Morphometric subaxial lateral mass evaluation allows for preoperative optimal screw trajectory planning

Area of Science:

• Orthopedic Surgery
• Neurosurgery
• Spine Surgery

Background:

• Lateral mass (LM) fixation is a standard treatment for cervical spine instability.
• Achieving maximal biomechanical stability with minimal morbidity remains a challenge.

Purpose of the Study:

• Evaluate bicortical screw placement and complication rates in a patient cohort.
• Investigate optimal screw trajectories using preoperative multiplanar computed tomography (CT) scans.

Main Methods:

• Retrospective evaluation of 55 patients undergoing LM fixation (subaxial cervical spine) with a modified Magerl technique.
• Analysis of postoperative CTs and clinical records for LM anatomy, screw length, bicortical purchase, and complications.
• Evaluation of 3D CT data from 45 subjects to assess LM geometry and optimal screw trajectories at C3-C7 segments.

Main Results:

• 284 LM screws placed with 88% bicortical purchase and a 3.8% malplacement rate.
• LM thickness varied significantly across subaxial levels and angulations.
• Optimal angulation for maximal LM thickness increased from C3 (14°) to C7 (38°), allowing for 8%-39% gains in screw length.

Conclusions:

• Optimal lateral mass screw trajectory is segment-specific in the subaxial cervical spine.
• Understanding LM geometry through preoperative sagittal CT imaging enables safe, long, bicortical screw placement.