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

Area of Science:

• Orthopedic surgery
• Spine biomechanics
• Biomaterials engineering

Background:

• Cervical spine fusion commonly uses lateral mass screw fixation.
• Optimizing construct strength is crucial to prevent hardware failure, complications, and revision surgeries.

Purpose of the Study:

• To evaluate how screw thread pitch, trajectory, and purchase depth affect screw pull-out strength in lateral mass fixation.
• To compare different screw types and trajectories for improved fixation stability.

Main Methods:

• Biomechanical testing on artificial bone models simulating the cervical spine lateral mass.
• Comparison of cortical and cancellous screws (3.5mm) at varying purchase depths (unicortical, bicortical, unicortical-backed-out) and trajectories (Roy-Camille, Magerl).
• Evaluation of 4.0mm unicortical screws versus 3.5mm bicortical screws, assessing thread pitch and trajectory effects.

Main Results:

• The Roy-Camille (RC) trajectory demonstrated superior fixation strength compared to the Magerl trajectory across all tested constructs.
• Cortical screws exhibited greater pull-out strength than cancellous screws with the RC technique.
• Using a 4.0mm unicortical screw resulted in significantly higher construct strength than a 3.5mm screw with reduced purchase depth.

Conclusions:

• Screw thread pitch, purchase depth, and trajectory are critical factors influencing the biomechanical strength of cervical lateral mass fixation.
• Spine surgeons must carefully consider these parameters and any modifications to fixation components to optimize surgical outcomes.