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

Lateral mass (LM) fixation has become a standard in cervical spine instability treatment; however, maximal biomechanical stability combined with low morbidity remains a challenge. We evaluated our own patient cohort for bicortical screw placement and complication rates and investigated optimal screw trajectories with preoperative multiplanar computed tomography (CT) scans.

Fifty-five patients were retrospectively evaluated after LM fixation at various subaxial cervical spine levels with a modified Magerl technique. Postoperative CTs and clinical records were used to determine LM anatomy, screw lengths, bicortical screw percentages, and complication rates. Additionally, 3D CT subaxial cervical spine data sets from 45 additional subjects with clinical indications for cervical spine imaging were evaluated. Subject LM geometries (thickness) were evaluated at different sagittal angulations (strict sagittal, 20°, 30° and the optimal angulation) for the optimal screw trajectories at the C3-C7 segments.

In total, 284 LM screws were placed, with a mean screw length of 16 mm and an 88% bicortical bone purchase. Additionally, a 3.8% malplacement rate was observed. LM thickness varied substantially between each subaxial cervical level and at each of the investigated angulations. The optimal angulation, at which LM thickness was maximal, increased continuously from C3 (14°) to C7 (38°). This increase permitted 8% (C3) to 39% (C7) gains in screw length compared with the strict sagittal plane assessments.

The optimal LM trajectory varied for each subaxial segment. The knowledge of LM geometry allows for safe, long and even bicortical screw placements using preoperative sagittal CT imaging evaluations.