Fig. 3

The full-length lateral films of the spine of two typical patients at immediate post-operative and at the last follow-up showed that PJK is more likely to occur for ASD patients with abnormal C2-FH at immediate post-operative (C2-FH < − 42.3 mm). The C2-FH and PJA of the first patient (A) were 0 mm and 22.2° at immediate post-operative (b) and 1.2 mm and 24.8° at follow-up (c). In another patient (B) with PJK, C2-FH and PJA were − 94.7 mm and 7.4° at immediate post-operative (e), while C2-FH and PJA were − 39.3 mm and 33° at follow-up (f). The preoperative full-length lateral films of the spine of patients A and B are shown in figure (a) and figure (d), respectively