Predicting Proximal Junctional Kyphosis: University of Toronto Study Reveals Key Sagittal Alignment Parameter

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Researchers at the University of Toronto have conducted a study on Mathematics to identify a significant preoperative sagittal alignment parameter that can predict the likelihood of proximal junctional kyphosis (PJK) in adult spinal deformity surgery. The study aimed to investigate the relationship between the preoperative slope of the level above the upper-instrumented vertebrae (UIV+1) and the risk of PJK development. By utilizing mathematical and musculoskeletal analyses with clinical correlation, the researchers were able to demonstrate that UIV+1 slope is a local sagittal alignment risk factor that affects shear force at UIV/UIV+1, influencing PJK risk.

Key Takeaways:

  • The study included 55 consecutive patients who underwent fusion surgery from lower thoracic (T9-T11) to the pelvis in a retrospective cohort study.
  • The researchers found that patients with a posterior UIV+1 slope had a lower incidence of PJK (28.6%) compared to those with an anterior UIV+1 slope (70.6%).
  • The average normalized shear difference of UIV+1 in PJK and non-PJK patients was significant, indicating a strong association between UIV+1 slope, shear force, and risk of PJK development.
  • The study concluded that 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.
  • The researchers utilized patient-specific musculoskeletal (MSK) models created from pre and immediate postoperative EOS images to compare pre and postoperative radiographic parameters and normalized vertebral loading at UIV and UIV+1.
  • The study received financial support from Schroeder Arthritis Institutes, University Health Network, and UHN foundation.
  • The research was conducted with the guidance of Christopher J. Nielsen, Dept. of Orthopedic Surgery, University of Toronto, University Health Network (UHN), Toronto, ON, Canada.
  • Additional authors for this research include Fatemeh Alavi, Raja Rampersaud, Angela M. Cheung, and Stephen Lewis.

Statistics:

  • 55 patients were included in the retrospective study.
  • 6/21 (28.6%) patients with posterior UIV+1 slope developed PJK.
  • 24/34 (70.6%) patients with anterior UIV+1 slope developed PJK.
  • 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.

Sources:

  • NewsRx. Studies Conducted at University of Toronto on Mathematics Recently Reported (Importance of UIV+1 Slope on Shear Force and Risk of PJK Development: Mathematical and Musculoskeletal Modeling With Clinical Confirmation). Medical Devices & Surgical Technology Week. November 2, 2025; p 1963.
  • Global Spine Journal. Importance of UIV+1 Slope on Shear Force and Risk of PJK Development: Mathematical and Musculoskeletal Modeling With Clinical Confirmation. 2025:21925682251387966.