title: Biomechanical comparison of 4 fixation techniques of sacral pedicle screw in osteoporotic condition.
abstract: METHODS
  An in vitro biomechanical cadaver study.
  
  
  OBJECTIVE
  To compare the subsidence displacement after cyclic loading among 4 sacral pedicle screw fixations of bicortical, tricortical, standard polymethylmethacrylate (PMMA) augmentation, and sub-endplate PMMA augmentation in osteoporotic condition.
  
  
  BACKGROUND
  Implant failure caused by screw loosening is a clinical problem for lumbosacral fusions, especially in osteoporotic patients. To improve sacral screw anchoring strength, the main fixation techniques need to be evaluated biomechanically.
  
  
  METHODS
  For this study, 11 fresh osteoporotic cadaver sacra were harvested and bone mineral density was measured with dual-energy radiograph absorptiometry. A 7 mm diameter monoaxial pedicle screw (S1) was randomly assigned by side (left vs. right) and placed bicortically or tricortically. The 2 screws, followed 2000 cyclic compression loading of 30 to 250 N, were removed. The screw tracts were filled up with PMMA, then, screws 5 mm shorter than the bicortical or tricortical fixation were reinserted (defined as standard and sub-endplate PMMA augmented sacral screw fixations, respectively). The PMMA augmented screws were then retested as before. Screw subsidence displacement after 2000 cyclic loading was measured and compared.
  
  
  RESULTS
  The average bone mineral density of 11 specimens was 0.71 g/cm, ranged from 0.65 to 0.78 g/cm. No significant difference of subsidence displacement was detected between tricortical and standard PMMA augmented screws (P>0.05), however, the 2 fixations exhibited markedly less subsidence than bicortical screw (P<0.05). Sub-endplate PMMA augmented screw showed the least subsidence among all the screws (P<0.05).
  
  
  CONCLUSIONS
  PMMA augmentation can increase the bonding strength of sacral screw-bone interface and the sub-endplate PMMA augmented sacral screw could obtain the highest stability among the 4 fixation techniques in osteoporotic condition.
authors.affiliation: Department of Spine Surgery, The First Affiliated Hospital, Sun Yat-sen University, Guangzhou, China.
authors.forename: Bin-Sheng
authors.initials: BS
authors.lastname: Yu
authors.forename: Xin-Ming
authors.initials: XM
authors.lastname: Zhuang
authors.forename: Zhao-Min
authors.initials: ZM
authors.lastname: Zheng
authors.forename: Jia-Fang
authors.initials: JF
authors.lastname: Zhang
authors.forename: Ze-Min
authors.initials: ZM
authors.lastname: Li
authors.forename: William W
authors.initials: WW
authors.lastname: Lu
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena / Q000502:physiology; D015519:Bone Density / Q000502:physiology; D001863:Bone Screws; D005260:Female; D006801:Humans; D016268:Internal Fixators; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D010024:Osteoporosis / Q000503:physiopathology / Q000601:surgery*; D012447:Sacrum / Q000503:physiopathology / Q000601:surgery*; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D016474:Weight-Bearing