title: Effects of posterior distraction forces on anterior column intradiscal pressure in the dual growing rod technique.
abstract: BACKGROUND
  Little evidence is available addressing biomechanical properties of posterior distraction forces and their effects on anterior spinal column in the growing rod technique. The question is often asked if posterior distraction forces may be kyphogenic. The goal of this study is to determine whether posterior distraction forces transmitted anteriorly through different foundation constructs (i.e., screws vs. hooks) affect intradiscal pressure.
  
  
  METHODS
  Six skeletally immature porcine spines were harvested leaving soft tissues and rib heads intact. Pedicle screws served as the lower foundation on a L3-L4 motion segment while pedicle screws and laminar hooks were randomly used at T3-T4 levels. Proximal constructs (hook vs. screw) were switched after initial distraction testing. The dual rod distractor was instrumented with strain gauges and calibrated using a custom force transducer. During distraction, intradiscal pressures immediately inferior to the superior foundation and the level equidistant between foundations were measured using needle pressure transducers. Maximum distraction force and maximum anterior disc pressure change were compared between hook and pedicle screw anchors using one-way ANOVA (p < 0.05).
  
  
  RESULTS
  Upper foundations with pedicle screws had significantly greater distraction forces (416 ± 101 N) than those with upper level hooks (349 ± 100 N). There were no significant differences in disc pressures between levels or between upper foundation constructs. Disc pressures adjacent to the upper foundation demonstrated greater reduction (disc expansion) than the level equidistant within the construct. Pedicle screw constructs demonstrated greater endplate separation (distraction) compared to hook constructs.
  
  
  CONCLUSIONS
  Posterior distraction forces result in anterior disc separation (distraction) and are distributed across multiple levels rather than delivered to the disc immediately adjacent to a foundation. Constructs with upper foundation hooks had lower distraction forces possibly due to hook motion during distraction. The load distribution at multiple levels may assist with curve control and may affect vertebral growth. The distraction forces may not be kyphogenic as is commonly believed.
authors.affiliation: Orthopedic Biomechanics Research Center, Rady Children's Hospital San Diego, San Diego, CA, USA, andrew.mahar@gmail.com.
authors.forename: Andrew
authors.initials: A
authors.lastname: Mahar
authors.forename: Nima
authors.initials: N
authors.lastname: Kabirian
authors.forename: Behrooz A
authors.initials: BA
authors.lastname: Akbarnia
authors.forename: Michael
authors.initials: M
authors.lastname: Flippin
authors.forename: Tucker
authors.initials: T
authors.lastname: Tomlinson
authors.forename: Patricia
authors.initials: P
authors.lastname: Kostial
authors.forename: Ramin
authors.initials: R
authors.lastname: Bagheri
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D016268:Internal Fixators; D007403:Intervertebral Disc; D008159:Lumbar Vertebrae / Q000601:surgery*; D019857:Osteogenesis, Distraction / Q000295:instrumentation*; D011312:Pressure; D012600:Scoliosis / Q000503:physiopathology / Q000601:surgery*; D013314:Stress, Mechanical; D013552:Swine; D013904:Thoracic Vertebrae / Q000601:surgery*