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Low bone mineral density (BMD) is believed to influence the outcome of instrumented spinal surgery and can lead to reoperation. Purpose of this observational and case-control study was to investigate the association of BMD with the risk of reoperation following instrumented lumbar spinal fusion (LSF). For the observational study, 81 patients were included who received LSF with and without augmentation. For the case-control study, 18 patients who had reoperation following LSF were matched to 26 patients who did not have reoperation (matched by sex, age +/- 5 years, fused levels and PMMA-augmentation). Opportunistic BMD screening was performed in perioperative CT scans using asynchronous calibration. Mean BMD was compared between patients with and without reoperation in augmented and non-augmented surgeries. In the observational study, prevalence of osteoporosis (BMD < 80 mg/cc) was 29% in non-augmented and 85% in augmented LSF. Seven of 48 patients with non-augmented (15%) and 4 of 33 patients with augmented LSF (12%) had reoperation. In non-augmented LSF, patients with reoperation had significantly lower BMD than patients without reoperation (P = 0.005). The best cut-off to predict reoperation after non-augmented LSF was BMD < 83.7 mg/cc. In the case-control study, patients with reoperation presented numerically lower BMD of 78.8 +/- 33.1 mg/cc than patients without reoperation with BMD of 89.4 +/- 39.7 mg/cc (P = 0.357). Despite much lower BMD surgeries with PMMA-augmentation showed no higher reoperation rate compared to non-augmented surgeries. Patients with reoperation following LSF showed slightly lower BMD compared to matched patients without reoperation, but the difference was not statistically significant. Opportunistic BMD screening can be performed in preoperative CT, thus informing about osteoporotic bone, a potential risk factor of surgery failure., Comment: 16 pages, 2 figures, 4 tables