Simple Ratio May Predict Fracture Risk in Obese Patients

Miriam E. Tucker

May 18, 2018

BOSTON — The ratio of bone mineral density (BMD) to body mass index (BMI) may be a simple and reliable tool for assessing fracture risk in obese individuals, new research suggests. 

The findings were presented May 17 here at the American Association of Clinical Endocrinologists (AACE) 2018 Annual Scientific & Clinical Congress by Mikiko Watanabe, MD, and Carla Lubrano, MD, PhD, both of Sapienza University, Rome, Italy.

"Traditionally obesity has been considered protective against bone fractures but recent evidence suggests it's not true. From our experience, we find that obese patients tend to have fractures quite a lot," Watanabe told Medscape Medical News.

And in fact, Watanabe and Lubrano note in their poster that some data suggest excess fat mass, especially visceral adipose tissue, is a risk factor for low BMD and fragility fractures. But because BMD is directly correlated with BMI, the measurement of BMD alone is unreliable as a marker of fracture risk in people with high BMI.

Recently, the trabecular bone score (TBS) has been introduced as an assessment of bone microarchitecture at the lumbar spine. The TBS, a textural image extracted from the 2-D lumbar spine, predicts fracture risk but requires special software added to the dual‐energy X‐ray absorptiometry (DEXA) image and isn't widely available. And, TBS doesn't assess hip fracture risk.

"So, we were trying to find an index that's readily available and easy to obtain, and that's what we found: the ratio of BMD obtained by DEXA scan and simple BMI," Watanabe said.

Asked to comment, Gregory Dodell, MD, assistant clinical professor of medicine, endocrinology, diabetes, and bone disease at the Icahn School of Medicine at Mount Sinai, New York City, called the findings "very interesting," noting that "right now DEXA is the only tool that's widely used to assess fracture risk, and if the tool you're using is not reliable in a given population, that's certainly a problem...You can get false reassurance in a population with a high BMI."

Regarding the BMD/BMI ratio, he said, "Simple is good. And I think the more data we have in evaluating a patient the better."

Could BMD/BMI Ratio Be Added to Guidelines?

The study involved 2225 overweight-obese patients with an average BMI of 36.5 kg/m2. They were an average age of 45 years and 82% were women. All underwent DEXA scans from which body composition, lumbar spine BMD, and TBS were obtained.

As has been previously reported, BMD increased with increasing BMI while TBS significantly decreased. The BMD/BMI ratio correlated with TBS more strongly than BMD alone.

Among the 46% of patients with metabolic syndrome, postulated to increase fracture risk, lumbar spine BMD was comparable with that of patients without metabolic syndrome (1.067 vs 1.063 g/cm2, P = .51), while TBS (1.21 vs 1.31) and BMD/BMI (0.28 vs 0.30) were significantly lower (P < 0.001 for both).

"Given its strong correlation with TBS, a simple ratio such as BMD/BMI is an interesting potential tool to easily assess the risk of fracture in obese subjects, where impaired metabolic health may play a detrimental role on bone strength," the authors note in the poster.

Dodell pointed out that the next step is to establish a cut-off score for the ratio, and the standard deviation, to assess which patients might benefit from osteoporosis treatment. 

Indeed, Watanabe told Medscape Medical News that her team is now conducting a longitudinal study to obtain that information, as well as to establish an association between the BMD/BMI ratio and actual fracture outcomes.

They're also exploring use of the ratio for predicting hip fracture risk and hope to investigate its use in patients with diabetes, who are at elevated fracture risk.

"We are very excited about the results...If we can confirm them [the BMD/BMI ratio] may get into the guidelines," she said.   

Watanabe, Lubrano, and Dodell have reported no relevant financial relationships.

American Association of Clinical Endocrinologists (AACE) 2018 Annual Scientific & Clinical Congress. May 17, 2018; Boston, Massachusetts. Abstract 610.

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