Bone Structure and Mechanics BME 615 Fall 2006 Note: Some of the figures in this presentation are from the internet and are for educational purposes only.

Bone Structure and MechanicsBone Structure and Mechanics

BME 615BME 615

Fall 2006Fall 2006

Note: Some of the figures in this presentation are from the internet and are for educational purposes only

Bone FunctionsBone Functions

Bone Functions:

• Support• Protection• Movement• Mineral Storage• Blood Cell

Formation

Bone ClassificationBone Classification

Classification:• Axial• Appendicular

Shapes:• Long• Short• Flat• Irregular

Long Bone AnatomyLong Bone Anatomy

Short, Flat, and Irregular Short, Flat, and Irregular Bone AnatomyBone Anatomy

Bone Development:Bone Development:Intramembraneous OssificationIntramembraneous Ossification

Bone Development:Bone Development:Endochondral OssificationEndochondral Ossification

Bone GrowthBone Growth

Bone GrowthBone Growth

• Cortical or compact boneCortical or compact bone

• Cancellous or trabecular boneCancellous or trabecular bone

• Woven or immature bone Woven or immature bone

Bone MesostructureBone Mesostructure

Bone MesostructureBone Mesostructure

• Cortical/CompactCortical/Compact– 80% of adult skeleton80% of adult skeleton– More resistant to bending and torsional forcesMore resistant to bending and torsional forces

• Trabecular/CancellousTrabecular/Cancellous– 20% adult skeleton20% adult skeleton– Concentrated where compressive forces Concentrated where compressive forces

predominatepredominate

Cancellous BoneCancellous Bone

Type of Bone (meso)Type of Bone (meso)

Microscopic Bone AnatomyMicroscopic Bone Anatomy

Bone MicrostructureBone Microstructure• Lamellar (mature)Lamellar (mature)

– normal bonenormal bone– stress-orientedstress-oriented– Organized around Organized around

Haversian systems into Haversian systems into osteonsosteons

– Cortical and cancellous Cortical and cancellous varietiesvarieties

• Woven (immature)Woven (immature)– More flexibleMore flexible– Higher turnoverHigher turnover– Not stress oriented Not stress oriented – Collagen more randomly Collagen more randomly

arrangedarranged• PathologicPathologic• Fracture repairFracture repair• Fetal boneFetal bone

Lamellar boneLamellar bone

• OsteonsOsteons parallel to parallel to long axislong axis

• Haversian CanalHaversian Canal– Arteries, venules, Arteries, venules,

lymphatics, etclymphatics, etc

• Volkmanns’ CanalsVolkmanns’ Canals– Connects haversian Connects haversian

systems to endosteal and systems to endosteal and periosteal surfacesperiosteal surfaces

Lamellar BoneLamellar Bone

• Neighboring Neighboring osteocytes osteocytes communicate via communicate via canliculicanliculi

• Limits of each osteon Limits of each osteon demarcated by cement demarcated by cement lineslines

Osteon

OsteonsOsteons

Skeletal Tissue Mechanics, Martin et al., 1998

Primary OsteonsPrimary Osteons

Skeletal Tissue Mechanics, Martin et al., 1998

Secondary OsteonsSecondary Osteons

Skeletal Tissue Mechanics, Martin et al., 1998

Osteon Cutting ConeOsteon Cutting Cone

Skeletal Tissue Mechanics, Martin et al., 1998

Osteon FibrilsOsteon Fibrils

Skeletal Tissue Mechanics, Martin et al., 1998

Woven BoneWoven Bone• Immature or pathologicImmature or pathologic

• Collagen arranged Collagen arranged irregularilyirregularily

• IsotropicIsotropicmechanical mechanical properties independent of properties independent of orientation of stresses orientation of stresses appliedapplied

• Exists in:Exists in:– Fetal skeletonFetal skeleton

– Fx callusFx callus

– Heterotopic OssificationHeterotopic Ossification

– Tooth socketsTooth sockets

– Bone forming tumorsBone forming tumors

– Stages of accel bone formationStages of accel bone formation

Compact Bone (cortical)Compact Bone (cortical)• 80% skeleton80% skeleton

• Osteons connected by Osteons connected by Haversian canalsHaversian canals

• Cement lines define outer Cement lines define outer border of osteonborder of osteon

• Nutriation via intraosseous Nutriation via intraosseous circulation.circulation.

• Slow turnoverSlow turnover

• Relatively high Young’s Relatively high Young’s modulusmodulus

• Higher resistance to Higher resistance to torsion and bending than torsion and bending than cancellous.cancellous.

Cancellous Bone (trabecular/spongy)Cancellous Bone (trabecular/spongy)

• Less denseLess dense• More remodeling More remodeling

along lines of stress along lines of stress (Wolff’s law)(Wolff’s law)

• Much larger surface Much larger surface areaarea

• Higher turnoverHigher turnover• Lower Lower apparentapparent

modulusmodulus• More elasticMore elastic• More resistance to More resistance to

compressive forcescompressive forces

Trabecular Bone and Yellow Trabecular Bone and Yellow Bone MarrowBone Marrow

Cellular PlayersCellular Players• OsteoblastOsteoblast– Bone forming cells that Bone forming cells that

line bone surfacesline bone surfaces– Derived from undiff. Derived from undiff.

Mesenchymal cellsMesenchymal cells• Differentiation Differentiation

influenced by influenced by interleukins, PDGF, interleukins, PDGF, IGFIGF

– More ER, Golgi, and More ER, Golgi, and mitochondria than other mitochondria than other cellscells

– Make Type I collagen and Make Type I collagen and osteocalcinosteocalcin

– Numerous receptors for Numerous receptors for regulatory factorsregulatory factors

• PTH, 1, 25 Vit D, PTH, 1, 25 Vit D, Glucocorts, Glucocorts, Prostoglandin E, Prostoglandin E, EstrogenEstrogen

Cellular PlayersCellular Players• Osteocytes:Osteocytes:

– 90% of cells in mature 90% of cells in mature skeletonskeleton

– Represent former Represent former osteoblasts that have been osteoblasts that have been trapped in matrixtrapped in matrix

– Maintain structural integrity Maintain structural integrity of boneof bone

– May assist in bone nutritionMay assist in bone nutrition– Serve to maintain bone and Serve to maintain bone and

help control extracellular help control extracellular concentrations of Ca and concentrations of Ca and Phos.Phos.

– Communicate via canaliculiCommunicate via canaliculi– Directly stimulated by Directly stimulated by

calcitonincalcitonin– Directly inhibited by PTHDirectly inhibited by PTH

Cellular PlayersCellular Players

• OsteoclastsOsteoclasts– Resorb boneResorb bone

– Multinucleated giant cellsMultinucleated giant cells

– Originate from monocyte Originate from monocyte progenitor cell linesprogenitor cell lines

– Ruffled border Ruffled border (osteopetrosis)(osteopetrosis)

– Resorption in Howship’s Resorption in Howship’s lacunaelacunae

– Synthesize tartrate resistant Synthesize tartrate resistant acid phosphate.acid phosphate.

– Bind to bone via integrinsBind to bone via integrins

• Osteoclasts:Osteoclasts:– Lower pH by producing Lower pH by producing

Hydrogen ionsHydrogen ions• Increases solubility of Increases solubility of

hydroxyapatite crystals.hydroxyapatite crystals.

– Possess specific receptors Possess specific receptors for calcitoninfor calcitonin

– Responsible for bone Responsible for bone resorption in metastatic resorption in metastatic disease and multiple disease and multiple myelomamyeloma

– IL-1 stimulates osteoclastsIL-1 stimulates osteoclasts

– IL-10 inhibits osteoclastsIL-10 inhibits osteoclasts

Overall Composition of BoneOverall Composition of Bone

• Two major componentsTwo major components– Organic matrixOrganic matrix

• Type I collagenType I collagen

• Amorphous ground Amorphous ground substancesubstance

– Inorganic matrixInorganic matrix• Calcium hydroxyapatitie.Calcium hydroxyapatitie.

Organic MatrixOrganic Matrix

• ~40% dry weight~40% dry weight1.1. collagen (90%)collagen (90%)

2.2. proteoglycans proteoglycans

3.3. non-collagenous matrix proteins non-collagenous matrix proteins

4.4. growth factorsgrowth factors

5.5. cytokinescytokines

CollagenCollagen

• 90% organic matrix90% organic matrix• Provides Provides tensile strengthtensile strength

to boneto bone• Primarily type IPrimarily type I• StructureStructure

– Triple helixTriple helixfibrilfibril– Hole zones b/w fibrilsHole zones b/w fibrils– Pores b/w sides of Pores b/w sides of

parallel moleculesparallel molecules• Minerals laid down in Minerals laid down in

holes and gapsholes and gaps

– X-linking decreases X-linking decreases solubility and increases solubility and increases the tensile strength.the tensile strength.

ProteoglycansProteoglycans

• Composed of Composed of glycosaminoglycansglycosaminoglycans complexescomplexes

• Inhibit mineralizationInhibit mineralization

• Numerous functions ranging from growth Numerous functions ranging from growth factors to binding propertiesfactors to binding properties

• Partially responsible for Partially responsible for compressive compressive strengthstrength of bone. of bone.

Matrix ProteinsMatrix Proteins(non-collagenous)(non-collagenous)

• Promote mineralization and bone formationPromote mineralization and bone formation– OsteocalcinOsteocalcin

• Produced by osteoblastsProduced by osteoblasts

• Directly related to regulation of bone densityDirectly related to regulation of bone density

• Most abundant non-collagen matrix proteinMost abundant non-collagen matrix protein

• Inhibited by PTHInhibited by PTH

• Activated by 1,25 Vitamin DActivated by 1,25 Vitamin D

• Can measure in urine or serum as marker of bone Can measure in urine or serum as marker of bone turnoverturnover

Matrix ProteinsMatrix Proteins(non-collagenous)(non-collagenous)

• OsteonectinOsteonectin– Secreted by platelets and osteoblastsSecreted by platelets and osteoblasts– Possible role in regulation of calcium and/or Possible role in regulation of calcium and/or

organization of mineral within matrix.organization of mineral within matrix.

• OsteopontinOsteopontin– Cell binding proteinCell binding protein

Growth Factors and CytokinesGrowth Factors and Cytokines

• Aid in differentiation, activation, Aid in differentiation, activation, growth and turnover of bonegrowth and turnover of bone– TGF-TGF-ßß– IGFIGF– IL-1IL-1– IL-6IL-6– VEGFVEGF– BMP’sBMP’s

Inorganic MatirxInorganic Matirx

• 60% dry weight60% dry weight

• Almost entirely Almost entirely Calcium HydroxyapatiteCalcium Hydroxyapatite

CaCa55(PO(PO44))33(OH) (OH)

Calcium HydroxyapatiteCalcium Hydroxyapatite

• Provides the Provides the compressive strengthcompressive strength of bone of bone

• Responsible for the mineralization of boneResponsible for the mineralization of bone

• Primary mineralization occurs in holes and Primary mineralization occurs in holes and porespores

MarrowMarrow

• Red marrowRed marrow– Active in hematopoiesisActive in hematopoiesis– Fetal skeleton Fetal skeleton – As adult only found in flatbones and vertebrae, As adult only found in flatbones and vertebrae,

but long bones can convert if need arisesbut long bones can convert if need arises

• Yellow marrowYellow marrow– Mainly fat cells most commonly found in adult Mainly fat cells most commonly found in adult

long boneslong bones

PeriosteumPeriosteum• Condensed fibrous Condensed fibrous

tissue outer layer.tissue outer layer.

• Inner cambium layerInner cambium layer

• Highly vascularHighly vascular

• Hold osteoprogenitor Hold osteoprogenitor cells that differentiate cells that differentiate into osteoblasts during into osteoblasts during growth or repair (in the growth or repair (in the cambium layer)cambium layer)

• Sharpey’s fibers bind Sharpey’s fibers bind periosteum to boneperiosteum to bone

• Very important in Fx Very important in Fx repairrepair

PeriosteumPeriosteum

• NOT PRESENTNOT PRESENT at: at:– Articular surfacesArticular surfaces– Sites of tendon/ligament insertionsSites of tendon/ligament insertions– Surfaces of sesamoid bonesSurfaces of sesamoid bones– Other specific regionsOther specific regions

• Subcapular femoral neckSubcapular femoral neck

• Lines all internal Lines all internal cavities of bonescavities of bones

• Single layer of Single layer of flattened (squamous) flattened (squamous) osteoprogenitor cellsosteoprogenitor cells

EndosteumEndosteum

Bone CirculationBone Circulation

• 5-10% of cardiac 5-10% of cardiac outputoutput

• Long bones receive Long bones receive blood from three blood from three sources.sources.– Nutrient arteryNutrient artery

– Metaphyseal-Metaphyseal-epiphyseal systemepiphyseal system

– Periosteal systemPeriosteal system

MechanicsMechanics

Trabecular MechanicsTrabecular Mechanics

Apparent DensityTrabecular bone is a porous material with

material properties that vary widely depending upon the amount of bone versus the amount of marrow in a region. We characterize it by its “homogenized” or apparent properties based on the volume fraction of bone Vf in the region.

app= tissVf

Apparent ModulusApparent Strength

Tensile TestingTensile Testing

Compression TestingCompression Testing

Torsional Testing for ShearTorsional Testing for Shear

Structural MechanicsStructural Mechanics

• Strain gauges Strain gauges • IndentationIndentation• NanoindentationNanoindentation• UltrasoundUltrasound

–Speed of soundSpeed of sound

–Density from Archimedes principleDensity from Archimedes principle

Other testing methodsOther testing methods

wettiss water

wet sub

m

m m

Mass of wet bone is obtained out of and submerged in water. Density is computed

2lE

Bone Anatomy – Wolff’s LawBone Anatomy – Wolff’s Law

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Bone Remodeling: Bone Remodeling: OsteoporosisOsteoporosis