Osteoporosis is characterised by low bone mass and micro-architectural deterioration. Bone mass is most often quantified by DXA. QCT is also used; a major disadvantage being radiation dose. The WHO T score criteria of osteoporosis are not applicable to spine QCT and interpretation is usually based on American College of Radiology guidelines. QCT bone density output of the proximal femora is similar to DXA of the hip and WHO T score criteria may be applied. There are few large longitudinal studies on QCT in predicting fracture but DXA hip data is supportive. QCT has a potential advantage of measuring structural parameters (e.g cortical thickness) but incorporation of these parameters into clinical practise remains difficult.
Assessment of the second characteristic of osteoporosis, deterioration of bone micro-architecture, has proved more difficult than measuring bone mass. HR-pQCT can derive micro-architectural parameters in the extremities, usually tibia or radius, but radiation dose limits its application in central sites. The high resolution of HR-pQCT allows measurements of cortical porosity and thickness, and parameters of trabecular microarchitecture, which can be used to derive estimates of bone strength using voxel-based finite element analysis. HR-pQCT has proved to be a major research tool. Cost, availability, and limited data confirming validity of extrapolation to prediction of axial skeleton microarchitecture, are factors limiting current clinical application
Trabecular bone score (TBS) derived from DXA spine images is a new parameter providing an indirect index of trabecular microarchitecture by evaluating pixel grey-scale variations. TBS is an independent predictor of fracture and has been shown to improve fracture prediction. Limitations of TBS include decreased reliability in obese patients. Recently, FRAX® has incorporated TBS into its absolute fracture risk algorithm facilitating an expansion of its role into clinical practise.
Optimal fracture prediction in the future will integrate measurements of bone mass, microarchitecture and clinical risk factors of osteoporosis.