title: Biomechanical evaluation of a novel fenestrated pedicle screw augmented with bone cement in osteoporotic spines.
abstract: METHODS
  Comparative biomechanical study was conducted in osteoporotic human cadaveric spines.
  
  
  OBJECTIVE
  Determine the influence of the volume of polymethyl methacrylate injected through a fenestrated pedicle screw on the pullout strength and on the ability to safely remove the implant.
  
  
  BACKGROUND
  Pedicle screw fixation in the osteoporotic spine can be improved by the addition of bone cement. Various injection techniques have been used. While improvement has been shown for the pullout strength, the optimal volume of cement to inject has not been previously studied.
  
  
  METHODS
  Seven osteoporotic spines were instrumented with a standard and a fenestrated pedicle screw augmented with polymethyl methacrylate at each level (T7-L5). Three volumes of bone cement were randomly injected and stratified to the thoracic (0.5 cc, 1.0 cc, and 1.5 cc) and lumbar spine (1.5 cc, 2.0 cc, and 2.5 cc). Axial pullout strength and removal torque of the pedicle screws were quantified.
  
  
  RESULTS
  The pullout strength of the fenestrated screw was normalized with respect to its contralateral control. Student paired t tests were conducted and a statistically significant increase was noted for 1.0 cc (186 ± 45%) and 1.5 cc (158 ± 46%) in the thoracic spine and for 1.5 cc (264 ± 193%), 2.0 cc (221 ± 93%), and 2.5 cc (198 ± 42%) in the lumbar spine. There was no significant difference with higher volumes of cement. The median removal torque was 0.34 Nm for the standard and 1.83 Nm for the augmented screws. When the augmented implants were removed, the bone cement sheared completely off at the fenestrations in 15 of the 17 cases.
  
  
  CONCLUSIONS
  Significant increases in pullout strength can be accomplished by injecting a limited quantity of bone cement through a fenestrated screw while minimizing the risks associated with higher volume. The majority of implants were removed without damaging the vertebra as the bone cement sheared off at the fenestrations.
authors.affiliation: Medtronic, Spinal and Biologics, Memphis, TN, USA. philippe.pare@medtronic.com
authors.forename: Philippe E
authors.initials: PE
authors.lastname: Paré
authors.forename: James L
authors.initials: JL
authors.lastname: Chappuis
authors.forename: Raja
authors.initials: R
authors.lastname: Rampersaud
authors.forename: Amit O
authors.initials: AO
authors.lastname: Agarwala
authors.forename: Joseph H
authors.initials: JH
authors.lastname: Perra
authors.forename: Serkan
authors.initials: S
authors.lastname: Erkan
authors.forename: Chunhui
authors.initials: C
authors.lastname: Wu
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D001843:Bone Cements / Q000627:therapeutic use*; D001863:Bone Screws; D002102:Cadaver; D003131:Combined Modality Therapy; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000187:drug effects / Q000503:physiopathology / Q000601:surgery; D008297:Male; D008875:Middle Aged; D010024:Osteoporosis / Q000188:drug therapy / Q000503:physiopathology* / Q000601:surgery; D019904:Polymethyl Methacrylate / Q000627:therapeutic use; D013122:Spinal Diseases / Q000188:drug therapy / Q000503:physiopathology* / Q000601:surgery; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013904:Thoracic Vertebrae / Q000187:drug effects / Q000503:physiopathology / Q000601:surgery
chemical_list: D001843:Bone Cements; D019904:Polymethyl Methacrylate