You might know that as we grow older, our bones become more brittle and more susceptible to fracture. The most talked about reason is the loss of mass that occurs as the years pass—after the age of 30, we don’t build more bone, we simply replace lost cells. And when we lose cells faster than we can replace them, bone health weakens.
To date, most medical treatments have focused on slowing down this loss. Now new research from scientists at the US Department of Energy (DOE)‘s Lawrence Berkeley National Laboratory (Berkeley Lab) shows that at microscopic dimensions, the age-related loss of bone quality can be every bit as important as the loss of quantity when it comes to breaking bones.
Using a combination of x-ray and electron based analytical techniques as well as macroscopic fracture testing, the researchers showed that advancing age ushers in a degrading of healthy bone properties over a range of different size scales. As a result, the bone’s ability to resist fracture becomes increasingly compromised. They found that this age-related loss of bone quality is independent of age-related bone mass loss and that it occurs as a result of a series of changes starting at the core level.
Compact bone is a composite of collagen molecules and nanocrystals of a mineralized form of calcium called hydroxyapatite (HA). Mechanical properties of bone such as stiffness, strength and toughness come from both the characteristic structure at the nanoscale and at multiple length scales through the hierarchical architecture of the bone. These length scales extend from the molecular level to the osteonal structures at near-millimeter levels. An osteon is the basic structural unit of compact bone, comprised of a central canal surrounded by concentric rings of lamellae plates, through which bone remodels.
"Mechanisms that strengthen and toughen bone can be identified at most of these structural length scales and can be usefully classified, as in many materials, in terms of intrinsic toughening mechanisms at small length scales, promoting non-brittle behavior, and extrinsic toughening mechanisms at larger length scales acting to limit the growth of cracks," Ritchie says. "These features are present in healthy, young human bone and are responsible for its unique mechanical properties. However, with biological aging, the ability of these mechanisms to resist fracture deteriorates leading to a reduction in bone strength and fracture toughness."
Working with the exceptionally bright beams of x-rays at Berkeley Lab’s Advanced Light Source (ALS), Ritchie and his colleagues analyzed bone samples that ranged in age between 34 and 99 years. Various types of x-rays were used to characterize the mechanical response of the collagen and mineral at the sub micrometer level. A combination of other imaging devices was used to characterize effects at micrometer levels. The team found that biological aging diminishes plasticity, that collagen fibrils no longer slide with respect to one another as a way to absorb energy from an impact and that the remodeling of bone can lead the osteons to triple in number, which means the channels become more closely packed and less effective at deflecting the growth of cracks. “Thus, age-related changes occur across many levels of the structure to increase the risk of fracture with age," he concludes.
More study is needed to better understand the loss of bone quality as we age and to find ways to preserve it. In the meantime, the best way to protect your bones, no matter what age you are, is to get needed calcium and vitamin D, from dairy foods and limited amounts of sunlight respectively, and through supplements if your doctor advises.
About the researchers. Ritchie, who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the University of California (UC) Berkeley’s Materials Science and Engineering Department, is the senior author of the study, "Age-related changes in the plasticity and toughness of human cortical bone at multiple length scales," published in the Proceedings of the National Academy of Science (PNAS). Co-authoring the PNAS paper with Ritchie were Elizabeth Zimmermann, Eric Schaible, Hrishikesh Bale, Holly Barth, Simon Tang, Peter Reichert, Björn Busse, Tamara Alliston and Joel Ager. The research was supported by a grant from the National Institutes of Health. The Advanced Light Source is a national user facility supported by the DOE Office of Science. Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 12 Nobel prizes. The University of California manages Berkeley Lab for the US Department of Energy’s Office of Science.