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.

Purpose
Pull-out of pedicle screw in posterior pedicle fixation for thoracic and lumbar burst fractures causes delayed rehabilitation, persistant pain, and imblance of sagittal plane. In this study we try to analyse the factors that cause the pull-out of pedicle screw.
Materials and Methods
From March 01, 2006 to December 31, 2009, we assorted into two group; Group I for pullout pedicle, Group II for control. Plane lateral x-ray view film of thoracolumbar spine was taken on preoperation, postoperation, the first time when screw was pulled out and last follow up. we measure inserted angle for the upper endplate of screw, convergency angle and change of body height loss and kyphotic angle. We analysed corelation between these measuring values and pedicle screw pull-out by Mann-Whitney test and T-test.
Results
Pull-out of pedicle screw was found at mean 5weeks among nine cases. For inserted pedicle screws, which place in upper and lower vertebral body of fractured one, Value of inserted angle for upper end plate and convergency angle was found non-significant(p>0.05, Mann-Whitney test). Restoration of height loss and kyphotic angle of fractured vertebral body was statically significant(p<0.05, T-test).
Conclusion
In posterior pedicle fixation for thoracic and lumbar burst fractures, sufficient restoration of height loss and kyphotic angle is important factor for prevention of screw pull-out than inserted angle for upper end plate and convergency angle at a short period of time. Therefore we think that sufficient anterior fixation of vertebral body and restoration of kyphotic angle have a decisive effect on prognosis of patients.