title: Safety evaluation of freehand lateral mass screw fixation in the subaxial cervical spine: evaluation of 1256 screws.
abstract: METHODS
  Prospective study.
  
  
  OBJECTIVE
  To evaluate the safety of the lateral mass screw (LMS) fixation.
  
  
  BACKGROUND
  LMS fixation has been known to have risk of injury to vertebral artery, nerve root, or facet joints.
  
  
  METHODS
  A consecutive series of 178 patients undergoing planned LMS fixation were studied. Screw fixation was performed using the freehand technique. Entry point of screws was 2-mm medial from the center of the lateral mass. Planned divergent angle of the screw was 30º. Bicortical fixation was tried in all cases. After the surgery, all patients underwent 3-dimensional computed tomographic scan of the cervical spine. We checked the number of screws in each level, divergent angle of the screws, and violation of foramen transversarium (FT), intervertebral foramen, or facet joint by screws. A reliability test was performed.
  
  
  RESULTS
  Total 1256 screws were fixed with 269 at C3, 318 at C4, 331 at C5, and 338 screws at C6. Mean divergent angle of the screws were 34.7º/33.1º at C3, 33.9 º/32.1º at C4, 34.7º/32.7º at C5, and 33.6º/30.7º at C6. Incidence of FT violation was 0.876%. FT violation was most common at C6 (6/11 violations). Mean divergent angle in cases of FT violation was 15.0º and was significantly smaller than that of safe cases. There was no injury to vertebral artery. There was no violation of intervertebral foramen. Incidence of facet violation was 1.433%. Seventeen facet violations were within fusion segment. Only 1 screw violated healthy facet. Facet violation was most common at C3 (12/18 violations).
  
  
  CONCLUSIONS
  LMS fixation is a safe stabilizing technique with very low incidence of violation of FT, intervertebral foramen, and facet. There is a possibility of FT violation if the divergent angle was small. FT violation was most common at C6. Facet violation was most common at C3.
authors.affiliation: *Department of Orthopaedic Surgery, Yonsei University College of Medicine, Seoul, Korea †Department of Orthopaedic Surgery, Kyung Hee University College of Medicine, Seoul, Korea; and ‡Department of Orthopaedic Surgery, Hemin Hospital, Seoul, Korea.
authors.forename: Hak-Sun
authors.initials: HS
authors.lastname: Kim
authors.forename: Kyung-Soo
authors.initials: KS
authors.lastname: Suk
authors.forename: Seong-Hwan
authors.initials: SH
authors.lastname: Moon
authors.forename: Hwan-Mo
authors.initials: HM
authors.lastname: Lee
authors.forename: Kyung Chung
authors.initials: KC
authors.lastname: Kang
authors.forename: Sang-Hun
authors.initials: SH
authors.lastname: Lee
authors.forename: Jin-Soo
authors.initials: JS
authors.lastname: Kim
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001863:Bone Screws / Q000009:adverse effects*; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D013122:Spinal Diseases / Q000000981:diagnostic imaging / Q000601:surgery*; D013123:Spinal Fusion / Q000009:adverse effects* / Q000379:methods; D014057:Tomography, X-Ray Computed; D055815:Young Adult