L5-S1 interbody fusion may not be necessary with multipoint pelvic fixation and quad rods across the lumbosacral junction for deformity constructs: a cadaveric biomechanics study

Introduction: Long segment constructs that span the lumbosacral junction commonly include interbody fusion at L5-S1 to add biomechanical support and to enhance fusion. However, the importance of adding L5-S1 interbody in the setting of more robust distal fixation with multipoint pelvic fixation and quad rods is not well understood.

Methods: Standard nondestructive flexibility tests (7.5 Nm) were performed on cadaveric specimens (L1-pelvis) to assess intervertebral flexibility (ROM), rod strains, and screw bending moments along a posterior fusion construct (PSR) spanning L2-S, supplemented by bilateral primary S2AI fixation, followed by bilateral accessory fixation fusion spanning L4-S2AI and added L5-S anterior fusion (ALIF). Five conditions were tested in each group: (1) intact; (2) L2-S1 PSR; (3) L2-S2AI PSR; (4) L2-S2AI+L4-S2AI, and (5) (5) L2-S2AI+L4-S2AI+L5-S1 ALIF. Data were analyzed using t-tests.

Results: No difference was observed between specimens with or without L5-S1 anterior interbodies in S1 screw strain or S2AI screw strain with flexion, extension, lateral bending, axial rotation, or compression (P > 0.05 for all comparisons). There was also no significant difference in L5-S1 rod strain for all comparisons (P > 0.05) as well as for range of motion across the fixated L5-S1 segment (P > 0.05).

Conclusion : When four pelvic fixation points and four rods are used, L5-S1 interbody fusion does not offer additional protection of L5-S1 rod strain, S1 screw strain, or L5-S1 ROM. Added morbidity of L5-S1 interbody fusion may not be justified from a biomechanics perspective.