title: Frozen storage affects the compressive creep behavior of the porcine intervertebral disc.
abstract: METHODS
  A biomechanical study of the compressive creep behavior of the porcine intervertebral disc before and after frozen storage.
  
  
  OBJECTIVE
  To determine whether frozen storage alters the creep response, hydration, and nuclear swelling pressure of the intact intervertebral disc.
  
  
  BACKGROUND
  The mechanical response of the disc is dominated by swelling and fluid flow, whose effects are time-dependent. Because fluid content, which may change during storage, plays a significant role in the disc's time-dependent behavior, changes in mechanical response due to freezing may have been missed in previous studies that focused on time-independent behavior only.
  
  
  METHODS
  Porcine intervertebral discs were tested in repeated cycles of compressive creep either immediately postmortem or after 3 weeks of frozen storage. Swelling pressure and nuclear hydration were also measured in fresh and frozen discs. A fluid transport model was used to analyze the creep data.
  
  
  RESULTS
  The creep behavior of the intact porcine intervertebral disc is dramatically affected by frozen storage. The apparent permeability of the frozen discs was 82% higher than that of the fresh discs, and the swelling pressure of frozen discs was 25% lower in frozen discs (P < 0.01). The behavior of fresh and frozen discs became more dissimilar with repeated cycles of creep.
  
  
  CONCLUSIONS
  In vitro tests of frozen porcine intervertebral discs do not represent fresh behavior. Frozen storage appears to permanently alter disc behavior. The precise nature of any freezing-induced damage, and whether frozen storage similarly affects human discs, remains to be seen.
authors.affiliation: Department of Mechanical Engineering, University of California at Berkeley, USA.
authors.forename: E C
authors.initials: EC
authors.lastname: Bass
authors.forename: N A
authors.initials: NA
authors.lastname: Duncan
authors.forename: J S
authors.initials: JS
authors.lastname: Hariharan
authors.forename: J
authors.initials: J
authors.lastname: Dusick
authors.forename: H U
authors.initials: HU
authors.lastname: Bueff
authors.forename: J C
authors.initials: JC
authors.lastname: Lotz
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D019245:Compressive Strength; D015925:Cryopreservation; D005260:Female; D007403:Intervertebral Disc / Q000503:physiopathology*; D008159:Lumbar Vertebrae / Q000503:physiopathology; D008954:Models, Biological; D009997:Osmotic Pressure; D010539:Permeability; D013552:Swine; D016474:Weight-Bearing