title: Flow-related mechanics of the intervertebral disc: the validity of an in vitro model.
abstract: METHODS
  An in vitro mechanical study on porcine motion segments.
  
  
  OBJECTIVE
  To test the validity of in vitro studies of the flow-related mechanics of the intervertebral disc and, in particular, to investigate whether fluid flows back into the disc during unloading after a loading cycle.
  
  
  BACKGROUND
  In vivo studies show both the inflow and outflow of fluid in the intervertebral disc. The resistance to flow out of the disc is higher than to inflow. The fluid flow is regulated via unbalance between the external load and the osmotic pressure of the nucleus pulposus.
  
  
  METHODS
  There were 8 porcine lumbar motion segments (without posterior elements) and 8 isolated discs tested in a physiologic saline bath (39 degrees C). The specimens were preloaded at 0.025 MPa for 15 minutes. Three 15-minute loading periods at 2.0 MPa were applied, each followed by an unloading period of 30 minutes. Loads, axial displacements, and nucleus pressure were recorded online.
  
  
  RESULTS
  Over the 3 loading and unloading periods, all specimens showed a net loss of height and mass. The time series of specimen height during the 3 unloading periods showed virtually identical responses. The pressure in the nucleus decreased in the subsequent loading periods and showed no increase during unloading.
  
  
  CONCLUSIONS
  The data show the limitations of an in vitro model for studying fluid flow-related intervertebral disc mechanics. During loading, outflow of fluid occurred, but inflow appears to be virtually absent during unloading. Poro-elastic behavior cannot be reproduced in an in vitro model.
authors.affiliation: Department of Physics and Medical Technology, VU University Medical Center, Amsterdam, The Netherlands. aj.veen@vumc.nl
authors.forename: Albert J
authors.initials: AJ
authors.lastname: van der Veen
authors.forename: Margriet
authors.initials: M
authors.lastname: Mullender
authors.forename: Theo H
authors.initials: TH
authors.lastname: Smit
authors.forename: Idsart
authors.initials: I
authors.lastname: Kingma
authors.forename: Jaap H
authors.initials: JH
authors.lastname: van Dieën
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D001826:Body Fluids / Q000378:metabolism*; D066298:In Vitro Techniques; D007403:Intervertebral Disc / Q000378:metabolism / Q000502:physiology*; D008159:Lumbar Vertebrae; D009997:Osmotic Pressure; D013552:Swine; D016474:Weight-Bearing / Q000502:physiology