The article “Hounsfield unit change in root and alveolar bone during canine retraction” focused on HU changes in the alveolar bone and root surface as the canines were moved by 2 treatment strategies—translation and controlled tipping—that were evaluated with cone-beam computed tomography (CBCT). The authors stated that although the use of HUs from CBCT images has not been considered a reliable way to quantify absolute bone mineral density, it is still the best method available to monitor changes of bone mineral density in terms of HUs through longitudinal studies as long as the same CBCT machine and the same scan settings are used.
Although the absolute values were not described in this study, the alterations found for root and bone mineral densities do not correspond to the variations of HUs and, therefore, should not be referred to as such.
The gray values obtained with CBCT show a linear relationship with the attenuation coefficients of the materials, and the HUs obtained with medical computed tomography and density values from dual energy x-ray absorptiometry. Errors are expected when CBCT images are used to define the quality of the scanned structures because these images show inconsistencies and arbitrariness in the gray values, particularly when related to abrupt changes in the density of the object, x-ray beam hardening effects, scattered radiation, projection data discontinuity-related effect, differences between CBCT devices, changes in the volume of the field of view (FOV), and changes in the relationships of size and position between the FOV and the object evaluated. According to the studies available to date, CBCT should not be considered the examination of choice for the determination of mineral density of osseous and soft tissues, especially when the values obtained are compared with predetermined standard values.
In the methodology used in the study, the standardization of the equipment, the image-acquisition setting, the patient’s head posture, and the FOV during image acquisition allowed the standardization of errors for image acquisition, minimizing the problems associated with the technique. However, the values obtained from the images were not from HUs, but of gray values. Our intention is not to criticize the methodology in the article, but to call attention to the appropriate nomenclature to be used.