In the clinical management of osteoporosis, the most common bone disease that is associated with reduced bone strength and increased risk of broken bones (fractures), it is useful to be able to assess bone strength in individual patients. Bone strength depends on the amount of bone (bone mass) as well as its structure. The heel bone is a very accessible site for assessing bone strength but, to date, the assessments have largely been undertaken with ultrasound-based devices that measure bone mass but only indirectly tell us something about bone structure. Recently, high-resolution x-ray imaging, using a device called the XtremeCT, has been developed to allow visualisation of the bone structure down to a resolution of approximately 0.1 millimetres, enabling us to see if the bone structure is thin and porous (weak) or thick and dense (strong). The aim of our work was to develop a procedure to image the heel bone using the XtremeCT and test its accuracy in assessing heel bone structure.
Ten human cadaveric feet were used to develop the method. Measurements of heel bone microstructure from the XtremeCT images were compared to ‘gold standard’ images, obtained in the laboratory by a scanner that is of such high resolution (0.02 millimetres) that it cannot be used in living human samples. We experimented with short and long scan durations, to see if the scan intensity (longer duration) could improve the accuracy of the structure assessment.
We found that the measurement of heel bone mass by the XtremeCT accurately reflected that measured by the ‘gold standard’ method, regardless of the scan duration. In contrast, the number and spacing of the structures within the heel bone was more strongly correlated with the ‘gold standard’ when the longer scan durations were used. Importantly, the accuracy of these measurements in the latter scans was similar to those reported at the wrist and ankle in previous research studies using the XtremeCT. The accuracy of the structural measurements was most accurate when the region measured lay closer to the upper surface of the bone (see the figure), because of the higher bone mass in this region and the lower quantity of surrounding bones and soft tissues (e.g. fat, muscle, tendons and ligaments).
In summary, our method produced accurate measurements of heel bone microstructure using the XtremeCT device, particularly in the upper regions of the heel bone. Using longer scan durations might be warranted for measurements in clinical studies in an attempt to improve measurement accuracy, but this may not be possible as even minor movements of the heel might detract from the accuracy.
Metcalf, L. M., Dall’Ara, E., Paggiosi, M. A., Rochester, J. A., Vilayphiou, N., Kemp, G. J., McCloskey, E. V. (2018), “Validation of calcaneus trabecular microstructure measurements by HR-pQCT”, Bone
Figure. XtremeCT slice through the centre of the heel bone.