in grams (g), respectively, of the tested specimens, and ΔH ° is the heat of fusion of 299% crystalline polyethylene expressed in J / g (291 J / g or 69.2 cal / g). The temperature corresponding to the endothermic peak was taken as the melting point. In some cases where there are multiple endothermic peaks, multiple melting points corresponding to these endothermic peaks were recorded. The crystallinity and melting point for the samples described in Examples 8 and 10 are reported in Tables 10 and 11 below.

TABLE 10

DSC at a heating rate of 10 ° C./min for the samples of Example 8 (CIR-SM)

TABLE 11

DSC at a heating rate of 10 ° C./min for the sample of Example 10 (WIR-AM)

The data indicate that crystallinity does not change significantly up to the absorbed dose of 20 Mrad. Therefore, the elastic properties of the crosslinked material must remain virtually unchanged upon crosslinking. On the other hand, the elastic properties can be adjusted by changing the crystallinity with a high dose. The data also shows that the WIR-AM treated material showed three melting peaks.

C. Pin-on-Disk Experiment on Wear Rate

In order to test the polymer pins with the polishing action of the pins against the highly polished Co-Cr discs, pin-on-disk (POD) experiments were carried out on a biaxial pin-on-disk tester at a frequency of 2 Hz. Prior to manufacturing the cylindrical shaped pins (height 13 mm, diameter 9 mm), 1 mm was machined from the surface of the puck to remove the oxidized outer layer during irradiation and before and after processing. The pins were then machined from the core of the puck and tested on a POD, with the electron beam incident surface facing the Co-Cr disk. Abrasion tests were performed up to 2,000,000 cycles in bovine serum. The weight of the pin was measured every 500,000 cycles and the average value (wear rate) of the weight loss for the specimens obtained from Examples 8 and 10 are reported in Tables 12 and 13, respectively.

TABLE 12

POD wear rate for the sample of Example 8 (CIR-SM)

TABLE 13

POD wear rate for the sample of Example 10 (WIR-AM)

The results indicate that the crosslinked UHMWPE had much better wear resistance than the unirradiated control.

D. Gel Content and Expansion Ratio

Samples were cut into 2 × 2 × 2 mm ³ size cubes, which were immersed in xylene at 130 ° C. for 24 hours. An antioxidant (1% N-phenyl-2-naphthylamine) was added to the xylene to prevent degradation of the sample. Sample weights were measured before inflation, after inflation for 24 hours, and after the expanded sample was vacuum dried to calculate the expansion ratio and gel content. The results for the samples obtained from Examples 8 and 10 are shown in Tables 14 and 15 below.

TABLE 14

Gel Content and Expansion Ratio for Specimen of Example 8 (CIR-SM)

TABLE 15

Gel content and expansion ratio for the sample of Example 10 (WIR-AM)

The results indicate that the expansion ratio decreases with an increase in the absorption dose indicating an increase in the crosslinking density. Increased gel content indicates the formation of crosslinked structures.

Example 12 Free Radical Concentration for UHMWPE Prepared by Cold Irradiation (CIR-SM) with and Without Subsequent Melt

This example illustrates the melting effect following the cold irradiation of UHMWPE on free radical concentration. Electron paramagnetic resonance (EPR) was performed at room temperature on samples after being placed in a nitrogen atmosphere in hermetic quartz tubes. The apparatus used was a Bruker ESP 300 EPR spectrometer, and the tube used (obtained from the Wilmad Glass Company, Buena, NJ) was a taperlock EPR sample tube.

The unirradiated sample did not have any detectable free radicals in the sample. During the course of the investigation, free radicals were formed that could last for at least several years under suitable conditions.

When tested using EPR technology, cold irradiated UHMWPE specimens showed strong free radical signals. When the same sample was detected by EPR after the melting cycle, it was found that the EPR signal was reduced to an undetectable level. The absence of free radicals in cold irradiated and subsequently molten (recrystallized) UHMWPE means that no further oxidative degradation can occur through attack on the trapped radicals.

Example 13 : Crystallinity and Melting Point at Different Depths for UHMWPE Prepared by Cold Irradiation and Subsequent Melting (CIR-SM)

This example provides the crystallinity and melting point at different depths of the crosslinked UHMWPE specimen obtained from Example 8 with a total dose of 20 Mrad. Samples were taken at various depths from the crosslinked specimens. Crystallinity and melting point were measured using a Perkin Elmer differential scanning calorimeter as described in Example 10 (B). The results are shown in Table 16 below.

TABLE 16

DSC at a heating rate of 10 ° C./min, for a sample of Example 8 (CIR-SM) irradiated with a total dose of 20 Mrad

The results indicate that the crystallinity changed with depth from the surface. The sudden drop at 16mm is due to the cascade effect. The peak at the absorbed dose was located near 16 mm, where the dose level was as high as 27 Mrad.

Example 14 Comparison of UHMWPE Prepared by CIR-SM Using Melting in Air versus Melting in Vacuum

This example illustrates that the oxidation level of the UHMWPE puck manufactured by CIR-SM is the same as the unirradiated puck at a depth of 3 mm below the surface of the puck, regardless of whether it melts in air or in vacuum.

A conventional UHMWPE ram extruded bar stock (Hosk Celanes GUR 415 bar stock, obtained from Westlake Plastics, Lenny, Pennsylvania) was used. The GUR 415 resin used for the bar stock had a molecular weight of 5,000,000 g / mol and contained 500 ppm calcium stearate. The bar stock was cut into a "hockey puck" shaped cylinder (height 4 cm, diameter 8.5 cm).

Two pucks were irradiated with a total absorbed dose of 17.5 Mrad at an irradiation rate of 2.5 Mrad per bar pass, measured on the top surface (surface where the electron beam was incident) with a scan width of 30 cm and a conveyor speed of 0.07 cm / sec at room temperature. [Using electron beam accelerators obtained from AECL, Manitoba, Canada, Canada]. Irradiated in air without packaging the puck. After irradiation, one puck is heated to 150 ° C. for 2 hours under vacuum and the other puck is heated to 150 ° C. for 2 hours in air so that no detectable residual crystalline content and no detectable free radicals are present. It was. The pucks were then cooled to room temperature at a cooling rate of 5 ° C./min. The degree of oxidation of the puck was then analyzed as described in Example 11 (A). In Table 11 below, the results obtained for the degree of oxidation are described.

TABLE 17

Oxidation degree of samples melted in air versus samples melted in vacuum

The results indicate that the oxidation level in the UHMWPE specimens irradiated within 3 mm below the free surface was lowered to the oxidation level observed in UHMWPE of the unirradiated control. This is the case irrespective of the melting atmosphere (air or vacuum) after irradiation. Thus, after irradiation, melting could be carried out in an air convection oven without oxidizing the core of the irradiated puck.

Example 15 Method for Producing UHMWPE by Cooled Irradiation and Subsequent Melting (CIR-SM) Using Gamma Irradiation

This example illustrates a method for producing UHMWPE having a crosslinked structure and virtually no free radicals is detected by cooling and irradiating UHMWPE with gamma irradiation.

A conventional UHMWPE ram extruded bar stock (Hosk Celanes GUR 415 bar stock, obtained from Westlake Plastics, Lenny, Pennsylvania) was used. The GUR 415 resin used for the bar stock had a molecular weight of 5,000,000 g / mol and contained 500 ppm calcium stearate. The bar stock was cut into a "hockey puck" shaped cylinder (height 4 cm, diameter 8.5 cm).

The puck was irradiated with a total absorbed dose of 4 Mrad at an irradiation rate of 0.05 Mrad / min, measured at room temperature, on the top surface (the side where gamma rays entered) (Isomedix, Northboro, Mass.). Irradiated in air without packaging the puck. After irradiation, the puck was heated to 150 ° C. under vacuum for 2 hours to melt the polymer to recombine free radicals so that virtually no free radicals were detected.

Example 16 I. Process for Making UHMWPE Using Warm Irradiation and Partial Adiabatic Melt (WIR-AM), Subsequently Completely Melt

This example produces a thermally insulative partial melting of the UHMWPE by irradiating a UHMWPE heated to a temperature below the melting point and subsequently melting the UHMWPE, thus having a cross-linked structure and allowing two individual melt endotherms in a differential scanning calorimeter (DSC). It illustrates a method of making a UHMWPE that is shown as well as virtually no free radicals detected.

GUR 4050 bar stock (manufactured from ram extruded Hoeskist Celanese GUR 4050 resin obtained from Westlake Plastics, Rennie, Pennsylvania) was machined into a hockey puck 8.5 cm in diameter and 4 cm thick. 25 puck, 25 aluminum holders, and 25 20 cm x 20 cm fiberglass blankets were preheated to 125 ° C. overnight in an air convection oven. The preheated puck was each placed in a preheated aluminum holder covered with a preheated fiberglass blanket to minimize heat loss to the environment during irradiation. The puck was then irradiated in air using a 10 MeV, 1 kW electron beam having a scan width of 30 cm (using an electron beam accelerator obtained from AECL, Manitoba, Canada). The conveyor speed was 0.07 cm / sec, giving an irradiation rate of 70 kGy per pass. The puck was irradiated twice under an electron beam to bring the total absorbed dose to 140 kGy. In the second pass, the conveyor belt was reversed as soon as the puck exited the electron raster area so that there was no heat loss from the puck. After warm irradiation, 15 pucks were heated to 150 ° C. for 2 hours to completely melt the crystals and substantially remove free radicals.

A. Thermal Properties (DSC) of Samples Prepared in Example 16

Perkin-Elmer DSC 7 was used at a cooling rate of 10 ° C./minute with an ice water heat sink and with continuous nitrogen cleaning. The crystallinity of the sample obtained from Example 16 was calculated from the weight of the sample and the heat of fusion of polyethylene crystals (69.2 cal / gm). The temperature corresponding to the endothermic peak was taken as the melting point. In the case of multiple endothermic peaks, multiple melting points were recorded.

Table 18 shows the changes obtained in the melt behavior and crystallinity of the polymer with depth from the electron beam incident surface. 8 shows a representative DSC melt endotherm obtained at 2 cm below the electron beam incident surface obtained for both before and after subsequent melting.

TABLE 18

WIR-AM GUR 4050 Bar Stock, Total Dose = 140kGy, 75kGy / Pass

These results show that the melting behavior of UHMWPE is significantly changed after the subsequent melting step in this embodiment of the WIR-AM method. Prior to subsequent melting, the polymer showed three melting peaks, whereas after subsequent melting, two melting peaks.

B. Electronic Paramagnetic Resonance (EPR) of Samples Prepared in Example 16

After placing the sample in a gas-tight quartz tube in a nitrogen atmosphere, EPR was performed on the sample obtained from Example 16 at room temperature. The apparatus used was a Bruker ESP 300 EPR spectrometer, and the tube used was a taperlock EPR sample tube (obtained from the Wilmad Glass Company, Buena, NJ).

The unirradiated sample did not have any detectable free radicals inside. During the investigation, free radicals were formed that could last for at least several years under suitable conditions.

Prior to subsequent melting, the EPR results showed a complex free radical peak consisting of both peroxy and primary free radicals. After subsequent melting, the EPR free radicals signal was reduced to undetectable levels. These results indicated that the free radicals caused by the irradiation process were virtually removed after the subsequent melting step. Thus, UHMWPE showed high oxidation resistance.

Example 17: II. Method for Making UHMWPE Using Warm Irradiation and Partial Adiabatic Melting (WIR-AM), Subsequently Fully Melted

This example, by irradiating UHMWPE heated to a temperature below the melting point to generate adiabatic partial melting of the UHMWPE and subsequently melting the UHMWPE, has a crosslinked structure and exhibits two individual melt endotherms in the DSC and is virtually free Illustrates a method of making UHMWPE in which no radicals are detected.

GUR 4020 bar stock (made from ram extruded Hoeskist Celanese GUR 4020 resin obtained from Westlake Plastics, Rennie, Pennsylvania) was machined into a hockey puck 8.5 cm in diameter and 4 cm thick. 25 puck, 25 aluminum holders, and 25 20 cm x 20 cm fiberglass blankets were preheated to 125 ° C. overnight in an air convection oven. The preheated pucks were each placed in a preheated aluminum holder covered with a preheated fiberglass blanket to minimize heat loss to the surroundings during irradiation. The puck was then irradiated in air using a 10 MeV, 1 kW electron beam having a scan width of 30 cm (using an electron beam accelerator obtained from AECL, Manitoba, Canada). The conveyor speed was 0.07 cm / sec, giving an irradiation rate of 70 kGy per pass. The puck was irradiated twice under an electron beam to give a total absorbed dose of 140 kGy. For the second pass, to avoid any heat loss from the puck, the conveyor belt was reversed as soon as the puck exited the electron beam raster area. After warm irradiation, 15 pucks were heated to 150 ° C. for 2 hours to completely melt the crystals and virtually eliminate free radicals.

Example 18 : III. Method for Making UHMWPE Using Warm Irradiation and Partial Adiabatic Melting (WIR-AM), Subsequently Completely Melting

The present example not only exhibits two individual melt endotherms in the DSC while having a crosslinked structure by irradiating UHMWPE heated to a temperature below the melting point to generate adiabatic partial melting of the UHMWPE and subsequently melting the UHMWPE. Illustrates a method of making a UHMWPE in which no free radicals are detected.

GUR 1050 bar stock (manufactured from ram extruded Hoeskist Celanese GUR 1050 resin obtained from Westlake Plastics, Rennie, Pennsylvania) was machined into a hockey puck 8.5 cm in diameter and 4 cm thick. 18 puck, 18 aluminum holders, and 18 20 cm x 20 cm glass fiber blankets were preheated to 125 ° C, 90 ° C, or 70 ° C, respectively, in an air convection oven overnight. Six pucks were used for each different preheat temperature. The preheated puck was each placed in a preheated aluminum holder covered with a preheated fiberglass blanket to minimize heat loss to the environment during irradiation. The puck was then irradiated in air using 10 MeV and 1 kW electron beams with a scan width of 30 cm (using electron beam accelerators obtained from AECL, Manitoba, Canada). The conveyor speed was 0.06 cm / sec, giving an irradiation rate of 70 kGy per pass. The puck was irradiated twice under an electron beam to give a total absorbed dose of 150 kGy. For the second pass, the conveyor belt's motion was reversed as soon as the puck exited the electron raster area so that there was no heat loss from the puck. After warm irradiation, half of the puck was heated to 150 ° C. for 2 hours to completely melt the crystals and virtually eliminate free radicals.

A. Thermal Properties of Samples Prepared in Example 18

Perkin-Elmer DSC 7 was used at a cooling rate of 10 ° C./minute with an ice water heat sink and with continuous nitrogen cleaning. The crystallinity of the sample obtained from Example 18 was calculated from the weight of the sample and the heat of fusion of polyethylene crystals (69.2 cal / gm). The temperature corresponding to the endothermic peak was taken as the melting point. In the case of multiple endothermic peaks, multiple melting points were recorded.

Table 19 describes the effect of preheat temperature on the melt behavior and crystallinity of the polymers. 9 shows the DSC profile of the puck treated with the WIR-AM method at a preheat temperature of 125 ° C. for both before and after subsequent melting.

TABLE 19

WIR-AM GUR 1050 Bar Stock, Total Dose = 150kGy, 75kGy / Pass

These results show that the melting behavior of UHMWPE is significantly changed after the subsequent melting step in this embodiment of the WIR-AM method. Prior to the subsequent melting, the polymer showed three melting peaks, while after the subsequent melting, two melting peaks.

Example 19 IV. Method for Making UHMWPE Using Warm Irradiation and Partial Adiabatic Melting (WIR-AM), Subsequently Completely Melting

This example, by heating to a temperature below the melting point, irradiates the UHMWPE to generate adiabatic partial melting of the UHMWPE and subsequently to melt the UHMWPE, thus having a cross-linked structure and exhibiting two individual melt endotherms in the DSC as well as virtually free Illustrates a method of making UHMWPE in which no radicals are detected.

GUR 1020 bar stock (manufactured from Ram extruded Hoeskist Celanese GUR 1050 resin obtained from Westlake Plastics, Rennie, Pennsylvania) was machined into a hockey puck 7.5 cm in diameter and 4 cm thick. Ten puck, ten aluminum holders, and ten 20 cm × 20 cm glass fiber blankets were preheated to 125 ° C. overnight in an air convection oven. The preheated puck was each placed in a preheated aluminum holder covered with a preheated fiberglass blanket to minimize heat loss to the surroundings during irradiation. The puck was then irradiated in air using a 10 MeV and 1 kW linear electron accelerator (AECL, Manitoba Pinawa, Canada). Scan width and conveyor speed were adjusted to achieve the desired irradiation rate per pass. The pucks were then irradiated with total absorbed doses of 61, 70, 80, 100, 140, and 160 kGy. Irradiation was completed in one pass for 61, 70 and 80 kGy uptake doses, while irradiation was completed in two passes for 100, 140 and 160 kGy doses. For each absorbed dose level, six pucks were examined. During the two pass experiment for the second pass, the conveyor belt's motion was reversed as soon as the puck exited the electron beam raster area to ensure no heat loss from the puck. After irradiation, half of the puck was heated to 150 ° C. for 2 hours in an air convection oven to completely melt the crystals and virtually eliminate free radicals.

Example 20 V. Method for Producing UHMWPE Using Warm Irradiation and Partial Adiabatic Melting (WIR-AM), Subsequently Completely Melting

The present example not only exhibits two individual melt endotherms in the DSC while having a crosslinked structure by irradiating UHMWPE heated to a temperature below the melting point to generate adiabatic partial melting of the UHMWPE and subsequently melting the UHMWPE. Illustrates a method of making a UHMWPE in which no free radicals are detected.

GUR 4150 bar stock (made from ram extruded Hoeskist Celanese GUR 4150 resin obtained from Westlake Plastics, Rennie, Pennsylvania) was machined into a hockey puck 7.5 cm in diameter and 4 cm thick. Ten puck, ten aluminum holders, and ten 20 cm × 20 cm glass fiber blankets were preheated to 125 ° C. overnight in an air convection oven. The preheated puck was each placed in a preheated aluminum holder covered with a preheated fiberglass blanket to minimize heat loss to the environment during irradiation. The puck was then irradiated in air using a 10 MeV and 1 kW linear electron accelerator (AECL, Manitoba Pinawa, Canada). The scan width and conveyor speed were adjusted to achieve the desired irradiation rate per pass. The pucks were then irradiated with total absorption doses of 61, 70, 80, 100, 140 and 160 kGy. For each absorbed dose level, six pucks were examined. Irradiation was completed in one pass for absorbed doses of 61, 70 and 80 kGy, while irradiation was completed in two passes for absorbed doses of 100, 140 and 160 kGy.

After irradiation, three puck deviating from each different absorbed dose level were heated to 150 ° C. for 2 hours to completely melt the crystals and reduce the free radical concentration to an undetectable level.

A. Properties of Specimens Prepared in Example 20

Perkin-Elmer DSC 7 was used at 10 ° C./min heating and cooling rate with ice water heat sink and continuous nitrogen wash. The crystallinity of the sample obtained from Example 20 was calculated from the weight of the sample and the heat of fusion of polyethylene crystals (69.2 cal / gm). The temperature corresponding to the endothermic peak was taken as the melting point. In the case of multiple endothermic peaks, multiple melting points were recorded.

The results are listed in Table 20 below as a function of the total absorbed dose level. These results indicate that crystallinity decreases with increasing dose level. At the absorbed dose levels studied, the polymer showed two melting peaks (T ₁ = -118 ° C., T ₂ = -137) after the subsequent melting step.

TABLE 20

WIR-AM GUR 4150 Bar Stock

Example 21 Temperature Rise During WIR-AM Process

This example illustrates the temperature rise during the warm irradiation process, causing adiabatic partial melting or complete melting of the UHMWPE.

GUR 4150 bar stock (manufactured from extruded Hoeskist Celanese GUR 4150 resin obtained from Westlake Plastics, Rennie, Pennsylvania) was machined into a hockey puck 8.5 cm in diameter and 4 cm thick. One hole was made in the puck's body-center. A K-type thermocouple was placed in the hole and the puck was preheated to 130 ° C. in an air convection oven. The puck was then irradiated using 10 MeV and 1 kW electron beam accelerators (AECL, Manitoba Pinawa, Canada). Irradiation was performed in air with an injection width of 30 cm. The irradiation rate was 27 kGy / min and the puck was stationary under the electron beam. The temperature of the puck was measured continuously during the irradiation.

10 illustrates the temperature rise in the puck obtained during the irradiation process. The initial temperature was the preheat temperature (130 ° C.). As soon as the electron beam was activated, the temperature increased, during which the UHMWPE crystals melted. Starting from 130 ° C., smaller size crystals melted, indicating that partial melting occurs during heating. Complete melting is achieved at a temperature near 145 ° C. where a sudden and unexpected change in heating behavior exists. After passing through that point, the temperature of the molten material continues to rise.

This example demonstrates that the dose level (during irradiation) absorbed during the WIR-AM process can be adjusted to partially or completely melt the polymer. In the former case, melting can be completed using an additional melting step in the oven to remove free radicals.

Example 22 : Method for Producing UHMWPE Using Cold Irradiation and Adiabatic Melting (CIR-AM), Subsequently Completely Melt

This example illustrates a method for producing UHMWPE having a crosslinked structure and virtually free of free radicals by irradiating UHMWPE at a sufficiently high irradiation rate to generate adiabatic partial melting of UHMWPE and subsequently to melt UHMWPE. do.

GUR 4150 bar stock (made from ram extruded Hoeskist Celanese GUR 4150 resin obtained from Westlake Plastics, Rennie, Pennsylvania) was machined into a hockey puck 8.5 cm in diameter and 4 cm thick. Twelve puck were irradiated at 60 kGy / min in air using 10 MeV and 30 kW electron beam (E-beam service in Cranberry, NJ) at rest. Six pucks were irradiated at a total dose of 170 kGy, while the remaining six pucks were irradiated at a total dose of 200 kGy. At the end of the irradiation, the puck temperature exceeded 100 ° C.

After irradiation, one puck in each heat was heated to 150 ° C. for 2 hours to melt all crystals to reduce the concentration of free radicals to an undetectable level.

A. Thermal Properties of Samples Prepared in Example 22

Perkin-Elmer DSC 7 was used with an ice water heat sink and at a heating rate and cooling rate of 10 ° C./min with continuous nitrogen wash. The crystallinity of the sample obtained from Example 22 was calculated from the weight of the sample and the heat of fusion of polyethylene crystals (69.2 cal / gm). The temperature corresponding to the endothermic peak was taken as the melting point.

Table 21 below describes the effect of total absorbed dose on the thermal properties of CIR-AM UHMWPE both before and after the subsequent melting process. The result obtained shows one single peak both before and after the subsequent melting step.

TABLE 21

CIR-AM GUR 4150 Bar Stock

Example 23 Comparison of Tensile Strain Behavior of Unirradiated UHMWPE, Cold Irradiated Subsequent Melt (CIR-SM) UHMWPE, and Warm Irradiated Partially Adiabatic Melted and Subsequent Melted (WIR-AM) HUMWPE

This example compares the tensile strain behavior of UHMWPE in unirradiated form and of UHMWPE in irradiated form through CIR-SM and WIR-AM methods.

Dog bone specimens for tensile testing were prepared using the ASTM D638 Type V standard. Tensile tests were performed on an Instron 4120 Universal Tester at a cross-head speed of 10 mm / min. Engineering stress deformation behavior was calculated from rod replacement data according to ASTM D638.

Dog bone specimens were machined from a GUR 4150 hockey puck (made from ram extruded Hoeskist Celanese GUR 4150 resin obtained from Lenny West Plastics, PA) treated by the CIR-SM and WIR-AM methods. The method described in Example 8 was followed for CIR-SM, while the method described in Example 17 was followed for WIR-AM. In both cases, the total dose administered was 150 kGy.

FIG. 11 shows the tensile behavior obtained for unirradiated controls, CIR-SM treated samples, and WIR-AM treated samples. Although irradiation was performed at 150 kGy in both methods, there was a difference in tensile strain behavior in CIR-SM and WIR-AM treated UHMWPE. This difference is due to the two phase structures generated by using the WIR-AM method.

Those skilled in the art will be able to ascertain many equivalents of the specific embodiments of the invention described herein only by routine experimentation. These and all other equivalents are included in the scope of the claims.

Claims (100)

Intracorporeal medical prosthesis formed from radiation treated ultra high molecular weight polyethylene having 2-3 melt peaks formed by heat generated by irradiation, and crosslinking. 2. The prosthesis of claim 1 wherein the ultrahigh molecular weight polyethylene has a crosslinked structure to reduce particle generation from the prosthesis during wearing of the prosthesis. The prosthesis of claim 1 wherein the ultrahigh molecular weight polyethylene is virtually free of oxidation. 2. The prosthesis of claim 1 wherein the ultrahigh molecular weight polyethylene is substantially oxidation resistant. The prosthesis of claim 1 wherein the ultrahigh molecular weight polyethylene has three melt peaks. The prosthesis of claim 1 wherein the ultrahigh molecular weight polyethylene has two melting peaks. 2. The prosthesis according to claim 1, wherein the ultra high molecular weight polyethylene is heat treated by irradiation. The prosthesis according to claim 1, wherein the polymer structure is extensively crosslinked such that a substantial portion of the polymer structure does not dissolve in xylene at 130 ° C. or decalin at 150 ° C. over 24 hours. The prosthesis of claim 1 wherein the initial average molecular weight of the ultrahigh molecular weight polyethylene is greater than 1,000,000. 2. The prosthesis of claim 1 wherein the portion of the prosthesis is in the form of a cup or tray shaped article with a load bearing surface. 11. The prosthesis of claim 10 wherein the rod bearing surface is in contact with a second portion of the prosthesis having a mating rod bearing surface of metal or ceramic material. 2. The prosthesis of claim 1 wherein the prosthesis is comprised and disposed of a replacement of a joint selected from the group consisting of hip, knee, elbow, shoulder, ankle and finger joints. The prosthesis according to claim 1, wherein the ultrahigh molecular weight polyethylene has a polymer structure having a crystallinity of less than 50% and a tensile modulus of less than 940 MPa to reduce the generation of fine particles from the prosthesis during wearing of the prosthesis. 14. The prosthesis of claim 13 wherein the hardness of the ultrahigh molecular weight polyethylene is less than 65 on the Shore D scale. 14. The prosthesis of claim 13 wherein the ultra-high molecular weight polyethylene that has been treated is of a highly entangled structure. 14. The prosthesis of claim 13 wherein the ultrahigh molecular weight polyethylene has a polymer structure having a crystallinity of 40% to 50%. Radiation treated ultrahigh molecular weight polyethylene having two to three melt peaks formed by heat generated by irradiation, and crosslinking. 18. The ultrahigh molecular weight polyethylene of claim 17, wherein the polyethylene is substantially oxidation resistant. 18. The ultrahigh molecular weight polyethylene of claim 17, having three melt peaks. 18. The ultrahigh molecular weight polyethylene of claim 17, having two melting peaks. 18. The ultrahigh molecular weight polyethylene according to claim 17, which is heat treated by irradiation. 18. The ultrahigh molecular weight polyethylene according to claim 17, having a unique polymer structure characterized by less than 50% crystallinity and a tensile modulus of less than 940 MPa. 23. The ultrahigh molecular weight polyethylene according to claim 22, having a high light transmittance. 23. The ultrahigh molecular weight polyethylene according to claim 22, which is a transparent and wear resistant film or sheet. 24. An assembled article formed from the ultra-high molecular weight polyethylene treated according to any one of claims 17 to 23. 27. The assembled article of claim 25, wherein the article is in the form of a bar stock that can be shaped into a second article by machining. 26. The assembly article of claim 25, provided on the surface of the rod bearing. A process for producing crosslinked ultrahigh molecular weight polyethylene having from two to three melt peaks formed by heat generated by irradiation, Providing an ultra high molecular weight polyethylene having a polymer chain; Irradiating the ultrahigh molecular weight polyethylene to crosslink the polymer chain; And Cooling the heated ultra high molecular weight polyethylene. 29. The method of claim 28, wherein the ultrahigh molecular weight polyethylene in the step of providing the ultrahigh molecular weight polyethylene is heated to a temperature above room temperature and below the melting point of the ultrahigh molecular weight polyethylene. As a method of preparing crosslinked ultra high molecular weight polyethylene, Providing ultra high molecular weight polyethylene with polymer chains at or below room temperature; Irradiating the ultrahigh molecular weight polyethylene to generate sufficient heat to (1) crosslink the polymer chain and (2) melt the partially or entirely ultra high molecular weight polyethylene; And Cooling the heated ultra high molecular weight polyethylene. As a method of preparing crosslinked ultra high molecular weight polyethylene, Providing ultra high molecular weight polyethylene at a temperature of no greater than 90 ° C .; Irradiating the ultrahigh molecular weight polyethylene to generate sufficient heat to crosslink the ultrahigh molecular weight polyethylene and to melt the ultrahigh molecular weight polyethylene partially or fully; And Cooling the irradiated ultra high molecular weight polyethylene. As a method of preparing crosslinked ultra high molecular weight polyethylene, Providing ultra high molecular weight polyethylene at a temperature of 90 ° C. to below the melting point; Irradiating the ultrahigh molecular weight polyethylene to generate sufficient heat to crosslink the ultrahigh molecular weight polyethylene and to melt the ultrahigh molecular weight polyethylene partially or fully; And Cooling the irradiated and heated ultra high molecular weight polyethylene. A process for preparing crosslinked ultrahigh molecular weight polyethylene that is substantially free of detectable free radicals, Providing an ultrahigh molecular weight polyethylene wool having a polymer chain and present at a temperature below the melting point; Irradiating ultrahigh molecular weight polyethylene with radiation greater than 5 Mrad to crosslink the polymer chain; And Cooling the heated ultra high molecular weight polyethylene. 30. The method of claim 29, wherein the final temperature of the ultrahigh molecular weight polyethylene after the irradiation step is higher than the melting point of the ultrahigh molecular weight polyethylene. 35. The method of any of claims 28 to 34, further comprising heating the irradiated ultrahigh molecular weight polyethylene such that the final temperature of the ultrahigh molecular weight polyethylene after such further heating is higher than the melting point of the ultrahigh molecular weight polyethylene. How to. 35. The method of any one of claims 28 to 34, wherein electron beam irradiation is used in the irradiation step. 34. The method of claim 33, wherein gamma radiation is used in the irradiation step. 35. The method of any of claims 28, 29, 33 or 34, characterized in that it generates sufficient heat to partially or wholly melt the ultrahigh molecular weight polyethylene by irradiation. 35. The method according to any one of claims 28 to 34, wherein electron beam irradiation is used in the irradiation step and an irradiation rate of 4 Mrad / min or more is used. 35. The method of any one of claims 28 to 34, wherein electron beam irradiation is used in the irradiation step and an electron beam irradiation rate of 0.05 to 3,000 Mrad / min is used. 35. The method of any of claims 28-34, wherein the irradiation rate from the irradiating step is 0.05-5 Mrad / min. 35. The method of any of claims 28-34, wherein the irradiation rate from the irradiating step is 0.05-10 Mrad / min. 35. The method of any of claims 28-34, wherein the irradiation rate from the irradiating step is 4-5 Mrad / min. 35. The method according to any one of claims 28 to 34, wherein the irradiation rate from the irradiating step is 2 to 3,000 Mrad / min. 35. The method of any of claims 28-34, wherein the irradiation rate from the irradiating step is 7-25 Mrad / minute. 35. The method of any one of claims 28-34, wherein the irradiation rate from the irradiating step is 7 Mrad / minute. 35. The method of any one of claims 28-32 or 34, wherein the total absorbed dose of radiation is between 0.5 and 1,000 Mrad. 35. The method of any one of claims 28-32 or 34, wherein a dose of greater than 20 Mrad is delivered to the heated ultrahigh molecular weight polyethylene by the irradiating step. 35. The method according to any one of claims 28 to 32 or 34, wherein the total absorbed dose to radiation is 1 to 100 Mrad. 35. The method of any of claims 28-34, wherein the total absorbed dose to radiation is 4 to 30 Mrad. 35. The method of any one of claims 28-34, wherein the total absorbed dose to radiation is 20 Mrad. 35. The method of any one of claims 28 to 34, wherein the total absorbed dose to radiation is 15 Mrad. 35. The method of any of claims 28-34, wherein the total absorbed dose to radiation is between 5 and 22 Mrad. 34. The method of claim 33, wherein the ultrahigh molecular weight polyethylene is irradiated with electron beam at an irradiation rate greater than 4 Mrad / min. 34. The method of claim 33, wherein the ultra high molecular weight polyethylene is treated at a dosage greater than 10 Mrad. 34. The method of claim 33, wherein the ultrahigh molecular weight polyethylene is treated at a dosage greater than 15 Mrad. 34. The method of claim 33, wherein the ultra high molecular weight polyethylene is treated at a dosage greater than 20 Mrad. 30. The method of claim 29, wherein the heated temperature of the ultrahigh molecular weight polyethylene is from 20 to 135 ° C. 30. The method of claim 29, wherein the heated temperature of the ultrahigh molecular weight polyethylene is 50 ° C. 30. The method of claim 29, wherein the heating is performed in a non-inert atmosphere. 30. The method of claim 29, wherein the heating is performed in an inert atmosphere. 30. The method of claim 29, wherein the ultrahigh molecular weight polyethylene before the irradiation step is between 100 and 135 ° C. 30. The method of claim 29, wherein the heated temperature of the ultrahigh molecular weight polyethylene prior to the irradiation step is 120 ° C. 35. The method of claim 34, wherein the final temperature is from 137 to 300 ° C. 35. The method of claim 34, wherein the final temperature is between 145 and 190 ° C. 35. The method of claim 34, wherein the final temperature is 150 ° C. 36. The process of claim 35 wherein the ultrahigh molecular weight polyethylene after further heating is 137-300 ° C. 36. The process of claim 35 wherein the ultrahigh molecular weight polyethylene after further heating is between 145 and 190 ° C. 36. The method of claim 35, wherein the ultrahigh molecular weight polyethylene after further heating is 150 ° C. 33. The method of claim 31 or 32, further comprising enclosing the ultrahigh molecular weight polyethylene with an inert material substantially free of oxygen. 34. The method of claim 33, wherein the temperature of the provided ultrahigh molecular weight polyethylene is below room temperature. 34. The method of claim 33, wherein the temperature of the provided ultrahigh molecular weight polyethylene is no greater than 130 ° C. 34. The method of claim 33, wherein the temperature of the provided ultrahigh molecular weight polyethylene is no greater than 120 ° C. 34. The method of claim 33, wherein the temperature of the provided ultrahigh molecular weight polyethylene is higher than room temperature. 75. The method of any one of claims 28-34, 37, 54-69, 71, or 72-74, further comprising sterilizing the cooled ultrahigh molecular weight polyethylene. Characterized in that. 75. The method of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein the ultrahigh molecular weight polyethylene in the providing step is bar stocked, shaped. Method selected from the group consisting of bar stocks, coatings and assembled articles. 75. The method of any of claims 28-34, 37, 54-69, 71 or 72-74, wherein the ultrahigh molecular weight polyethylene in the providing step is a cup for a prosthesis or Characterized in that the tray-shaped article. 78. The process of any of claims 28-34, 37, 54-69, 71, or 72-74, further comprising the step of machining the cooled ultrahigh molecular weight polyethylene. Including method. 75. The method of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein the ultrahigh molecular weight polyethylene in the providing step has an initial average molecular weight of 1,000,000 Method characterized by greater than. 78. The method of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein the irradiating step is carried out in an inert atmosphere. . 78. The method of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein the irradiating step is carried out in an inert atmosphere. 78. The method of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein electron beam irradiation is used in the irradiation step, and the energy of the electrons is Method characterized in that 0.5MeV to 12MeV. 78. The process of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein the cooling step is performed at a cooling rate greater than 0.1 ° C / min. Characterized by the above. 78. The process of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein the cooling step is performed at a cooling rate greater than 0.5 ° C / min. Characterized by the above. 78. The process of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein the cooling step is performed at a cooling rate of 0.5 ° C / min. How to feature. 78. The method of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein the ultrahigh molecular weight polyethylene in the providing step is present in the insulating material Heat loss from ultra high molecular weight polyethylene during processing is reduced. 78. The method of any of claims 28-34, 37, 54-69, 71, or 72-74, wherein electron beam irradiation is used in the irradiation step. A method of making a medical prosthesis from irradiated ultrahigh molecular weight polyethylene having two to three melt peaks, which reduces particle production from the prosthesis during wearing of the prosthesis, Providing an irradiated ultrahigh molecular weight polyethylene having two to three melt peaks formed by heat generated by irradiation; And Forming a medical prosthesis from ultra high molecular weight polyethylene forming a rod bearing of the prosthesis to reduce particle generation from the prosthesis during wearing of the prosthesis. A method of making a medical prosthesis from radiation treated ultra high molecular weight polyethylene, which reduces particle production from the prosthesis during wearing of the prosthesis, Providing crude high molecular weight polyethylene irradiated at a temperature above room temperature with an irradiation rate of at least 2 Mrad / hour; And Forming a medical prosthesis from ultra high molecular weight polyethylene forming a rod bearing of the prosthesis to reduce particle generation from the prosthesis during wearing of the prosthesis. A method of making a medical prosthesis from irradiated ultrahigh molecular weight polyethylene having two to three melt peaks, which reduces particle production from the prosthesis during wearing of the prosthesis, 77. Prepared by the method according to any one of claims 28 to 34, 37, 54 to 69, 71 or 72 to 74, formed by heat generated by irradiation. Providing a radiation treated ultrahigh molecular weight polyethylene having from 3 to 3 melt peaks; And Forming a medical prosthesis from ultra high molecular weight polyethylene forming a rod bearing of the prosthesis to reduce particle generation from the prosthesis during wearing of the prosthesis. 90. The method of claim 89, wherein the ultrahigh molecular weight polyethylene has a polymer structure having a crystallinity of less than 50% and a tensile modulus of less than 940 MPa. A method of making a medical prosthesis from radiation treated ultra high molecular weight polyethylene, which reduces particle production from the prosthesis during wearing of the prosthesis, Providing an ultra high molecular weight polyethylene irradiated at a temperature above room temperature with an irradiation rate of at least 4 Mrad / hour; And Forming a medical prosthesis from ultra high molecular weight polyethylene forming a rod bearing of the prosthesis to reduce particle generation from the prosthesis during wearing of the prosthesis. A medical prosthesis comprising radiation treated ultra high molecular weight polyethylene having two to three melt peaks, Providing an ultra high molecular weight polyethylene irradiated at a temperature above room temperature with an irradiation rate of at least 2 Mrad / hour; And A prosthesis produced by the step of forming a prosthesis of radiation treated ultrahigh molecular weight polyethylene having two to three melt peaks formed by heat generated by irradiation. A medical prosthesis comprising radiation treated ultra high molecular weight polyethylene having two to three melt peaks, 77. Prepared by the method according to any one of claims 28 to 34, 37, 54 to 69, 71 or 72 to 74, formed by heat generated by irradiation. Providing a radiation treated ultrahigh molecular weight polyethylene having from 3 to 3 melt peaks; And A prosthesis produced by forming a prosthesis of radiation treated ultra high molecular weight polyethylene having two to three melt peaks. 94. The medical prosthesis of claim 93 wherein the ultra-high molecular weight polyethylene treated with radiation has a polymer structure having a crystallinity of less than 50% and a tensile modulus of less than 940 MPa. A medical prosthesis comprising crosslinked ultrahigh molecular weight polyethylene having a polymer structure having a crystallinity of less than 50% and a tensile modulus of less than 940 MPa, thereby reducing the generation of fine particles from the prosthesis during wearing of the prosthesis. As a method of preparing crosslinked polyethylene, Providing polyethylene at a temperature below the melting point; Irradiating the polyethylene to generate (1) crosslinking the polymer chain and (2) generating sufficient heat to partially or wholly melt the polyethylene; And Cooling the heated polyethylene. 98. The method of claim 97, wherein the polyethylene is heated above its melting point to substantially remove free radicals. 99. The method of claim 98, wherein the polyethylene is melted by irradiation. 99. The method of claim 98, wherein the polyethylene is melted by a heat source other than radiation.