title: Biomechanics of thoracic short versus long fixation after 3-column injury.
abstract: OBJECTIVE
  Posterior screw-rod fixation for thoracic spine trauma usually involves fusion across long segments. Biomechanical data on screw-based short-segment fixation for thoracic fusion are lacking. The authors compared the effects of spanning short and long segments in the thoracic spine.
  
  
  METHODS
  Seven human spine segments (5 segments from T-2 to T-8; 2 segments from T-3 to T-9) were prepared. Pure-moment loading of 6 Nm was applied to induce flexion, extension, lateral bending, and axial rotation while 3D motion was measured optoelectronically. Normal specimens were tested, and then a wedge fracture was created on the middle vertebra after cutting the posterior ligaments. Five conditions of instrumentation were tested, as follows: Step A, 4-level fixation plus cross-link; Step B, 2-level fixation; Step C, 2-level fixation plus cross-link; Step D, 2-level fixation plus screws at fracture site (index); and Step E, 2-level fixation plus index screws plus cross-link.
  
  
  RESULTS
  Long-segment fixation restricted 2-level range of motion (ROM) during extension and lateral bending significantly better than the most rigid short-segment construct. Adding index screws in short-segment constructs significantly reduced ROM during flexion, lateral bending, and axial rotation (p < 0.03). A cross-link reduced axial rotation ROM (p = 0.001), not affecting other loading directions (p > 0.4).
  
  
  CONCLUSIONS
  Thoracic short-segment fixation provides significantly less stability than long-segment fixation for the injury studied. Adding a cross-link to short fixation improved stability only during axial rotation. Adding a screw at the fracture site improved short-segment stability by an average of 25%.
authors.affiliation: Division of Neurological Surgery, Barrow Neurological Institute, Phoenix, Arizona 85013, USA.
authors.forename: Bruno C R
authors.initials: BC
authors.lastname: Lazaro
authors.forename: Fatih Ersay
authors.initials: FE
authors.lastname: Deniz
authors.forename: Leonardo B C
authors.initials: LB
authors.lastname: Brasiliense
authors.forename: Phillip M
authors.initials: PM
authors.lastname: Reyes
authors.forename: Anna G U
authors.initials: AG
authors.lastname: Sawa
authors.forename: Nicholas
authors.initials: N
authors.lastname: Theodore
authors.forename: Volker K H
authors.initials: VK
authors.lastname: Sonntag
authors.forename: Neil R
authors.initials: NR
authors.lastname: Crawford
mesh_terms: D001863:Bone Screws; D006801:Humans; D008422:Materials Testing; D011183:Postoperative Complications / Q000503:physiopathology; D016059:Range of Motion, Articular / Q000502:physiology*; D016103:Spinal Fractures / Q000503:physiopathology / Q000601:surgery*; D013123:Spinal Fusion / Q000379:methods*; D013904:Thoracic Vertebrae / Q000293:injuries* / Q000503:physiopathology / Q000601:surgery*; D016474:Weight-Bearing / Q000502:physiology