title: Postoperative changes in spinal rod contour in adolescent idiopathic scoliosis: an in vivo deformation study.
abstract: METHODS
  Prospective case series.
  
  
  OBJECTIVE
  To evaluate the change in spinal rod contour from before implantation to after surgical correction of thoracic curves in patients with adolescent idiopathic scoliosis.
  
  
  BACKGROUND
  With segmental pedicle screw spinal instrumentation and vertebral derotation, many authors have reported a loss of thoracic kyphosis postoperatively. Although surgeons anticipate some flattening of the preimplantation rod contour in the sagittal plane, the magnitude of this change in shape has not been documented.
  
  
  METHODS
  The concave and convex rod shapes of 5.5-mm ultrahigh-strength steel spinal rods (200 KSI) from patients with thoracic adolescent idiopathic scoliosis (n = 27), which were contoured with benders by the surgeon, were traced prior to insertion. Postoperative (average, 5 weeks) sagittal rod shape was determined from lateral 2-dimensional radiographs. Maximal rod deflection and angle of the tangents to rod end points (Cobb) were measured. Repeated measures analysis of variance assessed differences between pre- and postoperation.
  
  
  RESULTS
  The scoliosis of 55° ±14° was corrected 72% to 15° ± 5°. The preinsertion rod shapes were more kyphotic for the concave (45.6°) than for the convex (31.4°) rods. Following correction, the concave rods flattened, with decrease in deflection of 13 mm and reduction in angle of 21° (both P < 0.001). The convex rods increased 1.5 mm in deflection and 2° in angle (P < 0.01, P = 0.18). The sagittal profile was maintained postoperatively as measured from T5-T12: 19° ±14° versus 22° ± 6° (pre vs. post, P > 0.1).
  
  
  CONCLUSIONS
  We found a significant difference between pre- and postoperative rod contour, particularly for concave rods. Rod overcontouring (by ~20° for concave rod) resulted in high degrees of correction without loss of sagittal alignment. The resulting deformations are likely associated with substantial in vivo deforming forces.
authors.affiliation: Rady Children’s Hospital and Health Center San Diego, CA, San Diego, CA, USA.
authors.forename: Krishna R
authors.initials: KR
authors.lastname: Cidambi
authors.forename: Diana A
authors.initials: DA
authors.lastname: Glaser
authors.forename: Tracey P
authors.initials: TP
authors.lastname: Bastrom
authors.forename: Thomas N
authors.initials: TN
authors.lastname: Nunn
authors.forename: Takashi
authors.initials: T
authors.lastname: Ono
authors.forename: Peter O
authors.initials: PO
authors.lastname: Newton
mesh_terms: D000293:Adolescent; D001863:Bone Screws; D002648:Child; D005260:Female; D006801:Humans; D016268:Internal Fixators; D007738:Kyphosis / Q000601:surgery; D008297:Male; D011184:Postoperative Period; D011446:Prospective Studies; D012600:Scoliosis / Q000601:surgery*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013904:Thoracic Vertebrae / Q000601:surgery*; D016896:Treatment Outcome