Multidirectional instability of the thoracic spine due to iatrogenic pedicle injuries during transpedicular fixation - A biomechanical investigation

Study Design. Pedicle fracture was simulated in an in vitro model, and its effect on multidirectional stability provided by pedicle instrumentation was quantified. Objectives. To quantify the multidirectional flexibility of pedicle instrumentation due to different iatrogenic pedicle injuries. Summary of Background Data. Misplacement of the screw and iatrogenic pedicle fracture are the main complications of pedicle instrumentation. Despite the increasing number of clinical studies dealing with this issue, there is little reliable information concerning the biomechanical effects of an intraoperative pedicle fracture. Methods. A burst fracture was created in 10 human cadaveric five-vertebrae spine specimens, from the middle and lower thoracic spine regions. The fracture was stabilized with a semirigid pedicle screw fixation device. To simulate an intraoperative pedicle fracture, the pedicles of the instrumented segments were resected in four steps. After each pedicle injury, three-dimensional flexibility in the form of range of motion and neutral zone of the construct was determined and compared with the intact values. Results. Resection of the pedicles had little effect on the multidirectional flexion-extension stability provided by the instrumentation. There were significant increases of axial rotation in the middle thoracic spine when the lateral wall was resected (range of motion, 8.2 degrees vs. 3.6 degrees; neutral zone, 4.2 degrees vs. 1.7 degrees), whereas in the lower thoracic spine, significant increases occurred only when all the pedicles were resected (range of motion, 3.8 degrees vs. 1.4 degrees neutral zone, 1.1 degrees vs. 0.4 degrees). Lateral resection of the pedicle resulted in significant increases of range of motion and neutral zone for lateral bending in both the middle thoracic spine (range of motion, 19.0 degrees vs. 10.0 degrees; neutral zone, 2.4 degrees vs. 1.1 degrees) and the lower thoracic spine (range of motion, 4.3 degrees vs. 2.5 degrees; neutral zone, 0.9 degrees vs. 0.3 degrees). Conclusions. Resection of the pedicles results in a significant decrease in axial rotation and lateral bending stability provided by the instrumentation. This effect was higher in the middle than in the lower thoracic spine and may be relevant to pedicle fractures produced by pedicle screws used in these regions.