Biomechanical advantages of antero-posterior accessory rods configuration in 4-rod constructs after a pedicle subtraction osteotomy

The pedicle subtraction osteotomy (PSO) is an invasive surgical technique aimed to restore the sagittal balance lost in patients with spinal deformities. Posterior fixation with pedicle screws and rods is required to stabilize the spine and facilitate bone healing. In the clinical practice, standard 4-rod constructs involving the medio-lateral positioning of accessory rods with respect to the primary rods are known to decrease the risk of rod failure, which is one of the major complications of posterior instrumentation. The present study proposes a novel 4-rod construct, involving the antero-posterior positioning of accessory rods with respect to primary rods. The aim is to evaluate and compare stiffness, flexibility (i.e. maximum rotation and neutral zone) and principal strains between standard and novel 4-rod constructs, in order to establish whether the novel construct could further reduce the risk of the posterior instrumentation failure. Five porcine specimens, each consisting of 7 vertebrae, underwent PSO at the central vertebra and the two vertebrae above and below the osteotomy site were posteriorly instrumented. Using an established spine simulator, biomechanical tests were carried out in flexion and torsion to evaluate the constructs’ stiffness and flexibility. Strains on the primary rods were acquired through triaxial rosettes. Statistical analysis was performed with paired t-tests. Stiffness and flexibility in both flexion and torsion didn’t statistically differ between the two 4-rod constructs, as well as the posterior rod strains in torsion. Conversely, the strains in flexion in the novel 4-rod constructs were almost half the ones in the standard 4-rod constructs, and this reduction was statistically significant. This suggests that, at least in flexion, a further reduction of the risk of rod failure could be achieved when adopting the antero-posterior configuration of accessory rods.