Upper lumbar standardized subcutaneous fat index: a novel sex-independent indicator of bone mineral density and musculoskeletal fat infiltration

OBJECTIVE: A recent study demonstrated that subcutaneous fat tissue thickness (SFTT) at the L1-L2 level is predicting paraspinal muscle fatty infiltration. Given that fat distribution patterns differ between males and females, we calibrated the SFTT index to obtain a standardized subcutaneous fat index (SSFI). METHODS: A total of 175 patients admitted between January 2023 and July 2024 were included in this study. Based on lumbar CT attenuation values, patients were classified into an osteoporosis group (HU ≤ 110) and a normal bone density group (HU > 110). Demographic characteristics and clinical variables, including age, BMI, sex, medical history, and the standardized subcutaneous fat index (SSFI), were compared between the two groups. Pearson correlation analysis was subsequently performed to evaluate the associations between SSFI1 and other variables. Multiple linear regression analyses were conducted to further assess the relationships between SSFI1 and multifidus fatty infiltration, erector spinae fatty infiltration, frailty index, and vertebral bone quality (VBQ). Finally, receiver operating characteristic (ROC) curve analysis was performed to evaluate the ability of SSFI1 to discriminate osteoporosis. RESULTS: Patients with osteoporosis were older than the control group, while no significant differences were observed in smoking, alcohol consumption, diabetes, or hypertension between the two groups. Compared with controls, patients with osteoporosis showed lower SSFI1 values, greater paraspinal muscle fatty infiltration, and higher frailty index and VBQ scores. Moreover, SSFI was lower in both male and female patients with osteoporosis. Pearson correlation analysis further demonstrated that SSFI1 was significantly associated with paraspinal muscle fatty infiltration, lumbar bone mineral density, femoral bone mineral density, frailty index, and VBQ. Multiple linear regression analysis revealed that SSFI1 was associated with VBQ. ROC curve analysis indicated that SSFI1 had potential value in distinguishing osteoporosis from non-osteoporotic individuals. When the cutoff value of SSFI1 was < 0.29, the area under the curve (AUC) was 0.642, with a sensitivity of 0.696 and a specificity of 0.921, suggesting that SSFI1 may serve as a potential imaging marker for osteoporosis. CONCLUSION: SSFI1 was significantly reduced in patients with osteoporosis and was closely associated with paraspinal muscle fatty infiltration, bone mineral density, frailty index, and VBQ. In addition, SSFI1 demonstrated potential value in distinguishing osteoporosis from non-osteoporotic individuals. These findings suggest that SSFI1 may serve as a simple and accessible imaging biomarker for evaluating bone quality and may provide a novel approach for the early identification and risk stratification of osteoporosis.