Comparative biomechanical analysis of pedicle screws versus cortical bone trajectory fixation in the treatment of ankylosing spondylitis.

objectiveAnkylosing spondylitis (AS) is an inflammatory rheumatic disease, in which an imbalance between loss and formation of bone can lead to ligamentous calcification or osteoporosis. Patients with severe spinal deformities are typically treated with long-segment pedicle screw (PS) internal fixation after bone resection; however, issues with the operation, such as fracture and screw loosening or pulling out, might arise. For patients with low bone quality, the cortical bone trajectory (CBT) has been suggested and developed.

methodsIn order to cure AS, this study set out to look at the biomechanical reactions to PS and CBT fixation. Preoperative and postoperative three-dimensional spine models were reconstructed. A finite element analysis was performed to simulate the motion of the spine under six conditions: flexion, extension, left lateral bending, right lateral bending, left axial rotation, and right axial rotation. The analysis model predicted the stiffness, T12-S1 range of motion (ROM), and Von Mises stress of the vertebra, screw-rod systems, and ligaments.

resultsAxial stiffness was greater in the PS than in the CBT model, while the PS model performed better than the CBT model in terms of bending stiffness, except for the PS model under extension conditions. Under the conditions of bending, extension, and left and right lateral bending, the stress borne by the screws and rods in the PS model was greater than that in the CBT model; whereas, under the conditions of left and right rotation, the stress borne by the CBT model was greater than that in the PS model. The stress was concentrated at the bone resection level. Calcified ligaments experience significant stress during spinal movement. After implantation of the internal fixation, the stress values of the anterior longitudinal ligament, posterior longitudinal ligament, interspinous ligament, and supraspinous ligament decreased, whereas the stress values of some intervertebral ligaments increased.

conclusionUnder conditions of flexion, extension, and left and right lateral bending, the PS technique provided better biomechanical strength. Whereas, under conditions of left and right rotation, the CBT technique provided better biomechanical strength. Overall, compared to CBT, the current PS technique was sufficiently effective in maintaining spinal fusion stability after bone resection for AS.