# Bone: An Outstanding Composite Material
## 4 Macrostructure  
At the highest hierarchical level, long bones are constituted by the cortical bone (also known as compact bone) and trabecular bone (also known as cancellous or spongy bone). Although they have similar compositions, the different structure arrangements and porosity levels grant unique mechanical properties and functions to each structure. The cortical bone is a compact structure with porosity ranging from 5 to 30%. For example, adult human femoral cortical bone porosity can vary from as low as 5% at the age of 20 and up to 30% above the age of 80 [21; 28; 83]. The trabecular bone has a spongier, lightweight, and honeycomb structure with porosity values of approximately 70% with higher values in some cases, such as the elderly spine, where 95% porosity is reached [25; 84; 85; 86; 87]. In the tibia, the diaphysis is the central portion of the bone and is characterized by the lowest width dimension. It is responsible for the load transmission between the two extremities (epiphysis and metaphysis at both ends) and has stiff cortical bone walls [88; 89]. The tibia epiphysis is characterized by bulk trabecular bone (containing vessels, nerves, and bone marrow) coated with a thin layer of cortical tissue [90; 91]. Cortical bone carries a considerable share of the total skeleton load. As demonstrated in a study developed by [PERSON] et al. [92], there was a negligible variation in axial and torsional stiffness values prediction by the finite element model if only the cortical bone was considered in the simulation. This was justified by the considerable difference between the Young's modulus of the cortical (10-40 GPa) [93; 94; 95; 96; 97; 98] and the trabecular bone (0.05-0.5 GPa) [93]. This demonstrated that it is the cortical rather than the spongy bone that is mainly responsible for most of the weight support. In the epiphyseal and metaphyseal regions, where cortical bone thickness is reduced, weight-bearing is achieved by stress reduction with the increase in surface area and load diversion. Stress varies inversely with the loaded cross-sectional area, which indicates that stress on the joint will be reduced with the increase in the surface area. The spongy nature of the ends of the tibia segment confers weight-bearing distribution in local trabeculae, hence it dissipates loads and absorbs energy. As the weight-bearing is transferred from the metaphysis to the diaphysis, it becomes more dependent on the cortical layer, which, in turn, becomes thicker and more resistant [4; 99]. It is important to highlight that the expansion of the bone ends is limited by the reduction in the joint friction to a minimum value (e.g., reduced weight of the bone ends will reduce the normal reaction force, while the reduced length of the radius of curvature will reduce the magnitude of the moment of friction). Considering the interesting case of the tibia as a load-bearing bone example, it articulates proximally at the knee joint and distally at the ankle, being the weight-bearing bone of the leg [100]. The body weight-bearing load on the tibia plateau is assumed to be divided into a central concentric and a medial and lateral eccentric knee contact force (split around 60 and 40% on the medial and lateral condyles, respectively). The higher loads on the medial side relative to the lateral side (see Table 1) is consistent with the fact that the medial side of the tibia has a larger condyle, and the bone is also denser and stronger than that of the lateral side. Hence, there is a balance in the bending stress generated between the condyles. According to [PERSON] et al. [101], by studying the distribution of cancellous bone mechanical properties in the proximal tibia, they determined that average apparent modulus and strength were 1.7 and 3 times higher in the axial direction compared to the transverse directions, respectively. They also verified that in the dominant axial direction, the bone was 1.3 times stiffer in the medial condyle than the lateral condyle.