title: Fatigue Performance of Cortical Bone Trajectory Screw Compared With Standard Trajectory Pedicle Screw.
abstract: METHODS
  Cadaveric biomechanical study.
  
  
  OBJECTIVE
  To determine fatigue behavior of cortical bone trajectory (CBT) pedicle screws.
  
  
  BACKGROUND
  Cortical bone trajectory screws have been becoming popular in spine surgery; however, the long-term fatigue behavior of the new CBT screws remains understudied and limitations not well defined.
  
  
  METHODS
  Twelve vertebrae from six cadaveric lumbar spines were obtained. After bone mineral density (BMD) measurements, each vertebral body was instrumented with screws from each group, that is, CBT (4.5 × 25 mm) or standard pedicle screw (6.5 × 55 mm). A load (± 4 Nm sagittal bending) was applied under displacement control at 1 Hz. Each construct was loaded for 100 cycles or until 6° of loosening was observed. After fatigue testing, the screws were pulled out axially at 5 mm/min.
  
  
  RESULTS
  The standard pedicle screw showed better resistance against 100 cycle loading compared with the CBT screws (P < 0.001, 6.9° ± 4.8° vs. 15.2° ± 5.5°, respectively). The standard pedicle screw testing usually required more than 100 cycles of loading to achieve the critical loosening (3592 ± 4564 cycles), whereas the CBT screw never exceeded 100 cycles (84 ± 24 cycles) (P = 0.002). Increased BMD was significantly associated with a higher number of cycles and less loosening. The standard pedicle screw group had a higher postfatigue pullout load than the CBT screw group (P = 0.001, 776 ± 370 N and 302 ± 232 N, respectively).
  
  
  CONCLUSIONS
  The standard pedicle screw had a better fatigue performance compared with the CBT screw in vertebra with compromised bone quality. The proper insertion of the CBT screw might be prevented by the laminar anatomy depending on the screw head design. The CBT screw damaged the bone along its shaft by rotating around a fulcrum, located at either the pars, pedicle isthmus, or the junction of the pedicle and superior endplate, contingent upon the strength of the bone.
authors.affiliation: *Orthopedic Biomechanics Laboratory, Department of Orthopedics, Loma Linda University, Loma Linda, CA†School of Medicine, Loma Linda University, Loma Linda, CA‡Spine Section, Department of Orthopedic Surgery, Loma Linda University, Loma Linda, CA.
authors.forename: Yusuf Tahiri
authors.initials: YT
authors.lastname: Akpolat
authors.forename: Serkan
authors.initials: S
authors.lastname: İnceoğlu
authors.forename: Nolan
authors.initials: N
authors.lastname: Kinne
authors.forename: Devon
authors.initials: D
authors.lastname: Hunt
authors.forename: Wayne K
authors.initials: WK
authors.lastname: Cheng
mesh_terms: D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000502:physiology*; D015519:Bone Density / Q000502:physiology; D000071538:Cortical Bone / Q000601:surgery*; D004867:Equipment Design; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000601:surgery*; D008297:Male; D065289:Pedicle Screws