The effect of different insertion points on the security of pedicle screw fixation in bovine vertebrae

Study Design. Cyclic loading of pedicle screws inserted into bovine vertebrae was applied and the security of fixation was assessed as a function of the point of insertion of the screws. Objective. To determine which point of insertion is associated with more secure fixation using 2 commonly selected insertion points. Summary of Background Data. The strength of pedicle screw fixation depends on several factors, some of these have been studied. The present study looked at the difference in fixation security resulting from the use of 2 different insertion points to obtain a more stable fixation and to reduce the risk of loosening. Methods. Pedicle screws were inserted through a different insertion point on each side of the vertebra, one lateral to the superior articular process and one through the superior articular process and into the pedicle. They were tested by applying cyclic loading in a craniocaudal direction and the vertical displacement was measured. Permanent migration of the screw through the bone was compared using repeated measures analysis of variance. Results. Screws inserted lateral to the process were associated with significantly less displacement than those placed through the process (P = 0.02). Conclusion. Using bovine vertebrae, different points of insertion of pedicle screws were associated with different security of fixation. However, further tests using human vertebrae are recommended to confirm the above results.