Methods of Evaluation of Bone Density and Quality
Dual Energy X-ray Absorptiometry
DXA is obtained by aiming two radiograph beams with different energy levels at the patient's bones. After subtracting the soft tissue absorption, the BMD is determined. DXA is the most widely used noninvasive technique for measuring BMD in the general population. Low BMD by DXA is a robust and consistent risk factor for fracture and treatments that increase BMD reduce fracture risk. The 2009 KDIGO guidelines recommended DXA for the assessment of fracture risk in CKD stages 1 through early 3, and that it should not be performed routinely in CKD stages 3–5 due to lack of evidence for fracture prediction in these patients. This recommendation was based on the fact that there were no longitudinal studies demonstrating that low BMD predicted fracture risk and the fact that cross-sectional studies reported inconsistent associations between low BMD and fracture.
Since the KDIGO publication there have been studies reporting that low DXA can predict fractures in CKD. A 5-year prospective cohort study of 485 participants on hemodialysis found that low BMD at the total hip or femoral neck was associated with excess risk of incident fractures independent of age, sex, diabetes, and dialysis vintage. A longitudinal study that analyzed the effect of CKD on fracture risk prediction by DXA in 587 older adults with CKD enrolled in the Health, Aging, and Body Composition Study reported that, with a follow-up of 11.3 years, each SD decrease in BMD at the femoral neck was associated with greater than a two-fold increased risk of fracture in patients with CKD compared with those without. A prospective study of participants with stages 3–5 CKD found that over 2 years of follow-up, BMD by DXA at baseline was significantly lower at all sites among those with incident fractures versus those without. For example, after adjusting for age and weight, for every SD decrease in BMD at the lumbar spine there was a two-fold increase in the risk of fracture; the authors found a similar relationship between total hip BMD and fracture. Finally, a study in 238 transplant patients reported an increase in the risk of fracture by 2.7 fold when osteopenia was diagnosed and 3.5 fold when osteoporosis was diagnosed by DXA. Considered together, these data demonstrate that low BMD predicts fracture in predialysis, dialysis, and transplant CKD.
High-resolution Peripheral Quantitated Tomography
High-resolution peripheral quantitated tomography (HR-pQCT) has a resolution of 82 μm and provides details of bone geometry and microarchitecture by quantifying trabecular number and thickness and cortical porosity. Cross-sectional studies demonstrated that low HR-pQCT parameters at the distal radius and tibia were associated with fractures. However, only one recent study published prospective data showing that HR-pQCT predicts fracture. Moreover, in this cohort, HR-pQCT did not demonstrate a better performance than DXA in terms of fracture prediction. This fact, combined with the limited availability of this technique, currently limits the use of HR-pQCT for fracture risk assessment in CKD. Nevertheless, this method is able to detect structural changes, differentiating cortical from trabecular compartments, and could potentially provide important information in future interventional studies.