Assessing bone quality in hounsfield units using computed tomography: what value should be used to classify bone as normal or osteoporotic?

Purpose: The purpose of this study was to investigate threshold values for classifying bone as normal or osteoporotic based on Computed Tomography (CT) Hounsfield Units (HU) and to determine if clinically applicable values could be derived to aid spine surgeons evaluating bone quality using CT.

Methods: This literature review was completed using PubMed and Ovid (MedLine), using syntax specific to bone quality and CT. The included articles were original clinical studies assessing bone quality and utilized composite L1-L4 HU values compared against dual-energy X-ray absorptiometry (DEXA) values. Extracted data study descriptors, CT measurement technique, and CT threshold values. CTs were measured from L1-L4 using either axial or sagittal images, and must classify their bone quality findings for any of the following 3 categories: normal, osteopenia, or osteoporosis.

Results: This review located 34 studies measuring bone density using CT with threshold values, of which, 10 were included in the final review. Number of patients ranged from 74 to 283 and cohort ages from 20s to 70.6 years. CT threshold values for assessing normal and osteoporotic bone quality ranged from 150 to 179 and 87 to 155, respectively. From combining values across studies, a HU value of ≥ 170 HU was associated with normal bone and ≤ 115 HU with osteoporosis.

Conclusion: There is variation in HU values used to differentiate normal from compromised bone quality, even after limiting studies. For patients with HU values between or near 170 or 115 HU, a DEXA scan may be warranted for further evaluation. With ongoing investigation in this area, threshold values for classifying bone quality using CT will be continually refined.