title: Severe impingement of lumbar disc replacements increases the functional biological activity of polyethylene wear debris.
abstract: BACKGROUND
  Wear, oxidation, and particularly rim impingement damage of ultra-high molecular weight polyethylene total disc replacement components have been observed following surgical revision. However, neither in vitro testing nor retrieval-based evidence has shown the effect(s) of impingement on the characteristics of polyethylene wear debris. Thus, we sought to determine (1) differences in polyethylene particle size, shape, number, or biological activity that correspond to mild or severe rim impingement and (2) in an analysis of all total disc replacements, regardless of impingement classification, whether there are correlations between the extent of regional damage and the characteristics of polyethylene wear debris.
  
  
  METHODS
  The extent of dome and rim damage was characterized for eleven retrieved polyethylene cores obtained at revision surgery after an average duration of implantation of 9.7 years (range, 4.6 to 16.1 years). Polyethylene wear debris was isolated from periprosthetic tissues with use of nitric acid and was imaged with use of environmental scanning electron microscopy. Subsequently, particle size, shape, number, biological activity, and chronic inflammation scores were determined.
  
  
  RESULTS
  Grouping of particles by size ranges that represented high biological relevance (&lt;0.1 to 1-μm particles), intermediate biological relevance (1 to 10-μm particles), and low biological relevance (&gt;10-μm particles) revealed an increased volume fraction of particles in the &lt;0.1 to 1-μm and 1 to 10-μm size ranges in the mild-impingement cohort as compared with the severe-impingement cohort. The increased volume fractions resulted in a higher specific biological activity per unit particle volume in the mild-impingement cohort than in the severe-impingement cohort. However, functional biological activity, which is normalized by particle volume (mm3/g of tissue), was significantly higher in the severe-impingement cohort. This increase was due to a larger volume of particles in all three size ranges. In both cohorts, the functional biological activity correlated with the chronic inflammatory response, and the extent of rim penetration positively correlated with increasing particle size, number, and functional biological activity.
  
  
  CONCLUSIONS
  The results of this study suggest that severe rim impingement increases the production of biologically relevant particles from motion-preserving lumbar total disc replacement components.
  
  
  METHODS
  Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.
authors.affiliation: Implant Research Center, School of Biomedical Engineering, Science & Health Systems, Drexel University, 3401 Market Street, Suite 345, Philadelphia, PA 19104, USA. mjs348@drexel.edu
authors.forename: Ryan M
authors.initials: RM
authors.lastname: Baxter
authors.forename: Daniel W
authors.initials: DW
authors.lastname: Macdonald
authors.forename: Steven M
authors.initials: SM
authors.lastname: Kurtz
authors.forename: Marla J
authors.initials: MJ
authors.lastname: Steinbeck
mesh_terms: D006801:Humans; D007403:Intervertebral Disc / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000601:surgery*; D008855:Microscopy, Electron, Scanning; D011095:Polyethylenes / Q000627:therapeutic use*; D011475:Prosthesis Failure; D012086:Reoperation; D059145:Total Disc Replacement / Q000009:adverse effects*
chemical_list: D011095:Polyethylenes