title: Should we use cortical bone screws for cortical bone trajectory?
abstract: OBJECTIVE
  In 2009, Santoni et al. reported cortical bone trajectory (CBT) as a method of inserting pedicle screws to obtain more solid fixation, and proposed the use of cortical trajectory screws with a more closely placed thread (cortical screws) for CBT. Since the entry trajectory in CBT differs from that in the traditional trajectory, it is unclear whether the increased strength derives from the specific trajectory or the shape of the screw thread in contact with the cortical bone. Whether the use of cortical screws is always required with CBT thus remains unclear. The authors therefore investigated the relationship between screw entry trajectory and screw thread characteristics and pullout strength in animal experiments.
  
  
  METHODS
  Lumbar vertebrae (L1-L4) from 4-month-old female pigs were randomly assigned to one of 4 groups, with cancellous screws or cortical screws inserted via the traditional trajectory or CBT. For pullout strength testing, the screw was pulled out vertically against the direction of insertion. Rod pullout testing (toggle testing) was also performed, and the peak breaking strength was measured.
  
  
  RESULTS
  The maximum pullout strength was significantly greater for CBT using cortical screws than for the traditional trajectory using cancellous screws. Pullout strength tended to be higher when cortical screws were used in both CBT and the traditional trajectory, although the difference was not significant. Toggle testing showed no significant differences among the 4 groups.
  
  
  CONCLUSIONS
  The specific unconventional trajectory seemed to have a major impact on the increased strength obtained with CBT.
authors.affiliation: Department of Orthopaedic Surgery, Kitasato University School of Medicine; and 
authors.forename: Masaki
authors.initials: M
authors.lastname: Ueno
authors.forename: Rina
authors.initials: R
authors.lastname: Sakai
authors.forename: Kensei
authors.initials: K
authors.lastname: Tanaka
authors.forename: Gen
authors.initials: G
authors.lastname: Inoue
authors.forename: Kentaro
authors.initials: K
authors.lastname: Uchida
authors.forename: Takayuki
authors.initials: T
authors.lastname: Imura
authors.forename: Wataru
authors.initials: W
authors.lastname: Saito
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Nakazawa
authors.forename: Naonobu
authors.initials: N
authors.lastname: Takahira
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Mabuchi
authors.forename: Masashi
authors.initials: M
authors.lastname: Takaso
keywords: CBT = cortical bone trajectory; PMMA = polymethylmethacrylate; TT = traditional trajectory; cortical bone trajectory; pedicle screw; screw thread; spinal fusion; technique