Expert Commentary

Does bone size affect bone strength?

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To determine whether a strength index that accounts for both bone density and bone size might better predict the risk of fracture than bone mineral density (BMD) alone.

Methods and Results

For this prospective investigation, researchers recruited 241 white women, all 48 years of age. Analysis was based on the 108 women who were followed through age 67, the study’s endpoint.

For the duration of the trial, participants’ right and left forearms were regularly evaluated via single photon absorptiometry for bone mineral content and BMD. From these scans, investigators calculated periosteal diameter, medullary diameter, and cortical thickness of the distal radius. Serum estradiol levels were also regularly assessed.

Researchers found that, annually, medullary diameter increased by 1.1% and periosteal diameter rose by 0.7%. BMD, however, decreased an average of 1.9% each year. The net result was a 0.7% annual decrease in the strength index, which accounted for both bone mass and skeletal structure.

Postmenopausal serum estradiol was related inversely to periosteal diameter and directly to BMD.

Who may be affected by these findings?

Women at risk for osteoporosis.

Expert commentary

We know that BMD correlates well with fracture risk. However, bone architecture—not assessed by BMD studies—also plays a role in bone strength.

Indeed, a recent trial on risedronate use noted a probable relationship between early changes in bone resorption and reduced fracture risk.1 Risedronate reduces the risk of vertebral fracture within the first year of therapy, but some believe this effect occurs too rapidly to be solely attributable to BMD changes, which are maximum by the third year. The implication is that bone-strength factors beyond BMD are at work.

Clinical relevance of bone size. Few would argue the need to treat all patients with documented osteoporosis (World Health Organization definition: a T-score of -2.5 or less); however, from a public health perspective it is not cost-effective to treat all women with low bone mass/osteopenia—even though there is an increased incidence of fractures among these patients. Thus, a strength index such as that described in this article would—if effective—help further stratify postmenopausal women with low bone mass into low, medium, and high risk for future fractures.

Flaws in study’s bone size measurements. In this timely study by Ahlborg et al, the authors theorize that the observed postmenopausal increase in periosteal apposition and size partially preserves bone strength—but this hypothesis has not been proven. With the crude images rendered by large pixels, it is impossible to note true bone size; furthermore, no evidence exists to confirm that we can actually measure bone size with single photon densitometry. In addition, the authors here evaluated the distal third of the forearm; ultradistal forearm measurements would have yielded a more accurate assessment since these correlate better with Colle’s fractures of the wrist/forearm.

It is anticipated that, in the future, quantitative computed tomography scanning with ultrathin slices will allow clinicians to assess bone architecture in a way that will allow for more accurate bone-strength measurements.

The authors theorize that the observed postmenopausal increase in periosteal apposition and size preserves bone strength—but this has not been proven.

Bottom line

Correctly classifying women into the appropriate bone-risk category is even more critical in this post-Women’s Health Initiative era. Many women who are stopping hormone therapy may still be at risk for losing bone mass; others who already have low bone mass may not have been offered other bone therapies. The current study is important only for raising the issue of the relationship of bone strength to future fracture risk.

Dual-energy bone densitometry—a valuable tool for diagnosing osteoporosis, assessing a patient’s fracture risk, and following the effects of bone treatments—remains the standard of care.

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