How Much Variance Exists Among Published Definitions of Proximal Junctional Kyphosis: A Retrospective Cohort Study of Adult Spinal Deformity

Background/Objectives: We sought to characterize the variance and overlap among definitions of Proximal Junctional Kyphosis (PJK) used in Adult Spinal Deformity (ASD) literature. PJK is defined as excess in PJK angle, a Cobb angle between the upper-instrumented vertebra (UIV) and a supra-adjacent vertebra (SAV), either one (UIV+1) or two (UIV+2) levels rostral of the UIV. No expert consensus exists for threshold angle or which SAV to use. Methods: 116 thoracolumbar fusion patients ≥65 years old were reviewed. The UIV+1 and UIV+2 angles were measured. Six definitions of PJK from the literature were evaluated. Pearson's chi-squared and pairwise comparisons were performed to evaluate the distinctness and agreement rates among these definitions. Results: The six definitions of PJK were as follows: [PJK20] PJK angle > 20° with UIV+2 as the (SAV), [PJK10] PJK angle > 10° with a >10° change from pre-op with UIV+2 as the SAV, [PJK2SD] PJK angle > 2 standard deviations from average with UIV+1 as the SAV, [PJK10+10] PJK angle > 10° with a >10° change from pre-op with UIV+1 as the SAV, [PJK15] PJK angle > 15° with UIV+1 as the SAV, and [PJK30] PJK angle > 30° with UIV+2 as the SAV, or displaced rod fracture, or reoperation within 2 years for junctional failure, pseudoarthrosis, or rod fracture. [PJK10] and [PJK2SD] were the most distinct definitions while [PJK20], [PJK10+10], [PJK15], and [PJK30] showed no significant pairwise differences. [PJK2SD] was stringent, while definition [PJK30] included unique diagnostic information not captured by other definitions. Conclusions: Use of [PJK20], [PJK10+10], [PJK15], or [PJK30] is recommended for consistency, with [PJK15] presenting the best balance. Stringent [PJK2SD] may be beneficial for identifying severe PJK, though with low sensitivity. Overall, PJK definitions must be standardized for consistent reporting of clinical outcomes and research comparability.