Biomechanical comparison of multi-rod constructs by satellite rod configurations (in-line vs. lateral) and screw types (monoaxial vs. polyaxial) spanning a lumbar pedicle subtraction osteotomy (PSO): is there an optimal configuration?

Abstract

Purpose Multi-rod constructs are used commonly to stabilize pedicle subtraction osteotomies (PSO). This study aimed to evaluate biomechanical properties of different satellite rod configurations and effects of screw-type spanning a PSO. Methods A validated 3D spinopelvic finite element model with a L3 PSO (30°) was used to evaluate 5 models: (1) Control (T10–pelvis + 2 rods); (2) lateral satellite rods connected via offsets to monoaxial screws (LatSat-Mono) or (3) polyaxial screws (LatSat-Poly); (4) in-line satellite rods connected to monoaxial screws (InSat-Mono) or (4) polyaxial screws (InSat-Poly). Global and PSO range of motions (ROM) were recorded. Rods’ von Mises stresses and PSO forces were recorded and the percent differences from Control were calculated. Results All satellite rods (save InSat-Mono) increased PSO ROM and decreased primary rods’ von Mises stresses at the PSO. Lateral rods increased PSO forces (LatSat-Mono:347.1 N; LatSat-Poly:348.6 N; Control:336 N) and had relatively lower stresses, while in-line rods decreased PSO forces (InSat-Mono:280.1 N; InSat-Poly:330.7 N) and had relatively higher stresses. Relative to polyaxial screws, monoaxial screws further decreased PSO ROM, increased satellite rods’ stresses, and decreased PSO forces for in-line rods, but did not change PSO forces for lateral rods. Conclusion Multi-rod constructs using in-line and lateral satellite rods across a PSO reduced primary rods' stresses. Subtle differences in biomechanics suggest lateral satellite rods, irrespective of screw type, increase PSO forces and lower rod stresses compared to in-line satellite rods, which had a high degree of posterior instrumentation stress shielding and lower PSO forces. Clinical studies are warranted to determine if these findings influence clinical outcomes.

Biomechanical evaluation of multi-rod constructs to stabilize an S1 pedicle subtraction osteotomy (PSO): a finite element analysis

“…Numerous studies have demonstrated that multi-rod constructs across lumbar PSOs provide more robust biomechanical environments and decrease rod fracture fractures relatively to 2-rod constructs [ 14 – 23 ]. Creating multi-rod constructs across S1-PSOs can be more challenging given the relatively limited space/anatomy as well as limited available osseous fixation points distal to S1 (i.e., in the sacrum and pelvis).…”

“…The size and length of the pedicle screws were consistent in all models. Polyaxial screws were simulated using a methodology presented in the literature [ 14 , 27 ]. The primary (PR), accessory (AR), and horizontal rods (HR) were tied to the tulip in all the models.…”

“…In contrast to lumbar PSOs, S1-PSOs are considerably less common operations owing to their challenging surgical demands, high complication profile, and relatively limited indications [ 12 ]. However, similar to lumbar PSOs, S1-PSOs require robust stability consisting of multi-rod constructs to optimize durability of the operation (i.e., maintain deformity correction and decrease rod fractures) [ 14 – 23 ]. While these constructs have been evaluated comprehensively for lumbar PSOs, there is a paucity of information and comparative data on surgical stabilization strategies for S1-PSOs.…”