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

Introduction: Dual-energy x-ray absorptiometry (DEXA) is used pre-operatively to determine if poor bone mineral density (BMD) needs to be treated to maximize success with lumbar fusion. As ordering a DEXA can add cost and delay diagnosis, spine surgeons use Hounsfield Units (HU) in lumbar spine computed tomography scans (CT) as a measure of BMD.

Objective: The aim of our study was to evaluate associations between DEXA and HU on lumbar CT scans.

Methods: Forty-six patients (32F/10M, mean age=67.7 yrs) with DEXA and lumbar spine CT scans performed within one year of each other were identified. DEXA T-scores and Z-scores were collected from the hip, forearm and L1-4. HU was determined using the maximum region of interest (ROI) within the cancellous area in the mid-vertebral body from L1-4 in the sagittal (SagHU) and axial (AxHU) planes.

Results: Mean lowest SagHU was 121.3, mean lowest AxHU was 119.9, mean lowest T-score was -1.5 and mean lowest Z-score was -0.29. Using the lowest T-score, 8 (19%) cases were osteoporotic and 25 (60%) were osteopenic. Statistically significant differences in HU were seen in osteoporotic cases (AxHU=59.2, SagHU=61.1, p=0.006) compared to osteopenic (AxHU=119.8, SagHU=122.9) and normal cases (AxHU=141.2, SagHU=142.3). There were moderate associations between the spine Z-scores and CT HUs (AxHU:r=0.50, SagHU:r=0.49, p< 0.001), weak associations between the AxHU (r=0.48, p< 0.000) and SagHU (r=0.48, p< 0.000) with hip Z-scores, and no correlations with forearm Z-scores. There were strong associations between AxHU and SagHU (r=0.98, p< 0.001).

Conclusion: The results of the current study show a strong association between the sagittal and axial vertebral HU measurements, which supports the use of either technique. Consistent with prior literature, the current study shows that DEXA Z-scores and CT HU units of the same corresponding areas of interest are only moderately correlated (r=0.5). Future studies are needed to determine whether DEXA or CT HU is more predictive of postoperative outcomes.