Spongy bone structure and mechanical response

Abstract

Humans in space suffer from losses of bone mass accompanied by bone restructuration. The structural adaption depends on prevailing loading conditions. It can be assumed that the resulting structure is optimized with respect to the given restrictions, which are governed by biological mechanisms. It is not understood how structure and mechanical properties of cancellous bone are linked and how the structure can be characterized precisely. The challenges are the complex spongy architecture of the bone as well as the anisotropy of mechanical properties of the trabeculae due to their evolution from collagen fibrils. For achieving a better understanding of dependence between structure and mechanical response researchers from 3 DLR-Institutes work together. We expect to gain insight in remodelling mechanisms, which can be considered as load dependent structural optimization processes. The knowledge can be translated into adapting the load, e.g. by systematic physical training of astronauts, for avoiding losses of bone mass under microgravity. The gained knowledge will be also utilized for producing artificial bone material adapted to external mechanical loads. Subject of first investigations were bovine tail vertebrae. By means of X-ray tomography 3-dimensional images of samples from trabecular bone tissue were generated and analysed for identifying geometric parameters characterizing the microstructure. An approach was selected, which describes the microstructure as material with irregularly shaped pores characterized by largest and smallest diameters and orientation of the axis along the largest extension. Global parameters such as density and elastic constants of the bone samples were determined on 3-dimensional numerical models. Using the microstructural parameters we calculated new bone structure models and compared their global parameters with those derived from real bone samples. We found that the global properties of calculated and real bone structure models matched well, indicating that the calculated models are appropriate for further studies on the influence of microstructural parameters on mechanical properties. Further investigations will include mechanical tests on real bone tissue samples for identifying the anisotropy effect of the trabeculae. The results of these analyses will be utilized when designing artificial bones from the new material class of aerogels for further developing spongy nano cellulose felt materials resembling cancellous bone.