Comparison of the Sagittal Age-Adjusted Score and the T4-L1PA Mismatch in Discriminating Proximal Junctional Kyphosis and Failure in Adult Spinal Deformity Surgery

Retrospective single-center study. To compare the discriminative ability of the sagittal age-adjusted score (SAAS) and the T4-L1PA mismatch for proximal junctional kyphosis (PJK) and proximal junctional failure (PJF) after adult spinal deformity (ASD) surgery. The SAAS and the T4-L1-Hip axis are two contemporary alignment schemes, but no study has directly compared their discriminative ability for proximal junctional complications. We analyzed 401 patients (mean age, 70&#xa0;yr; 87% female) who underwent surgery for degenerative adult spinal deformity with &#x2265;6-vertebrae fusion to the pelvis and two-year follow-up. The SAAS, the T4-L1PA mismatch, and their components were evaluated as continuous variables. Discrimination for PJK/F, PJK, and PJF was assessed by area under the curve (AUC) and compared using the DeLong test. Nested multivariable logistic regression tested the incremental value of each scheme over clinical risk factors, expressed as the change in -2 log likelihood (&#x394;-2LL). Proximal junctional complications occurred in 140 patients (34.9%; 113 PJK, 27 PJF). For PJK/F and PJK, the two schemes discriminated comparably (DeLong P =0.401 and 0.962) and both independently improved the adjusted models (SAAS &#x394;-2LL 4.8 and 5.3; T4-L1PA mismatch &#x394;-2LL 12.7 and 7.9; all P <0.05). For PJF, the T4-L1PA mismatch was significantly superior (area under the curve: 0.702 vs . 0.503; DeLong P =0.019) and added value in the nested model (&#x394;-2LL 10.8; P =0.001), whereas the SAAS did not (&#x394;-2LL 0.3; P =0.580). Component analysis showed that only the PI-LL offset drove the SAAS, while the L1PA was associated with all outcomes; for PJF, the composite T4-L1PA mismatch outperformed the L1PA alone. The SAAS and the T4-L1PA mismatch discriminated PJK comparably, but the T4-L1PA mismatch was more discriminative for PJF, although the overall discrimination remained modest. Correcting the lumbar segment addresses PJK, whereas optimal restoration of the T4-to-L1 relationship is additionally required to prevent PJF.