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.

Minimally invasive TLIF has been reported to be a useful treatment option for the patients with various degenerative lumbar diseases. Many studies have reported the favorable clinical results of MIS TLIF. However it remains technically demanding, leading to higher complication rates and longer operative times during the early period of the learning curve. It showed some potential complications due to small working space and visual field. In this study, authors tried to find out various possible complications and some tips avoiding these complications through the review of various articles and authors’ clinical experiences. In many studies, the general complication fusion rates of MIS TLIF have been reported to be similar to that of open fusion. The technical difficulty of the procedure, combined with inadequate training, is evident in initial studies of MIS TLIF. A difficult learning curve of MIS TLIF demands that surgeons have sufficient preclinical training, and education is obtained before the application of MIS TLIF in clinical practice.