title: Viscoelastic properties of the ovine posterior spinal ligaments are strain dependent.
abstract: BACKGROUND
  The biomechanical role of the posterior spinal ligaments for spinal stability has been stated in previous studies. The investigation of the viscoelastic properties of human lumbar spinal ligaments is essential for the understanding of physiological differences between healthy and degenerated tissues. The stress-relaxation behavior of biological tissues is commonly described with the quasi-linear viscoelastic model of Fung, which assumes that the stress-relaxation response is independent of the applied strain. The goal of this study was to investigate the stress-relaxation response of ovine posterior spinal ligaments at different elongations to verify the above-mentioned hypothesis.
  
  
  METHODS
  Twenty-four ovine lumbar spinal segments, consisting of only the supraspinous and interspinous ligaments and adjoining spinous processes, were elongated uniaxially to different strain levels within the physiological elastic region (5-20%). The experimental data were described with a non-linear viscoelastic model: the modified superposition method of Findley.
  
  
  RESULTS
  A linear dependency of the relaxation rate to the applied strains was observed on intact segments, when both ligaments were considered, as well as on each individual ligament. This result can be applied to the human spinal ligaments, due to similarities observed between the sheep and human spinal segment under physiological loading.
  
  
  CONCLUSIONS
  The non-linear viscoelastic modified superposition method of Findley is an appropriate model for describing the viscoelastic properties of lumbar spinal ligaments in vitro due to its ability to address variation in applied strain during the force relaxation measurements.
authors.affiliation: Bern University of Applied Sciences, Biomedical Engineering, Quellgasse 21, Biel, Switzerland. sveva.ambrosetti@gmail.com
authors.forename: Sveva
authors.initials: S
authors.lastname: Ambrosetti-Giudici
authors.forename: Philippe
authors.initials: P
authors.lastname: Gédet
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Ferguson
authors.forename: Salman
authors.initials: S
authors.lastname: Chegini
authors.forename: Juergen
authors.initials: J
authors.lastname: Burger
mesh_terms: D000818:Animals; D003198:Computer Simulation; D055119:Elastic Modulus / Q000502:physiology; D066298:In Vitro Techniques; D008022:Ligaments / Q000502:physiology*; D008159:Lumbar Vertebrae / Q000502:physiology*; D008954:Models, Biological; D012756:Sheep; D013314:Stress, Mechanical; D014783:Viscosity