title: Biomechanical analysis of derotation of the thoracic spine using pedicle screws.
abstract: METHODS
  Biomechanical analysis of derotational load-to-failure of pedicle screw (PS) instrumentation in cadaveric thoracic spinal segments.
  
  
  OBJECTIVE
  To investigate the derotational torque that can be applied to the thoracic spine through different linked constructs and evaluate the modes of failure.
  
  
  BACKGROUND
  Thoracic derotation with PSs has been shown to provide better 3 plane correction than other methods but the effects of linked PS constructs has not been studied.
  
  
  METHODS
  Four groups of thoracic segments with different PS constructs were loaded to failure with a rotational torque applied to the construct to simulate the left to right derotational force applied to a typical idiopathic dextrorotary thoracic scoliosis curve. Single screw T4 segments instrumented on the medial (group 1M) and lateral (group 1L) sides, bilaterally-linked T5 segments (group 2), unilaterally-linked T6-T9 segments on the medial (group 3M) and lateral (group 3L) sides, and quadrangularly-linked T6-T9 segments (group 4) were loaded with MTS machine in a simulated thoracic derotation model.
  
  
  RESULTS
  Single T4 PSs on the medial and lateral sides failed at 4.0 +/- 1.4 Nm (group 1M) and 6.1 +/- 2.5 Nm (group 1L), respectively. Bilaterally-linked T5 screws failed at 11.9 +/- 3.1 Nm (group 2). Unilaterally linked T6-T9 PS constructs on the medial and lateral sides failed at 21.2 +/- 7.5 Nm (group 3M) and 17.9 +/- 11.1 Nm (group 3L), respectively. Quadrangularly-linked PSs failed at 42.5 +/- 14.5 Nm (group 4). CONCLUSION.: A near linear increase in relative torque applied before failure was found with each additional PS linked. Linked constructs allow for significantly greater torque with less risk of PS breach of the spinal canal.
authors.affiliation: Department of Orthopaedic Surgery, Stanford University Medical Center. ivan.cheng@stanford.edu
authors.forename: Ivan
authors.initials: I
authors.lastname: Cheng
authors.forename: David
authors.initials: D
authors.lastname: Hay
authors.forename: Alex
authors.initials: A
authors.lastname: Iezza
authors.forename: Derek
authors.initials: D
authors.lastname: Lindsey
authors.forename: Lawrence G
authors.initials: LG
authors.lastname: Lenke
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000502:physiology; D001863:Bone Screws / Q000592:standards*; D005260:Female; D006801:Humans; D016268:Internal Fixators / Q000592:standards; D008297:Male; D008875:Middle Aged; D008953:Models, Anatomic; D011183:Postoperative Complications / Q000209:etiology / Q000517:prevention & control; D016059:Range of Motion, Articular / Q000502:physiology; D012399:Rotation / Q000009:adverse effects; D012600:Scoliosis / Q000473:pathology* / Q000503:physiopathology / Q000601:surgery*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013314:Stress, Mechanical; D013904:Thoracic Vertebrae / Q000473:pathology* / Q000503:physiopathology / Q000601:surgery*; D019415:Torque