title: In vitro biomechanical study of pedicle screw pull-out strength based on different screw path preparation techniques.
abstract: BACKGROUND
  Poor screw-to-bone fixation is a clinical problem that can lead to screw loosening. Under-tapping (UT) the pedicle screw has been evaluated biomechanically in the past. The objective of the study was to determine if pedicle preparation with a sequential tapping technique will alter the screw-to-bone fixation strength using a stress relaxation testing loading protocol.
  
  
  METHODS
  Three thoracolumbar calf spines were instrumented with pedicle screws that were either probed, UT, standard-tapped (ST), or sequential tapped to prepare the pedicle screw track and a stress relaxation protocol was used to determine pull-out strength. The maximum torque required for pedicle screw insertion and pull-out strength was reported. A one-way ANOVA and Tukeys post-hoc test were used to determine statistical significance.
  
  
  RESULTS
  The pedicle screw insertion torques for the probed, UT, ST and sequentially tapped (SQT) techniques were 5.09 (±1.08) Nm, 5.39 (±1.61) Nm, 2.93 (±0.43) Nm, and 3.54 (±0.67) Nm, respectively. There is a significant difference between probed compared to ST (P ≤ 0.05), as well as UT compared to both ST and SQT (P ≤ 0.05). The pull-out strength for pedicle screws for the probed, UT, ST and SQT techniques was 2443 (±782) N, 2353(±918) N, 2474 (±521) N, and 2146 (±582) N, respectively, with no significant difference (P ≥ 0.05) between techniques.
  
  
  CONCLUSIONS
  The ST technique resulted in the highest pull-out strength while the SQT technique resulted in the lowest. However, there was no significant difference in the pull-out strength for the various preparation techniques and there was no correlation between insertion torque and pull-out strength. This suggests that other factors such as bone density may have a greater influence on pull-out strength.
authors.affiliation: Musculoskeletal Education and Research Center (MERC), A Division of Globus Medical, Inc., Audubon, PA 19403, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Moldavsky
authors.affiliation: Musculoskeletal Education and Research Center (MERC), A Division of Globus Medical, Inc., Audubon, PA 19403, USA.
authors.forename: Kanaan
authors.initials: K
authors.lastname: Salloum
authors.affiliation: Musculoskeletal Education and Research Center (MERC), A Division of Globus Medical, Inc., Audubon, PA 19403, USA.
authors.forename: Brandon
authors.initials: B
authors.lastname: Bucklen
authors.affiliation: Musculoskeletal Education and Research Center (MERC), A Division of Globus Medical, Inc., Audubon, PA 19403, USA.
authors.forename: Saif
authors.initials: S
authors.lastname: Khalil
authors.affiliation: Royal Orthopedic Hospital, Birmingham Spinal Surgery, West Midlands, UK.
authors.forename: Jwalant S
authors.initials: JS
authors.lastname: Mehta
keywords: Bone screws; Calf vertebral bodies; biomechanics; fracture fixation; insertion torque; pedicle; pull-out strength; spinal fractures; stress relaxation; tapping technique