title: Prediction of fatigue screw loosening in anterior spinal fixation using dual energy x-ray absorptiometry.
abstract: METHODS
  A biomechanical study was performed to investigate a relation between the bone mineral density of the vertebral body and the number of loading cycles to induce fatigue loosening of an anterior vertebral screw.
  
  
  OBJECTIVE
  The objective of this study was to investigate the potential usefulness of dual energy x-ray absorptiometry of measuring bone mineral density of the vertebral body in predicting the fatigue loosening of th anterior vertebral screw.
  
  
  BACKGROUND
  Loosening of the vertebral body screw is a well know failure in spinal instrumentation, and more commonly observed than pullout failure. The relation between bone mineral density and pullout strength of the screw has been investigated previously, but no studies are available on the fatigue loosening in anterior spinal fixation.
  
  
  METHODS
  Bone mineral density was measured using dual energy x-ray absorptiometry and the screw loosening was produce by a cyclic loading in the cephalad-caudal direction. Screw loosening was defined as 1 mm displacement of the screw relative to bone, and the number of loading cycles to induce the screw loosening was obtained and statistically correlated with bone mineral density.
  
  
  RESULTS
  There was a positive correlation between the number of loading cycles to induce screw loosening and bone mineral density (R = 0.8, P < 0.01). The average number of loading cycles to induce screw loosening was significantly less for specimens with bone mineral density < 0.45 g/cm2 compared to those with bone mineral density > or = g/cm2.
  
  
  CONCLUSIONS
  These findings suggest that bone mineral density may be a good predictor of anterior vertebral screw loosening. Bone mineral density < 0.45 g/cm2 may be critical value of loosening of the anterior vertebral body screw. However, further biomechanical and clinical studies are required before using threshold value clinically.
authors.affiliation: Department of Orthopaedic Surgery, Medical College of Wisconsin, Milwaukee, USA.
authors.forename: T H
authors.initials: TH
authors.lastname: Lim
authors.forename: H S
authors.initials: HS
authors.lastname: An
authors.forename: T
authors.initials: T
authors.lastname: Hasegawa
authors.forename: L
authors.initials: L
authors.lastname: McGrady
authors.forename: K Y
authors.initials: KY
authors.lastname: Hasanoglu
authors.forename: C R
authors.initials: CR
authors.lastname: Wilson
mesh_terms: D015502:Absorptiometry, Photon; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000009:adverse effects*; D006801:Humans; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D011237:Predictive Value of Tests; D012044:Regression Analysis; D013123:Spinal Fusion / Q000295:instrumentation*