title: Biomechanical evaluation of the pullout strength of cervical screws.
abstract: OBJECTIVE
  In the process of anterior cervical fusion, little is known about the biomechanics of anterior cervical screw pullout. In this study, three different aspects of cervical screw fixation were evaluated: self-tapping (ST) versus self-drilling (SD) screws, the effect of screw geometry (length, diameter, thread pitch), and the use of rescue screws.
  
  
  METHODS
  Nine screws consisting of different diameters, lengths, and thread pitch (cancellous and cortical) were tested in peak pullout force in an artificial bone model using an MTS 858 Mini Bionix test system. Rescue screws (4.5 mm) were then inserted in the failed holes of 4.0-mm screws and extracted to determine their holding strength.
  
  
  RESULTS
  Length of screws and thread pitch both had a significant effect on the pullout force. Each 1 mm of increased screw length translates to 16 N of increased force to pullout in the foam bone model. Pullout strength did not vary significantly according to screw diameter or between SD and ST screws. However, the SD screw has an advantage because it can decrease the length of surgery. A decrease in pullout force of between 43% and 70% was found when using rescue screws.
  
  
  CONCLUSIONS
  In situations in which the use of rescue/salvage screws is required, the surgeon should anticipate a significant decrease in the holding force compared with the original screw. Future directions for research include an evaluation of pullout force for screw and plate constructs.
authors.affiliation: Department of Orthopaedics and Rehabilitation, University of Florida, Gainesville, Florida 32610, USA. conrabp@ortho.ufl.edu
authors.forename: Bryan P
authors.initials: BP
authors.lastname: Conrad
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Cordista
authors.forename: MaryBeth
authors.initials: M
authors.lastname: Horodyski
authors.forename: Glenn R
authors.initials: GR
authors.lastname: Rechtine
mesh_terms: D001696:Biomechanical Phenomena / Q000379:methods; D001863:Bone Screws; D002574:Cervical Vertebrae / Q000503:physiopathology* / Q000601:surgery*; D019544:Equipment Failure Analysis; D017276:Friction; D006801:Humans; D013123:Spinal Fusion / Q000295:instrumentation*; D013718:Tensile Strength