title: A Surgical Method for Determining Proper Screw Length in ACDF.
abstract: OBJECTIVE
  We describe a surgical tool that uses the distractor pin as a reference for determining proper screw length in ACDF. It is critical that screw purchase depth be as deep as possible without violating or penetrating the posterior cortical wall, which ensures strong pull out strength.
  
  
  METHODS
  We enrolled 81 adult patients who underwent ACDF using an anterior cervical plate from 2010 to 2012. Patients were categorized into Groups A (42 patients: retractor pin used as a reference for screw length) and B (39 patients: control group). Intraoperative lateral x-rays were taken after screwing the retractor pin to confirm the approaching vertebral level. The ratio of retractor pin length to body anteroposterior (A-P) diameter was measured as a reference. Proper screw length was determined by comparison to the reference.
  
  
  RESULTS
  The average distance from screw tip to posterior wall was 3.0±1.4mm in Group A and 4.1±2.3mm in Group B. The ratio of screw length to body sagittal diameter was 86.2±5.7% in Group A and 80.8±9.0% in Group B. Screw length to body sagittal diameter ratios higher than 4/5 occurred in 33 patients (90%) in Group A and 23 patients (59%) in Group B. No cases violated the posterior cortical wall.
  
  
  CONCLUSIONS
  We introduce a useful surgical method for determining proper screw length in ACDF using the ratio of retractor pin length to body A-P diameter as a reference. This method allows for deeper screw purchase depth without violation of the posterior cortical wall.
authors.affiliation: Department of Neurosurgery, Spine and Spinal Cord Institute, Yonsei University College of Medicine, Seoul, Korea.
authors.forename: Hae-Gi
authors.initials: HG
authors.lastname: Park
authors.affiliation: Department of Neurosurgery, Spine and Spinal Cord Institute, Yonsei University College of Medicine, Seoul, Korea.
authors.forename: Moo-Sung
authors.initials: MS
authors.lastname: Kang
authors.affiliation: Department of Neurosurgery, Spine and Spinal Cord Institute, Yonsei University College of Medicine, Seoul, Korea.
authors.forename: Kyung-Hyun
authors.initials: KH
authors.lastname: Kim
authors.affiliation: Department of Neurosurgery, Spine and Spinal Cord Institute, Yonsei University College of Medicine, Seoul, Korea.
authors.forename: Jeong-Yoon
authors.initials: JY
authors.lastname: Park
authors.affiliation: Department of Neurosurgery, Spine and Spinal Cord Institute, Yonsei University College of Medicine, Seoul, Korea.
authors.forename: Keun-Su
authors.initials: KS
authors.lastname: Kim
authors.affiliation: Department of Neurosurgery, Spine and Spinal Cord Institute, Yonsei University College of Medicine, Seoul, Korea.
authors.forename: Sung-Uk
authors.initials: SU
authors.lastname: Kuh
keywords: Anterior cervical discectomy and fusion (ACDF); Screw length; Screw pullout strength