title: Effect of frozen storage on the creep behavior of human intervertebral discs.
abstract: METHODS
  A biomechanical study of the compressive creep behavior of the human intervertebral disc before and after frozen storage.
  
  
  OBJECTIVE
  To determine whether frozen storage alters the time-dependent response of the intact human intervertebral disc.
  
  
  BACKGROUND
  The biomechanical properties of the intervertebral disc are generally determined using specimens that have been previously frozen. Although it is well established that freezing does not alter the elastic response of the disc, recent data demonstrate that freezing permanently alters the time-dependent mechanical behavior of porcine discs.
  
  
  METHODS
  Twenty lumbar motion segments from 10 human spines were harvested between 12 and 36 hours postmortem. The specimens were randomly assigned to one of two groups: Group 1 was tested promptly, stored frozen for 3 weeks, then thawed, and tested a second time; Group 2 was stored frozen for 3 weeks, thawed, and then tested. Each specimen was subjected to 5 cycles of compressive creep under 1 MPa for 20 minutes, followed by a 40-minute recovery under no load. After testing each specimen was graded on a degeneration scale. A fluid transport model was used to parameterize the creep data.
  
  
  RESULTS
  There was no statistically significant effect of freezing on the elastic or creep response of the discs. The degree of pre-existing degeneration had a significant effect on the creep response, with the more degenerated discs appearing more permeable.
  
  
  CONCLUSIONS
  Frozen storage for a reasonable time with a typical method does not significantly alter the creep response of human lumbar discs. Freezing may produce subtle effects, but these potential artifacts do not appear to alter the discs' time-dependent behavior in any consequential way. These results may not apply to tissue kept frozen for long durations and with poor packaging.
authors.affiliation: Orthopaedic Bioengineering Laboratory, Department of Orthopaedic Surgery, University of California at San Francisco, San Francisco, California 94143-0514, USA.
authors.forename: N
authors.initials: N
authors.lastname: Dhillon
authors.forename: E C
authors.initials: EC
authors.lastname: Bass
authors.forename: J C
authors.initials: JC
authors.lastname: Lotz
mesh_terms: D016477:Artifacts; D019245:Compressive Strength / Q000502:physiology*; D003890:Desiccation; D004548:Elasticity; D005615:Freezing; D006801:Humans; D066298:In Vitro Techniques; D007403:Intervertebral Disc / Q000502:physiology*; D014867:Water
chemical_list: D014867:Water