title: Relationship of forces acting on implant rods and degree of scoliosis correction.
abstract: BACKGROUND
  Adolescent idiopathic scoliosis is a complex spinal pathology characterized as a three-dimensional spine deformity combined with vertebral rotation. Various surgical techniques for correction of severe scoliotic deformity have evolved and became more advanced in applying the corrective forces. The objective of this study was to investigate the relationship between corrective forces acting on deformed rods and degree of scoliosis correction.
  
  
  METHODS
  Implant rod geometries of six adolescent idiopathic scoliosis patients were measured before and after surgery. An elasto-plastic finite element model of the implant rod before surgery was reconstructed for each patient. An inverse method based on Finite Element Analysis was used to apply forces to the implant rod model such that it was deformed the same after surgery. Relationship between the magnitude of corrective forces and degree of correction expressed as change of Cobb angle was evaluated. The effects of screw configuration on the corrective forces were also investigated.
  
  
  RESULTS
  Corrective forces acting on rods and degree of correction were not correlated. Increase in number of implant screws tended to decrease the magnitude of corrective forces but did not provide higher degree of correction. Although greater correction was achieved with higher screw density, the forces increased at some level.
  
  
  CONCLUSIONS
  The biomechanics of scoliosis correction is not only dependent to the corrective forces acting on implant rods but also associated with various parameters such as screw placement configuration and spine stiffness. Considering the magnitude of forces, increasing screw density is not guaranteed as the safest surgical strategy.
authors.affiliation: Laboratory of Biomechanical Design, Division of Human Mechanical Systems and Design, Graduate School of Engineering, Hokkaido University, North 13 West 8 Kita-ku, Sapporo, Japan.
authors.forename: Remel Alingalan
authors.initials: RA
authors.lastname: Salmingo
authors.forename: Shigeru
authors.initials: S
authors.lastname: Tadano
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Fujisaki
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Abe
authors.forename: Manabu
authors.initials: M
authors.lastname: Ito
mesh_terms: D000293:Adolescent; D001696:Biomechanical Phenomena; D001863:Bone Screws; D020342:Finite Element Analysis; D006801:Humans; D016268:Internal Fixators; D008297:Male; D019637:Orthopedic Procedures / Q000295:instrumentation / Q000379:methods*; D012399:Rotation; D012600:Scoliosis / Q000601:surgery* / Q000628:therapy*; D013131:Spine / Q000601:surgery; D013314:Stress, Mechanical; D054159:Torsion, Mechanical