title: Biomechanical modeling and analysis of a direct incremental segmental translation system for the instrumentation of scoliotic deformities.
abstract: BACKGROUND
  Severe scoliosis deformities are usually treated by surgical instrumentation. Systems utilizing monoaxial pedicle screws with a tulip top design require rods to be fully seated and locked into the slot of each implant. The result is a limitation to the vertebra-positioning capability and the correction force control. Direct incremental segmental translation (DIST) was developed as a refinement of translational correction philosophies to provide better force control and vertebra-positioning capability. The objective of this study is to develop a biomechanical model for the spine instrumentation with the DIST system in order to simulate, analyze and compare different deformity correction strategies.
  
  
  METHODS
  The patient-specific spine geometry was reconstructed using calibrated coronal and sagittal radiographs. The spine biomechanical properties were adapted from experimental data and further adjusted using the patient's side-bending radiographs. Instrumentation constructs were modeled as rigid bodies connected by kinematic joints. The instrumentation maneuvers of 6 cases were simulated for which the simulation parameters were extracted from the surgery documentation and video. The correction maneuvers and resulting effects were analyzed.
  
  
  RESULTS
  The simulations agreed well with the real surgery (differences on Cobb angles <5°). The vertebral position relative to the rod was determined by 5 independent variables (position and orientation) vs. 2 for a monoaxial screw, thus increasing the possible correction of the connected vertebrae. The DIST system allows the spine deformity to be reduced by either gradually pulling the spine towards the rod through helical connections or translating it by pivoting the posts. Load at the vertebra-implant connection did not exceed 338N, and was well distributed (standard deviation<110N).
  
  
  CONCLUSIONS
  The study shows that the DIST system allows good control of the scoliotic vertebrae with good load sharing among implants.
authors.affiliation: Ecole Polytechnique de Montréal, Department of Mechanical Engineering, Downtown Station, Montreal (Quebec), Canada.
authors.forename: Xiaoyu
authors.initials: X
authors.lastname: Wang
authors.forename: Carl-Eric
authors.initials: CE
authors.lastname: Aubin
authors.forename: Dennis
authors.initials: D
authors.lastname: Crandall
authors.forename: Hubert
authors.initials: H
authors.lastname: Labelle
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D002138:Calibration; D003198:Computer Simulation; D006801:Humans; D008953:Models, Anatomic; D011859:Radiography / Q000379:methods; D015203:Reproducibility of Results; D012600:Scoliosis / Q000503:physiopathology*; D013131:Spine / Q000000981:diagnostic imaging / Q000473:pathology / Q000503:physiopathology