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Low muscle volume, also called sarcopenia, is a common condition of aging with a prevalence between 1 and 29% in populations over the age of 50 years. Previous studies have demonstrated evidence that both osteoporosis and sarcopenia coexist, called osteosarcopenia, and these patients are at greater risk for falls, fractures and mortality. In osteoporotic post-menopausal female patients, the prevalence of sarcopenia is around 50%. Recent studies showed that sarcopenic subjects had a 4-fold higher risk of coexisting osteoporosis compared with nonsarcopenic individuals. However, a limited number of studies have been conducted in spine surgery patients. The aim of this study is to investigate the associations between the lean muscle volume (LMV) in paraspinal and proas muscle on MRI and quantitative computed tomography (QCT) volumetric bone mineral density (vBMD) in the lumbosacral region. A cross-sectional analysis of a prospectively maintained database at a single academic institution. Patients undergoing posterior lumbar fusion surgery between 2014-2020. Volumetric BMD measured by QCT in the lumbar spine and sacral ala. We reviewed an institutional database of posterior lumbar fusion patients. Patients who had ongoing antiosteoporotic therapy with bisphosphonate, denosumab or teriparatide were excluded. Muscle measurements were conducted utilizing a free software program (ITK-SNAP version 3.8) setting regions of interest (ROI) in the bilateral psoas and paraspinal (erector spinae and multifundus) muscles on preoperative T2-weighted axial MR images at the mid-L3 level. The area of fat-free muscle (LMV: lean muscle volume) was calculated using a custom written program on Matlab™ (MATLAB version 2019a, The MathWorks, Inc, Natick, MA, USA). QCT measurements were performed on preoperative CT scans using the Mindways QCT Pro Software (Mindways Software, Inc., Austin, TX, USA). QCT-vBMDs in the standard L1-L2 and bilateral sacral ala were analyzed. Pearson's correlation and linear regression analyses adjusting with age and BMI were performed. Since muscle volume is strongly affected by biological sex, we stratified by sex for all analyses. Statistical significance was set <0.05. Out of 144 patients, 30 patients were excluded due to QCT incompatible CTs or poor quality MRIs for muscle volume measurements. An additional 9 patients were excluded due to ongoing anti-osteoporotic drug therapy. A total of 105 patients (50 male and 55 female) were included in the final analysis. In female patients, there were statistically significant mild to moderate positive correlations between L1-L2 vBMD and LMV in paraspinal and psoas muscles, as well as sacral ala vBMD and paraspinal LMV, whereas there was no significant correlation in male patients. After the adjustment with age and BMI, high paraspinal LMV was an independent factor for high sacral ala vBMD (β=0.04 (95%CI: 0.00-0.14), p=0.040) and high psoas LMV was significantly associated with high L1-L2 vBMD (β=0.04 (95%CI: 0.01-0.14), p=0.025). Our results demonstrated that higher lean muscle volume was significantly associated with higher vBMD in the lumbosacral lesions in women. This association was not observed in men. This might indicate gender differences should be considered for the assessment of osteosarcopenia in spine surgery patients. This abstract does not discuss or include any applicable devices or drugs. [ABSTRACT FROM AUTHOR]