Evaluation of the Fixation Strength of Pedicle Screws Using Cortical Bone Trajectory What Is the Ideal Trajectory for Optimal Fixation?

Study Design. In vivo analysis of insertional torque of pedicle screws using cortical bone trajectory (CBT) technique. Objective. To investigate factors contributing to the fixation strength of CBT screws and to clarify the ideal cortical trajectory for lumbar fusion. Summary of Background Data. CBT has developed as a new minimally invasive technique of lumbar instrumentation. Despite biomechanical studies demonstrating the superior characteristics of CBT, no study has elucidated the most suitable path for optimal fixation or compared the fixation within variations of trajectory. Methods. The insertional torque of pedicle screws using CBT was measured intraoperatively in 72 consecutive patients. The detailed positions of a total of 268 screws were confirmed using postoperative reconstruction computed tomographic scans and were analyzed to identify factors contributing to the level of insertional torque. Investigated factors were as follows: (1) age, (2) bone mineral density of the femoral neck and lumbar vertebrae by dual-energy x-ray absorptiometry; (3) the pedicle width and height, (4) the length of the implant, (5) total screw length within the vertebra, (6) the screw length within the vertebral body, (7) the screw length within the lamina, (8) the cephalad and lateral angle of the trajectory, and (9) the distance from the long axis of the screw to the inferior and medial borders of the pedicle. Results. Multiple regression analysis showed that bone mineral density of the femoral neck, screw length within the lamina, and cephalad angle were significant independent factors affecting torque. Conclusion. The fixation of CBT screws varied depending on technical factors (cephalad angle and screw length within the lamina) as well as the individual patient factor of bone mineral density. The ideal trajectory was directed 25 degrees to 30 degrees cranially along the inferior border of the pedicle so as to obtain maximum contact with the lamina and sufficient length within the vertebral body.