Optimal satellite rod constructs to mitigate rod failure following pedicle subtraction osteotomy (PSO): a finite element study

BACKGROUND CONTEXT Pedicle subtraction osteotomy (PSO) is a challenging restoration technique for sagittal imbalance and is associated with significant complications. One of the major complications is rod fracture and there exists a need for a biomechanical assessment of this complication for various instrumentation configurations. PURPOSE To evaluate and compare the global range of motion (ROM), rod stress distribution, and the forces on the pedicle subtraction site in various instrumentation configurations using finite element analysis. STUDY DESIGN/SETTING A computational biomechanical analysis. METHODS A previously validated osseoligamentous three-dimensional spinopelvic finite element model (T10-pelvis) was used to develop a 30° PSO at the L3 level. In addition to the standard bilateral cobalt chromium primary rod instrumentation of the PSO model, various multirod configurations including constructs with medially, laterally, and posteriorly affixed satellite rods and the short-rod technique were assessed in spinal physiological motions. T10-S1 global ROM, maximum von Mises stress on the rods and at the PSO level, factor of safety (yield stress of the rod material/maximum actual stress in the rod) and the load acting across the PSO site were compared between various instrumentation configurations. The higher the factor of safety the lesser the chances of rod failure. RESULTS Among all multirod constructs, posteriorly affixed satellite rod construct showed the greatest motion reduction compared to the standard bilateral rod configuration followed by medially and laterally affixed satellite rod constructs. Compared to the standard bilateral rod configuration, recessed short-rod technique resulted in 4% to 49% reduction in T10-S1 ROM recorded in extension and lateral bending motions, respectively, while the axial rotation motion increased by approximately 31%. Considering the maximum stress values on the rods, the recessed short-rod technique showed the greatest factor of safety (FOS = 4.1) followed by posteriorly (FOS = 3.9), medially (FOS = 3), laterally affixed satellite rod constructs (FOS = 2.8), and finally the standard bilateral rod construct (FOS = 2.7). By adding satellite rods, the maximum von Mises stress at the PSO level of the rods also reduced significantly and at this level resulted in the greatest FOS in the posteriorly affixed satellite rod construct. Compared to the standard bilateral rod construct, the load magnitude acting on the osteotomy site decreased by 11%, 16%, and 37% in the laterally, medially, and posteriorly affixed satellite rod constructs, respectively, and did not change with the short-rod technique. CONCLUSIONS Adding satellite rods increases the rigidity of the construct, which results in an increase in the stability and the reduction of the global ROM. Additionally, having satellite rods reduces the stress on the primary rods at the PSO level and shifts the stresses from this PSO region to areas adjacent to the side-by-side connectors. The data suggest a significant benefit in supplementing medial over lateral satellite rods at the PSO by reducing stress on the primary rods. Except the recessed short-rod technique, all other multirod constructs decrease the magnitude of the load acting across the osteotomy region, which could cause a delayed or non-union at the PSO site. CLINICAL SIGNIFICANCE The study evaluates the mechanical performance of various satellite rod instrumentation configurations following PSO to predict the risk factors for rod fracture and thereby mitigate the rate of clinically relevant failures.