Comparison of Pedicle Screw Fixation Strength Among Different Transpedicular Trajectories A Finite Element Study

Study Design: Comparative biomechanical study by finite element (FE) method. Objective: To investigate the pullout strength of pedicle screws using different insertional trajectories. Summary of Background Data: Pedicle screw fixation has become the gold standard for spinal fusion, however, not much has been done to clarify how the fixation strength of pedicle screws are affected by insertional trajectories and bone properties. Materials and Methods: Three-dimensional FE models of 20 L4 vertebrae were constructed from the computed tomographic data. Five different transpedicular trajectories were compared: the traditional trajectory, the vertical trajectory, and the 3 lateral trajectories with different sagittal directions (caudal, parallel, cranial). For a valid comparison, screws of the same shape and size were inserted into the same pedicle in each subject, and the pullout strength were compared with nonlinear FE analyses. In addition, the pullout strength was correlated with bone mineral density (BMD). Results: The mean pullout strength showed a 3.9% increase for the vertical trajectory relative to the traditional trajectory, 6.1% for the lateral-caudal trajectory, 21.1% for the lateral-parallel trajectory, and 34.7% for the lateral-cranial trajectory. The lateral-cranial trajectory demonstrated the highest value among all trajectories (P< 0.001). In each trajectory, the correlation coefficient between the pullout strength and BMD of the femoral neck (r= 0.74-0.83, P< 0.01) was higher than the mean BMD of all the lumbar vertebrae (r= 0.49-0.75, P< 0.01), BMD of the L4 vertebra (r= 0.39-0.64, P< 0.01), and regional BMD of the L4 pedicle (r= 0.53-0.76, P< 0.01). Conclusions: Regional variation in the vertebral bone density and the amount of denser bone-screw interface contribute to the differences of stiffness among different screw trajectories. BMD of the femoral neck is considered to be a better objective predictor of pedicle screw stability than that of the lumbar vertebra.