PREDICTION OF FATIGUE SCREW LOOSENING IN ANTERIOR SPINAL FIXATION USING DUAL-ENERGY X-RAY ABSORPTIOMETRY

Study Design. 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 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 instrumentation, 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 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 significant less specimens with bone mineral density < 0.45 g/cm(2) compared to those with bone mineral density greater than or equal to 0.45 g/cm(2). Conclusions. These findings suggest that bone mineral be a good predictor of anterior vertebral screw loosening. Bone mineral density < 0.45 g/cm(2) 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.