For a study, researchers sought to understand that in patients having lumbar fusion surgery, estimated the absolute value of L4 trabecular region-of-interest (t-ROI) computed tomography (CT) attenuation, which could indicate pedicle screw loosening, and the changes in value according to the number of fused levels and sagittal balance. Although preoperative dual x-ray absorptiometry did not reveal osteoporosis, investigators saw multiple cases of screw loosening within a year following lumbar fusion surgery. They studied factors associated with screw loosening in 478 individuals. The relationship between L4 t-ROI CT attenuation and screw loosening was investigated, and the optimal cutoff value for predicting screw loosening was identified. Screw loosening was linked to the number of fused levels, postoperative C7-S1 sagittal vertical axis (SVA), and L4 t-ROI CT attenuation. The best cutoff point of the L4 t-ROI CT attenuation predicting screw loosening was 106.5 Hounsfield unit, according to the number of fused levels and postoperative C7-T1 SVA (≥36.9 mm or <36.9 mm) in patients with 1 fusion and C7-S1 SVA less than 36.9 mm (HU). Attenuation of L4 t-ROI did not change until 2 fusions. The optimum cutoff threshold for the L4 t-ROI CT attenuation predicting screw loosening in patients with 3 fusions and C7-S1 SVA less than 36.9 mm was 159.0 HU. In patients with 3 fusions and C7-S1 SVA, more than or equal to 36.9 mm, the best cutoff point for L4 t-ROI CT attenuation was 191.0 HU. The number of fused levels and sagittal balance of the L4 t-ROI CT attenuation value was an accurate assessment method to predict screw loosening. To increase the fusion rate and prevent instrument-related problems of lumbar spine surgery in osteoporotic patients, spine surgeons should be aware of the L4 t-ROI attenuation before operation.