Heterogeneity in the Definitions of Proximal Junctional Kyphosis and Failure in Spinal Deformity Literature: A Tower of Babel.

Study Design: Systematic review.

Objective: To describe the various definitions of PJK and PJF used in spinal deformity literature and their utility over time.

Summary Of Background Data: Proximal junctional kyphosis or failure (PJK/PJF) is among the most common complication after long-segment fusions, but there is no consensus on their definitions. This presents challenges in understanding risk factors, management, and prevention strategies.

Methods: A systematic literature review was performed on studies specifying a definition of PJK and/or PJF. PJK definitions were categorized as radiographic vs non-radiographic and data was collected on PJK criteria including threshold for proximal junctional angle (PJA), change in PJA, vertebra selection for PJA measurement, and follow-up timepoints. PJF definitions were categorized as structural failure, need for revision, symptomatic failure, and radiographic (angular).

Results: 359 studies defining PJK and/or PJF were identified. While 56% of studies used the definition PJA>10° and PJA change from baseline>10°, the remainder expressed significant heterogeneity with respect to criteria for magnitude of PJA and degree of PJA change. The most common vertebrae assessed were UIV/UIV+2 (74%) and most common minimum follow-up (mFU) listed was 2 years (60%). Mean FUs for studies varied considerably even in studies with the same mFU, from 2.1-8.9 years (2-year mFU) and 1.1-4.0 years (1-year mFU). PJF definitions were most commonly structural (58%) or defined as a need for revision (48%), with a much less common use of PJA thresholds (23%).

Conclusions: The challenges faced in preventing proximal junctional complications are mired in the heterogenous groundwork by which PJK and PJF are defined. Most definitions of PJK use radiographic thresholds without consideration of clinical relevance and variations in individual alignment. Conversely, definitions of PJF are based on clinical criteria, which are often subjective. Future research should focus on understanding the mechanisms of PJK/PJF, as only then will we be able to accurately define and prevent these complications.