Proximal junctional kyphosis (PJK) in adult spinal deformity (ASD) surgery patients is a severe complication with potential need for revision surgery. While thoracic kyphosis (TK) is known to influence PJK, the role of TK flexibility is still unknown. We analyzed the influence of TK flexibility to predict postoperative sagittal alignment. Patients with ASD, 2-year follow-up, and upper-most instrumented vertebra (UIV) including and below T10 were included in this retrospective study. TK flexibility, defined as 10 difference of the TK in standing and supine imaging, was analyzed. Patient characteristics like age, sex, weight, total hip arthroplasty, and sagittal alignment parameters were studied. Sixty-five patients aged 66 8years were included in the study. Lowest instrumented vertebra was S1 or the ilium in 85% of them; the number of levels being fused averaged 7. Flexible TK was present in 31% (n = 20). These patients had a larger preoperative TK (p 0.01), but no PJK was found (p = 0.04). In contrast, patients who underwent revision surgery had a decreased TK flexibility (p = 0.04) and increased PJK angle at follow-up (p = 0.01). In the non-flexible patients, the PJK was found in 14% of patients. Based on our retrospective data, TK flexibility influences the outcome of ASD surgery. In patients demonstrating no TK flexibility, a more cephalad UIV-level should be considered because spontaneous curve correction in the sagittal plane might be low in these patients. This new parameter should be included in future prediction models. These slides can be retrieved under Electronic Supplementary Material.
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