Optimal mini-plate insertion for cervical laminoplasty would adequately prevent the reclosure of an open hinge without instrument-related complications such as screw pullout, plate breakage, screw facet violation, and plate impingement with approximate level.3,5,8,17 Several studies have demonstrated that aggravation of kyphosis, decreased range of motion, and postoperative neck pain are common after laminoplasty.10–12 Although injuries to the posterior neck musculature have been commonly discussed as a factor causing these adverse outcomes, inappropriate instrument positioning such as plate impingement or screw facet violation would also have a negative effect on axial symptoms and may have been underestimated.3,8,9 While transfacet fixation has been reported as a viable technique for fusion, screw facet violation by mini-screws does not limit facet joint motion and would accelerate the degenerative process at the involved level.18 Chen et al. demonstrated that screw facet joint violation during mini-plate fixation results in decreased range of motion and aggravated neck pain, although neurological recovery was not affected.9
Two previous studies have suggested safe mini-screw insertion points for laminoplasty.3,8 Chen et al. demonstrated the safe zone using 3D image rendering.8 However, the safe zone definition in this study is complex considering that intraoperatively, surgeons can only adjust the plate position in the cranial or caudal direction. Min et al. also demonstrated the minimal safe distance of mini-screw insertion.3 The limitation of this study is that the measuring method has not been objectively described. Furthermore, although these two studies suggest that a certain distance is needed from the inferior pole of the lateral mass to avoid screw facet violation, they did not consider the possibility of plate impingement when the plate is located too cranially.3,8 Therefore, the present study attempted to define the safe zone of mini-plate placement by considering both minimum (to avoid screw facet violation) and maximum distances (to avoid plate impingement) from the inferior pole of the lateral mass.
In this study, screw facet violation was more common at the distal levels, including C5 and C6, whereas plate impingement was more common at the proximal levels, such as C3 and C4. The results of radiological measurements demonstrate that this trend is consistent with the anatomical differences between each level. The possibility of plate impingement would be higher at the proximal level because the posterior surface length is shorter at these levels. However, a smaller inferior pole angle at the distal level signifies a thin lateral mass at these levels, which increases the possibility of screw facet joint violation. Considering such anatomical differences at each level, locating the mini-plate more caudally at the proximal levels and more cranially at the distal levels would help avoid instrument-related complications.
The safe zone was described based on the distance between the inferior pole of the lateral mass and the caudal edge of the mini-plate. Although previous reports have used the screw insertion area as the reference point, we used the caudal edge of the mini-plate because it is easier to identify intraoperatively. The suggested safe zone demonstrates that inserting the mini-plate with a plate-to-lateral mass inferior pole distance of 4–5 mm for the C3–C5 levels and 5–6 mm for the C6–C7 levels would avoid instrument-related complications. Min et al. also demonstrated that more distance from the inferior border of the lateral mass is needed at distal levels to avoid screw facet joint violation.3 It is known that the lateral mass is generally thin at C7, which makes pedicle screw a more preferred choice than lateral mass screw.19,20 The results of the present study also demonstrated that inserting a 7-mm screw for mini-plate fixation in C6–C7 would be unsafe owing to the thin lateral mass at these levels as demonstrated by the small inferior pole angle. This finding supports performing partial laminectomy rather than laminoplasty at C7 due to a higher chance of instrument-related complications at this level.20,21
Laminoplasty is often performed in patients with cervical spondylosis, which distorts the anatomical landmarks due to bony spurs and spondylolisthesis. Lee et al. demonstrated that screw facet joint violation is more common in severely degenerative cervical spine than in mildly degenerative spine.7 Although the suggested safe zone in the present study could be used as a reference while placing the mini-plate, such distortion of anatomic landmarks would make it difficult to identify appropriate insertion points. Therefore, individual assessment and preoperative planning with radiographic measurements used in this study would further enhance the safety of laminoplasty.
The results of the present study demonstrated that despite the occurrence of screw facet joint violation or plate impingement, clinical results such as neck pain VAS or NDI were not adversely affected, which is contradictory to the findings of Chen et al., which suggested that screw facet joint violation is related to increased neck pain and decreased range of motion.9 However, both studies included a small number of patients with instrument-related complications, which warrants further evaluation.
This study had several limitations. First, the lateral mass posterior surface is not a plane surface, but rather has a round curvature, and the measuring method of the present study would have limitations reflecting such curved surfaces. However, within the confinement of using two-dimensional images, the round curvature of the lateral mass cannot be completely measured. Furthermore, the minimal safety distance to avoid screw facet violation demonstrated in this study corresponds to that reported in other previous reports.3,8 Second, as previously discussed, the study has limited capacity to demonstrate the clinical impact of inappropriate instrument positioning due to the small sample size. Finally, the study is not free from the possibility of selection bias because it was a retrospective, single-center study.
In conclusion, 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 the anatomical differences at each level. The demonstrated safe zone can be used as a reference for plate positioning. Despite inappropriate positioning of the mini-plate, the clinical outcomes were not adversely affected.