Does The Direction Of Pedicle Screw Rotation Affect The Biomechanics Of Direct Transverse Plane Vertebral Derotation?

Introduction: Thoracic pedicle screws are thought to have better vertebral rotation control and segmental scoliosis correction compared to hook and wires. This study evaluated the biomechanical differences in transverse plane vertebral body derotation maneuvers of pedicle screws in medial and lateral directions.

Methods: Vertebral bodies (T4-L5) from twelve cadavers were instrumented with appropriate length pedicle screws while measuring insertion torque. Bodies were anchored for independent loading in medial or lateral directions. Screws were rotated around a rod using a constant length lever arm (30.5cm) fixed to the screw head simulating posterior vertebral derotation. Yield torque (Nm) was analyzed using a one-way ANOVA (p<0 .05).

Results: Yield torques for both directions were significantly related to screw insertion torque (both p).

Discussion: While yield torque was not significantly different between medial and lateral directions, the failure modes provide interesting clinical information. Medial tests rotating the shank towards the spinal canal produced bone failure in half of the specimens while 2/3 of lateral tests resulted in lateral body fracture. From these data, a surgeon performing a direct vertebral derotation using a 30cm lever would need to apply roughly 40N (18lbs) to cause a failure. Adolescent patients would likely tolerate a greater force without failure due to greater bone density, yet, extreme caution is still recommended to prevent screw rotation into the spinal canal or laterally into the chest.