Translating Cervical Mechanics to Spinopelvic Alignment: Comparing a Validated Whole-Spine Finite Element Model With Eos Radiographs Using Op–c2 and Pi–ll in Forward Head Posture

This study introduces OP–C2 as a novel upper cervical sagittal balance parameter and investigates its relationship with global spinopelvic alignment in forward head posture (FHP). A validated whole-spine finite element model (THUMS v4) was used to simulate progressive FHP conditions, incorporating anterior head translation and compensatory occipital rotation. Model-derived sagittal parameters were compared with EOS radiographs from a cohort of 201 subjects, stratified by craniovertebral angle into FHP and non-FHP groups. Results demonstrate progressive shortening of OP–C2 with increasing FHP severity, accompanied by corresponding increases in pelvic incidence–lumbar lordosis (PI–LL) mismatch. Close agreement between simulations and imaging data reveals a previously unreported coupling between upper cervical alignment and lumbopelvic balance. These findings provide the first quantitative characterisation of OP–C2 and highlight its potential as a clinically relevant metric for assessing cervical–global alignment interactions in forward head posture and sagittal imbalance.