Editing Trabecula (section)

===Structure===
Trabecular bone, also called [[Bone#Trabeculae|cancellous bone]], is porous bone composed of trabeculated bone tissue. It can be found at the ends of long bones like the femur, where the bone is actually not solid but is full of holes connected by thin rods and plates of bone tissue.<ref name="Trabeculae of Bone: Definition & Function">{{cite web|title=Trabeculae of Bone: Definition & Function|url=http://study.com/academy/lesson/trabeculae-of-bone-definition-function.html|website=Study.com|access-date=31 March 2017|ref=1}}</ref> The holes (the volume not directly occupied by bone trabecula) is the ''intertrabecular space'', and is occupied by red [[bone marrow]], where all the blood cells are made, as well as fibrous tissue. Even though trabecular bone contains a lot of intertrabecular space, its spatial complexity contributes the maximal strength with minimum mass. It is noted that the form and structure of trabecular bone are organized to optimally resist loads imposed by functional activities, like jumping, running and squatting. And according to [[Wolff's law]], proposed in 1892, the external shape and internal architecture of bone are determined by external stresses acting on it.<ref name="“Mechanical properties of cortical and trabecular bone”">{{cite book|last1=Hayes|first1=Wilson C.|last2=Keaveny|first2=Tony M.|title=Bone: A Treatise|date=1993|publisher=CRC Press|isbn=978-0849388279|pages=285–344|edition=7|url=https://www.researchgate.net/publication/272152350|access-date=31 March 2017|ref=2}}</ref> The internal structure of the trabecular bone firstly undergoes adaptive changes along stress direction and then the external shape of [[Bone#Cortical bone|cortical bone]] undergoes secondary changes. Finally bone structure becomes thicker and denser to resist external loading.

Because of the increased occurrence of total joint replacement and its impact on bone remodeling, understanding the stress-related and adaptive process of trabecular bone has become a central concern for bone physiologists. To understand the role of trabecular bone in age-related bone structure and in the design for bone-implant systems, it is important to study the mechanical properties of trabecular bone as a function of variables such as anatomic site, bone density, and age related issues. Mechanical factors including modulus, uniaxial strength, and fatigue properties must be taken into account.

Typically, the porosity percent of trabecular bone is in the range 75–95% and the density ranges from 0.2 to 0.8 g/cm<sup>3</sup>.<ref name="Biological Materials Science">{{cite book|last1=Meyers|first1=M. A.|last2=Chen|first2=P.-Y.|title=Biological Materials Science|date=2014|publisher=Cambridge University Press|location=Cambridge |isbn=978-1-107-01045-1 |ref=3}}</ref> It is noted that the porosity can reduce the strength of the bone, but also reduce its weight. The porosity and the manner that porosity is structured affect the strength of material. Thus, the micro structure of trabecular bone is typically oriented and <nowiki>''</nowiki>grain<nowiki>''</nowiki> of porosity is aligned in a direction at which mechanical stiffness and strength are greatest. Because of the microstructural directionality, the mechanical properties of trabecular bone are highly anisotropic. The range of [[Young's modulus]] for trabecular bone is 800 to 14,000 MPa and the strength of failure is 1 to 100 MPa.

As mentioned above, the mechanical properties of trabecular bone are very sensitive to apparent density. The relationship between modulus of trabecular bone and its apparent density was demonstrated by Carter and Hayes in 1976.<ref>{{Cite journal|last1=Carter|first1=D. R.|last2=Hayes|first2=W. C.|date=1976-12-10|title=Bone compressive strength: the influence of density and strain rate|journal=Science|volume=194|issue=4270|pages=1174–1176|issn=0036-8075|pmid=996549|doi=10.1126/science.996549|bibcode=1976Sci...194.1174C}}</ref> The resulting equation states:

<big><math> E = a + b\cdot\rho^c </math></big>

where <math>E</math> represents the modulus of trabecular bone in any loading direction, <math>\rho</math> represents the apparent density, and <math>a,</math> <math>b,</math> and <math>c</math> are constants depending on the architecture of tissue.

Using scanning electron microscopy, it was found that the variation in trabecular architecture with different anatomic sites lead to different modulus. To understand structure-anisotropy and material property relations, one must correlate the measured mechanical properties of anisotropic trabecular specimens with the stereological descriptions of their architecture.<ref name="“Mechanical properties of cortical and trabecular bone”" />

The compressive strength of trabecular bone is also very important because it is believed that the inside failure of trabecular bone arise from compressive stress. On the stress-strain curves for both trabecular bone and cortical bone with different apparent density, there are three stages in stress-strain curve. The first is the linear region where individual trabecula bend and compress as the bulk tissue is compressed.<ref name="“Mechanical properties of cortical and trabecular bone”" /> The second stage occurs after yielding, where trabecular bonds start to fracture, and the final stage is the stiffening stage. Typically, lower density trabecular areas offer more deformed staging before stiffening than higher density specimens.<ref name="“Mechanical properties of cortical and trabecular bone”" />

In summary, trabecular bone is very compliant and heterogeneous. The heterogeneous character makes it difficult to summarize the general mechanical properties for trabecular bone. High porosity makes trabecular bone compliant and large variations in architecture leads to high heterogeneity. The modulus and strength vary inversely with porosity and are highly dependent on the porosity structure. The effects of aging and small cracking of trabecular bone on its mechanical properties are a source of further study.