To test the vertical posterior vertebral angles (VPVA) of the most caudal lumbar segments measured on EOS to identify and classify the lumbosacral transitional vertebra (LSTV). We reviewed the EOS examinations of 906 patients to measure the VPVA at the most caudal lumbar segment (cVPVA) and at the immediately proximal segment (pVPVA), with dVPVA being the result of their difference. Mann-Whitney, Chi-square, and ROC curve statistics were used. 172/906 patients (19%) had LSTV (112 females, mean

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... ge: 43 ± 21 years), and 89/172 had type I LSTV (52%), 42/172 type II (24%), 33/172 type III (19%), and 8/172 type IV (5%). The cVPVA and dVPVA in non-articulated patients were significantly higher than those of patients with LSTV, patients with only accessory articulations, and patients with only bony fusion (all p < .001). The cVPVA and dVPVA in L5 sacralization were significantly higher than in S1 lumbarization (p < .001). The following optimal cutoff was found: cVPVA of 28.2° (AUC = 0.797) and dVPVA of 11.1° (AUC = 0.782) to identify LSTV; cVPVA of 28.2° (AUC = 0.665) and dVPVA of 8° (AUC = 0.718) to identify type II LSTV; cVPVA of 25.5° (AUC = 0.797) and dVPVA of - 7.5° (AUC = 0.831) to identify type III-IV LSTV; cVPVA of 20.4° (AUC = 0.693) and dVPVA of - 1.8° (AUC = 0.665) to differentiate type II from III-IV LSTV; cVPVA of 17.9° (AUC = 0.741) and dVPVA of - 4.5° (AUC = 0.774) to differentiate L5 sacralization from S1 lumbarization. The cVPVA and dVPVA measured on EOS showed good diagnostic performance to identify LSTV, to correctly classify it, and to differentiate L5 sacralization from S1 lumbarization.