Determining the best vertebra for measuring pelvic incidence and spinopelvic parameters in adult spinal deformity patients with transitional anatomy

OBJECTIVE The aim of this study was to determine if spinal deformity patients with L5 sacralization should have pelvic incidence (PI) and other spinopelvic parameters measured from the L5 or S1 endplate.METHODS This study was a multicenter retrospective comparative cohort study comprising a large database of adult spinal deformity (ASD) patients and a database of asymptomatic individuals. Linear regression modeling was used to determine normative T1 pelvic angle (TPA) and PI - lumbar lordosis (LL) mismatch (PI-LL) based on PI and age in a database of asymptomatic subjects. In an ASD database, patients with radiographic evidence of L5 sacralization had the PI, LL, and TPA measured from the superior endplate of S1 and then also from L5. The differences in TPA and PI-LL from normative were calculated in the sacralization cohort relative to L5 and S1 and correlated to the Oswestry Disability Index (ODI). Patients were grouped based on the Scoliosis Research Society (SRS)-Schwab PI-LL modifier (0, +, or ++) using the L5 PI-LL and S1 PI-LL. Baseline ODI and SF-36 Physical Component Summary (PCS) scores were compared across and within groups.RESULTS Among 1179 ASD patients, 276 (23.4%) had transitional anatomy, 176 with sacralized L5 (14.9%) and 100 (8.48%) with lumbarization of S1. The 176 patients with sacralized L5 were analyzed. When measured using the L5 superior endplate, pelvic parameters were significantly smaller than those measured relative to S1 (PI: 24.5 degrees +/- 11.0 degrees vs 55.7 degrees +/- 12.0 degrees, p = 0.001;TPA: 11.2 degrees +/- 12.0 degrees vs 20.3 degrees +/- 12.5 degrees, p = 0.001; and PI-LL: 0.67 degrees +/- 21.1 degrees vs 11.4 degrees +/- 20.8 degrees, p = 0.001). When measured from S1, 76 (43%), 45 (25.6%), and 55 (31.3%) patients had SRS-Schwab PI-LL modifiers of 0, +, and ++, respectively, compared with 124 (70.5%), 22 (12.5%), and 30 (17.0%), respectively, when measured from L5. There were significant differences in ODI and PCS scores as the SRS-Schwab grade increased regardless of L5 or S1 measurement. The L5 group had lower PCS functional scores for SRS-Schwab modifiers 0 and ++ relative to same grades in the S1 group. Offset from normative TPA (0.5 degrees +/- 11.1 degrees vs 9.6 degrees +/- 10.8 degrees, p = 0.001) and PI-LL (4.5 degrees +/- 20.4 degrees vs 15.2 degrees +/- 19.3 degrees, p = 0.001) were smaller when measuring from L5. Moreover, S1 measurements were more correlated with health status by ODI (TPA offset from normative: S1, R = 0.326 vs L5, R = 0.285; PI-LL offset from normative: S1, R = 0.318 vs L5, R = 0.274).CONCLUSIONS Measuring the PI and spinopelvic parameters at L5 in sacralized anatomy results in underestimating spinal deformity and is less correlated with health-related quality of life. Surgeons may consider measuring PI and spino-pelvic parameters relative to S1 rather than at L5 in patients with a sacralized L5. https://thejns.org/doi/abs/10.3171/2023.8.SPINE23432