title: Surface modifications induced by in-vitro wear and oxidation on γ-irradiated UHMWPE hip liners belonging to different commercial generations.
abstract: Single-step and three-step irradiated and annealed ultra-high molecular weight polyethylene (UHMWPE) hip liners have been studied by means of Raman spectroscopy (RS) and Fourier transform infrared spectroscopy (FT-IR), in order to clarify the microstructural modifications induced by in vitro oxidative degradation and wear. These spectroscopic techniques enabled us to measure profiles of oxidation index (OI), crystalline (αc), amorphous (αa), and third phase (αb) fractions along the subsurface of the acetabular cups as a function of in vitro oxidation time or after standard testing in hip simulator. Microtomed sections of the liners after accelerated aging (ASTM F2003-02) showed that oxidation profiles developed differently during the first two weeks, while all samples aged longer than 2 weeks revealed OI increasing with lower rates. The initial oxidation of the single-step-annealed material was higher than the one retrieved from the 3-step-annealed material and showed a peak of OI located at a depth of ~1mm below the exposed surface. The profiles of αc, calculated from the same sample cross-sections, followed trends similar to the respective OI profiles, which enabled a phenomenological (but quantitative) correlation between oxidation and crystallization processes to be obtained. Wear simulation under edge loading conditions was conducted on series of four samples of the above two types of irradiated and annealed materials, and for two different liner thicknesses (5.9 and 7.9 mm). The wear rates calculated at the end of the test were very low for all samples (max 2.08 mm(3)/mc for the thinner liners of the single-step irradiated and annealed material). The results indicated that there was a statistically significant increase in both wear rate and volume loss only for the thinner single-step irradiated and annealed liners. Surface analyses by Raman spectroscopy revealed distinct gradients of crystallinity, amorphous, and third phase fractions along the in-depth axis. In both types of UHMWPE materials, the worn area showed an increase of crystallinity at the expense of the third phase. Differences in crystallinity profiles observed at the wear zone of liners with different thicknesses were correlated to the higher contact stress experienced by thinner liners.
authors.affiliation: Department of Molecular Cell Physiology, Kyoto Prefectural University of Medicine, Kamigyo-ku Hirokoji agaru, Kawaramachi-dori, 602-8566 Kyoto, Japan. Electronic address: leonardo@koto.kpu-m.ac.j.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Puppulin
authors.affiliation: Department of Industrial Engineering and Information Technology University of Trieste, 34128 Trieste, Italy.
authors.forename: Sara Della
authors.initials: SD
authors.lastname: Negra
authors.affiliation: Department of Orthopedic Surgery, Osaka University Medical School, 2-2 Yamadaoka, Suita 565-0871, Osaka, Japan.
authors.forename: Nobuhiko
authors.initials: N
authors.lastname: Sugano
authors.affiliation: Department of Industrial Engineering and Information Technology University of Trieste, 34128 Trieste, Italy.
authors.forename: Orfeo
authors.initials: O
authors.lastname: Sbaizero
authors.affiliation: Ceramic Physics Laboratory and Research Institute for Nanoscience, Kyoto Institute of Technology, Sakyo-ku, Matsugasaki, 606-8585 Kyoto, Japan; The Center for Advanced Medical Engineering and Informatics, Osaka University, Yamadaoka, Suita 565-0871, Osaka, Japan.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Pezzotti
mesh_terms: D005720:Gamma Rays; D006622:Hip Prosthesis; D008422:Materials Testing; D055595:Mechanical Phenomena; D010084:Oxidation-Reduction; D011095:Polyethylenes / Q000737:chemistry*; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors
chemical_list: D011095:Polyethylenes; C111601:ultra-high molecular weight polyethylene
keywords: FT-IR; Hip replacement; Oxidation; Raman spectroscopy; UHMWPE; Wear.