title: Clear Zone Formation around Screws in the Early Postoperative Stages after Posterior Lumbar Fusion Using the Cortical Bone Trajectory Technique.
abstract: METHODS
  Retrospective study.
  
  
  OBJECTIVE
  To evaluate the initial fixation using the cortical bone trajectory (CBT) technique for posterior lumbar fusion through assessment of the clear zones around the screws and the risk factors involved.
  
  
  BACKGROUND
  Postoperative radiolucent zones (clear zones) are an indicator of poor conventional pedicle screw fixation.
  
  
  METHODS
  Between January 2013 and April 2014, 19 patients (8 men and 11 women) underwent posterior lumbar interbody fusion or posterior lumbar fusion using the CBT technique. A total of 109 screws were used for evaluation with measurement of the maximum insertional torque of last two screw rotations. Clear zone-positivity on plain radiographs was investigated 6 months after surgery. The relation between intraoperative insertional torque and clear zone-positivity was investigated by one-way analysis of variance. In addition, the correlation between clear zone-positivity and gender, age (<75 years old or >75 years old), or operative stabilization level (<2 or >3 vertebral levels) was evaluated using the chi-square test.
  
  
  RESULTS
  Clear zones were observed around six screws (5.50%) in five patients (26.3%). The mean insertional torque (4.00±2.09 inlbs) of clear zone-positive screws was lower than that of clear zone-negative screws (8.12±0.50 in-lbs), but the difference was not significant. There was a significant correlation between clear zone-positivity and operative level of stabilization.
  
  
  CONCLUSIONS
  The low incidence of clear zone-positive screws indicates good initial fixation using the CBT technique. Multilevel fusions may be risk factors for clear zone generation.
authors.affiliation: Department of Neurosurgery, Osaka University Graduate School of Medicine, Suita, Japan.
authors.forename: Koshi
authors.initials: K
authors.lastname: Ninomiya
authors.affiliation: Department of Neurosurgery, Osaka University Graduate School of Medicine, Suita, Japan.
authors.forename: Koichi
authors.initials: K
authors.lastname: Iwatsuki
authors.affiliation: Department of Neurosurgery, Osaka University Graduate School of Medicine, Suita, Japan.
authors.forename: Yu-Ichiro
authors.initials: Y
authors.lastname: Ohnishi
authors.affiliation: Department of Neurosurgery, Osaka University Graduate School of Medicine, Suita, Japan.
authors.forename: Toshika
authors.initials: T
authors.lastname: Ohkawa
authors.affiliation: Department of Neurosurgery, Osaka University Graduate School of Medicine, Suita, Japan.
authors.forename: Toshiki
authors.initials: T
authors.lastname: Yoshimine
keywords: Clear zone; Cortical bone trajectory; Initial fixation