Chapter 4 Chapter 4 Tissue Biomechanics and Tissue Biomechanics and

AdaptationAdaptation

Modification of an organism or its Modification of an organism or its parts that makes it more fit for parts that makes it more fit for

existence under the conditions of existence under the conditions of its environment (Mish, 1984)its environment (Mish, 1984)

In vitro, in situ or in vivo?In vitro, in situ or in vivo?

In vitro: in a glass (artificial environment)In vitro: in a glass (artificial environment) Allow direct measurementsAllow direct measurements InvasiveInvasive

In situ: in its normal placeIn situ: in its normal place some elements of the natural environment are some elements of the natural environment are

preservepreserve Artificial testingArtificial testing

In vivo: done within the living body (ideal)In vivo: done within the living body (ideal) Difficult to obtain (invasive), few human modelsDifficult to obtain (invasive), few human models

Testing ProceduresTesting Procedures

Same testing principles used Same testing principles used for testing materialsfor testing materials

Materials can be tested Materials can be tested under:under: compressioncompression tensiontension torsiontorsion bendingbending shearshear

Sample of material of known Sample of material of known dimension is testeddimension is tested

Load-deformation curveLoad-deformation curve

Elastic regionElastic region Proportional limit Proportional limit

(yield point)(yield point) Elastic limitElastic limit Plastic regionPlastic region Ultimate strengthUltimate strength Energy storedEnergy stored

Structural vs. material propertiesStructural vs. material properties

Material properties are Material properties are the characteristics of the characteristics of the material regardless the material regardless of size, density etc.of size, density etc.

The femur and The femur and phalange can have the phalange can have the same material same material properties but different properties but different structural properties structural properties (maximal load, bending (maximal load, bending stiffness)stiffness)

GeometryGeometry

Moment of InertiaMoment of Inertia I=mrI=mr22

Example A: smaller Example A: smaller moment of inertia, moment of inertia, bending will occurbending will occur

Example B: larger (I) Example B: larger (I) greater cross-greater cross-sectional more sectional more stiffnessstiffness

A

B

Bone geometryBone geometry

d= 2.0

d = 2.5

I II

III

Exam I Exam II Exam IIIPeriostial 2 2 2.5Endosteal 0.5 0 2Bone area 2.95 3.14 1.77Area I 0.78 0.79 1.13Force 20 20 20Stress 256 253 221

Increase in stiffness without adding mass

Why not solid bones?

Mechanical properties of cortical boneMechanical properties of cortical bone

AnisotropicAnisotropic Stiffness: Stiffness:

calcium/porositycalcium/porosity Poisson ratio(Poisson ratio())

High: High: << 0.6 0.6

Absorbs Absorbs ME before ME before fracturefracture

Ductile: Allows Ductile: Allows deformationdeformation

Cortical Bone PropertiesCortical Bone Properties

ViscolelasticViscolelastic Strain-rate sensitiveStrain-rate sensitive

rate rate ultimate strength ultimate strength also also

Fatigue: cyclic loadsFatigue: cyclic loads Remodeling outpaced by Remodeling outpaced by

damage microcracks damage microcracks develop, stress fracturesdevelop, stress fractures

Microcracks: most likely Microcracks: most likely to occur in the highly to occur in the highly mineralized part of the mineralized part of the bonebone

Trabercular BoneTrabercular Bone

Mesh network: Mesh network: different densities different densities and patternsand patterns

Nonlinear elastic Nonlinear elastic modulus and modulus and strengthstrength

Marrow: Enhances Marrow: Enhances Load bearing effectLoad bearing effect

Bone AdaptationBone Adaptation

Modeling: addition of Modeling: addition of new bonenew bone different ratesdifferent rates continuoscontinuos any bone surfaceany bone surface growing years (fast)growing years (fast) initiation ?initiation ?

Remodeling: resorption Remodeling: resorption and formation of boneand formation of bone Activation, resorption and Activation, resorption and

formationformation

Osteoclast resorptionOsteoclast resorption new bone (osteoblast)new bone (osteoblast) Longer processLonger process Initiated Initiated

functional strainfunctional strain fatigue damage theory fatigue damage theory

(Burr)(Burr)

AgeAge

BMC: Bone mineral BMC: Bone mineral contentcontent

PHV: Peak height PHV: Peak height velocity (growth)velocity (growth)

Period of bone Period of bone weakness PHV and weakness PHV and BMCBMC

Maximal BMC 20-30 Maximal BMC 20-30 yearsyears

AgeAge

Men > BMC then womenMen > BMC then women cortical bonecortical bone

50’s decline in BMC50’s decline in BMC cortical same ratecortical same rate women lose trabercular women lose trabercular

bone at a faster ratebone at a faster rate rate rate after menopause after menopause

(3%)(3%)

Importance of reaching Importance of reaching high BMC during high BMC during adolescenceadolescence

OsteoporosisOsteoporosis

Reduction of bone mineral Reduction of bone mineral mass and changes in mass and changes in geometry leading to geometry leading to fractures (hip, spine, wrist)fractures (hip, spine, wrist)

Bone mass loss increases Bone mass loss increases after menopauseafter menopause

Nutrition Nutrition

Mineral balanceMineral balance vitamin D metabolitesvitamin D metabolites

parathyroid hormoneparathyroid hormone calcitonincalcitonin

99% of Calcium is found 99% of Calcium is found in the skeleton (1% in in the skeleton (1% in extracellular fluid)extracellular fluid)

Vitamin DVitamin D calcium absorptioncalcium absorption sun exposuresun exposure

Dietary protein helps control Dietary protein helps control urinary calcium handlingurinary calcium handling deficiency deficiency calcium calcium

absorption, osteopeniaabsorption, osteopenia excess excess calcium loss calcium loss

causing imbalancecausing imbalance excess dietary fats excess dietary fats

calcium absorptioncalcium absorption

Physical ActivityPhysical Activity

Exercise can stimulate Exercise can stimulate bone growth bone growth growing bone: low-growing bone: low-

moderate activitymoderate activity threshold threshold Moderate-intense Moderate-intense BMC BMC

(1-3%) in men and women(1-3%) in men and women Intense activity 11% in tibia Intense activity 11% in tibia

of young adultsof young adults Must continue exerciseMust continue exercise depend of initial bone massdepend of initial bone mass

Exercise related Exercise related conditions conditions amenorrheaamenorrhea oligomenorrheaoligomenorrhea dietary restrictionsdietary restrictions female triad female triad

eating disorderseating disorders disrupted hormone disrupted hormone

levelslevels low BMClow BMC

Type of exerciseType of exercise high intensity and impacthigh intensity and impact

Bone exerciseBone exercise

DisuseDisuse

Immobilization, bed Immobilization, bed rest, space flightrest, space flight

Space flight: lack of Space flight: lack of loadsloads depositiondeposition resorptionresorption affect more weight bearing affect more weight bearing

trabercular bonestrabercular bones Mostly reversible process: Mostly reversible process:

recovery is much slowerrecovery is much slower Early mobilizationEarly mobilization

fracture braces etc.fracture braces etc.

Articular CartilageArticular Cartilage

Type II collagenType II collagen Different fibers Different fibers

orientationorientation Shear forcesShear forces tensile resistance to tensile resistance to

swellingswelling CreepCreep

constant loadconstant load compression loadcompression load

Cyclic loadingCyclic loading Benefits vs. damageBenefits vs. damage

Articular Cartilage lubricationArticular Cartilage lubrication

Synovial jointsSynovial joints Low coefficients of Low coefficients of

friction .01-.04friction .01-.04

Theories of lubricationTheories of lubrication BoundaryBoundary Fluid filmFluid film

hydrodynamic (non hydrodynamic (non deformable)deformable)

elastohydrodynamicelastohydrodynamic Squeeze FilmSqueeze Film

right angle movementright angle movement short durationshort duration

molecules

Fluid

Articular Cartilage lubricationArticular Cartilage lubrication

Boosted LubricationBoosted Lubrication combination of combination of

elastohydrodynamic elastohydrodynamic and squeezeand squeeze

AC is deformed AC is deformed matrix fluid is forced matrix fluid is forced out in the space out in the space between the between the surfaces surfaces fluid fluid viscosityviscosity

Rigid

Deformable

Articular Cartilage: PermeabilityArticular Cartilage: Permeability

How easy a fluid flows How easy a fluid flows through a permeable through a permeable membranemembrane

Inversely proportional to Inversely proportional to

frictional dragfrictional drag High loads decreases High loads decreases

permeability of ACpermeability of AC

Articular Cartilage: AdaptationArticular Cartilage: Adaptation

Active loading & Active loading & unloadingunloading

Degenerative changes Degenerative changes (OA)(OA)

AgingAging water contentwater content PGPG collagen contentcollagen content

Articular Cartilage: Use & DisuseArticular Cartilage: Use & Disuse

Exercise: swelling of AC, Exercise: swelling of AC, increase PG’sincrease PG’s

Long term: wear & tear, Long term: wear & tear, degradation, OAdegradation, OA

OA: cause ?OA: cause ? excessive loadsexcessive loads inferior biomaterialsinferior biomaterials

Some FactorsSome Factors heredityheredity chemical changeschemical changes altered joint mechanics altered joint mechanics

(ACL- laxity)(ACL- laxity) obesityobesity

Articular Cartilage: Use & DisuseArticular Cartilage: Use & Disuse

DisuseDisuse atrophyatrophy reduction of synthesisreduction of synthesis PGPG fibrillationfibrillation mechanical propertiesmechanical properties

deforms rapidlydeforms rapidly

Changes are reversibleChanges are reversibleB

iolo

gica

l pro

pert

ies

Lack Nonstrenuous Strenuous

Control

Tendon & LigamentTendon & Ligament

Ultimate tensile stress of Ultimate tensile stress of tendon considerably high tendon considerably high (50-100 MPa)(50-100 MPa)

Viscoelastic behaviorsViscoelastic behaviors creep, stress-relaxationcreep, stress-relaxation strain rate sensitivity, strain rate sensitivity,

different from bonedifferent from bone fast strain rate ligament fast strain rate ligament

injuries, slow rate (avulsion injuries, slow rate (avulsion fracture)fracture)

Partial failurePartial failure GeometryGeometry

Tendon & LigamentTendon & Ligament

AgeAge before maturity: more before maturity: more

viscous & compliantviscous & compliant maturity: maturity: stiffness & stiffness &

modulus of elasticitymodulus of elasticity After middle age: After middle age:

viscosity, less compliant, viscosity, less compliant, weak insertions (avulsion weak insertions (avulsion fractures)fractures)

Tendon & LigamentTendon & Ligament

Sensitive to training and disuseSensitive to training and disuse Hypertrophy: increase in size Hypertrophy: increase in size

and mechanical strengthand mechanical strength Exercise can produce increases Exercise can produce increases

up to 20% in ligament strengthup to 20% in ligament strength Increase in number of collagen Increase in number of collagen

fibrils and cross-sectional area of fibrils and cross-sectional area of tendons, collagen synthesistendons, collagen synthesis

DisuseDisuse deterioration of both deterioration of both

mechanical and biochemical mechanical and biochemical propertiesproperties

strength, GAG, water, strength, GAG, water, collagen synthesis, masscollagen synthesis, mass

Skeletal MuscleSkeletal Muscle

Force productionForce production twitchtwitch tetanustetanus depends on # cross-bridgesdepends on # cross-bridges rate force: sarcomeres in seriesrate force: sarcomeres in series

High power outputHigh power output

Skeletal Muscle: strainsSkeletal Muscle: strains

TearsTears bone tendon junctionsbone tendon junctions muscle bellymuscle belly myotendinous junctionmyotendinous junction

Contracting muscles Contracting muscles required more force and required more force and energy to reach failureenergy to reach failure

Skeletal Muscle AdaptationSkeletal Muscle Adaptation

# muscle fibers set at birth ?# muscle fibers set at birth ? Muscle length associated with Muscle length associated with

addition of sarcomeres at addition of sarcomeres at myotendinous junctionmyotendinous junction

Muscle adaptations in children Muscle adaptations in children strength no increase in size strength no increase in size (neural factors)(neural factors)

Maximal strength 20-30 yearsMaximal strength 20-30 years Plateau age 50 with a declinePlateau age 50 with a decline Loss of strength Loss of strength # fibers, fast # fibers, fast

twitchtwitch Gender: women 75% total cross-Gender: women 75% total cross-

sectional aresectional are Same relative strengthSame relative strength

Skeletal Muscle AdaptationSkeletal Muscle Adaptation

Hypertrophy vs. HyperplasiaHypertrophy vs. Hyperplasia Neurological componentsNeurological components Specific demandsSpecific demands

strength vs. endurancestrength vs. endurance AtrophyAtrophy

immobilizationimmobilization bed restbed rest sedentary lifesedentary life weightlessnessweightlessness

Changes in fiber sizeChanges in fiber size lower protein synthesislower protein synthesis increase degradation increase degradation Slow twitch fibers more Slow twitch fibers more

affectedaffected