Age effects on the material and mechanical properties of bone at the osteocyte lacuna
This is an exploratory study aimed at characterizing the microenvironment of bone tissue around osteocyte lacunae, referred to as peri-lacunar tissue, as a function of age. Comprising nearly 90% of all bone cells, osteocytes are ideally situated to sense mechanical strain and translate mechanical signals into biochemical signals to regulate bone modeling and remodeling. Previous work has shown that there is an amplification of globally applied macroscopic bone strains at the microstructural level of the osteocyte due to a strain concentrating effect around the osteocyte lacuna and surrounding extracellular matrix. This peri-lacunar region is believed to have different material and mechanical properties than bone tissue not associated with an osteocyte lacuna. These alterations in bone tissue at the osteocyte lacuna are believed to have a significant impact on the tissue strains sensed by the osteocyte.
The global hypotheses are two-fold. First, peri-lacunar tissue is significantly different from non peri-lacunar bone tissue. Second, the mechanotransduction signal---the translation of mechanical stimuli to cellular signal---acting on the osteocyte is altered as the peri-lacunar tissue's mechanical and material properties change with aging. These hypotheses are tested using Raman spectroscopy, two dimensional displacement/strain mapping, and nanoindentation. Using these three techniques the mechanical and material properties of the peri-lacunar tissue were found to be significantly different from the surrounding non peri-lacunar tissue in its young crystalline structure, but not in its old crystalline structure, mineralization, carbonate substitution, collagen cross-linking, effective strains, and elastic modulus. Age effects on the bone tissue irrespective of location with respect to the lacuna were significant as carbonate substitution, crystallinity, collagen cross-linking, and effective strains near the loaded crack tip showed significant differences. These differences imply bone changes both with age and with proximity to an osteocyte lacuna. These findings further the understanding of how age effects and the osteocyte microenvironment affect the osteocyte mechanotransduction process.