The Association Between Adjacent Forces and the Development of Proximal Junctional Kyphosis: A Musculoskeletal Analysis.

Study Design: Biomechanical modeling study.

Objectives: Despite several recognized risk factors for proximal junctional kyphosis (PJK), its causative mechanism remains unknown. There are limited biomechanical studies to elucidate the association of sagittal alignment with PJK. Our aim is to determine the association of adjacent forces with PJK development.

Methods: ASD patients operated on with posterior fusion from T9-T11 to the pelvis and had 2 year follow-up were included in this retrospective study. Patient-specific musculoskeletal models were created from EOS images. High vs low normalized vertebral forces were defined based on a threshold of 0.2 for shear and 0.5 for compression. The load ratio was defined as the ratio of immediate postoperative to preoperative vertebral forces.

Results: A total of 35 patients (19 PJK, 16 Non-PJK) were included. PJK patients had statistically significant global malalignment with respect to global tilt (GT), T1 pelvic angle (TPA) and C2-UIV+1PA. A high-risk zone for the development of PJK is defined as high normalized shear and/or compression with a high load ratio. While none of the non-PJK patients were in the high-risk zone, 5 PJK patients were not in that zone, suggesting factors other than alignment may have contributed to this adverse event. Despite low shear for 3 of these non-PJK patients, they experienced high shear ratio.

Conclusions: The proposed thresholds were accurate in 86% of patients who developed PJK. These findings suggest postoperative shear at UIV+1 as an important risk factor for PJK. Keeping shear force low in alignment will help surgeons reduce PJK development.