title: Intraoperative techniques to reduce the potential of set-screw loosening in long spinal constructs: a static and fatigue biomechanical investigation.
abstract: METHODS
  The purpose of this study was to investigate the effects of implant selection and set-screw tightening technique on the loosening torques in long scoliosis constructs after long-term biaxial fatigue loading.
  
  
  BACKGROUND
  Expanded use of pedicle screws in the correction of long scoliotic curves and the mechanical demands on segmental fixation systems requires surgeon awareness of revisiting set screws to ensure full screw/rod engagement and minimize the potential of set-screw loosening and/or rod slippage postoperatively.
  
  
  METHODS
  Biomechanical tests were performed to evaluate the effect of set-screw tightening techniques and rod approximation on screw/rod interface strength.
  
  
  RESULTS
  Rod reduction test shows the force required to approximate a rod to a pedicle screw is statistically lower with uniplanar or polyaxial screws, when compared with monoaxial screws. This ease of approximation in both polyaxial and uniplanar screws directly correlate to improvement in the axial slippage resistance. In the simulated spinal model construct, rod/screw securement can vary based on the number of tightening torques applied to the system.
  
  
  CONCLUSIONS
  Sequential revisiting of sets crews in long scoliosis constructs resulted in a statistically significant increase in loosening torque for monoaxial and polyaxial screw systems. Intraoperative securement assessment of set screws is recommended. The use of polyaxial and uniplanar screws at the distal ends in long constructs is recommended to increase the screw/rod interface strength.
authors.affiliation: DePuy Spine, Raynham, MA 02767, USA. hserhan@its.jnj.com
authors.forename: Hassan
authors.initials: H
authors.lastname: Serhan
authors.forename: Kim
authors.initials: K
authors.lastname: Hammerberg
authors.forename: Michael
authors.initials: M
authors.lastname: O'Neil
authors.forename: Peter
authors.initials: P
authors.lastname: Sturm
authors.forename: Steven
authors.initials: S
authors.lastname: Mardjetko
authors.forename: Alvin
authors.initials: A
authors.lastname: Crawford
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D008422:Materials Testing; D012600:Scoliosis / Q000601:surgery*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013131:Spine / Q000601:surgery*; D013314:Stress, Mechanical; D019415:Torque