Importance of UIV+1 Slope on Shear Force and Risk of PJK Development: Mathematical and Musculoskeletal Modeling With Clinical Confirmation

Study design Retrospective cohort study. Objectives Despite several recognized risk factors, PJK remains a common postoperative occurrence in adult spinal deformity surgery. This study aims to identify a significant preoperative sagittal alignment parameter that is associated with vertebral loading at the proximal junction and can predict the likelihood of proximal junctional kyphosis (PJK). Methods Fifty-five consecutive patients who underwent fusion surgery from lower thoracic (T9-T11) to the pelvis were included in this retrospective study. Based on the preoperative slope of the level above the upper-instrumented vertebrae (UIV+1), patients were stratified into two groups: posterior and anterior UIV+1 slope. Patient-specific musculoskeletal (MSK) models were created from pre and immediate postoperative EOS images. Pre and postoperative radiographic parameters, and normalized vertebral loading at UIV and UIV+1 were compared. Results 6/21 (28.6%) patients with posterior UIV+1 slope developed PJK, while the incidence of PJK for anterior UIV+1 slope was 24/34 (70.6%). Average normalized shear difference of UIV+1 in PJK and non-PJK patients was 0.19 and 0.1 for posterior slope patients, whereas for anterior slope patients, they were 0.3 and 0.18 respectively. Statistically significant associations were revealed between UIV+1 slope, shear force and risk of PJK development. Conclusion Utilizing mathematical and musculoskeletal analyses with clinical correlation, we were able to demonstrate UIV+1 slope as a local sagittal alignment risk factor that affects shear force at UIV/UIV+1, influencing PJK risk. Considering UIV+1 slope as a local alignment risk factor for PJK development may provide insight regarding UIV selection and alignment goals during preoperative planning.