Bone density measured on sagittal reconstructed CT is highly correlated with axial CT but both measurements are only moderately correlated with DEXA T-scores

Abstract

BackgroundDuring the preoperative evaluation of a patient being considered for spinal surgery, Dual-energy x-ray absorptiometry (DEXA) has been traditionally used to diagnose poor bone mineral density (BMD) as a risk factor. As ordering a DEXA can add cost and delay diagnosis, spine surgeons have more recently began to use Hounsfield Units (HU) measured on computed tomography scans (CT) as a measure of BMD. The aim of our study was to evaluate associations between DEXA and HU on lumbar spine CT scans. MethodsForty-two patients (32 female, 10 male, mean age = 67.7 years) with DEXA and lumbar spine CT scans performed within one year of each other were identified. DEXA T-scores were collected from the hip, forearm and L1-L4. HU was determined using the maximum region of interest within the cancellous area in the mid-vertebral body from L1-L4 in the sagittal and axial planes. ResultsUsing the lowest T-score, 8 (19 %) cases were osteoporotic and 25 (60 %) were osteopenic. Statistically significant differences in HU were seen in osteoporotic cases (Axial HU = 59.2, Sagittal HU = 61.1, p = 0.006) compared to osteopenic (Axial HU = 119.8, Sagittal HU = 122.9) and normal cases (Axial HU = 141.2, Sagittal HU = 142.3). There were moderate associations between the spine T-scores and CT HUs (Axial HU:r2 = 0.50, Sagittal HU:r2 = 0.49, p < 0.001), weak associations between the Axial HU (r2 = 0.48, p < 0.000) and Sagittal HU (r2 = 0.48, p < 0.000) with hip T-scores, and no correlations with forearm T-scores. There were strong associations between Axial HU and Sagittal HU (r2 = 0.98, p < 0.001). Clinical relevanceThe results of the current study show a strong association between the sagittal and axial vertebral HU measurements, which supports the clinical use of either measurement technique. The weak correlation between T-scores and HU is consistent with prior studies and warrants future studies to determine which modality will better predict postoperative mechanical failures in patients undergoing spinal surgery. Level of EvidenceIII.