Does Additional Distal S2 Fixation Improve Lumbosacral Stability Compared with Standard L4-S1 Fixation in High-Grade Spondylolisthesis? A Prospective Randomised Comparative Study.

OBJECTIVE: To compare standard L4-S1 fixation with an identical construct extended by bilateral S2 fixation in patients undergoing posterior lumbar interbody fusion for high-grade L5-S1 spondylolisthesis.

SUMMARY OF BACKGROUND DATA: High-grade L5-S1 spondylolisthesis subjects posterior fixation to lumbosacral shear that may exceed the tolerance of a standard L4-S1 construct. Extending fixation distally to S2 - using either a conventional S2 pedicle screw or, where sacral purchase is inadequate, an S2 alar-iliac (S2AI) screw - has been proposed to mitigate this shear, but comparative prospective data remain limited.

METHODS: Seventy consecutive patients with Meyerding grade III/IV spondylolisthesis undergoing posterior lumbar interbody fusion between May 2020 and November 2022 were randomly allocated 1:1 to standard L4-S1 fixation (Group A, n=35) or bilateral S2 extension (Group B, n=35; conventional S2 pedicle screws in 29, intraoperative conversion to S2-alar-iliac fixation in 6). Co-primary outcomes were the Visual Analogue Scale (VAS) and Oswestry Disability Index (ODI) at 36 months, compared using a Bonferroni-adjusted significance threshold.

RESULTS: All 70 patients (100%) completed follow-up. Group B achieved lower VAS (1.9±0.8 vs. 3.1±1.0, P<0.001) and ODI (21.4±5.6 vs. 30.1±6.0, P<0.001). Fusion rate (88.6% vs. 77.1%, P=0.34), pseudarthrosis (11.4% vs. 22.9%, P=0.34), and screw pull-out (2.9% vs. 14.3%, P=0.20) all numerically favoured Group B without reaching statistical significance. S1 screw breakage occurred exclusively in Group A (65.7% vs. 0%, P<0.001).

CONCLUSIONS: These findings suggest that distal fixation, through S2 or S2AI, may play a substantial role in mitigating the shear stress forces at the lumbosacral junction and the hardware failures commonly seen with S1-only anchorage in this complex pathology. Given the potential for performance bias and technique heterogeneity (conventional S2 vs. S2AI), multicentre randomised trials are warranted to confirm these findings and define which patients benefit most from S2 extension.

STUDY DESIGN: Prospective, randomised, single-centre comparative study.