The Pros and Cons of DXA-What Are the Alternatives?

A wide range of techniques are used in research to assess bone density and quality, but dual-energy X-ray absorptiometry (DXA) is likely to remain the standard procedure used in clinical practice for the foreseeable future for most patients, predicts the co-author of a recent review of imaging-based techniques for assessing bone in rheumatology.

DXA is cheap, widely available and its standardization and good reproducibility allows reasonable assessment of the impact of treatments for osteoporosis, says Thomas Link, professor of radiology, chief of musculoskeletal imaging and clinical director of musculoskeletal and quantitative imaging research at the department of radiology and biomedical imaging at UCSF School of Medicine, San Francisco.

But it does have some limitations, particularly in patients with inflammatory arthroscopies that should be clearly highlighted, he says.

Assessments of bone density using DXA will not provide an accurate reflection of fracture risk in patients with degenerative changes, those treated with corticosteroids, and those with a small or large frame or body weight.

Corticosteroids and degenerative changes tend to make the bones denser, so DXA underestimates the risk of fractures, he says. Conversely, risk of fracture is underestimated by DXA in people with small bones.

The technique also does not fully account for obesity in very obese individuals, he adds.

“That is why the FRAX [fracture risk assessment] tool was devised which also includes clinical risk factors,” he says.
Although DXA is now the most widely used technique for assessing bone density, it was not the first - that was quantitative CT (QCT). QCT is still a useful technique for evaluating patients with certain rheumatologic conditions, including advanced degenerative disease of the spine, diffuse idiopathic skeletal hyperostosis (DISH), Link says.

QCT allows volumetric measurements of the lumbar spine and proximal femur, which are independent of body size, and is able to separate trabecular and cortical compartments of the bone.

“When you want to look at changes over time, especially in patients who are treated with corticosteroids or immunosuppressant agents, then you can see changes in the trabecular bone faster than you can in the cortical bone,” he explains.

But QCT also has its issues, including lack of standardisation, Link warns, so it is a research tool and “a problem solver” and not to be recommended as a technique for routine use.

The obvious limitations of measurements of bone mineral density for assessing fracture risk has led to the development of measures of bone architecture, because the trabecular bone structure is an indication of strength.

“Bone is built like a stepladder,” he says. “If you have a very good interconnections between your rods, then it’s going to be a very stable system. However, if your system is not very well connected, even if your rods are strong, then it’s going to fail because the architecture is just not right.” He offers Eiffel Tower in Paris as an analogy - a seemingly frail graceful structure but extraordinary stable.

While most of these techniques will remain research tools because of their complexity and cost, one that can and is being used clinical practice is the trabecular bone score, he says.

“It’s a very cheap way to assess texture,” he explains, “and it is also easily available.” Some DXA scanners come with software that can calculate the trabecular bone score automatically, and in others it can be installed.

“It provides some additional information about the macro architecture of the bone. It is by no means a very sophisticated technique, it’s really extremely crude,” he emphasises. But there is some evidence that the trabecular bone score is useful for certain rheumatologic disorders, particularly rheumatoid arthritis and patients using glucocorticosteroids, and the International Society for Clinical Densitometry (ICSD) has embraced the technique, he says. “It is not a major quantum leap forward but it may help to provide a better diagnosis.”

Unfortunately, there is not much else on the horizon for routine clinical practice, Link says. Some sophisticated techniques being used in the research setting are assessing, factors such as bone marrow composition and cortical porosity, he adds, “but they are not methods where you would say okay these could be introduced in the clinic in a couple of years.”

What is most important, he emphasises, is for rheumatologists to have a relatively low threshold for ordering a bone mineral density test because patients  rheumatologic disorders, especially inflammatory disorders, are so much at risk of fractures, and it vital that these patients get treated early and in the correct way to avoid devastating fractures. “I always have this impression that sometimes it is just a little overlooked.”