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This is retrospective study.Our purpose is to examine the state of the L5–S1 disk after long fusion arthrodesis stopping at L5 in adult scoliosis, and to identify the risk factors for disk degeneration.The long fusion arthrodesis surgery for adult scoliosis has shown to have good clinical results. However, there is still some controversy with reference to the potential caudal extents of the fusion, that is, L5 or S1. One of the most common problems related to the L5 fusion is whether subsequent L5–S1 disk degenerations occurs because of stress concentration. In this study, 1.5-T MRI was used to evaluate the L5–S1 disk degeneration according to Pfirrmann classification.In total, 43 patients with adult scoliosis after posterior long fusions arthrodesis surgery were reviewed retrospectively with the average follow-up period of 4.17±1.55 years (range, 2–9 y). The 1.5 T MRI and standing long x-ray were performed preoperatively and at last follow-up visit. Clinical outcomes were evaluated according to Japanese Orthopedic Association (JOA) scores and Oswestry Disability Index (ODI).Subsequent L5–S1 disk degeneration was observed in 33 patients (51.56%). Preoperative average Pfirrmann grade was 1.98±0.58, whereas during last follow-up average Pfirrmann grade was 2.63±0.83 (P<0.001). Preoperative Pfirrmann grade in patients with subsequent disk degeneration was 1.94±0.66, and in patients without subsequent disk degeneration was 2.06±0.51 (P=0.278). Greater ODI showed in patients with subsequent degeneration (P=0.008). Multivariate logistic regression analysis revealed that long follow-up period, heavy labor, and preoperative imbalance were the risk factors.Subsequent L5–S1 disk degeneration was common after long fusions arthrodesis was stopped at L5. The recovery of neurological status was similar in patients with or without subsequent L5–S1 disk degeneration, whereas patients with subsequent degeneration complained more about low back pain. Patients with long follow-up period, heavy labor, and preoperative imbalance were more likely to suffer subsequent L5–S1 disk degeneration.