title: Enhancing pedicle screw fixation in the lumbar spine using allograft bone plug interference fixation.
abstract: METHODS
  A within-subjects controlled laboratory study.
  
  
  OBJECTIVE
  To examine a biological alternative to cement augmentation for pedicle screw fixation comparing bilateral axial pullout tests of augmented and nonaugmented (controls) pedicle screws.
  
  
  BACKGROUND
  Fixation in the osteoporotic spine remains a difficult challenge with failure by loosening or backout. Pedicle screw augmentation has been attempted using polymethylmethacrylate and bioabsorbable calcium cements; however, the potential for extravasation and embolization of cement are becoming increasingly concerning and merit the search for alternative methods to improve screw-anchoring strength.
  
  
  METHODS
  Twenty-four (24) fresh human lumbar vertebrae were tested to compare the pullout strength of augmented and nonaugmented pedicle screws. Two different augmentation strategies were employed using allograft bone plugs (ABPs) and evaluated using 12 specimens per group. Bone mineral density of each specimen was obtained using dual-energy x-ray absorptiometry. The augmented versus nonaugmented pedicle was randomized for each vertebra, and bilateral testing enabled paired statistical analyses. Axial pullout tests were performed using an materials testing system servohydraulic test system, and peak force, failure displacement, and stiffness was obtained for each test and correlated with bone mineral density.
  
  
  RESULTS
  Augmentation using 6-mm-diameter ABPs with 6.25-mm-diameter pedicle screws resulted in statistically weaker average pullout strength (775±455 N) than the nonaugmented controls (1233±826 N). When using smaller (5 mm diameter) AGPs with the same diameter screws, there was no statistical difference between average pullout strength for the augmented pedicle screws (1772±652 N) and the nonaugmented screws (1780±575 N).
  
  
  CONCLUSIONS
  Preliminary study of pedicle screw augmentation using cannulated ABPs showed no improvement of fixation with pedicles in the spine. This was even true in osteoporotic specimens, where augmentation would seem to be of considerable benefit.
authors.affiliation: *Applied Biomechanics Laboratory, Department of Mechanical Engineering †Department of Orthopaedics and Sports Medicine, University of Washington, Seattle, WA.
authors.forename: Bopha
authors.initials: B
authors.lastname: Chrea
authors.forename: Harsha
authors.initials: H
authors.lastname: Malempati
authors.forename: Jeffrey R
authors.initials: JR
authors.lastname: Campbell
authors.forename: Sonja
authors.initials: S
authors.lastname: Khan
authors.forename: Randal P
authors.initials: RP
authors.lastname: Ching
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Lee
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D064591:Allografts; D001696:Biomechanical Phenomena; D001863:Bone Screws; D016025:Bone Transplantation; D005260:Female; D005593:Fracture Fixation, Internal / Q000379:methods*; D006801:Humans; D008159:Lumbar Vertebrae / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008422:Materials Testing; D008875:Middle Aged; D014184:Transplantation, Homologous; D016474:Weight-Bearing; D055815:Young Adult