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
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