Beyond Kyphosis: Modes of Failure at The Proximal Junction in Adult Spinal Deformity

Abstract

Study design: Retrospective cohort study based on a multicenter adult spinal deformity (ASD) database.

Objective: To characterize distinct patterns of proximal junctional failure (PJF) beyond kyphosis-based definitions and evaluate their morphology, timing, and clinical implications.

Summary of background data: Proximal junctional kyphosis (PJK) is commonly defined by angular measurements, yet many patients develop other junctional complications-such as vertebral fractures, disc degeneration, or instrumentation failure-without measurable kyphosis. These "non-kyphotic" failures are underrecognized in current classification systems, limiting clinical decision-making and preventive strategies.

Methods: Data from 185 ASD patients who either met Lovecchio's radiographic PJK criteria or underwent revision surgery with proximal extension were retrospectively reviewed. Three independent readers qualitatively classified failure morphology at the proximal junction. Free-text reports were standardized and categorized into vertebral, soft-tissue/disc, or diffuse degenerative failure modes. An unsupervised cluster analysis was used to identify failure patterns based on imaging features and time to onset. Relationships between failure types, UIV level, prophylactic measures, and timing were analyzed.

Results: Among 1,506 enrolled patients, 185 (12.3%) developed proximal junctional complications (median age 67.5 years; 86.5% female). Failure modes included vertebral (66%), soft tissue/disc (64.9%), and diffuse degeneration (22.2%). Notably, 27% of patients exhibited no kyphotic angulation. Cluster analysis identified three patterns: Type 1 (acute vertebral fracture, median onset 73.5 days), Type 2 (disc/soft tissue failure, 368 days), and Type 3 (degeneration, 670 days).

Conclusion: Proximal junctional failure is not synonymous with kyphosis. This study identifies three distinct, temporally and morphologically defined failure modes, including a significant proportion of non-kyphotic cases. These findings support expanding PJF definitions and adopting individualized, mechanism-based preventive strategies in ASD surgery.

Level of evidence: 3.

Keywords: Adult Spinal Deformity; Cluster Analysis; Mode of Failure; Proximal Junctional Failure; Proximal Junctional Kyphosis.