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

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.