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.Objectives
The Objective of this study was to investigate the potential usefulness of dual energy x-ray absorptiometry for measuring bone mineral density of the vertebral body in predicting the fatigue loosening of the anterior vertebral screw.Summary of Background Data
Loosening of the vertebral body screw is a well known failure in spinal in strumentation, and is 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 produced by a cyclic loading in the cephalad-caudal direction. Screw loosening was defined as 1mm 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 < 0.45 g/cm2.Conclusion
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 for loosening of the anterior vertebral body screw. However further biomechanical and clinical studies are required before using this threshold value clinically.