title: The effect of simulated microgravity on lumbar spine biomechanics: an in vitro study.
abstract: Disc herniation risk is quadrupled following spaceflight. This study tested the hypothesis that swelling-induced disc height increases (comparable to those reported in spaceflight) stiffen the spine and elevate annular strain and nuclear pressure during forward bending.
  
  
  
  Eight human lumbar motion segments were secured to custom-designed testing jigs and subjected to baseline flexion and compression and pure moment flexibility tests. Discs were then free-swelled in saline to varying supraphysiologic heights consistent with prolonged weightlessness and re-tested to assess biomechanical changes.
  
  
  
  Swelling-induced disc height changes correlated positively with intradiscal pressure (p < 0.01) and stiffening in flexion (p < 0.01), and negatively with flexion range of motion (p < 0.05). Swelling-induced increases in disc height also led to increased annular surface strain under combined flexion with compression. Disc wedge angle decreased with swelling (p < 0.05); this loss of wedge angle correlated with decreased flexion range of motion (R (2) = 0.94, p < 0.0001) and decreased stiffness fold change in extension (p < 0.05).
  
  
  
  Swelling-induced increases in disc height decrease flexibility and increase annular strain and nuclear pressure during forward bending. These changes, in combination with the measured loss of lordotic curvature with disc swelling, may contribute toward increased herniation risk. This is consistent with clinical observations of increased disc herniation rates after microgravity exposure and may provide the basis for future countermeasure development.
authors.affiliation: University of California, Box 0514, 513 Parnassus Avenue, Medical Sciences Bldg., San Francisco, CA, 94143-0514, USA.
authors.forename: Cory J
authors.initials: CJ
authors.lastname: Laws
authors.affiliation: University of California, Box 0514, 513 Parnassus Avenue, Medical Sciences Bldg., San Francisco, CA, 94143-0514, USA.
authors.forename: Britta
authors.initials: B
authors.lastname: Berg-Johansen
authors.affiliation: UCSD Medical Center, University of California, 350 Dickinson St., Suite 121, Mail Code 8894, San Diego, CA, 92103-8894, USA.
authors.forename: Alan R
authors.initials: AR
authors.lastname: Hargens
authors.affiliation: University of California, Box 0514, 513 Parnassus Avenue, Medical Sciences Bldg., San Francisco, CA, 94143-0514, USA. lotzj@orthosurg.ucsf.edu.
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Lotz
mesh_terms: D001696:Biomechanical Phenomena; D006801:Humans; D007403:Intervertebral Disc / Q000502:physiology*; D007405:Intervertebral Disc Displacement / Q000503:physiopathology; D008159:Lumbar Vertebrae / Q000502:physiology*; D011187:Posture / Q000502:physiology; D011312:Pressure; D016059:Range of Motion, Articular / Q000502:physiology; D012106:Research; D013026:Space Flight; D013314:Stress, Mechanical; D014893:Weightlessness
keywords: Biomechanics; Herniation; Intervertebral disc; Microgravity; Spaceflight