title: Biomechanical Comparisons of Pull Out Strengths After Pedicle Screw Augmentation with Hydroxyapatite, Calcium Phosphate, or Polymethylmethacrylate in the Cadaveric Spine.
abstract: OBJECTIVE
  In vertebrae with low bone mineral densities pull out strength is often poor, thus various substances have been used to fill screw holes before screw placement for corrective spine surgery. We performed biomechanical cadaveric studies to compare nonaugmented pedicle screws versus hydroxyapatite, calcium phosphate, or polymethylmethacrylate augmented pedicle screws for screw tightening torques and pull out strengths in spine procedures requiring bone screw insertion.
  
  
  METHODS
  Seven human cadaveric T10-L1 spines with 28 vertebral bodies were examined by x-ray to exclude bony abnormalities. Dual-energy x-ray absorptiometry scans evaluated bone mineral densities. Twenty of 28 vertebrae underwent ipsilateral fluoroscopic placement of 6-mm holes augmented with hydroxyapatite, calcium phosphate, or polymethylmethacrylate, followed by transpedicular screw placements. Controls were pedicle screw placements in the contralateral hemivertebrae without augmentation. All groups were evaluated for axial pull out strength using a biomechanical loading frame.
  
  
  RESULTS
  Mean pedicle screw axial pull out strength compared with controls increased by 12.5% in hydroxyapatite augmented hemivertebrae (P = 0.600) and by 14.9% in calcium phosphate augmented hemivertebrae (P = 0.234), but the increase was not significant for either method. Pull out strength of polymethylmethacrylate versus hydroxyapatite augmented pedicle screws was 60.8% higher (P = 0.028).
  
  
  CONCLUSIONS
  Hydroxyapatite and calcium phosphate augmentation in osteoporotic vertebrae showed a trend toward increased pedicle screw pull out strength versus controls. Pedicle screw pull out force of polymethylmethacrylate in the insertion stage was higher than that of hydroxyapatite. However, hydroxyapatite is likely a better clinical alternative to polymethylmethacrylate, as hydroxyapatite augmentation, unlike polymethylmethacrylate augmentation, stimulates bone growth and can be revised.
authors.affiliation: Department of Neurosurgery, Spine and Spinal Cord Institute, Yonsei University College of Medicine, Seoul, Republic of Korea.
authors.forename: Seong
authors.initials: S
authors.lastname: Yi
authors.affiliation: Department of Neurosurgery, Stanford University Medical Center, Stanford, California, USA.
authors.forename: Dae-Cheol
authors.initials: DC
authors.lastname: Rim
authors.affiliation: Department of Neurosurgery, Stanford University Medical Center, Stanford, California, USA.
authors.forename: Seoung Woo
authors.initials: SW
authors.lastname: Park
authors.affiliation: Department of Neurosurgery, Stanford University Medical Center, Stanford, California, USA.
authors.forename: Judith A
authors.initials: JA
authors.lastname: Murovic
authors.affiliation: Department of Neurosurgery, Stanford University Medical Center, Stanford, California, USA.
authors.forename: Jesse
authors.initials: J
authors.lastname: Lim
authors.affiliation: Department of Neurosurgery, Stanford University Medical Center, Stanford, California, USA. Electronic address: jonpark1@stanford.edu.
authors.forename: Jon
authors.initials: J
authors.lastname: Park
mesh_terms: D015502:Absorptiometry, Photon; D001672:Biocompatible Materials; D001696:Biomechanical Phenomena; D001843:Bone Cements; D002102:Cadaver; D002130:Calcium Phosphates; D017886:Durapatite; D006801:Humans; D010024:Osteoporosis / Q000473:pathology / Q000601:surgery; D065289:Pedicle Screws; D019904:Polymethyl Methacrylate; D013131:Spine / Q000469:parasitology / Q000473:pathology / Q000601:surgery*
chemical_list: D001672:Biocompatible Materials; D001843:Bone Cements; D002130:Calcium Phosphates; D019904:Polymethyl Methacrylate; D017886:Durapatite; C020243:calcium phosphate
keywords: Biomechanics; Calcium phosphate; Human cadaver; Hydroxyapatite; Osteoporosis; Pedicle screw; Polymethylmethacrylate; Pull out strength; Spine