264. Effect of postmenopausal state on distribution of bone mineral density in the lumbar spine

BACKGROUND CONTEXT Hounsfield units (HU), typically measured at L1-L2, have been identified as a surrogate marker for lumbar Bone Mineral Density (BMD). However, concern remains that L1-L2 BMD may not fully capture BMD differences throughout the lumbosacral spine and inadvertently provide inaccurate assessment. PURPOSE To evaluate the impact of how conventional HU evaluation relates to local HU at commonly instrumented levels, and to evaluate the impact of age and postmenopausal state on HU in the lumbar spine. STUDY DESIGN/SETTING Cross sectional comparative study. PATIENT SAMPLE Consecutive female patients presenting to a spine clinic between April 2017 and January 2019 who underwent lumbar CT were selected. Previous lumbar surgery, tumor, fracture or infection were excluded. OUTCOME MEASURES HU were assessed on lumbar CT by placing an elliptical region of interest confined to the medullary space of the vertebral body, and avoiding cortical bone or sclerosis. HU measurements were performed for L1-S1 in 7 regions of each vertebral body: mid-sagittal; axial-superior, waist, inferior; axial at the waist–anterior, middle, posterior. METHODS Patients were divided based on whether they were pre- or postmenopausal. HU were compared between the pre- and postmenopausal cohorts using student t-test or Mann-Whitney u-test. The difference between routinely used L1-L2 average HU and local HU in each region was calculated and compared between groups. Association of age and menopausal state with HU was assessed using regression analyses. RESULTS Forty-eight patients, 18 premenopausal and 30 postmenopausal were included. The postmenopausal group was significantly older (67 versus 41 years; p 0.05). L1-L2 average was also more than 10 units greater than L5 measurements in 17–78% of premenopausal and 40–70% of postmenopausal women (p>0.05, except sagittal region p=0.038). Age, but not BMI showed a significant negative correlation with HU, which was strong in premenopausal and moderate in postmenopausal women. Age and menopausal state account for 35-60% of the variability in HU, except in the posterior region, where they account for 25-45%. CONCLUSIONS Age and postmenopausal state were negatively associated with HU in the lumbosacral spine, and the conventionally used L1-L2 average did not adequately capture local HU variations. Local BMD assessment using HU may provide a more accurate assessment of morphometric characteristics and trabecular milieu in the operative region. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.