Failure mechanisms of pedicle screws and cortical screws fixation under large displacement: A biomechanical and microstructural study based on a clinical case scenario

Study design: Cadaveric biomechanical with imaging analysis.

Objective: This study aims to compare the fixation failure between pedicel screws (PS) and cortical screws (CS), thus to investigate their failure mechanisms under vertical migration.

Summary of background data: Due to their minimal invasive nature, CS are gaining popularity. However, contradictions exist in the literature regarding whether CS may have superior fixation failure resistance compared to PS under vertical migration.

Methods: Human vertebral specimens were examined under Dual-energy X-ray. For each specimen, PS were inserted on the left and CS on the right with rods secured. Vertical force-displacement tests were applied to rods. MicroCT images were taken pre and post-MTS® for microstructural analysis.

Results: The average T-scores of the specimens were -4±0.25. Three phases of force-displacement behaviour featuring different PS and CS failure-resistance were discovered. For phase I, the force required to migrate PS tended to be slightly higher than CS. However, during phase II, a fixation instability occurred for PS and the CS fixation strength was superior. For phase III under large displacement, CS did not require increased force to displace, whereas PS re-stabilised and revealed improved displacement resistance. Both force analysis and microstructural analysis indicated that PS migrated along the direction of the vertical loading, whereas CS had a force component in the longitudinal axis of the screw.

Conclusions: Different failure mechanisms underlay PS and CS under large vertical displacement. PS fail with trabecular bone compaction possibly altering the initial material property surround the screw. CS fail with screw cut-out due to the force component along the screw axis.