### OBJECTIVE

The subtraction of lumbar lordosis (LL) from the pelvic incidence (PI) offers an estimate of the LL required for a given PI value. Relative LL (RLL) and the lordosis distribution index (LDI) are PI-based individualized measures. RLL quantifies the magnitude of lordosis relative to the ideal lordosis as defined by the magnitude of PI. LDI defines the magnitude of lower arc lordosis in proportion to total lordosis. The aim of this study was to compare RLL and PI − LL for their ability to predict postoperative complications and their correlations with health-related quality of life (HRQOL) scores.

### METHODS

Inclusion criteria were ≥ 4 levels of fusion and ≥ 2 years of follow-up. Mechanical complications were proximal junctional kyphosis/proximal junctional failure, distal junctional kyphosis/distal junctional failure, rod breakage, and implant-related complications. Correlations between PI − LL, RLL, PI, and HRQOL were analyzed using the Pearson correlation coefficient. Mechanical complication rates in PI − LL, RLL, LDI, RLL, and LDI interpreted together, and RLL subgroups for each PI − LL category were compared using chi-square tests and the exact test. Predictive models for mechanical complications with RLL and PI − LL were analyzed using binomial logistic regressions.

### RESULTS

Two hundred twenty-two patients (168 women, 54 men) were included. The mean age was 52.2 ± 19.3 years (range 18–84 years). The mean follow-up was 28.8 ± 8.2 months (range 24–62 months). There was a significant correlation between PI − LL and PI (r = 0.441, p < 0.001), threatening the use of PI − LL to quantify spinopelvic mismatch for different PI values. RLL was not correlated with PI (r = −0.093, p > 0.05); therefore, it was able to quantify divergence from ideal lordosis for all PI values. Compared with PI − LL, RLL had stronger correlations with HRQOL scores (p < 0.05). Discrimination performance was better for the model with RLL than for PI − LL. The agreement between RLL and PI − LL was high (κ = 0.943, p < 0.001), moderate (κ = 0.455, p < 0.001), and poor (κ = −0.154, p = 0.343), respectively, for large, average, and small PI sizes. When analyzed by RLL, each PI − LL category was further divided into distinct groups of patients who had different mechanical complication rates (p < 0.001).

### CONCLUSIONS

Using the formula of PI − LL may be insufficient to quantify normolordosis for the whole spectrum of PI values when applied as an absolute numeric value in conjunction with previously reported population-based average thresholds of 10° and 20°. Schwab PI − LL groups were found to constitute an inhomogeneous group of patients. RLL offers an individualized quantification of LL for all PI sizes. Compared with PI − LL, RLL showed a greater association with both mechanical complications and HRQOL. The use of RLL and LDI together, instead of PI − LL, for surgical planning may result in lower mechanical complication rates and better long-term HRQOL.