Anatomical considerations for appropriate mini-plate positioning in open-door laminoplasty to avoid plate impingement and screw facet violation

This study aimed to describe a safe zone for mini-plate positioning that can avoid instrument-related complications in laminoplasty. Fifty-one patients who underwent laminoplasty and were followed up for at least 1 year were retrospectively reviewed. The posterior surface length and inferior pole angle of the lateral mass were measured at each level using computed tomography. The safe zone was defined based on these measurements. Incidences of screw facet violation and plate impingement were recorded. Patient-reported outcome measures were compared between the appropriate position (AP) and inappropriate position (IP) groups. Among 40 patients included, 15 (37.5%) had inappropriate plate positioning, causing screw facet violation or plate impingement, which more commonly occurred at distal (C5, C6) and proximal (C3, C4) levels, respectively. Lateral mass posterior surface length was shorter at the proximal levels, and the inferior pole angle of the lateral mass was smaller at the distal levels, signifying that the lateral mass became thin and long at the distal levels. Patient-reported outcome measures were not significantly different between the two groups. However, cervical range of motion at the final follow-up was significantly less in the IP group (p = 0.01). The suggested safe zone demonstrates that inserting the mini-plate with plate-to-lateral mass inferior pole distances of 4–5 mm and 5–6 mm at the C3–C5 and C6–C7 levels, respectively, would avoid instrument-related complications. The risk of plate impingement was higher at the proximal level, whereas the risk of screw facet violation was higher at the distal level in open-door cervical laminoplasty. These risks coincide with anatomical differences at each level. Despite inappropriate positioning of the mini-plate, clinical outcomes were not adversely affected.