title: Quantification of polyethylene degradation in mobile bearing knees: a retrieval analysis of the Anterior-Posterior-Glide (APG) and Rotating Platform (RP) Low Contact Stress (LCS) knee.
abstract: BACKGROUND
  Routine qualitative observations of more than 850 polyethylene fixed and mobile bearings at our institution have noted minimal wear of mobile bearings. The APG mobile bearing is the most recent design variant of the LCS knee, allows multi-directional movement at the tibiofemoral articulation, and is posterior cruciate sparing. Even though it is difficult to perform, quantitative wear measurement is important in determining the likely longevity of new arthroplasty devices, and is especially relevant because of increasing numbers of new mobile bearing designs.
  
  
  METHODS
  We analyzed 10 retrieved APG and 7 retrieved RP tibial bearings (De Puy) with a mean implantation period of 33 (9-70) months. We used coordinate-measuring techniques to quantitatively determine linear penetration, and optical and scanning electron microscopy to assess wear mechanisms qualitatively.
  
  
  RESULTS
  The mean total volume loss (superior and inferior articulations) of the APG and RP designs was 85 mm((3))/year and 77 mm((3))/year, respectively. Burnishing was the predominant wear mechanism, and to a lesser extent scratching, abrasion and pitting. Multidirectional scratching and abrasion were noted on the APG inserts inferiorly, whereas there was circumferential scratching on the RP inserts.
  
  
  CONCLUSIONS
  Our short-term results for the APG and RP mobile bearing designs are similar and compare more than favorably with reported values for fixed-bearing designs. However, increased backside wear due to multidirectional movement may predispose the APG design to greater wear in the long term.
authors.affiliation: Department of Medical Engineering and Physics, Royal Perth Hospital, Perth, Western Australia. alan.kop@health.wa.gov.au
authors.forename: Alan M
authors.initials: AM
authors.lastname: Kop
authors.forename: Eric
authors.initials: E
authors.lastname: Swarts
mesh_terms: D019645:Arthroplasty, Replacement, Knee; D006801:Humans; D007720:Knee Prosthesis; D008422:Materials Testing; D020959:Polyethylene / Q000032:analysis*; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012399:Rotation; D013314:Stress, Mechanical
chemical_list: D020959:Polyethylene