title: A comparison of the torsional stiffness of the lumbar spine in flexion and extension.
abstract: OBJECTIVE
  The main mechanism of injury to the spine is torsion especially when coupled with compression. In this study, the in vitro torsional stiffness of the lumbar spine segments is compared in flexion and extension positions by cyclic and failure testing.
  
  
  METHODS
  Fifteen lumbar spines were sectioned from fresh cadavers into 15 L2/3 and 15 L45 motion segments. Each vertebral segment was then potted superiorly and inferiorly in polymethylmethacrylate, effectively creating a bone-disk-bone construct. The potted spinal segments were mounted in a mechanical testing system, preloaded in compression to 300 N, and axially rotated to 3 degrees in both directions at a load rate of 1 degrees /s. This was done over 3 cycles for each motion segment in the flexion and extension positions. Each specimen was then tested to torsional failure in either flexion or extension. Stiffness, torque, and energy were determined from cyclic and failure testing.
  
  
  RESULTS
  The results showed that in all cases of cyclic testing, the higher segment extension resulted in higher torsional stiffness. In relative extension, the lumbar specimens were stiffer, generated higher torque values, and generally absorbed more energy than the relative flexion condition. There were no differences found in loading direction or failure testing.
  
  
  CONCLUSIONS
  Increasing the effective torsional stiffness of the lumbar spine in extension could provide a protective mechanism against interverbral disk injury. Restoration of segmental extension through increasing the lumbar lordosis may decrease the strain and reinjury of the joints, which can help reduce the extent of pain in the lumbar spine.
authors.affiliation: Department of Orthopaedic Surgery and Rehabilitation, Division of Spine Surgery, University of Texas Medical Branch, Galveston, Texas, USA. kjgarges@earthlink.net
authors.forename: Kim J
authors.initials: KJ
authors.lastname: Garges
authors.forename: Ali
authors.initials: A
authors.lastname: Nourbakhsh
authors.forename: Randall
authors.initials: R
authors.lastname: Morris
authors.forename: Jinping
authors.initials: J
authors.lastname: Yang
authors.forename: Milan
authors.initials: M
authors.lastname: Mody
authors.forename: Rita
authors.initials: R
authors.lastname: Patterson
mesh_terms: D000704:Analysis of Variance; D001696:Biomechanical Phenomena / Q000502:physiology; D002102:Cadaver; D019245:Compressive Strength / Q000502:physiology; D004548:Elasticity; D004734:Energy Metabolism; D005260:Female; D006801:Humans; D007403:Intervertebral Disc / Q000293:injuries; D008141:Lordosis / Q000000981:diagnostic imaging / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control; D017116:Low Back Pain / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000293:injuries / Q000502:physiology* / Q000503:physiopathology; D008297:Male; D008953:Models, Anatomic; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology*; D012399:Rotation; D054159:Torsion, Mechanical; D016474:Weight-Bearing