Global Alignment Masks Residual Segmental Malalignment in Short-Segment Lumbar Fusions

Area of Science:

• Spine surgery
• Orthopedic surgery
• Biomechanical analysis

Background:

• Pelvic incidence-lumbar lordosis (PI-LL) mismatch is a common metric for sagittal alignment after lumbar fusion.
• However, PI-LL may mask segmental under-correction, potentially leading to iatrogenic flatback deformity.
• Assessing segmental alignment is crucial for accurate evaluation of surgical outcomes.

Purpose of the Study:

• To evaluate iatrogenic sagittal malalignment following short-segment lumbar fusions (L4-5, L5-S1, L4-S1).
• To compare segmental alignment criteria with PI-LL mismatch for assessing fusion outcomes.
• To identify preoperative predictors of sagittal malalignment after lumbar fusion.

Main Methods:

• Retrospective cohort study analyzing a multicenter database of 516 patients undergoing short-segment lumbar fusion.
• Global alignment defined as PI-LL <10°; segmental alignment defined by specific lordosis criteria for L4-5, L5-S1, and L4-S1 levels.
• McNemar's test and logistic regression were used to compare alignment measures and identify predictors.

Main Results:

• Segmental criteria identified significantly more malalignment than PI-LL at L5-S1 (57.7% vs. 17.3%) and L4-S1 (66.4% vs. 28.0%).
• Postoperative worsening rates were substantially higher with segmental assessment at L5-S1 (44.2%) and L4-S1 (55.1%) compared to PI-LL.
• Anterior interbody approaches and greater preoperative segmental lordosis predicted restoration of alignment.

Conclusions:

• Segmental alignment assessment reveals higher rates of malalignment and worsening than PI-LL alone, particularly in fusions involving L5-S1.
• Anterior interbody techniques and preoperative lordosis are associated with better sagittal correction.
• Incorporating segmental alignment targets is recommended for planning distal lumbar fusions to avoid flatback deformity.