Biomechanical pullout strength and stability of the cervical artificial pedicle screw

This study used cadaveric specimens to compare the biomechanical performance of artificial pedicle screws (APS) versus lateral mass screws (LMS). The goal of this study was to biomechanically characterize APS range-of-motion and pullout strength in surgical instances that preclude LMS insertion. Posterior approaches used in instances of ventral spine tumors often necessitate complete facetectomy, thereby removing fixation points for LMS and requiring longer constructs with fewer segmental fixations to span the resected levels. Recently, APS were developed to overcome this obstacle. Although APS have been used successfully in clinical cases, they have yet to be biomechanically validated. Seven fresh-frozen cervical spine segments (C2-C7) were harvested from human donors (F = 1, M = 6; 65 +/- 5 years old, range: 50-72 years old). Nondestructive range-of-motion tests were conducted on each specimen in its intact and surgically destabilized states, and after each of 3 different APS and LMS surgical stabilizations. After nondestructive bending tests, a final pullout test of APS and LMS at the C4 level was performed for each specimen. The pullout strength of the APS was twice as strong on average as that for the LMS (503.4 +/- 338.3 vs.- 254.3 +/- 142.3 N); this difference approached but was not statistically significant (P = 0.07). There was no significant difference in specimen stiffness between the APS- and LMS-instrumented configurations in all ranges of primary and off-axis motions (P > 0.05). However, all fixation methods increased specimen stiffness in comparison with the intact conditions (1.7-36.5 times increase; P < 0.05). Our results demonstrate that APS provide comparable stability to LMS and can therefore be considered a viable alternative in surgical scenarios requiring the complete removal of lateral masses. Moreover, APS may provide some enhanced strength in the face of destructive pullout forces.