Effect of Screw Thread Pitch, Purchase Depth, and Trajectory on Cervical Lateral Mass Fixation Strength

Biomechanical study.

Determine the effect of screw thread pitch, trajectory, and purchase depth on the screw pull-out strength in lateral mass fixation constructs.

Fusion of the cervical spine is routinely performed with lateral mass screw fixation. It is imperative to optimize the lateral mass fixation construct strength to minimize the risk of hardware failure and subsequent complications and reoperations.

Biomechanical testing was performed using bicortical artificial bone models to replicate the lateral mass of the cervical spine. Cortical and cancellous screws of 3.5 mm diameter were compared at 3 purchase depths: unicortical, bicortical, and bicortical backed-out to unicortical, and 2 trajectories: Roy-Camille (RC) and Magerl. In a second construct, bicortical 3.5 mm screws were replaced with unicortical 4.0 mm screws. Both thread pitches and trajectories were also compared.

Fixation with the RC technique was stronger than with Magerl in all constructs. Roy-Camille fixation using cortical screws was stronger than cancellous screws in all purchase depths. Magerl fixation using cancellous screws was stronger in all purchase depths but only statistically significant for the group where the bicortical screw was backed-out to unicortical.The construct where the bicortical 3.5 mm screw was replaced with a 4.0 mm unicortical screw was stronger than when the screw was backed-out to a unicortical depth. This was significant for cortical screws in both trajectories but only significant for cancellous screws using the RC technique.

Biomechanical strength of cervical lateral mass fixation was shown to be directly influenced by thread pitch, depth, and trajectory. Thus, spine surgeons should be cognizant of their fixation constructs and any changes made to their components.