Senile bone fragility is not only osteoporosis

Everyone knows that the fragility of bones increases with age and, accordingly, the risk of fractures. It is also a generally recognized fact that the main reason for this is a decrease in bone mass – osteopenia, which gradually leads to the development of osteoporosis. The goal of all medical interventions aimed at reducing the likelihood of fractures in the elderly has so far been to slow down this process. However, new data obtained by researchers at the National Laboratory named after Lawrence at Berkeley, working under the direction of Robert Ritchie, indicate that an equally important factor contributing to the increase in bone fragility is a change in the structure of bone tissue at all structural levels.

The results of the study, which included analysis of the structure of bone tissue using X-ray and electronic analytical methods, as well as macroscopic examination of fractures, showed that aging leads to deterioration of the mechanical properties of the cortical substance (outer layer) of bone at different structural levels. Moreover, this happens regardless of the decrease in bone mass.

 
Changes occurring at each structural level contribute to the age-related increase in bone fragility.

The cortical, or compact, bone substance consists of collagen fibers and mineral nanocrystals of calcium hydroxyapatite. The mechanical properties of bone, such as strength and stiffness, are determined by the structural features both at the nanoscale and at other structural levels of bone structure, including at the molecular level and at the level of osteones. An osteon is a structural unit of compact bone tissue consisting of concentrically arranged bone plates surrounding a central canal. The size of osteons is slightly less than a millimeter.

Using highly sensitive X-ray, computed tomography and scanning electron microscopy, the researchers analyzed the bone structure of people aged 34 to 99 years at the micro and submicro levels. Analysis of the data obtained showed that the process of biological aging increases the number of enzymatic cross-linking between collagen molecules, which reduces the elasticity of bone at the nanoscale. This means that the collagen fibers lose the ability to slide relative to each other, while absorbing the energy of an external impact. In addition, biological aging leads to an increase in the number of osteons and the density of their placement, which limits the possibility of repairing cracks at the micro level.

In combination, these two mechanisms significantly reduce the strength of the bone. This is manifested by the fact that instead of collagen that has lost elasticity, the impact energy is absorbed by rigid structures, which leads to deformations at higher structural levels and the appearance of microcracks.

 
The graphs show the change in the mechanical properties of the compact substance of human bone as it ages:
A – strength, B – fracture resistance.

With age, the number of osteons can increase threefold, which leads to an excessively dense arrangement of channels and a decrease in the effectiveness of mechanisms for blocking the growth of cracks. The result of this growing inefficiency of damage repair is the formation of many microcracks at higher structural levels. Thus, age-related changes occur at all levels of the structural organization of the cortical substance, which is one of the reasons for the increased predisposition to fractures.

Article by E. A. Zimmermann et al. Age-related changes in the plasticity and toughness of human cortical bone at multiple length was published on August 22 in the preliminary on-line version of Proceedings of the National Academy of Sciences.

Evgeniya Ryabtseva
Portal "Eternal youth" www.vechnayamolodost.ru based on the materials of Lawrence Berkeley National Laboratory:
The Brittleness of Aging Bones – More than a Loss of Bone Mass.

31.08.2011