Quantifying apex mismatch: a new method to assess spinal and rod apex position in sagittal realignment

The objective of this study was to develop and validate a radiographic method to quantify the lumbar apex on calibrated EOS imaging, to compare the anatomical lumbar apex with the rod apex, and to evaluate the impact of pedicle subtraction osteotomy (PSO) on apex location.

This retrospective cohort included 53 patients (mean age 52 ± 16 years) with a 2-year follow-up who underwent L4 or L5 PSO with sacral fixation using 40°, 65°, or 90° pre-bent rods. Pre- and immediate postoperative EOS images were assessed to measure spinopelvic parameters and spinal lordosis ratio (SLR). Differences between the lumbar and rod apices (ΔApex_LL-Rod) and between pre- and postoperative spinal apices (ΔApex_Preop-Postop) were calculated. Analyses used paired t-tests, ANOVA, and Pearson correlations.

The lumbar apex was on average 6.1 mm (p < 0.001) higher than the rod apex. PSO induced a significant distal migration of the spinal apex of - 28.6 mm (p < 0.001). SLR increased by 92.7% postoperatively (p = 0.05). No significant subgroup differences for both ΔApex were found. Both ΔApex values correlated significantly with preoperative pelvic incidence, pelvic tilt, lumbar lordosis, and SLR. Reproducibility was excellent, with ICC values over 0.87.

The spinal apex lies on average higher than the rod apex, and PSO induces a marked distal migration of the anatomical apex. Combined with SLR, apex analysis refines the description of early postoperative sagittal construct geometry. Clinically meaningful thresholds and associations with longer-term outcomes (mechanical events, radiographic maintenance, and PROMs) remain unknown and require longitudinal validation.

Retrospective, single-center observational cohort study, Level of evidence IV.