Bone Structure and Physiology & Fatigue Properties of Bone and Stress Fractures.

Bone Structure and Bone Structure and PhysiologyPhysiology

&&Fatigue Properties of Fatigue Properties of

Bone and Stress Bone and Stress FracturesFractures

BoneBone Structural support of the bodyStructural support of the body

Connective tissue that has the potential to Connective tissue that has the potential to repair and regeneraterepair and regenerate

Comprised of a rigid matrix of calcium salts Comprised of a rigid matrix of calcium salts deposited around protein fibersdeposited around protein fibers

• Minerals provide rigidityMinerals provide rigidity

• Proteins provide elasticity and strengthProteins provide elasticity and strength

ShapeShape

www.sirinet.net/ ~jgjohnso/skeleton.html

Long, short, flat, and irregularLong, short, flat, and irregular• Long bones are cylindrical and “hollow” to Long bones are cylindrical and “hollow” to

achieve strength and minimize weightachieve strength and minimize weight

Bone Physiology. Courtesy Gray's Anatomy 35th edit Longman Edinburgh 1973

Cancellous Bone

Cortical Bone

Osteon

Periosteum

Microstructure of the BoneMicrostructure of the Bone

(a) (b) (c)

Microstructure of Bone Microstructure of Bone (Cont’d)(Cont’d)

Composition of Bone: Composition of Bone: CellsCells

OsteocytesOsteocytes

OsteoblastsOsteoblasts

OsteoclastsOsteoclasts

Controlling Factors Controlling Factors

HormonesHormones

• Estrogen Estrogen

• TestosteroneTestosterone

• CytokinesCytokines Growth factors,Growth factors,

Interleukins (1, 6, and 11), Interleukins (1, 6, and 11),

Transforming growth factor-b Transforming growth factor-b

Tumor necrosis factor-a Tumor necrosis factor-a

of osteoclasts and of osteoclasts and osteoblastsosteoblasts

MacrophageMacrophage

• Phagocytose invading pathogensPhagocytose invading pathogens Cell alters shape to surround bacteria or debrisCell alters shape to surround bacteria or debris

Process: Chemotaxis, adherence, phagosome Process: Chemotaxis, adherence, phagosome formation, phagolysosome formationformation, phagolysosome formation

• Secrete Interleukin-1 Secrete Interleukin-1

(IL-1)(IL-1)

• Involved in bone Involved in bone resorptionresorption

Controlling Factors Controlling Factors of osteoclasts and of osteoclasts and osteoblastsosteoblasts

http://saints.css.edu/bio/schroeder/macrophage.htmlhttp://academic.brooklyn.cuny.edu/biology/bio4fv/page/phago.htmhttp://www.allsciencestuff.com/mbiology/research/osteoporosis

BacteriumNuclei

Ingested bacterium

Composition of Bone: Composition of Bone: MatrixMatrix

Cortical/ Compact Cortical/ Compact

Bone Bone

Cancellous/ Cancellous/

Trabecular/ Spongy Trabecular/ Spongy

BoneBone

CorticalCortical CancellouCancellouss

Physical Physical DescriptioDescriptio

nnDense protective shellDense protective shell

Rigid lattice designed for Rigid lattice designed for strength; Interstices are strength; Interstices are filled with marrow filled with marrow

LocationLocation

Around all bones, Around all bones, beneath periosteum; beneath periosteum; Primarily in the shafts Primarily in the shafts of long bonesof long bones

In vertebrae, flat bones In vertebrae, flat bones (e.g. pelvis) and the ends (e.g. pelvis) and the ends of long bonesof long bones

% of % of Skeletal Skeletal

MassMass80%80% 20%20%

CorticalCortical CancelloCancellousus

First Level First Level StructureStructure

OsteonsOsteons TrabeculaeTrabeculae

PorosityPorosity 5-10%5-10% 50-90%50-90%

CirculatioCirculationn

Slow circulation of Slow circulation of nutrients and wastenutrients and waste

Haversian system allows Haversian system allows diffusion of nutrients and diffusion of nutrients and waste between blood waste between blood vessels and cells; Cells vessels and cells; Cells are close to the blood are close to the blood supply in lacunaesupply in lacunae

CorticalCortical CancelloCancellousus

StrengthStrength Withstand greater Withstand greater stressstress Withstand greater Withstand greater strainstrain

Direction Direction of of

StrengthStrength

Bending and torsion, Bending and torsion, e.g. in the middle of e.g. in the middle of

long boneslong bones

Compression; Young’s Compression; Young’s modulus is much greater modulus is much greater

in the longitudinal in the longitudinal directiondirection

StiffnessStiffness HigherHigher LowerLower

Fracture Fracture PointPoint

Strain>2%Strain>2% Strain>75%Strain>75%

Properties of Cortical and Properties of Cortical and Cancellous BonesCancellous Bones

Load TypeLoad Type Elastic modulusElastic modulus (10(1099N/mN/m22))

Ultimate stressUltimate stress (10(1066N/mN/m22))

Bone TypeBone Type Cortical Cortical

CancellousCancellous Cortical Cortical

CancellousCancellous

TensionTension 11-1911-19 ~0.2-5~0.2-5 107-146107-146 ~3-20~3-20

CompressionCompression 15-2015-20 0.1-30.1-3 156-212156-212 1.5–501.5–50

ShearShear 73-8273-82 6.6+/-1.66.6+/-1.6

http://www.orthoteers.co.uk/Nrujp~ij33lm/Orthbonemech.htm

Bone RemodelingBone Remodeling

Bone RemodelingBone Remodeling Bone structural integrity is Bone structural integrity is

continually maintained by remodelingcontinually maintained by remodeling

• Osteoclasts and osteoblasts Osteoclasts and osteoblasts assemble into assemble into Basic Basic MulticellularMulticellular Units ( Units (BMUsBMUs))

• Bone is completely remodeled in Bone is completely remodeled in approximately 3 yearsapproximately 3 years

• Amount of old bone removed equals Amount of old bone removed equals new bone formednew bone formed

http://www.elixirindustry.com/resource/osteoporosis/jilka.htm

BMU Remodeling BMU Remodeling SequenceSequence

Activation

Resorption

Reversal

Quiescence

Formation & Mineralization

www.ifcc.org/ejifcc/ vol13no4/130401004n.htm

Osteocytes

Load Characteristics of Load Characteristics of BoneBone

Load characteristics of a bone include:Load characteristics of a bone include:

Direction of the applied forceDirection of the applied force• TensionTension• CompressionCompression• BendingBending• TorsionTorsion• ShearShear

Magnitude of the loadMagnitude of the load

Rate of load applicationRate of load application

Material Properties Material Properties Comparison*Comparison*

MaterialMaterial Compressive Compressive Strength (MPa)Strength (MPa)

Modulus Modulus (GPa)(GPa)

Cortical Cortical 10-16010-160 4-274-27

TrabelcularTrabelcular 7-1807-180 1-111-11

ConcreteConcrete ~ 4~ 4 3030

SteelSteel 400-1500400-1500 200200

WoodWood 100100 1313

Pink: http://www.engineeringtoolbox.com/24_417.html Yellow: http://www.brown.edu/Departments/EEB/EML/background/Background_Bone.htmGreen: http://ttb.eng.wayne.edu/%7Egrimm/BME5370/Lect3Out.html#TrabecularBone

*Variability of *Variability of PropertiesProperties

Material properties listed may vary widely due to test methods used to determine them

Variances of the following can effect results: Orientation of sample

Bone and wood are elastically anistropic; steel is not

Condition of sample Dry or wet with various liquids

Specifics of sample Bone: age of donor, particular bone studied

Wood: species of tree

Steel/Concrete: preparation methods, componentshttp://silver.neep.wisc.edu/~lakes/BoneAniso.html

Function of BoneFunction of Bone Mechanical supportMechanical support

HematopoiesisHematopoiesis

Protection of vital structuresProtection of vital structures

Mineral homeostasisMineral homeostasis

Fatigue of BoneFatigue of Bone Microstructural damage due to repeated Microstructural damage due to repeated

loads below the bone’s ultimate strength loads below the bone’s ultimate strength

• Occurs when muscles become fatigued and Occurs when muscles become fatigued and less able to counter-act loads during less able to counter-act loads during continuous strenuous physical activitycontinuous strenuous physical activity

• Results in Progressive loss of strength and Results in Progressive loss of strength and stiffnessstiffness

Cracks begin at discontinuities within the Cracks begin at discontinuities within the bone bone (e.g. haversian canals, lacunae)(e.g. haversian canals, lacunae)

• Affected by the magnitude of the load, Affected by the magnitude of the load, number of cycles, and frequency of loadingnumber of cycles, and frequency of loading

Fatigue of Bone (Cont’)Fatigue of Bone (Cont’) 3 Stages of fatigue fracture3 Stages of fatigue fracture

• Crack InitiationCrack Initiation Discontinuities result in points of increased local Discontinuities result in points of increased local

stress where micro cracks formstress where micro cracks form • Often bone remodeling repairs these cracksOften bone remodeling repairs these cracks

• Crack Growth (Propagation)Crack Growth (Propagation) If micro cracks are not repaired they grow until they If micro cracks are not repaired they grow until they

encounter a weaker material surface and change encounter a weaker material surface and change directiondirection

• Often transverse growth is stopped when the crack Often transverse growth is stopped when the crack turns from perpendicular to parallel to the loadturns from perpendicular to parallel to the load

• Final FractureFinal Fracture Occurs only when the fatigue process progresses faster than Occurs only when the fatigue process progresses faster than

the rate of remodelingthe rate of remodelinghttp://www.orthoteers.co.uk/Nrujp~ij33lm/Orthbonemech.htm Simon, SR. Simon, SR. Orthopaedic Basic ScienceOrthopaedic Basic Science. Ohio: American Academy of Orthopaedic Surgeons; 1994.. Ohio: American Academy of Orthopaedic Surgeons; 1994.

Process to Fatigue Process to Fatigue FailureFailure

Road to Failure: Region 1Road to Failure: Region 1

1.1.Crack initiationCrack initiation

2.2.AccumulationAccumulation

3.3.GrowthGrowth

Characteristics:Characteristics:• Matrix damage in regions of Matrix damage in regions of

High stress concentration High stress concentration Low strengthLow strength

• Relatively rapid loss of stiffness Relatively rapid loss of stiffness

• Bear less loadBear less load

• Absorb more energy ( can sustain larger Absorb more energy ( can sustain larger

deflections)deflections)

• Cracks develop rapidly Cracks develop rapidly May stabilize quickly without much May stabilize quickly without much

propagationpropagation

Process to Fatigue Failure Process to Fatigue Failure (cont’d)(cont’d)

Process to Fatigue Failure Process to Fatigue Failure (Cont’d)(Cont’d)

• Cracks occur first iCracks occur first in regions of high n regions of high strain strain Accumulate with either Accumulate with either

Increased number of cycles Increased number of cycles Increased strainIncreased strain

• Cracks develop perpendicular to the Cracks develop perpendicular to the load axisload axis

Road to Failure: Region 2Road to Failure: Region 2

1.1.Crack growthCrack growth

2.2.CoalescenceCoalescence

3.3.Delamination and debondingDelamination and debonding

Characteristics:Characteristics:

• After a crack formsAfter a crack forms Interlamellar tensile and shear Interlamellar tensile and shear

stresses are generated at its tip stresses are generated at its tip Tend to separate and shear lamellae Tend to separate and shear lamellae

at the fiber-matrix interface at the fiber-matrix interface

Process to Fatigue Failure Process to Fatigue Failure (cont’d)(cont’d)

• Secondary cracks may extend between Secondary cracks may extend between

lamellae in the load directionlamellae in the load direction

• Cracks tend to grow parallel to the load Cracks tend to grow parallel to the load

• Delamination along the load axisDelamination along the load axis Elevated and probably unidirectional strain Elevated and probably unidirectional strain

redistributions redistributions Along the fibers parallel to the load axisAlong the fibers parallel to the load axis

Process to Fatigue Failure Process to Fatigue Failure (cont’d)(cont’d)

Process to Fatigue Failure Process to Fatigue Failure (Cont’d)(Cont’d)

Road to Failure: Region 3Road to Failure: Region 3

• Stiffness declines rapidly Stiffness declines rapidly • End of a material’s fatigue lifeEnd of a material’s fatigue life• Fiber failure Fiber failure

Coalescence of accumulated damage Coalescence of accumulated damage Crack propagation along interfacesCrack propagation along interfaces

• Rapid process Rapid process • Ultimate failure of the structureUltimate failure of the structure

Stress FracturesStress Fractures Stress fractures are Stress fractures are

• Partial or complete fractures of bonePartial or complete fractures of bone

• Repetitive strain during sub-maximal Repetitive strain during sub-maximal activityactivity

There are two main types:There are two main types:

1.1. Fatigue fracture Fatigue fracture

2.2. Insufficiency fractureInsufficiency fracture

Fatigue FractureFatigue Fracture

A fatigue fracture may be caused by:A fatigue fracture may be caused by:• Abnormal muscle stressAbnormal muscle stress

Loss of shock absorptionLoss of shock absorption Strenuous or repeated activityStrenuous or repeated activity

• Torque Torque bone with normal elastic resistancebone with normal elastic resistance

• Associated with new or different activityAssociated with new or different activity Abnormal loadingAbnormal loading Abnormal stress distributionAbnormal stress distribution

Fatigue Micro DamageFatigue Micro Damage

Insufficiency FracturesInsufficiency Fractures

Due to normal muscular activity stressing Due to normal muscular activity stressing

the bonethe bone Seen in post-menopausal and/or Seen in post-menopausal and/or

amenhorroeic women whose bones areamenhorroeic women whose bones are• Deficient in mineral Deficient in mineral

• Reduced elastic resistanceReduced elastic resistance

Occurs if osteoporosis or some other Occurs if osteoporosis or some other

disease weakens the bones disease weakens the bones

Signs and SymptomsSigns and Symptoms Pain that develops graduallyPain that develops gradually

Increases with weight-bearing activityIncreases with weight-bearing activity Diminishes with restDiminishes with rest

Swelling on the top of the foot or the Swelling on the top of the foot or the

outside ankleoutside ankle

Tenderness to touch at the site of the Tenderness to touch at the site of the

fracture fracture

Possible bruisingPossible bruising

Causes of Stress Causes of Stress FracturesFractures

There are two theories about the origin of There are two theories about the origin of

stress fractures: stress fractures:

1.1. Fatigue theoryFatigue theory

2.2. Overload theoryOverload theory

Fatigue TheoryFatigue Theory• During repeated efforts (as in running) During repeated efforts (as in running)

Muscles become unable to support during Muscles become unable to support during impact impact

Muscles do not absorb the shockMuscles do not absorb the shock

Load is transferred to the boneLoad is transferred to the bone

As the loading surpasses the capacity of the As the loading surpasses the capacity of the bone to adaptbone to adapt

A fracture developsA fracture develops

Overload TheoryOverload Theory

Certain muscle groups contract Certain muscle groups contract •Cause the attached bones to bendCause the attached bones to bend

After repeated contractions and bendingAfter repeated contractions and bending

Bone finally breaks Bone finally breaks

Risk Factors for Stress Risk Factors for Stress FracturesFractures

Age:Age:• The risk increases with age The risk increases with age

• Bone is less resistant to fatigue in older peopleBone is less resistant to fatigue in older people

Training errors:Training errors:• Sudden, drastic increase in running mileage or Sudden, drastic increase in running mileage or

intensityintensity• Running with an unequal distribution of weight Running with an unequal distribution of weight

across the footacross the foot• Intense training after an extended period of restIntense training after an extended period of rest• Beginning training too great in quantity or intensityBeginning training too great in quantity or intensity

Fitness history:Fitness history: • Sedentary people entering a sports Sedentary people entering a sports

program are prone to injury program are prone to injury • Gradual increase in training loads is Gradual increase in training loads is

importantimportant

Footwear:Footwear: • Only significant factor is the condition of Only significant factor is the condition of

the running shoethe running shoe• Newer shoes lead to fewer fracturesNewer shoes lead to fewer fractures

Risk Factors for Stress Fractures Risk Factors for Stress Fractures (Cont’d)(Cont’d)

Endocrine status:Endocrine status: • Women athletes suffering from amenorrhea are at Women athletes suffering from amenorrhea are at

especially high risk especially high risk

• Heavy endurance training may also compromise Heavy endurance training may also compromise

androgen status in menandrogen status in men

Nutritional factors:Nutritional factors: • Recommended calcium intake in post-puberty is Recommended calcium intake in post-puberty is

800mg/day800mg/day

• Stress-fracture patients are encouraged to consume Stress-fracture patients are encouraged to consume

1500mg of calcium daily1500mg of calcium daily

Risk Factors for Stress Risk Factors for Stress Fractures (Cont’d)Fractures (Cont’d)

Biomechanical factors:Biomechanical factors: • Incidence of stress fractures* are due to Incidence of stress fractures* are due to

Tibial torsion (twisting/bending)Tibial torsion (twisting/bending)

Degree of external rotation at the hipDegree of external rotation at the hip

• When neither were presentWhen neither were present Incidence was 17%Incidence was 17%

• When both were presentWhen both were present Incidence was 45%Incidence was 45%

Risk Factors for Stress Risk Factors for Stress Fractures (Cont’d)Fractures (Cont’d)

* - Gilati and Abronson (1985)* - Gilati and Abronson (1985)

Other factors include: Other factors include: •High arched footHigh arched foot•Excessive pronation of foot Excessive pronation of foot

(turning inward) (turning inward) •Excessive supination of foot Excessive supination of foot

(turning outward) (turning outward) •Longer second toe Longer second toe •Bunion on the great toeBunion on the great toe

Risk Factors for Stress Risk Factors for Stress Fractures (Cont’d)Fractures (Cont’d)

Prevention of Stress Prevention of Stress FracturesFractures

Avoid abrupt increases in overall training load and Avoid abrupt increases in overall training load and intensity intensity

Take adequate restTake adequate rest

Replace running shoes Replace running shoes Tend to lose their shock-absorbing capacity by 400 milesTend to lose their shock-absorbing capacity by 400 miles

Bony alignment may be modified to some extent by the Bony alignment may be modified to some extent by the use of orthotics use of orthotics

Women athletes should pay careful attention to Women athletes should pay careful attention to • TrainingTraining• Hormonal statusHormonal status• Nutrition and eating disordersNutrition and eating disorders

Treatment of Stress Treatment of Stress FracturesFractures

Discontinue the activity Discontinue the activity

Rest Rest

Ice Ice

Elevate the affected part Elevate the affected part

Non-impact aerobic activity (e.g. swimming Non-impact aerobic activity (e.g. swimming

and cycling) and cycling)

Cast (if necessary) Cast (if necessary)

CrutchesCrutches

The EndThe End

OsteonOsteon Major structural Major structural

unit of cortical unit of cortical bonebone• Concentric Concentric

cylinders of bone cylinders of bone matrix around matrix around haversian canalshaversian canals

http://www.nd.edu/~humosteo/OsteonModel.gi

Haversian Canal

PeriosteumPeriosteum

Capillary-rich, fibrous membrane Capillary-rich, fibrous membrane coating exterior bone surface coating exterior bone surface

• Responsible for nourishing boneResponsible for nourishing bone

The osteoclast is a large cell with multiple

nuclei

nuclei

cytoplasm

OsteoclastsOsteoclasts Located in lacunaeLocated in lacunae Derive from Derive from pluripotent cells of the bone marrowpluripotent cells of the bone marrow Responsible for bone resorptionResponsible for bone resorption

• Bind to bone via integrinsBind to bone via integrins• Enzymes digest bone matrixEnzymes digest bone matrix• Controlled by hormonal and growth factorsControlled by hormonal and growth factors

Identifying traits Identifying traits • Large size Large size • Mulitple nuclei Mulitple nuclei • Ruffled edge Ruffled edge

Location of active resorptionLocation of active resorption

OsteoblastsOsteoblasts Bone forming cells Bone forming cells

• Line the surface of the boneLine the surface of the bone• Surrounded by unmineralized bone matrixSurrounded by unmineralized bone matrix• Derived from osteoprogenitor cell lineDerived from osteoprogenitor cell line

Produce type I collagenProduce type I collagen

• Secretion is polarized towards the bone surfaceSecretion is polarized towards the bone surface

Attract Ca salts and P to precipitate to mineralize Attract Ca salts and P to precipitate to mineralize the bonethe bone

Osteoblasts (Cont’d)Osteoblasts (Cont’d)

Upon completion of bone formation, Upon completion of bone formation, • Remains on the surface of boneRemains on the surface of bone

• Covered by non-calcified osteoidCovered by non-calcified osteoid

Identifying traits: Identifying traits:

• Outer membrane surface coated in alkaline Outer membrane surface coated in alkaline phosphatesphosphates

• Polarized (nucleus away from bone surface)Polarized (nucleus away from bone surface)

• Basophilic stainsBasophilic stains

OsteocytesOsteocytesOsteoblasts surrounded by mineralized bone Osteoblasts surrounded by mineralized bone matrixmatrix

• Most numerous bone cellMost numerous bone cell

Positioned between lamellae in a concentric Positioned between lamellae in a concentric pattern around the central lumen of osteonspattern around the central lumen of osteons

Regulate extracellular concentration of calcium Regulate extracellular concentration of calcium and phosphateand phosphate

Osteocytes (Cont’d)Osteocytes (Cont’d) Mechanosensory cellsMechanosensory cells

• Respond to deformationRespond to deformation

• Flow of interstitial fluid through the osteocytic Flow of interstitial fluid through the osteocytic canalicular networkcanalicular network

Directed away from regions of high strainDirected away from regions of high strain

Initiates electrokinetic and mechanical signalsInitiates electrokinetic and mechanical signals

Growth Facors (intercellular signal molecules)Growth Facors (intercellular signal molecules)

• Insulin-like growth factor, IGF-1,Insulin-like growth factor, IGF-1,

• Prostaglandins G/H synthase Prostaglandins G/H synthase

• PGE2 and Nitric oxidePGE2 and Nitric oxide

(a) First Level(a) First Level Hydroxyapatite Hydroxyapatite

crystals embedded crystals embedded between collagen between collagen fibrilfibril

(b) Second Level(b) Second Level

Fibrils are arranged into Fibrils are arranged into lamellaelamellae

• Sheets of collagen fibers Sheets of collagen fibers with a preferred with a preferred orientationorientation

(c) Third Level(c) Third Level

Lamellae are Lamellae are arranged into arranged into tubular osteons tubular osteons

Osteoclast

Osteocytes

Osteoblast

Basic Multicellular Basic Multicellular UnitsUnits

““The Basic Multicellular Unit (BMU) is a The Basic Multicellular Unit (BMU) is a wandering team of cells that dissolves a pit wandering team of cells that dissolves a pit in the bone surface and then fills it with in the bone surface and then fills it with new bone.”new bone.” http://http://uwcme.org/site/courses/legacy/bonephys/physiology.phpuwcme.org/site/courses/legacy/bonephys/physiology.php

• BMUs are BMUs are discrete temporary anatomic discrete temporary anatomic structuresstructures organized as functional unit organized as functional unit

Osteoclasts remove old bone, then Osteoclasts remove old bone, then osteoblasts synthesize new boneosteoblasts synthesize new bone

• old bone is replaced by new bone in quantized old bone is replaced by new bone in quantized packetspackets

Basic Multicellular Units Basic Multicellular Units (cont’d)(cont’d)

A photomicrograph of bone showing osteoblasts and osteoclasts together in one Bone Metabolic Unit

http://uwcme.org/site/courses/legacy/bonephys/physiology.php

ActivationActivation Occurs when bone experiences micro Occurs when bone experiences micro

damage or mechanical stress, or at random damage or mechanical stress, or at random

A BMU originates and travels along the bone A BMU originates and travels along the bone surfacesurface

• Differentiated cells are recruited from stem cell Differentiated cells are recruited from stem cell populationspopulations

• Pre-osteoclasts merge to form multi-nucleated Pre-osteoclasts merge to form multi-nucleated osteoclastsosteoclasts

http://uwcme.org/site/courses/legacy/bonephys/physiology.php

Bone ResorptionBone Resorption Newly differentiated osteoclasts are Newly differentiated osteoclasts are

activated and begin to resorb boneactivated and begin to resorb bone

• Minerals are dissolved and the matrix is digested Minerals are dissolved and the matrix is digested by enzymes and hydrogen ions secreted by the by enzymes and hydrogen ions secreted by the osteoclastic cells osteoclastic cells

• Move longitudinally on bone surfaceMove longitudinally on bone surface

This process is more rapid than formation, This process is more rapid than formation, though it may last several daysthough it may last several days

http://uwcme.org/site/courses/legacy/bonephys/physiology.phphttp://www.britannica.com/ebc/article?tocId=41887

ReversalReversal Transition from osteoclastic to osteoblastic Transition from osteoclastic to osteoblastic

activityactivity

Takes several days Takes several days

Results in a cylindral space (tunnel) between Results in a cylindral space (tunnel) between

the resorptive region and the refilling regionthe resorptive region and the refilling region

Forms the cement lineForms the cement line

Bone FormationBone Formation Following Resorption, osteoclasts are replaced by Following Resorption, osteoclasts are replaced by

osteoblasts around the periphery of the tunnel osteoblasts around the periphery of the tunnel Attracted by cytokines and growth factorsAttracted by cytokines and growth factors

Active osteoblasts secrete and produce layers of osteoid, Active osteoblasts secrete and produce layers of osteoid, refilling the tunnelrefilling the tunnel

Osteoblasts do not completely refill the tunnelOsteoblasts do not completely refill the tunnel

• Leaves a Haversian canal Leaves a Haversian canal • Contains capillaries to support the metabolism of Contains capillaries to support the metabolism of

the BMU and bone matrix cells the BMU and bone matrix cells • Carries calcium and phosphorus to and from the Carries calcium and phosphorus to and from the

bonebone

http://uwcme.org/site/courses/legacy/bonephys/physiology.php

MineralizationMineralization When the osteoid is about 6 microns thick, it begins to When the osteoid is about 6 microns thick, it begins to

mineralize mineralize

Formation of the initial mineral deposits at multiple Formation of the initial mineral deposits at multiple

discrete sites (initiation)discrete sites (initiation)

• Mineral is deposited within and between the collagen Mineral is deposited within and between the collagen

fibersfibers

• This process, also, is regulated by the osteoclastsThis process, also, is regulated by the osteoclasts

Mineral maturation Mineral maturation

• Once the cavity is full the mineral crystals pack Once the cavity is full the mineral crystals pack

together, increasing the density of the new bonetogether, increasing the density of the new bone

http://uwcme.org/site/courses/legacy/bonephys/physiology.php

QuiescenceQuiescence After the tunneling and refilling After the tunneling and refilling

• Some osteoblasts become osteocytesSome osteoblasts become osteocytes Remain in bone, sense mechanical stresses Remain in bone, sense mechanical stresses

on boneon bone• Remaining osteoblasts become lining cellsRemaining osteoblasts become lining cells

Calcium release from bonesCalcium release from bones

Period of relative inactivity Period of relative inactivity • Secondary osteon and its associated cells carry Secondary osteon and its associated cells carry

on their mechanical, metabolic and homeostatic on their mechanical, metabolic and homeostatic functionsfunctions

http://uwcme.org/site/courses/legacy/bonephys/physiology.php

Mechanical SupportMechanical Support

Provides strength and stiffness Provides strength and stiffness Hollow cylinder: Strong and lightHollow cylinder: Strong and light Have mechanisms for avoiding fatigue Have mechanisms for avoiding fatigue

fracturefracture

HematopoiesisHematopoiesisDevelopment of blood cells Development of blood cells

• Occurs in the marrow of boneOccurs in the marrow of bone

These regions are mainly composed of These regions are mainly composed of trabecular bone trabecular bone

• (e.g. The iliac crest, vertebral body, (e.g. The iliac crest, vertebral body, proximal and distal femur)proximal and distal femur)

Protection of Vital Protection of Vital StructuresStructures

Flat bones in the head protect the Flat bones in the head protect the

brainbrain

Protects heart and lungs in chestProtects heart and lungs in chest

Vertebrae in the spine protect the Vertebrae in the spine protect the

spinal cord and nervesspinal cord and nerves

Mineral HomeostasisMineral Homeostasis Primary storehouse of calcium and Primary storehouse of calcium and

phosphorusphosphorus

Trabecular bone are rapidly formed Trabecular bone are rapidly formed

or destroyedor destroyed

• In response to shifts in calcium stasis In response to shifts in calcium stasis

without serious mechanical without serious mechanical

consequencesconsequences

Fatigue CurveFatigue Curve

Probability of Injury