Tool gauges bone’s fracture risk

UC SANTA BARBARA (US)—Physicists have developed a new tool to measure the strength and quality of bone.

Paul Hansma, a professor of physics at the University of California, Santa Barbara, and colleagues have validated the diagnostic tool—known as the Reference Point Indentation (RPI) instrument—using live human test subjects. They report their findings in the Journal of Bone and Mineral Research.

“This is a revolutionary breakthrough,” Hansma says. “People get excited when they hear about this.”

The study documents the first clinical trials of the RPI, which uses a mechanically driven test probe enclosed in a microscopically small cylinder about the size of a hypodermic needle.

The test probe is driven like a tiny piston into the bone of the test subject for about 10 seconds, creating microscopic indentations. The indentations are repeated and the microfractures are measured to determine the bone’s strength and quality.

“The properties of bone materials are an important part of fracture risk,” Hansma says. “In normal, everyday life, you wonder about whether something is going to break or not. It depends. How big is it? How thick is it? Is it made of balsa wood, or walnut? In the field of bone fracture risk, there has not been any instrument that can measure the material properties of bone relative to fracture risk, so it’s been ignored.”

Conventional measurement of bone mineral density is conducted with X-rays, called DEXA (dual energy X-ray absorptiometry).

“This is a measure of how much bone you have,” Hansma says. “As people age, they lose bone. This bone loss can be monitored with DEXA. If you’ve lost a certain amount, you’ll typically be prescribed drugs like Fosamax or Boniva. They help the bones get bigger, help mitigate bone loss and, in some cases, rebuild bone. It’s a $6 billion market right now, growing at 25 percent per year. All they’re doing is dealing with bone quantity. They’re dealing with half the problem.”

What’s been missing is a way to measure bone quality. Using the RPI, a doctor would be able to determine bone quality by studying the microfractures created in the bones of tested patients.

“There’s been no way to measure this until now,” Hansma says. “This is just a local measurement of how easily the bone is fractured. Since there was no way to measure this, there was no way to develop therapies. But with the RPI, we can measure how easily a bone could be fractured.

“So now we have a goal—let’s try to do something to the bone to lower the total indentation distance as measured by the RPI, because we would like the bone to be less easily fractured.”

All clinical trials have taken place in Barcelona, Spain, under the supervision of Hansma and Adolfo Diez-Perez, a physician at Hospital del Mar in Barcelona and the study’s first author.

Hospital patients—victims of hip fractures and other ailments—were injected with a local anesthetic in their tibia. The leg bone was then tested using the RPI instrument. The RPI’s probe creates microfractures in the tibia, measuring the force that’s being applied and the distance that the probe goes into the bone.

“What you can see from the tests is that everyone whose bone was easily fractured by the RPI’s indentations—very easily fractured at a probe depth of 40 microns—every single one was someone who had previously suffered a bone fracture,” Hansma says. “And below 30 microns of probe depth, no one had suffered a fracture. It’s very statistically significant. Clearly, there’s a big difference.”

So far, the only site approved for clinical trials is in Spain. The Food and Drug Administration (FDA) has not approved the RPI for tests in the United States.

Researchers from UC Santa Barbara; Hospital del Mar in Barcelona; RETICEF, Instituto Carlos III in Madrid, Spain; UC Berkeley; and Active Life Scientific contributed to the work.

More news from UC Santa Barbara: www.ia.ucsb.edu/pa/

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