title: The aging micromechanisms of alumina matrix composite (AMC) used in total hip arthroplasty.
abstract: BACKGROUND
  The aim of the present study is to simulate and better understand the long-term interplay between alumina matrix composite (AMC) femoral head and aqueous environment. In particular, we focused on clarifying the role of alumina grains on zirconia phase stability and mechanical equilibrium in AMC head during aging processes in a clinically-relevant time frame.
  
  
  METHODS
  The tested AMC head consists of an alumina matrix (82 vol.%) reinforced by yttria-stabilized zirconia (17 vol.%), chromium oxide (0.5 vol.%), and strontium aluminate (0.5 vol.%). The accelerated hydrothermal aging testing (at 134 °C, 2-bars pressure) was conducted up to 20 h. According to ASTM standard, 1-h aging under these conditions corresponds approximately to 2 yrs in vivo exposure. Confocal Raman and fluorescence spectroscopy were applied to quantify surface tetragonal-to-monoclinic phase transformation of zirconia and surface stresses in the AMC head. The Mehl-Avrami-Johnson (MAJ) theory was applied, which allowed modeling the micromechanisms of nucleation and growth of monoclinic sites during the transformation process. The obtained results were compared to those of monolithic zirconia (3Y-TZP) femoral heads.
  
  
  RESULTS
  The 3Y-TZP head showed a rapid increase of transformation rate beyond the aging time of 5 h (simulated as ∼10 yrs in vivo), suggesting the initiation of the transformation cascade toward the neighboring zirconia grains (growth mechanism). On the other hand, MAJ analysis revealed that the growth mechanism was completely absent and the nucleation of the monoclinic phase was partially prevented in the AMC head even after the 20-h aging (∼40 yrs in vivo). In addition, the stress accumulation in the AMC head was restricted at a quite low level throughout the aging simulation.
  
  
  CONCLUSIONS
  Those results suggest that the presence of stable and hard alumina in the AMC can play a considerable role in slowing down the destabilization processes by spatially encompassing zirconia grains.
authors.affiliation: Department of Orthopedic Surgery, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan.
authors.forename: Shinji
authors.initials: S
authors.lastname: Horie
authors.affiliation: Department of Orthopedic Surgery, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan; Department of Bone and Joint Biomaterial Research, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan. Electronic address: yasuhito@tokyo-med.ac.jp.
authors.forename: Yasuhito
authors.initials: Y
authors.lastname: Takahashi
authors.affiliation: Department of Orthopedic Surgery, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan.
authors.forename: Takaaki
authors.initials: T
authors.lastname: Shishido
authors.affiliation: Department of Orthopedic Surgery, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan.
authors.forename: Toshinori
authors.initials: T
authors.lastname: Masaoka
authors.affiliation: Department of Orthopedic Surgery, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan.
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Tateiwa
authors.affiliation: Department of Orthopedic Surgery, Tokyo Medical University, 6-7-1 Nishishinjuku, Shinjuku-ku, Tokyo 160-0023, Japan.
authors.forename: Kengo
authors.initials: K
authors.lastname: Yamamoto
mesh_terms: D000537:Aluminum Oxide / Q000737:chemistry*; D019644:Arthroplasty, Replacement, Hip / Q000379:methods*; D005270:Femur Head / Q000601:surgery*; D006622:Hip Prosthesis; D006801:Humans; D008422:Materials Testing; D011474:Prosthesis Design; D013499:Surface Properties
chemical_list: D000537:Aluminum Oxide