Differences across various ideal lumbar lordosis measurement formulas for patient-specific sagittal alignment goals.

Objective: Multiple studies in the past have developed equations to determine the ideal lumbar lordosis (ILL) in the sagittal plane. These equations differ but all look to accomplish the same goal of providing the surgeon with specific alignment targets during surgery. To date, no study has compared the different equations of ILL against each other. This study compares 5 target alignment formulas in patients with high, normal, and low pelvic incidence (PI).

Methods: The authors conducted a retrospective chart review from January 2015 through April 2022 and reviewed available full-spine standing spine radiographs. They classified patients by their PI: high (65°-95°), normal (50°-60°), and low (30°-45°) and included 5 patients for each classification. They calculated ILL based on the measured PI using 5 different formulas (Global Alignment and Proportion [GAP] score [ILL = 0.62 × PI + 29], Le Huec [ILL = 0.5 × PI + 28], Hyun [ILL = 0.62 × PI + 27.6], Hamamatsu University [ILL = 0.45 × PI + 31.8], and Kelly [ILL = 0.6 × PI + 30]). They compared the different ILL equations to determine if there were significant differences and considered any measurement ± 3° as equivalent to account for measurement variability.

Results: Fifteen patients were included in the data analysis (5 patients for each PI classification). The mean PI measurements in the patients were as follows: high, 77.8°; normal, 54.6°; and low, 37.8°. The ILL measurements using the GAP formula were high, 77.2°; normal, 62.9°; and low, 52.4°. The ILL measurements using the Le Huec formula were high, 66.9°; normal, 55.3°; and low, 46.9°. The ILL measurements using the Hyun formula were high, 75.8°; normal, 61.5°; and low, 51.0°. The ILL measurements using the Hamamatsu University formula were high, 66.8°; normal, 56.4°; and low, 48.8°. Finally, the ILL measurements using the Kelly formula were high, 76.7°; normal, 62.8°; and low, 52.7°. Two-way ANOVA using Tukey Honestly Significant Difference post hoc multiple comparisons showed that the GAP, Hyun, and Kelly formulas for calculating ILL significantly differed from Le Huec and Hamamatsu University formulas (p < 0.001).

Conclusions: Variation exists among the 5 different ILL equations, with 3 of the formulas being statistically different from the others. One must take note of these differences when considering patient-specific sagittal alignment goals. Further discussion is needed to determine which ILL equation should be widely used.