Surface crosslinked polyethylene

Abstract

A method for producing a wear resistant polyethylene medical implant includes forming a medical implant, such as an orthopedic implant, made at least partially of ultra high molecular weight polyethylene (UHMWPE). The polyethylene may be irradiated with gamma ray or e-beam radiation to form free radicals and then crosslinked to eliminate free radicals prior to exposure to oxygen. The so treated bearing surface of the crosslinked polyethylene is then coated with a photoinitiator. Thereafter the bearing material is photocrosslinked with ultra-violet (UV) radiation. The photocrosslinking process can also be applied to non-crosslink UHMWPE.

Claims

1. A method for producing a wear resistant ultra-high molecular weight polyethylene medical implant comprising: forming a medical implant made at least partially of ultra-high molecular weight polyethylene (UHMWPE) with a bearing surface of the medical implant made of UHMWPE; irradiating the medical implant with gamma ray or e-beam radiation to form free radicals and crosslinking the formed free radicals prior to exposure to air; evaporating a photoinitiator to deposit the photoinitiator into the UHMWPE bearing surface, wherein the photoinitiator is penetrated into the UHMWPE bearing surface to a depth of less than about 1 mm; breaking carbon-hydrogen bonds in the UHMWPE by exposing the UHMWPE to ultra-violet (UV) radiation; and removing any residual photoinitiator from the UHMWPE.

2. The method of claim 1, wherein the photoinitiator is Benzophenone.

3. The method of claim 1, wherein the UV radiation has a wavelength of 300 to 400 mm.

4. The method of claim 1, wherein the intensity of the UV radiation is up to 100 mW/cm.sup.2.

5. The method of claim 1, wherein the medical implant is an orthopedic implant.

6. The method of claim 1, wherein the medical implant is an acetabular cup or tibial insert.

7. The method of claim 1, wherein the irradiation with gamma ray or e-beam radiation and crosslinking are performed in at least two sequential radiation doses.

8. The method of claim 7, wherein each radiation dose is less than 3.0 MRads.

9. The method of claim 1, wherein the implant is allowed to cool between gamma ray or e-beam crosslinking and the photocrosslinking.

10. The method of claim 1, wherein the photoinitiator selected from the group consisting of 2-methylanthraquinone, 2-ethylanthraquinone, 2-chloroanthraquinone, p-chloranil, benzyl sulfide, benzyl sulfoxide, phenyl sulfoxide, 4-acetylbiphenyl, anthrone, hexachlorobenzene, benzophenone, 4,4′-dimethoxybezophenone, 4-Nitrobenzophenone, 2-chlorobenzophenone, 4-chlorobenzophenone and 4,4′-dichlorobenzophenone.

11. The method of claim 1, wherein the depth that the photoinitiator penetrated into the virgin UHMWPE bearing surface is between 200 microns and 1 mm.

12. The method of claim 1, wherein the irradiating the medical implant with the gamma ray or e-beam radiation is performed after the removing the residual photoinitiator.

13. The method of claim 1, wherein the irradiating the medical implant with the gamma ray or e-beam radiation is performed before the depositing a photoinitiator.

14. The method of claim 1, wherein the depositing the photoinitiator into the virgin UHMWPE bearing surface of the medical implant is performed before the irradiating the medical implant.

15. The method of claim 1, wherein the breaking carbon-hydrogen bonds in the UHMWPE is performed before the irradiating the medical implant.

16. The method of claim 1, wherein the removing any residual photoinitiator from the UHMWPE is performed before the irradiating the medical implant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing the gravimetric wear of cobalt chrome on a sequentially crosslinked polyethylene (×3) and the same sequentially crosslinked polyethylene including photocrosslinking (PXL);

(2) FIG. 2 shows a pair of comparative photographs of the photocrosslinked cup of FIG. 1 prior to wear testing on the left and after wear testing of 2.5 million cycles on the right as obtained using an interferometricprofilometer;

(3) FIG. 3 shows the gravimetric wear of a PEEK surface on a photocrosslinked ultra high molecular weight polyethylene previously crosslinked in an inner atmosphere by gamma irradiation and a similar cup not photocrosslinked after one million cycles;

(4) FIG. 4 shows an Izod impact energy before and after photocrosslinking of virgin ultra high molecular weight polyethylene showing no fracture toughness dropped after photocrosslinking with UV radiation;

(5) FIG. 5 is a graph showing the volumetric wear of a cobalt chrome prosthetic femoral head on an N.sub.2 VAC treated (3 Mrad gamma ray dose in an atmosphere of <1% oxygen and no subsequent annealing) UHMWPE bearing;

(6) FIG. 6 is a graph showing the volumetric wear of a PEEK (polyether ether Ketone) prosthetic femoral head on an N.sub.2 VAC treated UHMWPE bearing compared to the same bearing with UV photocrosslinking; and

(7) FIG. 7 is a graph showing the volumetric wear of Virgin Antioxident doped UHMWPE compared to the same antioxidant treated UHMWPE with UV surface crosslinking at 3.5 million cycles (Mc).

DETAILED DESCRIPTION

Example 1

(8) Commercially available polyethylene acetabular cups (N2Vac according to the process of Stryker U.S. Pat. No. 5,414,049), or Virgin GUR 1020 UHMWPE which was sequentially crosslinked three times per Stryker U.S. Pat. No. 7,517,919 and obtained from Stryker orthopaedics) were used. The UV light with a wavelength of 350 nm was used as a light source. Benzophenone from Aldrich was used as a photoinitiator.

(9) The UV crosslinking process is described below. Several already bulk crosslinked UHMWPE acetabular cups were immersed in an acetone solution of benzophenone. After one minute, each cup was taken out from the solution and dried under vacuum for an hour. The cups were immersed in deionized water in a glass jar that was purged with an inert gas (nitrogen) to produced oxygen free water. The glass jar was sealed and placed in a water bath at 65° C. Each cup was then irradiated by the UV light for up to 2 hours. After photoirradiation (which produces photocrosslinking (PXL)), each cup was washed with acetone and water. No visual changes were observed during this process. The photocrosslinked (PXL) cups were tested in a hip stimulator machine for its wear performance. Cobalt chrome and plastic ball heads (PEEK) were used against one of the UV crosslinked cups. Untreated cups were used as control. Wear testing was conducted in house following the ASTM standard method. Gravimetric wear was obtained using ASTM F2025. The soak time was 1 minute in 10 mg benzophenone per ml of acetone. The UV radiation was 50 milliwatts per cm.sup.2 and wavelength of 350 nm (average).

(10) FIG. 1 shows gravimetric wear (mg) of a cobalt chrome femoral head on a sequentially crosslinked (CoCr/X3®) cup and a similar cup with photocrosslinking and CoCr/X3®-PXL after 2.5 million cycles. No wear from CoCr/X3®-PXL was detected. The UHMWPE was initially crosslinked three times by the process described in U.S. Pat. No. 7,517,919. As shown in FIG. 2 it was also found that machine marks on the photocrosslinked cup were still visible after 2.5 million cycles.

(11) Similar results were observed in the wear testing of PEEK head on N2Vac treated cup and PEEK head on N2Vac plus PXL (FIG. 3). The cup was 28 mm in diameter with PEEK heads in a standard hip simulator. No wear was detected in PEEK/N2Vac-PXL after 1.0 million cycles while a non-photocrosslinked N2Vac cup produced more than 10 mg of gravimetric wear.

(12) FIG. 4 shows an Izod impact energy before and after photocrosslinking of virgin ultra high molecular weight polyethylene showing no fracture toughness dropped after photocrosslinking with UV radiation.

(13) FIG. 5 is a graph showing the ASTM F-648-04 as tested by the volumetric wear of a cobalt chrome prosthetic femoral head on an N.sub.2 VAC treated (3 Mrad gamma ray dose in an atmosphere of <1% oxygen and no subsequent annealing) UHMWPE bearing.

(14) FIG. 6 is a graph showing the volumetric wear of a PEEK (polyether ether ketone) prosthetic femoral head on an N.sub.2 VAC treated UHMWPE bearing.

(15) FIG. 7 is a graph showing the volumetric wear of Virgin Antioxidant doped (with anthocyanin) UHMWPE compared to the same antioxidant treated UHMWPE with UV surface crosslinking both at 3.5 million cycles with the wear rate is in mm.sup.3/million cycles.

(16) An alternate method of coating the UHMWPE with benzophenone utilizes Physical Vapor Deposition process (PVD). The photointiator is evaporated into a gas by heating over its boiling temperature, then condensed onto UHMWPE bearing surface to form a uniform thin film. UV light is applied on the photoinitiator coated bearing surface and crosslinks the surface. Non-bearing surfaces may be masked to prevent being coated with a photoinitiator. Also a CVD method may be used to uniformly coat sensitizer (photoinitiator) on UHMWPE bearing surface. An alternative bearing combination could be UV crosslinked UHMWPE on UV crosslinked UHMWPE.

(17) Any residual photoinitiator may advantageously be removed from preformed UHMWPE after photocrosslinking prior to fabrication of the implant. Alternately, if the implant is photocrosslinked after fabrication, the residual photoinitiator may be removed.

(18) Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.