Abstract
To prevent aseptic loosening resulting from osteolysis induced by polyethylene (PE) wear particles in THA, it is necessary to develop a high wear-resistance bearing material. We have investigated the bearing surface mimicking the articular cartilage; grafting a biocompatible polymer, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), onto the PE surface. High wear-resistance of PMPC-grafted surface has been revealed in the hip simulator wear test of 20 million cycles. Additionaly, in THA, oxidation degradation induced by residual free radicals resulting from gamma-ray irradiation for cross-linking or sterilization is also regarded as serious issue. Recently, gas plasma (GP) sterilization has been used as a less residual radical sterilization method. In this study, we ask a question: the GP sterilization would affect to PMPC surface and/or PE substrate? Hence, we investigated surface chemical, wear, mechanical, physical and oxidation properties of GP sterilized PMPC-grafted highly cross-linked PE (CLPE).
GP-sterilized CLPE and PMPC-grafted CLPE (CLPE (GP) and PMPC-CLPE (GP), respectively; GUR 1020 resin, 75 kGy irradiation), and 25 kGy-gamma-sterilized PMPC-grafted CLPE (PMPC-CLPE (g); GUR 1020 resin, 50 kGy irradiation) were evaluated. Surface property of PMPC layer was evaluated by X-ray photoelectron spectroscopy (XPS), fourier-transform infrared (FT-IR) spectroscopy, fluorescence microscope and cross-sectional transmission electron microscope (TEM) observations. Wettability and lubrication of the PMPC-CLPE surface were evaluated by static water contact angle measurement and ball-on-plate friction test, respectively. Wear properties of the acetabular cups were examined by using hip simulator in the combination with Co-Cr-Mo femoral heads. To evaluate the GP sterilization effect to the CLPE substrate, tensile test, izod impact test, small punch test, gel content, residual radical concentration and oxidation degradation were conducted. Oxidation degradation was evaluated as oxidation index by using a FT-IR spectroscopy.
By the XPS and FT-IR measurements, phosphorus peak and P-O peak attributed to grafted PMPC were observed, respectively. Uniform PMPC layer (100–200 nm thick) was observed on both surfaces of PMPC-CLPE (g) and PMPC-CLPE (GP) [Fig. 1]. Water contact angle of CLPE (GP) was almost 100 degree, while those for PMPC-CLPE (g) and PMPC-CLPE (GP) decreased dramatically to almost 10 degree. Dynamic coefficient of friction of PMPC-CLPE (g) and PMPC-CLPE (GP) was lower than that for CLPE (GP). In the hip simulator wear test, PMPC-CLPE (g) and PMPC-CLPE (GP) cups showed significantly lower amount of wear than that of CLPE (GP) [Fig. 2]. The number of the wear particles was extremely less in PMPC-CLPE (g) and PMPC-CLPE (GP), though the size was not different of all cases. Water thin film might be formed at the grafted PMPC layer, which acted as significantly efficient lubricant. There was no difference in the mechanical and physical properties among three groups. Oxidation index for PMPC-CLPE (GP) after acceleration of aging was lower than that of PMPC-CLPE (g). The GP sterilization might affect only to the PMPC-grafted surface, whereas gamma irradiation affects also to the PE substrate.
From these results, the PMPC-CLPE (GP) is expected to be one of the great bearing materials having not only high-wear resistance but also high-oxidation resistance, which could give further longevity of implantation.
