title: Pullout strength of thoracic pedicle screws improved with cortical bone ratio: a cadaveric study.
abstract: BACKGROUND
  The application of pedicle screw constructs for the osteoporotic vertebrae remains a serious clinical challenge for spinal surgeons and has been intensely studied recently. However, the exact role of the pedicular cortical bone composition and the screw-bone gap on the screw fixation failure has yet to be quantitatively documented. The current study aims to address this gap in our knowledge and elucidate possible relationships.
  
  
  METHODS
  Twelve fresh-frozen human cadaveric thoracic spine vertebrae (T9-T12) were harvested from six human cadavers (five males; one female; 63.5 ± 17 years). A three-dimensional reconstruction of the individual vertebrae was firstly rendered from computed tomography (CT) scan images to allow calculation of the cortical bone ratio. Specimens were then subdivided into three groups: Intact, 1-mm screw-bone gap, and 2-mm screw-bone gap. The gap groups were subjected to a standard cyclic fatigue-loading protocol. The pullout strength of the pedicle screws for all specimens were then determined.
  
  
  RESULTS
  The pullout strength of the 1-mm and 2-mm groups were significantly reduced when compared with the intact group. A moderate to excellent positive correlation was identified between the cortical bone area ratio and pullout strength for all groups (r > 0.55). A cortical shell ratio of 0.73 or higher was also found to be a safe cut-off index for screw fixation failure, even with an observable 1-mm screw-bone gap.
  
  
  CONCLUSIONS
  The current in vitro cadaveric spine study identified a significant correlation between cortical bone area ratio and the thoracic pedicle screw pullout strength. The presented results also demonstrate that the fatigue-loading-induced screw-bone gap of 1-mm was sufficient to cause a significant decrease in the pullout strength. However, a cortical bone area ratio of 0.73 or higher in this group was able to preserve most of the screw-bone interfacial strength, and subsequently may prevent a complete implant failure.
authors.affiliation: Institute of Biomedical Engineering, College of Medicine and College of Engineering, National Taiwan University, 602 Jen-Shu Tower, 1, Section 4, Roosevelt Road, Taipei, 10617, Taiwan, ROC.
authors.forename: Wen-Kai
authors.initials: WK
authors.lastname: Chou
authors.forename: Andy
authors.initials: A
authors.lastname: Chien
authors.forename: Jaw-Lin
authors.initials: JL
authors.lastname: Wang
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D002102:Cadaver; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D008422:Materials Testing / Q000379:methods*; D008875:Middle Aged; D010024:Osteoporosis / Q000000981:diagnostic imaging / Q000601:surgery*; D065289:Pedicle Screws; D013123:Spinal Fusion / Q000379:methods*; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D014057:Tomography, X-Ray Computed