Two new studies presented at the 2010 AAOS conference in March are trying to understand why.

“For the first time we’re having some explanation for what up until now has only been a hypothesis,” Dr. Schneider says.

Melvin Rosenwasser, MD, a surgeon who is the director of the orthopaedic hand and trauma service at New York-Presbyterian Hospital, is the lead author of one of those studies. He found that long-term bisphosphonate treatment changes the structure of bone and leads to fractures in some patients.

“It’s a drug that works. But at some point a burden has been assumed by use of it that may be leading some patients to have a complication of the treatment,” he says. “We may be homing in on a group of patients who’ve been on the drug for a long time and may be vulnerable to the fractures.”

Dr. Rosenwasser and his colleagues identified 112 postmenopausal women with primary osteoporosis. Sixty-two had been taking bisphosphonates for at least four years, while 50 in a control group took only calcium and vitamin D supplements. Doctors used a series of X-ray scans to evaluate the structure of women’s femurs over time, looking at measures of strength and structural integrity.

They discovered that the longer the treatment, the more structural integrity declined. It went from 3.8 percent above pretreatment baseline to 1.3 percent after at least four years on the medication. There were also declines in the control group, which was expected since those patients were only taking supplements.

Dr. Rosenwasser says his team’s conclusion is that bone properties change after long-term bisphosphonate use in a way that makes them more likely to fracture.

“We think something is happening over time that may be changing the character of the bone,” Dr. Rosenwasser says.

“I don’t want to be too alarmist. It’s not 100 percent of patients at seven years are breaking their femurs. But some are,” Dr. Rosenwasser explains. “They are all losing something, but not at the same rate or speed or level. It’s not homogenous. They are individuals. That’s why some people’s would break and someone else who’s weak but not critically weak won’t break.”

Rosenwasser says he and his colleagues have not yet identified the point someone is likely to fracture. He says that will require further study.

“There is some magic number that we don’t have yet. Is five years OK, but six years bad? I don’t know. I just know as they go farther and farther into a treatment plan we see this effect more and at some point with some patients, the bone breaks,” Dr. Rosenwasser says.

For the second study, Joe Lane, MD, an orthopaedic trauma surgeon at the Hospital for Special Surgery in New York City, studied patients who had already fractured or broken their legs.