Why Aging Bones Become Brittle: More Than Meets The Eye

We have known for some time that aging causes our bones to become brittle and in turn makes them more susceptible to fractures. So far the reason attributed to brittleness of bones was the loss of mass or quantity of bone in the senior population. Because of this reason the focus of most bone-health research was to come up with solutions to retard if not reduce this quantity of bone loss. However, scientists from the Lawrence Berkeley National Laboratory (U.S Department of Energy) have come up with some very interesting findings from their new research (supported by a grant from the National Institutes of Health). They say that it is not only the loss in the quantity or mass of bone but also in the quality of bone that occur at the microscopic level which is as responsible for the ageing population’s being prone to bone fractures. ⁽¹⁾

The researchers used three tools and techniques to study the changes that occur at the microscopic level of the bones, namely, x-ray devices, macroscopic fracture testing tools and electron-based analytical techniques. This helped them take a close look at the loss of quality in the mechanical properties the human cortical bones serve to provide.

Cortical bone forms the solid mass of bone and contributes to approximately 80% of the body’s skeletal weight. It’s primary function is to provide support to the body, protect the internal organs, help in body movement, and store and release calcium. The outer shell of most bones is formed by the cortical bone.⁽²⁾

The new study examined cortical bone over a range of different size scales all of which showed degradation of quality in the mechanical properties bone irrespective of the fact whether there had been a corresponding loss of bone mass yet, thus making fractures more possible. The age-related changes that were found to be occurring were degrading both the external toughness of the bone as well as the intrinsic quality of the bone. As per Berkeley Lab materials scientist Robert Ritchie, “Based on multi-scale structural and mechanical tests, we attribute this degradation to a hierarchical series of coupled mechanisms that start at the molecular level.” ⁽³⁾

The mechanical properties offered by the human cortical bone are those of stiffness, toughness and strength. This feature is possible due to the characteristic structure of the cortical bone at the nanoscale, and at multiple length scales through the hierarchical architecture of the bone. The lengths of scale that were studied in the research ranged from the molecular level up to the osteonal structures at millimetre levels. Osteons are the basic functional and structural unit of the bone at microscopic level.

The expected healthy observation of cortical bones would include the seeing of intrinsic strengthening mechanisms which occur at small length scales thus promoting non-brittle properties. The extrinsic toughness mechanisms would be occurring at longer scales inhibiting the growth of cracks.

With age, there is an increase in the non-enzymatic cross-linking between collagen molecules. This brings down the plasticity at the nanoscale level. The collagen fibrils are not able to slide in correspondence with one another as a response to absorb energy from an impact. As per Ritchie, “We also found that biological aging increases osteonal density, which limits the potency of crack-bridging mechanisms at micrometer scales.” ⁽⁴⁾

Also, with ageing, the osteons triple in number thus making the channels present more packed and unable to deflect cracks. This ‘wrong’ is tried to be offset at the at higher structural levels by increased micro cracking and eventually compromising extrinsic strength and increasing fracture risk over a period of time.

SOURCES:

1. The Brittleness of Aging Bones – More than a Loss of Bone Mass; Berkeley Lab – News Center; Lynn Yarris; August, 2011;
http://newscenter.lbl.gov/news-releases/2011/08/29/the-brittleness-of-aging-bones-%E2%80%93-more-than-a-loss-of-bone-
mass/
2. Musculoskeletal System: Anatomy, Physiology, And Metabolic Disorders; Summit, New Jersey: Ciba-Geigy Corporation; Netter,
Frank H; 1987; ISBN 0-914168-88-66
3. The Brittleness of Aging Bones: More Than Loss of Bone Mass; Science Daily- Science News; August 2011;
http://www.sciencedaily.com/releases/2011/08/110829114727.htm
4. Age-related changes in the plasticity and toughness of human cortical bone at multiple length scales; Cellectis; Elizabeth A.
Zimmermann, Eric Schaible, Hrishikesh Bale, Holly D. Bartha, Simon Y. Tang, Peter Reichert, Bjoern Busse, Tamara Alliston,
Joel W. Ager III, and Robert O. Ritchiea; August, 2011; http://www.pnas.org/content/108/35/14416