Purpose
The purpose of this retrospective study was to evaluate the results and complications of lateral mass screw fixation on C7 in a single spinal center.
Materials and Methods
During a 7-year period, a total of 104 lateral mass screws were placed on C7 in 56 patients with cervical disorders. A review of the hospital records and radiographs of these patients was conducted. Followup plain X-rays and computed tomography (CT) including sagittal reconstruction were obtained to analyze screw positions.
Results
A total of 104 screws were placed on C7 lateral mass using the modified Magerl’s method. Most patients had two screws on both lateral masses, but eight underwent unilateral lateral mass screw fixation due to fracture (n=4), lateral mass deficit due to tumor removal (n=1), and unilateral fixation for additional stabilization of an anterior fixation (n=3). Most cases were connected to the subaxial spine (91.1%). All patients underwent more than 1 year of follow-up. The only complication was one wound infection. On follow-up CT, one transverse foramen invasion (2 mm), one spinal canal invasion, and four facet joint violations developed post-operatively. However, no symptoms related to these issues were occurred, so reoperations for instrument repositioning were not required. Screw loosening and pseudoarthrosis did not develop during the follow-up period. Also, there was no case of vertebral artery or neural injury requiring reoperation due to the position of the lateral mass screw. Almost patients showed successful bone fusion on follow-up images, with the exception of one patient.
Conclusion
Despite the small size of the lateral mass, risk of pseudoarthrosis, and steep angle of the lamina, lateral mass screw fixation can be a safe, easy and less complicated method of posterior cervical stabilization. If the lateral mass is suitable, lateral mass screw fixation may enable C7 stabilization during posterior subaxial cervical surgery.

Objective
To compare the pull-out strength of polyaxial general screws and rescue screws when inserted into the lateral mass through cadaveric biomechanical experiment
Materials and Methods
Twenty three segments of the human cervical spine (from C3 to C7) were prepared. Two biomechanical studies were progressed. In the first experiment (13 segments), each segment was instrumented with 3.5×12 mm polyaxial screws on both sides. In one side, the inserted screw was removed and then the rescue screw was inserted to the same screw hole. In the second experiment (10 segments), all segments were instrumented with 3.5×12 mm polyaxial screws on both sides and all screws were removed. In one side, removed same screw was reinserted and in the other side, the rescue screw was inserted without change of the screw trajectory. All specimens were fixed to the specially designed frame with the cement. Universal Material Test Machine (Mini Bionix 858) was used to assess the pull-out strength of the screws. All data were compared with non-parametric paired test (Wilcoxon’s signed rank test).
Results
There was no crack or fracture around the screw hole. No significant difference was noted between the original screws (not reinserted) and the rescue screws in the first experiment (p=0.753). There is no significant difference between the same screw reinsertion and the conversion to the rescue screw (p=0.646).
Conclusions
The overall results of this study showed the conversion to the rescue screw with the same screw trajectory could offer no biomechanical advantage over reinsertion of the same screw. In case of secure screw hole after removal of the screw, the reinsertion of the same screw could be recommended.