Purpose <p>This study aimed to identify predictive factors for femoral bone mineral density (BMD) in adult patients with spinal deformity and to establish cutoff values for detecting osteoporosis using preoperative computed tomography (CT) scans.</p> Methods <p>A retrospective analysis was conducted on 90 patients with adult spinal deformity (ASD) who underwent preoperative bone density testing. Key parameters such as age, body mass index (BMI), and Hounsfield unit (HU) values from various vertebral levels were analyzed. Dual-energy X-ray absorptiometry (DXA) was used to measure BMD T-score, and HU values were measured at C2, C7, T8, T9, L1, L2, and L3.</p> Results <p>Multivariate regression analyses identified BMI and HU at C2 as independent predictors of femoral T-score. The following predictive equation was developed: femoral T-score = − 5.178 + 0.053 × BMI + 0.006 × C2 HU (R² = 0.50). The formula derived from these variables explained 50% of the variance in femoral T-score. Receiver operating characteristic analysis identified cutoff values for predicting osteoporosis as 281 for C2 HU and 22.9 for BMI. The areas under the curve were 0.751 and 0.638, respectively.</p> Conclusion <p>These findings suggest that HU values from preoperative CT scans can serve as reliable indicators of femoral T-score, offering an alternative to DXA in cases where DXA is unavailable or inaccurate due to factors like severe spinal degeneration. This approach could enhance the preoperative assessment of osteoporosis in ASD patients by facilitating more targeted use of DXA and improving overall clinical decision-making.</p> Level of evidence <p>IV.</p>