The role of the interstitial fluid content in bone remodeling

Bone is an extraordinary biological material able to modify dynamically its outer shape and inner microstructure in response to chemo-mechanical stimuli coming from the environment adapting its hierarchical microstructure to respond to static and dynamic loads for offering optimal mechanical features, in terms of stiffness and toughness. To date, many theories and mathematical models have been proposed by several authors to describe the remodeling phenomenon, all approaches starting from the adaptive elasticity and the bone maintenance theories. Within this framework, one of the most classical strategies employed in the studies is the so-called Stanford’s law, which allows uploading the effect of the time-dependent load-induced stress stimulus into a biomechanical model to guess the bone structure evolution. In the present work, we generalize this approach by introducing the bone poroelasticity, thus incorporating in the model the role of the fluid content that, by driving nutrients and contributing to the removal of wastes of bone tissue cells, synergistically interacts with the classical stress fields, in this way affecting growth and remodeling of the bone tissue. Two paradigmatic example applications, i.e. a cylindrical slice with internal prescribed displacements idealizing a tract of femoral diaphysis pushed out by the pressure exerted by a femur prosthesis and a bone element in a form of a bent beam, and a real study case of a patient subject to total hip replacement, and CT scanned at 24 hours after surgery and at 1 year post-surgery have been considered. It has to note that the proposed model is capable to catch more realistically both the transition between spongy and cortical regions and the expected nonsymmetrical evolution of bone tissue density in the medium-long term, unpredictable with the standard approach. Although limitations still characterize some hypotheses at the basis of the present approach, the proposed model overcomes the intrinsic-and unrealistic-independence of the bone remodeling from the stress sign and from the indirect effect of stress gradients driving nutrients through the flow of the fluid content in the tissue, allowing to predict important spatial asymmetries in bone mass density, so paving the way to more reliable mechanobiological strategies and engineering tools for the faithful prediction of bone remodeling, with implications in diagnosis of risk fracture, optimal design of scaffolds and bone prostheses.