A global sagittal deviation index (GSDI-5) incorporating upper cervical alignment: initial construct validation in forward head posture

Abstract Purpose Forward head posture (FHP) is commonly assessed using isolated cervical measures, potentially overlooking its relationship with global sagittal alignment. This study introduces the Global Sagittal Deviation Index (GSDI-5), a composite measure integrating multiregional sagittal alignment with explicit inclusion of the upper cervical spine. Methods Standing lateral EOS radiographs from 201 adults referred for evaluation of low back pain were analysed and classified into FHP and non-FHP groups. Standardized sagittal parameters across the cervical, thoracic, lumbar, and global spine were combined into a single score. Initial construct validity was examined through known-groups comparisons and convergent analyses with TPA and PI–LL. Results Individuals with FHP demonstrated higher GSDI-5 scores. GSDI-5 showed a large effect size for differentiating FHP from non-FHP (Hedges’ g = 0.85), slightly exceeding that of TPA (g = 0.81) and PI–LL (g = 0.32), with increasing severity associated with progressive changes in global alignment measures. Conclusion By integrating upper cervical alignment into a whole-spine framework, GSDI-5 summarizes cranio-cervical behaviour relevant to FHP and may serve as a complementary exploratory measure for global sagittal deviation in radiographic and postural assessment, alongside existing sagittal alignment metrics.