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

<b>Background/Objectives</b>: We sought to characterize the variance and overlap among definitions of Proximal Junctional Kyphosis (PJK) used in the 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. <b>Methods</b>: A total of 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. These definitions were selected based on citation frequency, historical relevance, and accessibility through commonly used databases. Pearson's Chi-squared and pairwise comparisons were performed to evaluate the distinctness and agreement rates among these definitions. <b>Results</b>: The six definitions of PJK were as follows: <b>[PJK20]</b> PJK angle ≥ 20° with UIV+2 as the (SAV), <b>[PJK10]</b> PJK angle ≥ 10° with a >10° change from pre-op with UIV+2 as the SAV, <b>[PJK2SD]</b> PJK angle > 2 standard deviations from average with UIV+1 as the SAV, <b>[PJK10+10]</b> PJK angle ≥ 10° with a >10° change from pre-op with UIV+1 as the SAV, <b>[PJK15]</b> PJK angle > 15° with UIV+1 as the SAV, and <b>[PJK30]</b> 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 <b>[PJK2SD]</b> were the most distinct definitions while <b>[PJK20]</b>, <b>[PJK10+10]</b>, <b>[PJK15]</b>, and <b>[PJK30]</b> showed no significant pairwise differences. [PJK2SD] was stringent, while definition <b>[PJK30]</b> included unique diagnostic information not captured by other definitions. <b>Conclusions</b>: The use of <b>[PJK20]</b>, <b>[PJK10+10]</b>, <b>[PJK15]</b>, or <b>[PJK30]</b> is recommended for consistency, with <b>[PJK15]</b> presenting the best balance. Stringent <b>[PJK2SD]</b> may be beneficial for identifying severe PJK, though with low sensitivity. Overall, PJK definitions must be standardized for the consistent reporting of clinical outcomes and research comparability.