The present invention discloses methods for enhancing the wear-resistance of polymers, the resulting polymers, and in vivo implants made from such polymers. One aspect of this invention presents a method whereby a polymer is irradiated, preferably with gamma radiation, then thermally treated, such as by remelting of annealing. The resulting polymeric composition preferably has its most oxidized surface layer removed. Another aspect of the invention presents a general method for optimizing the wear resistance and desirable physical and/or chemical properties of a polymer by crosslinking and thermally treating it. The resulting polymeric compositions is wear-resistant and may be fabricated into an in vivo implant.

Crosslinking of polyethylene for low wear using radiation and thermal treatments

The present invention discloses a method for enhancing the wear-resistance of polymers by crosslinking them, especially before irradiation sterilization. In particular, this invention presents the use of chemically crosslinked ultrahigh molecular weight polyethylene in in vivo implants.

Chemically crosslinked ultrahigh molecular weight polyethylene for artificial human joints

A medical implant of ultrahigh molecular weight polyethylene having an improved balance of wear properties and oxidation resistance is prepared by irradiating a preform of ultrahigh molecular weight polyethylene, annealing the irradiated preform in the absence of oxygen to a temperature at or above the onset of melting temperature, and forming an implant from the stabilized cross-linked polymer. Implants prepared according to the process of the present invention have comparable oxidation resistance and superior wear performance compared to unirradiated ultrahigh molecular weight polyethylene.

Process for medical implant of cross-linked ultrahigh molecular weight polyethylene having improved balance of wear properties and oxidation resistance

The most important factor affecting performance and longevity of hip joint and knee implants is the wear rate of the ultra-high-molecular-weight polyethylene (UHMWPE) component. UHMWPE wear debris has been linked to complications including tissue inflammation, bone loss (osteolysis) and implant loosening. Reduction of debris has been addressed by investigating new polyethylene formulations, manufacturing and finishing processes, including surface treatments and coatings both on plastic and metallic components. There have been many studies on the effect of surface treatments and hard coatings on cobalt chromium and titanium alloys for prosthetic applications. However, most of them have used laboratory tribo-testers without much correlation to articulating movements in human joints. In this work, hard coatings (TiN and DLC) and surface treatments (nitrogen ion implantation) have been investigated as potential candidates to reduce wear of UHMWPE, when applied on the counter face cobalt chromium alloy. Also the effect of applying nitrogen ion implantation on the UHMWPE surface itself has been investigated. To evaluate wear performance a special knee wear simulator has been used with a combined rolling-sliding movement that corresponds to the most unfavourable situation in the knee. As testing materials, conventional coated and uncoated Co–Cr as well as Al 2 O 3 femoral heads were used against UHMWPE plates to obtain comparative data. Testing was carried out at 50 MPa using distilled water at 37±2°C as a lubricant. Wear measurements on UHMWPE were made following ASTM F732 standard by weight loss measurements and also by laser profilometry. The results, up to 5 million wear cycles that represent approximately 3 years of implant life, clearly have demonstrated the beneficial effects of diamond-like carbon (DLC) and ion implantation (both on Co–Cr and UHMWPE) in reducing wear of UHMWPE. Similar values were also obtained for Al 2 O 3 ceramic material. A wear reduction up to approximately five times was obtained by the former materials, in comparison with the uncoated control specimen. However, the ion plated TiN coating increased significantly wear on the UHMWPE. Transference of UHMWPE to the ball was very evident with the uncoated control and TiN coated Co–Cr, as observed by SEM, but with DLC and the ceramic material this occurred to a much lesser extent. Some micro-delamination at the worn surface of the UHMWPE was observed when wearing against uncoated Co–Cr, but was not clear in the rest options. Ion implantation and DLC can thus be two good candidate treatments to reduce wear of UHMWPE in cobalt chromium knee and hip joint implants in substitution of a more expensive ceramic material. This DLC has passed all biocompatible tests in accordance with FDA regulations and ISO 10993 standards for implantable devices. An example of a uniformly DLC coated femoral head and knee implant is shown.

Wear reduction effect on ultra-high-molecular-weight polyethylene by application of hard coatings and ion implantation on cobalt chromium alloy, as measured in a knee wear simulation machine

A simple wear test was investigated for evaluating the wear and damage of material pairs when used in total knee replacement. The test consisted of an axially loaded metallic femoral indentor and a reciprocating ultrahigh molecular weight polyethylene (UHMWPE) flat disk that represented the tibial component. A number of variables were studied including the effect of conformity by varying the radii of the femoral surface, distilled water or serum as a lubricant, different femoral materials, and different types of UHMWPE. In general, the different morphologies of the surface wear of the UHMWPE were similar to those seen on retrieved total knee replacements. Increased conformity with a cylindrical indentor gave a reduced wear rate initially, compared with that of the lower conformity spherical indentor. However, the wear rates were similar subsequent to this initial wearing in phase. Transfer films of UHMWPE were observed on the cobalt-chrome indentors for both serum and distilled water lubrication, although this film was more extensive for distilled water. The lowest wear rate was observed when oxidized zirconium was used on the femoral side, which was attributed to greater wettability, surface hardness, and immunity to oxidative wear. Tests using cobalt-chrome femoral cylinders and different grades of UHMWPE showed different wear rates. Of these PEs, GUR 415 showed less wear than both RCH 1000 and UHMWPE containing numerous fusion defects. For the latter, wear was attributed to a fatigue mechanism, although in most cases it was associated with surface phenomena rather than subsurface cracking. However, in some specimens of UHMWPE subsurface crack propagations occurred with defects. The test method is discussed in relation to its applicability to the evaluation and comparison of bearing materials and surfaces, with particular application to total knee replacements.

Wear testing of materials and surfaces for total knee replacement.

The objective was to examine possible reasons for delamination observed in tibial inserts of the porous-coated anatomic (PCA) knee replacement. To date, 33 PCA inserts have been forwarded to the authors' labs. Of these 33, 52% showed severe delamination within four years of implantation. Visual, structural, and mechanical analyses were conducted and data compared on the heat-pressed PCA type and the common machined inserts. Twenty inserts of the two different types were examined. Visual data using polarized light microscopy showed the presence of a surface layer separated from the middle region of the heat-pressed inserts by a line of demarcation 250-580 microns beneath the articulating surface. This anomaly was not observed in machined inserts. Structural analysis showed the new heat-pressed inserts had increased crystallinity in the surface layer when compared to new machined inserts. The retrieved heat-pressed inserts showed increased crystallinity in the surface and middle regions. There was a slight increase in surface crystallinity in the retrieved machined inserts. Microhardness data showed that there was an increased hardness associated with the crystallinity seen on the heat-pressed inserts. Orthopedic surgeons should be aware of early delamination and surface failure in heat-pressed inserts.

Investigation of early surface delamination observed in retrieved heat-pressed tibial inserts.

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Investigation of early surface delamination observed in retrieved heat-pressed tibial inserts.