**2. Bone remodeling**

#### **2.1 Generalities**

The cell activity leading to bone renewing has been identified by Frost [9] as a bone multicellular unit (BMU). A mature BMU contains a group of osteoclast and osteoblast cells, in addition to blood supply associated with the connective tissue. Knowing that the lifespan of a BMU is higher than the cells one, new osteoclasts and osteoblasts have to continually adhere to the BMU space in order to maintain its operation [10]. The bone multicellular units are presented in our skeletal under the form of discrete foci that could take different shapes, such as the unidirectional, the branched, and the clustered forms [11]. These foci move forward in all bone compartment to assure the renewing of concerned places with a longitudinal advance rate of 25 μm/day [12].

In order to characterize and discover precisely the movement of bone cells during the remodeling process, researchers have identified four phases of this biological phenomenon. First, the activation phase where osteoblasts are excited to release resorbing cytokines, after receiving the biological signal transmitted by osteocytes. Osteocytes are a bone cells that are embedded in the bone matrix and have the ability to sense the mechanical force applied upon the bone. Resorbing cytokines are responsible of the osteoclasts' recruitment. The involvement of these cells engenders the start of resorption phase. During this remodeling step, active osteoclasts erode the damaged bone matrix by means of ions, acid, and enzymes [13, 14]. Thereafter, macrophages appear into the gap created and clean the surface from the remaining bone debris. This step is called the reversal phase and it is essential to prepare a clean surface for matrix formation. In order to fill in the bone lacunae, active osteoblasts migrate into the concerned surface and produce the osteoid, which gets gradually mineralized [15]. The final result of bone remodeling is, thus, a new, healthy, and stronger bone matrix.
