title: A novel method of screw placement for extremely small thoracic pedicles in scoliosis.
abstract: METHODS
  A retrospective clinical study.
  
  
  OBJECTIVE
  To introduce a novel method of pedicle screw placement for extremely small thoracic pedicles in scoliosis and evaluate the safety and accuracy of the method.
  
  
  BACKGROUND
  Few studies have provided technical guidelines for screw placement in patients with extremely small thoracic pedicles in scoliosis.
  
  
  METHODS
  In a severely rotated scoliotic spine, thoracic pedicle screw placement is challenging, and particularly more so for extremely small pedicles with a diameter less than 2 mm. The authors introduced a novel method of screw placement for these small pedicles: "medial margin targeting method."The C-arm fluoroscope is rotated until a true PA image of the rotated vertebral body is acquired and both pedicle shadows are symmetrically visualized en face. In extremely small pedicles, pedicle shadows appear as long, slender ellipses or lines. An imaginary pedicle outline is presumed with the elliptical or linear shadows being the medial margin of the pedicle. The entry point of a screw can be made at the 10-o'clock or 2-o'clock position on the presumed pedicle outline, and the screw can be safely inserted targeting the presumed medial margin with caution not to penetrate the medial cortex using the guidance of a true PA image. This is a kind of extrapedicular screw placement method.The safety and accuracy of this method were evaluated in 97 patients with scoliosis who had undergone posterior correction and instrumentation using postoperative computed tomography scans. A total of 1634 pedicle screws were inserted into thoracic pedicles, 128 of them (7.8%) being extremely small pedicles with a diameter less than 2 mm.
  
  
  RESULTS
  Among 128 extremely small thoracic pedicles with a diameter less than 2 mm, one screw (0.8%) violated the medial cortex and 22 screws (17.6%) violated the anterior cortex of the vertebral body. No screws violated the lateral cortex of the pedicle-rib unit. There were no complications associated with screw misplacement.
  
  
  CONCLUSIONS
  In scoliosis patients with extremely small thoracic pedicles, our pedicle screw placement method targeting the presumed medial margin in a true PA C-arm image allows easy application with accuracy and safety, which would not possible by any other method described so far.
authors.affiliation: Scoliosis Center, Asan Medical Center, University of Ulsan College of Medicine, 86 Asanbyeongwon-gil, Songpa-gu, Seoul, Korea.
authors.forename: Choon Sung
authors.initials: CS
authors.lastname: Lee
authors.forename: Soo-An
authors.initials: SA
authors.lastname: Park
authors.forename: Chang Ju
authors.initials: CJ
authors.lastname: Hwang
authors.forename: Dong-Joon
authors.initials: DJ
authors.lastname: Kim
authors.forename: Won-Jae
authors.initials: WJ
authors.lastname: Lee
authors.forename: Yung-Tae
authors.initials: YT
authors.lastname: Kim
authors.forename: Mi Young
authors.initials: MY
authors.lastname: Lee
authors.forename: So Jung
authors.initials: SJ
authors.lastname: Yoon
authors.forename: Dong-Ho
authors.initials: DH
authors.lastname: Lee
mesh_terms: D000293:Adolescent; D001863:Bone Screws; D005260:Female; D005471:Fluoroscopy / Q000379:methods; D006801:Humans; D008297:Male; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012600:Scoliosis / Q000000981:diagnostic imaging / Q000601:surgery*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D014057:Tomography, X-Ray Computed