Editing Trabecula

{{Short description|Tissue element that supports or anchors a framework of parts within a body or organ}}
{{Infobox anatomy
| Name        = Trabecula
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| Image       = Coxa-valga-norma-vara-000.svg
| Caption     = Alternation of trabecular pattern in the thigh bone reflects mechanical stress
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| Latin       =
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| PartOf = [[Bone]]
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{{more citations needed|date=May 2017}}
[[File:Bone-inside.jpg|right|thumb|200x200px|Inside of a bone showing the trabecular structure]]
[[File:Typical carcinoid tumor of the lung, trabecular pattern.jpg|thumb|A typical [[carcinoid tumor]] of the [[lung]] showing a [[histopathologic architecture|trabecular pattern]] of elongated groups of cells.]]
A '''trabecula''' ({{plural form}}: '''trabeculae''', from Latin for 'small beam') is a small, often microscopic, [[biological tissue|tissue]] element in the form of a small [[Beam (structure)|beam]], [[strut]] or rod that supports or anchors a framework of parts within a body or organ.<ref>{{Cite web|url=https://www.merriam-webster.com/dictionary/trabecula|title=Definition of TRABECULA|website=www.merriam-webster.com|language=en|access-date=2017-09-24}}</ref><ref>{{Cite journal|title=trabecula|url=http://medical-dictionary.thefreedictionary.com/trabecula|journal=The Free Dictionary}}</ref> A trabecula generally has a mechanical function, and is usually composed of dense [[collagen]]ous tissue (such as the [[trabeculae of spleen|trabecula]] of the [[spleen]]).  It can be composed of other material such as muscle and bone. In the [[heart]], [[muscle]]s form [[trabeculae carneae]] and [[septomarginal trabecula|septomarginal trabeculae]],<ref>{{Cite journal|last1=Goo|first1=Soyeon|last2=Joshi|first2=Purva|last3=Sands|first3=Greg|last4=Gerneke|first4=Dane|last5=Taberner|first5=Andrew|last6=Dollie|first6=Qaasim|last7=LeGrice|first7=Ian|last8=Loiselle|first8=Denis|date=October 2009|title=Trabeculae carneae as models of the ventricular walls: implications for the delivery of oxygen|journal=The Journal of General Physiology|volume=134|issue=4|pages=339–350|doi=10.1085/jgp.200910276|issn=0022-1295|pmc=2757768|pmid=19752188|url=https://researchspace.auckland.ac.nz/bitstream/2292/7825/2/Trabeculae%20carneae%20as%20models%20of%20the%20ventricular%20walls-%20implications%20for%20the%20delivery%20of%20oxygen.pdf}}</ref> and the [[left atrial appendage]] has a tubular trabeculated structure.<ref name="pmid26405642">{{cite journal | vauthors=Srivastava MC, See VY, Price MJ | title=A review of the LARIAT device: insights from the cumulative clinical experience | journal=[[SpringerPlus]] | volume=4 | pages=522 | year=2015 |  doi = 10.1186/s40064-015-1289-8 | doi-access=free | pmc=4574041 | pmid=26405642 }}</ref> 

[[Bone#Trabeculae|Cancellous bone]] is formed from groupings of trabeculated bone tissue. In cross section, trabeculae of a [[Bone#Trabeculae|cancellous bone]] can look like [[septum|septa]], but in three dimensions they are topologically distinct, with trabeculae being roughly rod or pillar-shaped and septa being sheet-like.

When crossing fluid-filled spaces, trabeculae may offer the function of resisting tension (as in the [[penis]], see for example [[trabeculae of corpora cavernosa of penis|trabeculae of corpora cavernosa]] and [[trabeculae of corpus spongiosum of penis|trabeculae of corpus spongiosum]]) or providing a cell filter (as in the [[trabecular meshwork of the eye]]).

==Bone trabecula==
===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.

===Clinical significance===
[[File:Spongy bone - Trabecular bone - Normal trabecular bone Trabecular bone etc -- Smart-Servier (cropped).jpg|thumb|Normal and pathological trabecular bone structures]]
Studies have shown that once a human reaches adulthood, bone density steadily decreases with age, to which loss of trabecular bone mass is a partial contributor.<ref name="Characterisation of Trabecular Bone Structure">{{cite book|last1=Parkinson|first1=Ian H.|last2=Fazzalari|first2=Nicola L.|title=Characterisation of Trabecular Bone Structure|date=12 January 2012|publisher=Springer-Verlag Berlin Heidelberg|location=Adelaide, SA, Australia|isbn=9783642180521|pages=31–51|url=https://www.springer.com/978-3-642-18052-1|access-date=31 March 2017|ref=4}}</ref> Loss of bone mass is defined by the World Health Organization as [[osteopenia]] if [[Bone density|bone mineral density (BMD)]] is one standard deviation below mean BMD in young adults, and is defined as [[osteoporosis]] if it is more than 2.5 standard deviations below the mean.<ref name="4BoneHealth">{{cite web|title=World Health Organization – WHO Criteria for Diagnosis of Osteoporosis|url=http://www.4bonehealth.org/education/world-health-organization-criteria-diagnosis-osteoporosis/|website=4BoneHealth|access-date=31 March 2017|ref=5}}</ref> A low bone density greatly increases risk for [[stress fracture]], which can occur without warning.<ref>{{cite web|title=Stress Fractures of the Foot and Ankle-OrthoInfo - AAOS|url=http://orthoinfo.aaos.org/topic.cfm?topic=a00379|website=orthoinfo.aaos.org|access-date=31 March 2017|ref=6}}</ref> The resulting low-impact fractures from osteoporosis most commonly occur in the [[Upper extremity of femur|upper femur]], which consists of 25-50% trabecular bone depending on the region, in the [[vertebra|vertebrae]], which are about 90% trabecular, or in the [[Carpal bones|wrist]].<ref name="UPenn">{{cite web|last1=Wehrli|first1=Felix W.|title=Role of Cortical and Trabecular Bone Architecture in Osteoporosis|url=http://cds.ismrm.org/protected/09MProceedings/files/Fri%20A22_07%20Wehrli.pdf|publisher=University of Pennsylvania School of Medicine|access-date=31 March 2017|ref=7}}</ref>

When trabecular bone volume decreases, its original plate-and-rod structure is disturbed; plate-like structures are converted to rod-like structures and pre-existing rod-like structures thin until they disconnect and resorb into the body.<ref name="UPenn" /> Changes in trabecular bone are typically gender-specific, with the most notable differences in bone mass and trabecular microstructure occurring within the age range for menopause.<ref name="Characterisation of Trabecular Bone Structure" /> Trabeculae degradation over time causes a decrease in bone strength that is disproportionately large in comparison to volume of trabecular bone loss, leaving the remaining bone vulnerable to fracture.<ref name="UPenn" />

With osteoporosis there are often also symptoms of [[osteoarthritis]], which occurs when [[cartilage]] in joints is under excessive stress and degrades over time, causing stiffness, pain, and loss of movement.<ref name="Osteoarthritis">{{cite journal|last1=Haq|first1=I.|last2=Murphy|first2=E.|last3=Dacre|first3=J.|title=Osteoarthritis|journal=Postgraduate Medical Journal|date=1 July 2003|volume=79|issue=933|pages=377–383|doi=10.1136/pmj.79.933.377|pmid=12897215|url= |ref=8|language=en|issn=0032-5473|pmc=1742743}}</ref> With osteoarthritis, the underlying bone plays a significant role in cartilage degradation. Thus any trabecular degradation can significantly affect stress distribution and adversely affect the cartilage in question.<ref name="MIT">{{cite web|last1=Lorna|first1=Gibson|title=Lecture 11: Trabecular Bone and Osteoporosis {{!}} Video Lectures {{!}} Cellular Solids: Structure, Properties and Applications {{!}} Materials Science and Engineering {{!}} MIT OpenCourseWare|url=https://ocw.mit.edu/courses/materials-science-and-engineering/3-054-cellular-solids-structure-properties-and-applications-spring-2015/video-lectures/Lecture11-Trabecular-Bone-and-Osteoporosis/|website=ocw.mit.edu|publisher=Massachusetts Institute of Technology|access-date=31 March 2017|ref=9|language=en}}</ref>

Due to its strong effect on overall bone strength,<ref>{{cite journal |first1=Elham |last1=Taghizadeh |first2=Vimal |last2=Chandran |first3=Mauricio |last3=Reyes |first4=Philippe |last4=Zysset |first5=Philippe |last5=Büchler |title=Statistical analysis of the inter-individual variations of the bone shape, volume fraction and fabric and their correlations in the proximal femur |year=2017 |journal=Bone |volume=103 |pages=252–261 |doi=10.1016/j.bone.2017.07.012 |pmid=28732775|url=https://boris.unibe.ch/102296/ }}</ref> there is currently strong speculation that analysis in patterns of trabeculae degradation may be useful in the near future in tracking the progression of osteoporosis.<ref name="Radiopaedia">{{cite web|last1=Shetty|first1=Aditya|title=Trabecular pattern of proximal femur {{!}} Radiology Reference Article {{!}} Radiopaedia.org|url=https://radiopaedia.org/articles/trabecular-pattern-of-proximal-femur|website=radiopaedia.org|access-date=31 March 2017|ref=10|language=en}}</ref>

===Birds===
The hollow design of bird bones is multifunctional. It establishes high [[specific strength]] and supplements open airways to accommodate the [[skeletal pneumaticity]] common to many birds. The [[specific strength]] and resistance to [[buckling]] is optimized through a bone design that combines a thin, hard shell that encases a spongy core of trabeculae.<ref name="Marc Meyers 2014, p. 504-506">{{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 |pages=504–506}}</ref> The [[allometry]] of the trabeculae allows the skeleton to tolerate loads without significantly increasing the bone mass.<ref name="ReferenceA">{{cite journal |first1=Michael |last1=Doube |first2=Michał M. |last2=Kłosowski |first3=Alexis M. |last3=Wiktorowicz-Conroy |first4=John R. |last4=Hutchinson |first5=Sandra J. |last5=Shefelbine |display-authors=1 |title=Trabecular bone scales allometrically in mammals and birds |year=2011 |journal=Proceedings of the Royal Society |volume=278 |issue=1721 |pages=3067–3073 |doi=10.1098/rspb.2011.0069 |pmid=21389033 |pmc=3158937 }}</ref> The [[red-tailed hawk]] optimizes its weight with a repeating pattern of V-shaped struts that give the bones the necessary lightweight and stiff characteristics. The inner network of trabeculae shifts mass away from the [[neutral axis]], which ultimately increases the resistance to [[buckling]].<ref name="Marc Meyers 2014, p. 504-506"/>

As in humans, the distribution of trabeculae in bird species is uneven and is dependent on load conditions. The bird with the highest [[density]] of trabeculae is the [[Kiwi (bird)|kiwi]], a flightless bird.<ref name="ReferenceA"/> There is also uneven distribution of trabeculae within similar species such as the [[great spotted woodpecker]] or [[grey-headed woodpecker]]. After examining a micro[[CT scan]] of the woodpecker's forehead, temporomandibulum, and occiput it was determined that there is significantly more trabeculae in the forehead and occiput.<ref name="ReferenceB">{{cite journal |first1=Lizheng |last1=Wang |first2=Xufeng |last2=Niu |first3=Yikun |last3=Ni |first4=Peng |last4=Xu |display-authors=1 |title=Effect of Microstructure of Spongy Bone in Different Parts of Woodpecker's Skull on Resistance to Impact Injury |year=2013 |journal=Journal of Nanomaterials |volume=2013 |pages=1–6 |doi=10.1155/2013/924564 |doi-access=free |hdl=10397/31085 |hdl-access=free }}</ref> Besides the difference in distribution, the [[aspect ratio]] of the individual struts was higher in woodpeckers than in other birds of similar size such as the [[Eurasian hoopoe]]<ref name="ReferenceB"/> or the [[lark]].<ref name="ReferenceC">{{cite journal |first1=L. |last1=Wang |last2=Zhang |first2=H. |last3=Fan |first3=Y. |title=Comparative study of the mechanical properties, micro-structure, and composition of the cranial and beak bones of the great spotted woodpecker and the lark bird |journal=Science China Life Sciences |year=2011 |volume=54 |issue=11 |pages=1036–1041 |doi=10.1007/s11427-011-4242-2 |pmid=22173310 |doi-access=free }}</ref> The woodpeckers' trabeculae are more plate-like while the hawk and lark have rod-like structures networked through their bones. The decrease in strain on the woodpecker's brain has been attributed to the higher quantity of thicker plate-like struts packed more closely together than the hawk or hoopoe or the lark.<ref name="ReferenceC"/> Conversely, the thinner rod-like structures would lead to greater deformation. A destructive mechanical test with 12 samples show the woodpecker's trabeculae design has an average ultimate strength of 6.38MPa, compared to the lark's 0.55MPa.<ref name="ReferenceB"/>

Woodpeckers' beaks have tiny struts supporting the shell of the beak, but to a lesser extent compared to the skull. As a result of fewer trabeculae in the beak, the beak has a higher stiffness (1.0 GPa) compared to the skull (0.31 GPa). While the beak absorbs some of the impact from pecking, most of the impact is transferred to the skull where more trabeculae are actively available to absorb the shock. The ultimate strength of woodpeckers' and larks' beaks are similar, inferring the beak has a lesser role in impact absorption.<ref name="ReferenceC"/> One measured advantage of the woodpecker's beak is the slight overbite (upper beak is 1.6mm longer than lower beak) which offers a [[bimodal distribution]] of force due to the asymmetric surface contact. The staggered timing of impact induces a lower strain on the trabeculae in the forehead, occiput, and beak.<ref name="Why Do Woodpecker's Resist Head Impact Energy">{{cite journal |first1=Lizheng |last1=Wang |first2=Jason Tak-Man |last2=Cheung |first3=Fang |last3=Pu |first4=Deyu |last4=Li |first5=Ming |last5=Zhang |first6=Yubo |last6=Fan |doi=10.1371/journal.pone.0026490 |pmid=22046293 |pmc=3202538 |journal=[[PLOS One]] |volume=6 |issue=10 |pages=e26490 |title=Why Do Woodpecker's Resist Head Impact Energy: A Biomechanical Investigation |year=2011 |doi-access=free }}</ref>

=== Trabecula in other organisms ===
{{unreferenced section|date=March 2018}}
The larger the animal, the higher the load forces on its bones. Trabecular bone increases stiffness by increasing the amount of bone per unit volume or by altering the geometry and arrangement of individual trabeculae as body size and bone loading increases. Trabecular bone scales [[Allometry|allometrically]], reorganizing the bones' internal structure to increase the ability of the [[skeleton]] to sustain loads experienced by the trabeculae.  Furthermore, scaling of trabecular geometry can moderate trabecular strain. Load acts as a [[Stimulus (physiology)|stimulus]] to the trabecular, changing its geometry so as to sustain or mitigate strain loads. By using finite element modelling, a study tested four different species under an equal apparent stress (σapp) to show that trabecular scaling in animals alters the strain within the trabecular. It was observed that the strain within trabeculae from each species varied with the geometry of the trabeculae. From a scale of tens of micrometers, which is approximately the size of [[osteocyte]]s, the figure below shows that thicker trabeculae exhibited less strain. The relative frequency distributions of element strain experienced by each species shows a higher [[Elastic modulus|elastic moduli]] of the trabeculae as the species size increases.

Additionally, trabeculae in larger animals are thicker, further apart, and less densely connected than those in smaller animals. Intra-trabecular [[osteon]] can commonly be found in thick trabeculae of larger animals, as well as thinner trabeculae of smaller animals such as [[cheetah]] and [[lemur]]s. The osteons play a role in the diffusion of nutrients and waste products of osteocytes by regulating the distance between osteocytes and bone surface to approximately 230 μm.

Due to an increased reduction of blood oxygen saturation, animals with high metabolic demands tend to have a lower trabecular thickness (Tb.Th) because they require increased vascular [[perfusion]] of trabeculae. The [[vascularization]] by tunneling osteons changes the trabecular geometry from solid to tube-like, increasing bending stiffness for individual trabeculae and sustaining blood supply to deep tissue osteocytes.

Bone volume fraction (BV/TV) was found to be relatively constant for the variety of animal sizes tested. Larger animals did not show a significantly larger mass per unit volume of trabecular bone. This may be due to an [[adaptation]] which reduces the physiological cost of producing, maintaining, and moving tissue. However, BV/TV showed significant positive scaling in avian [[Condyle (anatomy)|femoral condyle]]s. Larger birds present decreased flight habits due to avian BV/TV allometry. The flightless kiwi, weighing only 1–2&nbsp;kg, had the greatest BV/TV of the birds tested in the study. This shows that trabecular bone geometry is related to ‘prevailing mechanical conditions’, so the differences in trabecular geometry in the femoral head and condyle could be attributed to different loading environments of [[Hip joint|coxofemoral]] and [[Knee|femorotibial joints]].

The [[woodpecker]]'s ability to resist repetitive head impact is correlated with its unique micro/nano-hierarchical [[Composite material|composite]] structures.<ref name="ReferenceC"/> [[Microstructure]] and [[nanostructure]] of the woodpecker's [[skull]] consists of an uneven distribution of [[Bone#Trabeculae|spongy bone]], the organizational shape of individual trabeculae. This affects the woodpecker's mechanical properties, allowing the [[Skull|cranial bone]] to withstand a high [[ultimate strength]] (σu). Compared to the cranial bone of the [[lark]], the woodpecker's cranial bone is denser and less spongy, having a more plate-like structure rather than the more rod-like structure observed in larks. Furthermore, the woodpecker's cranial bone is thicker and more individual trabeculae. Relative to the trabeculae in lark, the woodpecker's trabecular is more closely spaced and more plate-like. [19] These properties result in higher ultimate strength in the cranial bone of the woodpecker.

==History==
The diminutive form of Latin ''trabs'', means a beam or bar. In the 19th century, the neologism ''trabeculum'' (with an assumed plural of ''trabecula'') became popular, but is less etymologically correct. ''Trabeculum'' persists in some countries as a synonym for the [[trabecular meshwork]] of the [[eye]], but this can be considered poor usage on the grounds of both etymology and descriptive accuracy.

==Other uses==
For the skull development component, see [[trabecular cartilage]].

==See also==
[[Arachnoid trabeculae]]

== References ==
{{Reflist}}
{{Wiktionary}}

{{Bone and cartilage}}

[[Category:Tissues (biology)]]