Biomechanical Analysis of an S1 Pedicle Screw Salvage Technique via a Superior Articulating Process Entry Point

Study Design. Biomechanical, cadaveric study. Objective. To compare the fixation strength of a novel S1 pedicle screw insertion technique in a revision setting to a standard S1 pedicle screw and an L5 pedicle screw. Summary of Background Data. Fusions to the sacrum remain a difficult clinical challenge. Very few salvage techniques exist when a nonunion occurs. Methods. The biomechanical integrity of three screw fixations, L5 pedicle screws, a standard S1 pedicle screw, and an S1 pedicle screw placed via a superior articulating process entry point (SAP S1), was characterized by performing pullout tests using cadaveric specimens including L5 and sacrum. Results. SAP S1 constructs (735.5 +/- 110.1 N/mm) were significantly stiffer than standard S1 (P = 0.005) and L5 (P = 0.02) constructs. There was no statistically significant difference between the L5 constructs and the standard S1 constructs for linear stiffness. There was no statistical difference between the three fixations for yield load, displacement at yield load, and energy absorbed to yield load. The ultimate pullout force for the SAP S1 was statistically higher than the standard S1 (1213.7 +/- 579.6 vs. 478.6 +/- 452.9N; P = 0.004). Displacement at ultimate load was significantly greater for L5 screw fixation (3.3 +/- 1.1 mm) compared to the other two constructs. Both the L5 (2277.4 +/- 1873.3 N-mm) and SAP S1 (2628.2 +/- 2054.4 N-mm) constructs had significantly greater energy absorbed to ultimate load than the standard S1 construct (811.7 +/- 937.6 N-mm), but there was no statistical difference between the L5 and SAP S1 constructs. Conclusion. S1 pedicle screw fixation via an SAP entry point provides biomechanical advantages compared to screws placed via the standard S1 or L5 entry point and may be a viable option for revision of a failed L5-S1 fusion with a compromised standard S1 entry point.