Predictive value of upper instrumented level for proximal junctional Kyphosis in Scheuermann's Kyphosis.

Retrospective study. This study aimed to determine whether selecting Th3 and more distal vertebrae as upper instrumented vertebrae (UIV) affects the incidence of proximal junctional kyphosis (PJK) compared with more proximal levels or among themselves. Patients who underwent surgery for Scheuermann’s kyphosis (SK) between 2020 and 2023 were retrospectively reviewed. Individuals without a previous history of spine surgery and with a minimum follow-up of 2 year were included. A total of 34 patients were included, with 29 (85%) being male. The mean age was 17.67 ± 3.68 year. Patients with higher preoperative thoracic kyphosis and greater pelvic incidence–lumbar lordosis (PI–LL) mismatch had an increased risk of developing PJK (p = 0.005 and p = 0.043, respectively). The first erect postoperative kyphosis was significantly greater in patients who developed PJK (p = 0.004). Compared with Th2, patients with Th3 as the UIV had a substantially higher risk of PJK (odds ratio [OR], 7.38; 95% confidence interval [95% CI], 1.53–43.54; p = 0.012). The model showed fair discrimination (area under the curve = 0.721). No significant difference was found when Th4 or lower vertebrae were selected as UIV compared with Th2 (OR, 1.15; 95% CI, 0.10–8.77; p = 0.899). The findings suggest that selecting Th3 as the UIV increases the risk of PJK, whereas using more distal vertebrae does not. However, due to the retrospective design and small sample size, further studies are required to validate these results.