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Boning Up on Fracture Mechanics

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Robert Ritchie has also explored the fracture mechanics of teeth.

As we age, our bones become brittle. According to the National Institutes of Health, one in two women and one in four men over 50 will have an osteoporosis-related fracture in her or his remaining lifetime. Often these fractures require surgery. Sometimes they prove fatal. But what can be done to toughen bones? The first step, says UC Berkeley materials science professor Robert Ritchie, is to find out how they break.

"The only way to possibly treat the brittleness of bone is to understand the mechanisms that make it fracture," says Ritchie, who is also the chair of the Department of Materials Science and Engineering and a scientist at Lawrence Berkeley National Laboratory.

X-ray computed tomography of human bone performed at Berkeley Lab's Advanced Light Source and the Stanford Linear Accelerator Center reveals nanoscale damage. In this image, shapes that appear solid are actually spaces — the blue structures are canals that carry blood and lymph vessels and nerves through the bone, while the pale green shape is a propagating crack. (courtesy Berkeley Lab) [view larger image]

Ritchie has spent his entire career analyzing the fatigue and fracture of various materials, from ceramics to silicon to exotic alloys. After "running out of materials to break and study," he jokes, he stumbled upon bone. Common knowledge said that an age-related reduction in the density of bone mineral is tied to brittleness. However, recent studies suggest that bone mineral density may only be a small part of the problem.

"Bone quality, rather than quantity, seems to be a bigger part of the problem," Ritchie says. "And I realized I could quantify this using fracture mechanics. Our approach is that the mechanisms of fracture depend on the structure of the bone, and that structure changes with age, disease or clinical treatment."

To conduct the research, Ritchie and his colleagues use advanced 3D visualization techniques such as X-ray computed tomography, made possible by Berkeley Lab's Advanced Light Source, to study the bones of cadavers. They examine the bone across a spectrum of size scales, from the macroscale to the nanoscale. The critical dimension, Ritchie says, is surprisingly quite large—on the scale of hundreds of microns. This is the dimension of osteons, channels that surround the blood vessels passing through the bone. When a large crack reaches the edge of an osteon, it's often deflected. That mechanism can toughen the material. These regions are also prone to microcracking, which surprisingly can also lead to toughening in bone.

Two-dimensionaly tomographs show fewer and smaller bridges in older bone. (courtesy the researchers) [view larger image]

"The small cracks actually open up ahead of any major crack," Ritchie says. "And between the microcracks are regions of unbroken bone. So when a major crack tries to open, these unbroken regions act as 'bridges' across the crack that sustain load and prevent the larger crack from propagating."

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With age though, the number of osteons increases. And that's "too much of a good thing," Ritchie says. Too many microcracks actually defeats the toughening mechanism. So armed with their newfound knowledge, the researchers are collaborating with groups at the UC San Francisco and UC Davis to determine whether therapeutic treatments can be developed to help offset this deterioration in the fracture resistance of bone as one ages.

"It fascinates me that you might be able to explain things as complicated as disease and aging in terms of mechanical factors," Ritchie says. "Indeed, it's doubly intriguing because we can also use nature's inspiration to develop new materials based on these mechanisms."

Robert O. Ritchie's home page

Ritchie Group

"Why Older People Suffer More Bone Fractures" by Dan Krotz (Science@Berkeley Lab, September 23, 2005)

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