lecture bone

8/3/2019 Lecture Bone

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BIOE 4710/5710 Bone Tissue

Function, physiology and composition ofbone tissue cortical

trabecular

Biomechanics of bone tissue mechanical properties

viscoelasticity

Textbook: Skeletal Tissue Mechanics, (Martin RB et al.)


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Bone: Structural Hierarchy


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Bone: Composition

collagen + water + mineral +proteoglycans + noncollagenousproteins

mineral: bioapatite

Ca10 (PO4)6-x (OH)2-y(CO3)x+y 6u x u0 and 2u y u0

substitutions include HPO4, CO3, Mg, Fl

rod or plate shaped (5x5x40 nm)

proteoglycans decorin

biglycan


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Bone: Composition


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Bone: Composition


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Bone: Composition

proteoglycans may control mineralization

decorin

collagen fibrillogenesis

protein core-GAG

biglycan

interaction with collagen ?

noncollagenous proteins osteocalcin, osteonectin, osteopontin

osteocalcin abundant

chemoattractant for bone cells

suppresses excess mineralization


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Bone: Trabecular vs. Cortical


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Bone: Trabecular Bone

Trabecular bone(a.k.a. cancellousor spongy bone)

found in cuboidalbones, flat bonesand the ends oflong bones

range of porosity75%-95%

interconnectedpores

filled with marrow


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Bone: Trabecular Bone

Trabecular bone (cont:)

formed by organization of plate- and rod-likestruts called trabeculae

trabeculae are about 200 Qm thick.


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Bone: Cortical Bone

Cortical bone (a.k.a. compact bone) shafts of long bones shell around cuboidal bones

porosity 5-10% Haversian canal

aligned with the long axis of bone contains capillaries and nerves 50 Qm in diameter

Volkmanns canal transverse canals connecting Haversian canals contains blood vessels

Resorption cavities temporary spaces created by osteoclasts 200 Qm in diameter


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Bone: Cortical Bone


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Bone: Cortical Bone

Cortical bone (cont) types of cortical bone

lamellar parallel layers of lamellae mineralized collagen fibers are parallel within each

lamella direction of fibers may alternate between adjacent

lamellae

woven bone quickly formed

poorly organized, fibers are more or less randomlyarranged more mineralized than lamellar weaker than mineralized


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Bone: Primary and secondary

primary bone: laiddown on existingbone surface

circumferentiallamellar

lamellae areparallel to bonesurface

primary osteonsaround bloodvessels


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primary bone:(cont)

plexiform

construction of atrabecularnetwork followedby filling in thegaps

mixture of wovenand lamellarbone

large and fastgrowing animals

(cows)


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secondary bone: resultsfrom resorption andreplacement of existingbone with lamellar bone

(remodeling) cortical bone: secondary

tissue consists ofcylindrical structurescalled secondaryosteons or Haversian

systems 200 Qm in diameter

16 concentriccylindrical lamellae

outer boundarycement line


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secondary bone:

(cont) trabecular bone:

remodelingproducestrenches on theexisting surfaces

filling of these

trenches createtrabecularpackets


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Bone: Modeling and remodeling

modeling customized the

shape of bones inaccordance with

mechanical needs metaphyseal

modeling to reducebone diameterduring growth

diaphysealmodeling toincrease bonediameter addition of bone

on theperiosteum

resorption ofbone at

endosteum


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modeling (cont) customized the shape of bones in accordance

with mechanical needs diaphyseal modeling to alter curvature

cross section drifts sideways relative to the endsof the bone

modeling of flat bones resorption on the inner surface and formation on

the outer surface of cranial bone to accommodatethe growth in size of brain


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remodeling removes older bone and replaces with new

bone prevents accumulation of fatigue damage

draws calcium from bones to be usedmetabolically elsewhere fine tunes mechanical properties accomplished by teams of about 10 osteoclasts

and several hundred osteoblasts that worktogether in basic multicellular units (BMUs)


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remodeling (cont.) three stages in BMUs lifetime (ARF)

Activation Resorption Formation

resorption in the form of a tunnel orditch about 200 Qm in diameter at arate of 40 Qm/day

mesenchymal cells differentiate intoosteoblasts


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remodeling (cont.) osteoblasts fill the tunnel with osteoid

tissue at a rate of 0.5 Qm/day

resorption lasts for3

weeks remodeling sequence lasts for 4months

BMUs replace 5% of cortical bone and25% of trabecular bone each year


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modeling-remodeling: differences action of osteoclasts and osteblasts are

independent in modeling and coupledin remodeling

modeling results in change of bonessize, shape or both whereas remodelingdoes not effect size or shape usually

rate of modeling reduced aftermaturation, remodeling occurs

throughout life modeling is continuous and prolonged

whereas remodeling is episodic


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Bone: Strength of cortical bone


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determinants of osteonal bonemechanical properties porosity

holes weaken structures

voids in bone range from a few to severalhundred micrometers

Schaffler and Burr (1988) (up to 31%porosity) E = 33.9 (1-p)10.9, p:porosity, E:modulus

Currey (1988) (up to 7.8% porosity) E = 23.4 (1-p)5.74


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determinants of osteonal bonemechanical properties (cont) mineralization

amount of mineral per volume of bonematrix (specific mineralization)

amount of mineral per unit volume ofwhole bone (volumetric mineralization,affected by porosity)

Schaffler and Burr (1988) E = 89.1 A3.91

A: percent ash by mass


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determinants of osteonal bone mechanicalproperties (cont) density

apparent density: mass per unit bulk volume

(function of porosity and mineralization) Carter and Hayes (1977)

: strain rate, E: modulus

d: density

apparent density of cortical bone 1.8-2.0 g/cm3

histologic architecture osteonal density

amount of primary lamellar bone

collagen fiber organization

I

306.03790 dE I!


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determinants of osteonal bone mechanicalproperties (cont)

fatigue damage

rate of deformation

osteoid tissue

fluid flow within interconnected spaces

cement lines

energy absorption capacity optimized in therange of 0.01-0.1 s-1


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Bone: Strength of Cancellous Bone


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determinants of cancellous bonemechanical properties

apparent density apparent density of trabecular bone 1.0-

1.4 g/cm3

the relationship given by Carter andHayes (1977) applies to trabecular bone

trabecular density

mean trabecular thickness


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determinants ofcancellous bonemechanicalproperties trabecular

orientation (meanintercept length)


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Bone: Viscoelastic models

Sedlin (1965)

three-parameter solid

a frictional element to account for plastic

deformation

Bargren et al. (1974)

Kelvin is good enough for physiological rates

Laird and Kingsbury (1973)

three-parameter solid cannot model thedependency on frequency


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Bone: Fatigue properties

lecture bone

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