According to an NIH Consensus Conference in 2001, bone quality is related to various aspects of bone: its micro- and macroarchitecture, turnover, resorption and mineralisation. Radiological imaging techniques can be used to visualise and quantify bone micro- and macroarchitecture in vivo.
The parameters of bone macrostructure can be obtained using various methods: X-rays, DXA, QUS, QCT and MR imaging.
The parameters derived from traditional radiological investigations (such as hip axis length, neck width and neck shaft angle), like Singh indices, have been shown to be of limited usefulness in the diagnosis of osteoporosis and, indeed, have never been accepted as standard diagnostic tools.
Geometrical parameters (e.g. hip axis length, length and width of the neck of the femur) have also been obtained from DXA images of the hip; it has been shown that an increase, of two standard deviations, in hip axis length triples the risk of hip fracture. These measures have been used in numerous studies, but none has been shown, convincingly, to add substantial information to BMD in predicting status or fracture risk.
QUS provides quantitative parameters that are used to establish the properties of bone tissue. This method offers a series of advantages: smaller dimensions, simple and rapid measurements, no need for ionising radiations, as well as low cost compared with DXA and QCT. Consequently, the QUS seems to generate more information on bone fragility, to the extent that, at present, QUS systems are the ones most used in osteoporotic fracture risk prediction. Given the availability of various techniques for evaluating risk fracture, the T-score approach to fracture risk assessment seems to present some shortcomings linked to discrepancies between examined sites and techniques used. Ten-year fracture probability is the best method for determining the threshold for intervention.
QCT images, too, have been used to measure geometrical parameters; it was found that patients with osteoporotic fractures had a greater vertebral axial cross-section than fracture-free patients, and that treatment with parathormone increases the vertebral area. Initial studies have been performed using MRI data to generate geometrical parameters.
The microarchitectural parameters of trabecular bone structure have proved to be more useful than bone macroarchitecture measurements in evaluation of bone quality and in distinguishing between patients with and without osteoporotic fractures. Clinical studies have been performed using high-resolution techniques to study trabecular bone architecture; these techniques include multidetector CT, magnetic resonance and in vivo micro-CT. Indeed, using multidetector CT, bone structure measurements were shown to be better than BMD in differentiating between the two patient groups. However, the radiation dose needed to obtain sufficiently high quality images was found to be necessarily rather high, which is thus a potential limitation of this technique. HR-MR imaging, on the other hand, does not involve the use of radiation and is therefore more attractive for scientific studies. It has been used to study trabecular bone architecture in a number of studies, showing a good ability to discriminate between patients with and without osteoporotic fractures. The sites most frequently studied using HR-MR imaging are the distal radius and heel. The disadvantage of MR-based techniques is that the use of standard 1.5 Tesla systems is limited to peripheral parts, like the heel, distal tibia and distal radius, whereas higher magnetic fields (3 Tesla) would allow better visualisation of the trabecular bone structure and examination of more central parts of the skeleton, such as the proximal femur.
In vivo micro-CT is a recently developed imaging technique; initial studies on its ability to quantify the bone microarchitecture of the peripheral skeleton have given good results in terms of reproducibility and also capacity to detect age- and disease-related changes.
In conclusion, in vivo imaging of bone macro- and microarchitecture is possible, and a certain number of studies, geared at the optimisation and clinical application of these techniques, have already been conducted. The NIH in the USA are promoting and supporting the concept of bone quality, which in the future could lead to new diagnostic standards and techniques for analysing bone structure and will probably change the definition of osteoporosis.