title: Axial pullout strength comparison of different screw designs: fenestrated screw, dual outer diameter screw and standard pedicle screw.
abstract: BACKGROUND
  The pullout strength of pedicle screws is influenced by many factors, including diameter of the screws, implant design, and augmentation with bone cement such as PMMA. In the present study, the pullout strength of an innovative fenestrated screw augmented with PMMA was investigated and was compared to unaugmented fenestrated, standard and dual outer diameter screw.
  
  
  METHODS
  Twenty four thoracolumbar vertebrae (T10-L5, age 60 to 70 years) from three cadavers were implanted with the four different pedicle screws. Twelve screws of each type were instrumented into either left or right pedicle with standard screw paired with unaugmented and dual outer diameter screw paired with augmented fenestrated screw in any given vertebra. Axial pullout testing was conducted at a rate of 5 mm/min. Force to failure (Newtons) for each pedicle screw was recorded.
  
  
  RESULTS
  The augmented fenestrated screws had the highest pullout strength, which represented an average increase of 149%, 141%, and 78% in comparison to unaugmented, standard, and dual outer diameter screws, respectively. Pullout strength of unaugmented screws was comparable to that of standard screws, however it was significantly lower than dual outer diameter screws.
  
  
  CONCLUSIONS
  Fenestrated screws augmented with PMMA improve the fixation strength and result in significantly higher pullout strength compared to dual outer diameter, standard and unaugmented fenestrated screws. Screws with dual outer diameter provided enhanced bone-screw purchase and may be considered as an alternative technique to increase the bone-screw interface in cases where augmentation using bone cement is not feasible. Unaugmented screws can be left in the pedicle even without cement and provide similar pullout strength to standard screws.
authors.affiliation: Asklepios Klinik Lindenlohe, Schwandorf, Germany.
authors.forename: Evangelos
authors.initials: E
authors.lastname: Christodoulou
authors.affiliation: Globus Medical Inc., Ave 2560 General Armistead Ave, Audubon, PA 19403 USA.
authors.forename: Suresh
authors.initials: S
authors.lastname: Chinthakunta
authors.affiliation: Globus Medical Inc., Ave 2560 General Armistead Ave, Audubon, PA 19403 USA.
authors.forename: Divya
authors.initials: D
authors.lastname: Reddy
authors.affiliation: Globus Medical Inc., Ave 2560 General Armistead Ave, Audubon, PA 19403 USA.
authors.forename: Saif
authors.initials: S
authors.lastname: Khalil
authors.affiliation: G. Papanikalaou Hospital Exohi, 570 10 Thessaloniki, Greece.
authors.forename: Thomas
authors.initials: T
authors.lastname: Apostolou
authors.affiliation: Universitätsmedizin Mainz, Langenbeckstr, 1 55131 Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Drees
authors.affiliation: Asklepios Klinik Lindenlohe, Schwandorf, Germany.
authors.forename: Konstantinos
authors.initials: K
authors.lastname: Kafchitsas
keywords: Biomechanical study; Fenestrated screws; Large diameter screws; Pedicle screws; Pullout strength