Global Alignment and Proportion (GAP) score in asymptomatic individuals: is it universal?

Abstract

Background: The global alignment and proportion (GAP) score was established based on American and European subjects, which might limit its applicability to the Chinese population due to ethnicity-related difference of sagittal alignment.

Purpose: To analyze the applicability of GAP score in the Chinese population and to investigate the age- and gender-associated differences of spinopelvic and GAP score parameters.

Study design: A prospective cross-sectional radiographic study.

Patients sample: Of 692 asymptomatic Chinese volunteers aged between 20 and 79 prospectively recruited between January 2017 and June 2019, 490 subjects were eventually included in this study.

Outcome measures: The pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), L1-S1 lordosis, L4-S1 lordosis, sagittal vertical axis (SVA), T1 pelvic angle (TPA) and global tilt (GT) were measured on lateral X-rays. The GAP scores and its parameters including relative pelvic version (RPV), relative lumbar lordosis (RLL), lordosis distribution index (LDI) and relative spinopelvic alignment (RSA) were calculated for each subject.

Methods: Subjects were divided into four groups: Group 1M: male subjects <60 years old; Group 1F: female subjects <60 years old; Group 2M: male subjects ≥60 years old and Group 2F: female subjects ≥60 years old. The GAP scores and categories were determined and compared between groups. The results of sagittal alignment were compared with the previous studies evaluating the normative sagittal alignment in other populations. Univariate linear regression analysis was carried out between pelvic incidence (PI) and sacral slope (SS), lumbar lordosis (LL) and global tilt (GT) in each group.

Results: The distributions of GAP categories and the updated Roussouly classification were statistically different from other populations. Significantly different distribution of GAP categories was observed between Group 1M and Group 2M, Group 1F and Group 2F, and Group 1M and Group 1F. Radiographic measurements and GAP parameters were significantly different between Group 1M and Group 2M, and Group 1F and Group 2F. Gender-related difference of parameters was more prominent between Group 1M and Group 1F. Linear relationship of PI with SS, LL and GT were different from the regression models of "ideal" sagittal alignment in GAP score.

Conclusions: The GAP score might be inappropriate in Chinese population due to ethnicity-related alignment difference. Worse feasibility of GAP score was observed in female and old subjects.

Keywords: Adult spinal deformity; Age; Asymptomatic; Ethnicity; Gender; Global alignment and proportion score; Sagittal alignment.