**4. Biomechanics of bone tissue**

The fracture behavior of bone is influenced by its viscoelastic, anisotropic, and heterogeneous mechanical properties. The stress-strain behavior of bone is dependent on the rate of loading, which is characteristic of a viscoelastic material [3, 14]. If the bone is loaded at a high rate, such as occurs with vehicular trauma or gunshot injury,

#### **Figure 10.**

*Illustration of the shear, tensile, and compressive stresses and strains at supraphysiological loads causing a spiral fracture pattern.*

its stiffness (Young's modulus), ultimate strain, and energy-to-failure increase. The clinical significance of the high toughness of healthy bone is that if a high-rate loading causes macroscopic failure or fracture, as opposed to just distributed microscopic interfacial failures, the large release of the absorbed energy will cause marked comminution and injury to surrounding soft tissues [15]. Bone is considered a material with anisotropic properties; as a consequence, the values of strength and stiffness are a function of the direction of applied loads regarding bone structure (**Figure 11**) [3, 14].

## **4.1 Cortical vs. cancellous bone material properties**

All bones are composed of a combination of cortical (compact) and cancellous (trabecular) bone. Both cortical and cancellous bones are formed from an inorganic mineralized matrix called hydroxyapatite, which is primarily calcium and phosphate. Hydroxyapatite (HA) is a naturally occurring calcium phosphate mineral characterized by the chemical formula Ca10(PO4)6(OH)2. HA-like compounds compose approximately 60–65% of bone's dry weight [16]. The inorganic matrix is combined with an organic nonmineralized matrix (35–40% of bone's dry weight) [16]. By contrast, the organic extracellular is significantly more complex and consists mainly of *Biomechanical Basis of Bone Fracture and Fracture Osteosynthesis in Small Animals DOI: http://dx.doi.org/10.5772/intechopen.112777*

#### **Figure 11.**

*Stress-strain curve depicting the anisotropic behavior of bone. Load forces of tension were applied in two different orientations: parallel and perpendicular to the longitudinal axis.*

collagen type I (90%) and noncollagenous proteins (10%) such as glycosaminoglycans, water, and cellular elements [16]. The inorganic matrix imparts strength and rigidity to the bone, and the organic matrix gives it flexibility and resiliency [1].

The cortical bone always surrounds the cancellous bone; however, the relative quantity of each type varies from one bone to another as well as according to the specific location within a particular bone (diaphysis vs. metaphysis or epiphysis); cortical bone is designed to give strength and stiffness to the bone [3]. From a mechanical standpoint, cancellous bone is designed to absorb a tremendous amount of energy and transmit load [1].

Both cortical and cancellous bones have inorganic and organic components; however, one of the primary differences between both bone types is the different percentages of organic versus inorganic matrix of each type. Structurally, this difference influences the porosity and apparent density and consequently the mechanical behavior of each type of bone when submitted to loads.

Porosity is defined as the volume of bone occupied by nonmineralized tissue. Cortical bone is composed primarily of inorganic mineralized matrix and therefore has low porosity. The porosity of cortical bone has been estimated to vary from 5% to 30% and in the cancellous bone, it can vary from as little as 30% to as much as 90% [17].

Apparent density is a measurement related to porosity and is directly related to its inorganic mineral content, being the mass of the bone tissue divided by the bulk unit volume of bone tissue, including mineralized bone and marrow space [17]. Cortical bone typically has a higher apparent density than cancellous bone tissue [17].

The differences in porosity, or apparent density, between cancellous and cortical bone dramatically affect their behavior when the two types of bone are submitted to loads (**Figure 12**). Cancellous bone initially exhibits elastic behavior followed by a yield, which occurs as bone trabeculae begin to fracture. After the yield point, a long plateau of plastic deformation occurs as a result of progressive fracture and collapse of additional trabecular bone and marrow spaces (**Figure 12**). Once the entire marrow

**Figure 12.** *Different stress/strain curve profiles for cortical and cancellous bone.*

space has filled with debris from fractured bone trabeculae, which is referred to as pore closure, there is a marked increase in stiffness before the ultimate failure point of cancellous bone is reached. Under compression loading, cancellous bone exhibits a stress-strain behavior similar to that of soft porous metal. When compression loading is applied, cancellous bone can absorb a large amount of energy (when compared to cortical bone) and can tolerate strain values up to 7% before structural failure.

On the contrary, cortical bone, due to its low porosity, presents a brittle behavior when subject to compressive loads, similar to glass. Cortical bone is characterized by a decreased plastic deformation phase before failure, absorbs less energy, and tolerates lower strain values (<2%) before fracture as compared with cancellous bone (**Figure 11**). However, cortical bone has greater ultimate strength and increased stiffness and can tolerate more force loads before fracture than cancellous bone.

The clinical implications of the relationship between bone's apparent density and its mechanical behavior are evident when large changes in the strength and modulus of bone can result from small changes in its apparent density. In the clinical setting, the reduction of apparent density is evident on radiographs only when lost by 30– 50%, and consequently, the reduction in bone density detected on radiographs is associated with greatly reduced stiffness and strength [1]. Conversely, greatly enhanced fracture zone stiffness and strength may be present even with minor increases in fracture zone density observed in radiographs.
