Prevalence of severe sagittal malalignment in patients undergoing short-segment lumbar fusion

Short-segment 1-2 level lumbar fusion is frequently performed to address localized degenerative spine conditions. However, the prevalence and impact of baseline sagittal malalignment in these patients are not well understood. This study aimed to determine the prevalence and severity of baseline sagittal malalignment using Schwab modifiers and segmental L4-S1 lordosis in patients undergoing short-segment lumbar fusion.

Adult patients (≥ 18 years) who underwent 1-2 level lumbar interbody fusion were included. Sagittal alignment was classified using SRS-Schwab modifiers: pelvic incidence-lumbar lordosis (PI-LL; 0: <10°, +: 10-20°, ++: >20°), pelvic tilt (PT; 0: <20°, +: 20-30°, ++: >30°), and sagittal vertical axis (SVA; 0: < 40 mm, +: 40-95 mm, ++: >95 mm). SVA was assessed in a sub-group with full-body radiographs. Schwab modifier distribution was described overall and compared across age groups (< 40, 40-59, ≥ 60 years), and surgical history (primary or revision) using chi-square analyses. Regional L4-S1 alignment was also compared across Schwab modifiers.

Among 1,023 patients (mean age = 59.0 years, 54.5% female), 20.1% met severe SRS-Schwab criteria overall. Individually, 12.1% had PT + + and 15.0% PI-LL++. In the subset with full-body radiographs (n = 199, 19.5%), 14.6% had SVA++ (reported as exploratory). The distribution of severe deformity differed significantly across age groups for PT (5.1% <40 yrs, 8.9% 40 to 59 yrs, 15.8% ≥60 yrs), PI-LL (3.8%, 10.0%, 20.5%), and SVA (0.0%, 4.7%, 21.1%) (all p < 0.01), with a significant increasing trend for PT and PI-LL (test for trend, p < 0.001). Revision fusions showed a higher burden of severe deformity than primary cases (PT++: 21.7% vs. 10.4%; PI-LL++: 24.3% vs. 13.3%; p < 0.01 for both). Decreased L4-S1 lordosis was associated with increasing PT (0: 33.5°, +: 30.9°, ++: 24.2°), PI-LL (0: 33.5°, +: 31.7°, ++: 23.7°), and SVA (0: 33.0°, +: 28.2°, ++: 21.3°).

In this study of over 1,000 patients undergoing 1-2 level lumbar fusion, over 20% had baseline severe SRS-Schwab + + sagittal malalignment. Rates of severe sagittal malalignment were higher in patients with increasing age and those undergoing revision fusion. These findings underscore the importance of routine preoperative assessment of spinopelvic alignment, particularly pelvic tilt and PI-LL mismatch, in short-segment fusion candidates.

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