Wednesday, September 26, 2018 7:35 AM–9:00 AM ePosters: P12. The interaction of satellite rod reconstruction, rod material and rod diameter on posterior rod strain in pedicle subtraction osteotomies for adult spinal deformity correction: a finite element analysis

BACKGROUND CONTEXT Pedicle subtraction osteotomy (PSO) is a prevailing technique used to correct adult spinal deformity; however, instrumentation failure of rods remains a significant concern at the PSO level or lumbosacral junction. Several options have been proposed in clinical literature to reduce mechanical demand of the rod, including multirod constructs (three- or four-rod vs. two rods), alternative rod materials [stainless steel (SS) or cobalt chrome (CoCr) vs. titanium (Ti)], or increased rod diameter (6.35 mm vs. 5.5 mm). Nevertheless, a comprehensive evaluation of these construct features is warranted and necessary to guide optimization of post-PSO surgical reconstruction. PURPOSE The current study investigates the effect of multirod techniques, and varied rod material and diameter, on motion-induced rod strain at the PSO and L5-S1. STUDY DESIGN/SETTING Finite element analysis. METHODS A lumbosacral finite element model (T12-S1) was developed and validated with cadaveric range of motion data. Vertebral segments were modeled as three-dimensional solid elements. Intervertebral discs (nucleus and annulus fibrosis) were structured as hyperelastic materials. The intact model underwent PSO at L3, and Ti pedicle screws and rods from T12-S1. Simulations assessed (1) multirod construction, with either a satellite rod on patient left (three-rod) or bilateral four-rod constructs; (2) posterior rod materials (SS or CoCr vs. Ti); and (3) 5.5 mm versus 6.35 mm posterior rods. A pure 10 Nm moment was applied at T12 to simulate flexion–extension. Motion-induced rod strain was normalized to the two-rod construct (100%), averaged between left and right rods, and evaluated at the PSO and L5-S1. RESULTS All rods, irrespective of number of accessory rods, material, or diameter, experienced less strain than two-rod construct across the PSO level and L5-S1. At the PSO, 5.5 mm three-rod Ti, SS, and CoCr constructs reduced primary rod strain to 65%, 41%, and 34%, respectively; 6.35 mm rods reduced strain to 47%, 29%, and 24%, respectively. Similarly, four-rod Ti, SS, and CoCr constructs reduced primary rod strain to 49%, 30%, and 25%, respectively; 6.35 mm rods reduced strain to 35%, 21%, and 17%, respectively. At L5–S1, 5.5 mm three-rod Ti, SS, and CoCr constructs reduced primary rod strain to 82%, 59%, and 51%, respectively; 6.35 mm rods reduced strain to 73%, 48%, and 30%, respectively. Similarly, four-rod Ti, SS, and CoCr constructs reduced primary rod strain to 85%, 60%, and 51%, respectively; 6.35 mm rods reduced strain to 75%, 48%, and 30%, respectively. Some interactions were observed. At the PSO, 6.35 mm three-rod construction was equivalent to 5.5 mm four-rod, across Ti (47% vs. 49%), SS (29% vs. 30%), and CoCr rods (24% vs. 25%). Similarly, at L5-S1, 5.5 mm three- and four-rod constructs were equivalent across Ti (82% vs. 85%), SS (59% vs. 60%), and CoCr rods (51% vs. 51%); similar trends with 6.35 mm rods occurred across Ti (73% vs. 75%), and SS rods (48% vs. 48%). CONCLUSIONS Overall trends in primary rod strain across the PSO and L5-S1 observed in the present study are as follows: (1) four-rod rod strain