May 4, 2011 -- The osteoporosis drugs known as bisphosphonates increase the risk of getting unusual thigh bone fractures, as experts have suspected, according to a new Swedish study.
But these fractures are infrequent and the risk is small the study shows.
When correctly prescribed, the overall benefit of the drugs preventing all types of fractures outweighs the risk of getting the unusual fractures, says study researcher Per Aspenberg, MD, PhD, a professor of orthopaedic surgery at Linkoping University, Sweden.
His research looked at 12,777 women aged 55 and older who suffered a fracture of the thigh bone in 2008. He found that the suspected link between the bone-building drugs and the unusual fractures is strong.
Experts have continued to debate whether there is a cause-and-effect relationship, Aspenberg tells WebMD. "The only thing agreed on so far [before his study] is that there is a weak association."
"Our study shows an extremely strong association, a dose-response relationship," he says. The link was consistent when he analyzed the findings in different ways. The link held up when he took into account other drugs the women were taking or other diseases they had.
"Our data demonstrates beyond a doubt there is a strong connection between the atypical fracture of the femoral shaft [thigh bone] and it is so strong one could argue that it is causative,” he says. “We can never prove that but it is highly likely."
The study is published in The New England Journal of Medicine.
Aspenberg evaluated the X-rays of the 12,777 women who had the thigh bone fractures. He found 59 of them had the unusual type of thigh bone fracture.
The researchers also compared the 59 patients with the unusual fractures with 263 others who had more ordinary thigh bone fractures. While 78% of those with the unusual fractures were on bisphosphonates, 10% of those with the other fractures were.
The researchers then looked at 1.5 million women in the National Swedish Patient Register who were 55 or older in 2008. He classified the women who had taken the osteoporosis drugs -- more than 83,000 -- into categories, depending on length of use. They took them for less than one year, one to 1.9 years, or two years or more.