The DXA Scan – Uses and Limitations

Posted by on Oct 29, 2014 in Uncategorized | No Comments

The DXA scanner has become a familiar feature of modern health screening. Most women over 50 have had a DXA scan to assess their bone mineral density or BMD, as have many older men. Using epidemiologically based statistics; this measure can give an estimate of a person’s relative risk for fracture. This post is intended to explain a bit more about what the test actually is, what it tells us about bone health, and what it does not. We will also introduce the concept of bone quality, and how our device the OsteoProbe is providing new information to researchers around the world.

The term DXA (pronounced dexa) is an acronym, which stands for dual x-ray absorptiometry, also called densitometry. The scan is essentially a 2-dimensional x-ray, which provides an estimate of the mineral density in the region studied. Common areas to examine are the total hip, femoral neck, spine and peripheral locations such as the distal radius (wrist). Since the test is based on 2-dimensional imaging, the result is expressed as grams/cm2 rather than grams/cm3, and it is an estimate of density rather than a direct measure.

The accuracy of this density estimate is affected by many factors. Smaller people with smaller bones will achieve lower scores, machines from different manufacturers use different algorithms and yield non-comparable results, different anatomical sites yield different results for men but not women, and anatomic abnormalities in the area, such as surgery or compression fractures, will also skew the output. The “areal-density” measure is generally reported as a T-score, or the amount of deviation from the mean value seen in a young adult of the same gender. It is an unusual output in the field of clinical medicine, and should be understood to represent not an absolute value, but one compared to population statistics.

The figure below, illustrates the cut-off values for normal, osteopenia and osteoporosis.
graph
Clinical definitions include: Normal (bone density ±1.0 standard deviation of the young adult mean), Osteopenia (density between -1 and -2.5 standard deviations below the young adult mean) and Osteoporosis (density more than 2.5 SD below the mean, less than -2.5).

All limitations considered, DXA scans provide useful information about bone health. While the T-score cannot tell an individual their absolute risk of fracture, it does give an estimate of relative risk, the risk compared to others with normal density. And currently, it is simply the best measure available to provide such information.

But bone mineral density is widely understood to represent just one of the critical measures of bone health. The DXA scanner tells us about the amount of bone, the quantity, but not the quality of the bone itself. And quality, the underlying strength of the bone material, is a key consideration. As with any structural material, both the quantity of that substance and its underlying quality are critical in determination of overall strength.

In our next installment we discuss the Quality issue; what is bone quality, and how does it influence bone strength?