Biomechanical Evaluation of Cross Trajectory Technique for Pedicle Screw Insertion: Combined Use of Traditional Trajectory and Cortical Bone Trajectory

Objective To introduce a novel double-screw (cross trajectory) technique that combines use of the traditional trajectory (TT) and cortical bone trajectory (CBT) and to investigate its fixation strength quantitatively by finite element (FE) analysis. Methods Three-dimensional FE models of 30 osteoporotic L4 vertebrae (patients' mean age: 77.3 ± 7.4 years, 11 men and 19 women) were computationally created. Each vertebral model was implanted with bilateral pedicle screws by TT (using 7.5 mm × 40 mm screws), CBT (using 5.5 mm × 35 mm screws) and cross trajectory (combined use of TT screws of 5.5 mm × 40 mm and CBT screws of 5.5 mm × 35 mm) and compared among three groups. The vertebral fixation strength of a bilateral-screw construct was examined by applying forces simulating flexion, extension, lateral bending, and axial rotation to the vertebrae by non-linear FE analyses. Results Fixation strength using the cross trajectory was the highest among the three different techniques (P < 0.01). The cross trajectory construct demonstrated 320% higher strength than the TT construct in flexion, 293% higher in extension, 102% higher in lateral bending, and 40% higher in axial rotation (P < 0.01). Similarly, the cross trajectory construct showed 268% higher strength than the CBT construct in flexion, 269% higher in extension, 210% higher in lateral bending, and 178% in axial rotation (P < 0.01). Conclusions The cross trajectory technique offered superior fixation strength over the TT and CBT techniques in each plane of motion. This technique may be a valid option for posterior fusion, especially in osteoporotic spine.