Rabeb Ben Kahla - Bone Remodeling Process: Mechanics, Biology, and Numerical Modeling

1. Tables in Chapter 1
2. Tables in Chapter 2
3. Tables in Chapter 4
4. Tables in Chapter 6

1. Figures in Introduction
2. Figures in Chapter 1
3. Figures in Chapter 2
4. Figures in Chapter 3
5. Figures in Chapter 4
6. Figures in Chapter 5

Rabeb Ben Kahla

Laboratoire de Systmes et de Mcanique Applique (LASMAP), Ecole Polytechnique de Tunis Universit De Carthage, La Marsa, Tunisie

Laboratoire de Mcanique Applique et Ingnierie (LR-MAI), Ecole Nationale dIngnieurs de Tunis Universit Tunis El Manar, Tunis, Tunisie

Abdelwahed Barkaoui

Laboratoire des Energies Renouvelables et Matriaux Avancs (LERMA), Universit Internationale de Rabat, Rabat-Sala El Jadida, Morocco

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The skeletal system plays an essential support role for the entire human body. It supports the gravity forces and the stresses produced by daily activities. The bone thus optimizes and adapts its mass and geometry through the remodeling process. Mechanically, bone is a living, nanocomposite material with a complex hierarchical structure that gives bone remarkable mechanical properties: light weight, high rigidity, toughness, and fracture resistance. The imbalance in bone remodeling is responsible for certain bone pathologies such as osteoporosis and Pagets disease. In particular, osteoporosis induces a loss of bone mass as well as a reduction in the quality of bone tissue (microarchitecture). The architecture and the structural properties of the bone are thus degraded, which causes a decrease in bone quality and therefore, an increase of fractures risk.

Throughout life, bone is constantly remodeled through the complementary resorption and formation activities, establishing what is known as bone remodeling process (). This process requires a highly coordinated regulation in time and space to consistently maintain bone amount and quality. This coordination mainly incorporates bone-resorbing osteoclasts and bone-forming osteoblasts, which are the major two actors in the remodeling event. The delicate balance between the resorbed bone amount and the subsequent deposited amount requires a strict coordination of the resorption and the formation activities, allowing to generate the appropriate osteoblast number in remodeling area, which is referred to as the coupling mechanism. Moreover, the coordination between osteoblast and osteoclast activities involves other cells from diverse origins, in addition to several hormones, cytokines, and growth factors that tightly interlink osteoblast- and osteoclast-lineage cells through a complex interaction network throughout the remodeling cycle.

Figure 1 Cartoon representing the bone remodeling process with the osteoblast-mediated bone formation and the osteoclast-mediated bone resorption driven by osteocytes. Bahia, M. T., Hecke, M. B., Mercuri, E. G. F., & Pinheiro, M. M. (2020). A bone remodeling model governed by cellular micromechanics and physiologically based pharmacokinetics. Journal of the Mechanical Behavior of Biomedical Materials, 104, 103657.

The remodeling process is an integral part of the calcium homeostatic system and provides a crucial mechanism for old bone removal, as well as for bone damage repair and adaptation to physical stress, allowing to maintain the skeleton mechanical integrity. The remodeling process manifests at anatomically distinct sites known as bone multicellular units, each unit functioning asynchronously and independently from other units throughout the skeleton. It should however be noted that the bone multicellular units in cortical and in trabecular bones greatly differ in their structure, as well as in the way the bone is removed and replaced.

The concept of bone remodeling compartment consists of initiating the remodeling process within a canopy, and intercellular communication occurs in this compartment among the component bone cells, from vascular and endothelial cells, and probably from immune cells reaching the remodeling sites via the blood supply.

Bones occupy about 15% of the whole body weight, a fraction that does not deserve any consequence interpretation. Interestingly, human body ambulation, ventilation, and protection are primarily associated with bone, which highlights bone mechanical function. Therefore bone represents a structural material with mechanical characteristics resembling any other material with a mineral-based structure, even if the discovery process and the investigation of the relation between microcomponents and bulk material is opposite for the two types of material. After more than 2000 years of improvements, we now have enough knowledge of the right components to make a high quality steel, but still do not know yet how to efficiently treat bone metabolic disorders and age-related diseases, including osteoporosis. Steel is inert material, whereas bone is a living material. Steel is a mineral material, whereas bone is a biological material. Steel structure alters with higher mechanical loads, whereas bone strengthens with higher mechanical loads. All of these are differential features clearly show that bone mechanics do not necessarily follow the same classic rules of continuum damage mechanics as the remaining structures. This goes back to several poorly known factors and mechanisms, according to which bone structure is maintained.