Chapter 4 Tissue Biomechanics and Adaptation Modification of

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Chapter 4 Tissue Biomechanics and Adaptation

• Modification of an organism or its parts that
  makes it more fit for existence under the
  conditions of its environment (Mish, 1984)

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In vitro, in situ or in vivo?

• In vitro in a glass (artificial environment)
• Allow direct measurements
• Invasive
• In situ in its normal place
• some elements of the natural environment are
  preserve
• Artificial testing
• In vivo done within the living body (ideal)
• Difficult to obtain (invasive), few human models

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Testing Procedures

• Same testing principles used for testing
  materials
• Materials can be tested under
• compression
• tension
• torsion
• bending
• shear
• Sample of material of known dimension is tested

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Load-deformation curve

• Elastic region
• Proportional limit (yield point)
• Elastic limit
• Plastic region
• Ultimate strength
• Energy stored

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Structural vs. material properties

• Material properties are the characteristics of
  the material regardless of size, density etc.
• The femur and phalange can have the same material
  properties but different structural properties
  (maximal load, bending stiffness)

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Geometry
A

• Moment of Inertia
• Imr2
• Example A smaller moment of inertia, bending
  will occur
• Example B larger (I) greater cross-sectional
  more stiffness

B
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Bone geometry
I
II
d 2.0
III
Increase in stiffness without adding mass
d 2.5
Why not solid bones?
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Mechanical properties of cortical bone

• Anisotropic
• Stiffness calcium/porosity
• Poisson ratio(?)
• High ? lt 0.6
• Absorbs ? ME before fracture
• Ductile Allows deformation

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Cortical Bone Properties

• Viscolelastic
• Strain-rate sensitive
• rate ? ultimate strength also ?
• Fatigue cyclic loads
• Remodeling outpaced by damage microcracks
  develop, stress fractures
• Microcracks most likely to occur in the highly
  mineralized part of the bone

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Trabercular Bone

• Mesh network different densities and patterns
• Nonlinear elastic modulus and strength
• Marrow Enhances Load bearing effect

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Bone Adaptation

• Modeling addition of new bone
• different rates
• continuos
• any bone surface
• growing years (fast)
• initiation ?
• Remodeling resorption and formation of bone
• Activation, resorption and formation

• Osteoclast resorption
• new bone (osteoblast)
• Longer process
• Initiated
• functional strain
• fatigue damage theory (Burr)

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Age

• BMC Bone mineral content
• PHV Peak height velocity (growth)
• Period of bone weakness PHV and BMC
• Maximal BMC 20-30 years

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Age

• Men gt BMC then women
• cortical bone
• 50s decline in BMC
• cortical same rate
• women lose trabercular bone at a faster rate
• rate ?? after menopause (3)
• Importance of reaching high BMC during adolescence

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Osteoporosis

• Reduction of bone mineral mass and changes in
  geometry leading to fractures (hip, spine, wrist)
• Bone mass loss increases after menopause

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Nutrition

• Mineral balance
• vitamin D metabolites
• parathyroid hormone
• calcitonin
• 99 of Calcium is found in the skeleton (1 in
  extracellular fluid)
• Vitamin D
• calcium absorption
• sun exposure

• Dietary protein helps control urinary calcium
  handling
• deficiency ? calcium absorption, osteopenia
• excess ?? calcium loss causing imbalance
• excess dietary fats ? calcium absorption

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Physical Activity

• Exercise can stimulate bone growth
• growing bone low-moderate activity
• threshold
• Moderate-intense ? BMC (1-3) in men and women
• Intense activity 11 in tibia of young adults
• Must continue exercise
• depend of initial bone mass

• Exercise related conditions
• amenorrhea
• oligomenorrhea
• dietary restrictions
• female triad
• eating disorders
• disrupted hormone levels
• low BMC
• Type of exercise
• high intensity and impact

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Bone exercise
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Disuse

• Immobilization, bed rest, space flight
• Space flight lack of loads
• ? deposition
• ? resorption
• affect more weight bearing trabercular bones
• Mostly reversible process recovery is much
  slower
• Early mobilization
• fracture braces etc.

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Articular Cartilage

• Type II collagen
• Different fibers orientation
• Shear forces
• tensile resistance to swelling
• Creep
• constant load
• compression load
• Cyclic loading
• Benefits vs. damage

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Articular Cartilage lubrication

• Synovial joints
• Low coefficients of friction .01-.04
• Theories of lubrication
• Boundary
• Fluid film
• hydrodynamic (non deformable)
• elastohydrodynamic
• Squeeze Film
• right angle movement
• short duration

molecules
Fluid
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Articular Cartilage lubrication

• Boosted Lubrication
• combination of elastohydrodynamic and squeeze
• AC is deformed matrix fluid is forced out in the
  space between the surfaces ? fluid viscosity

Rigid
Deformable
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Articular Cartilage Permeability

• How easy a fluid flows through a permeable
  membrane
• Inversely proportional to frictional drag
• High loads decreases permeability of AC

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Articular Cartilage Adaptation

• Active loading unloading
• Degenerative changes (OA)
• Aging
• ? water content
• ? PG
• ? collagen content

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Articular Cartilage Use Disuse

• Exercise swelling of AC, increase PGs
• Long term wear tear, degradation, OA
• OA cause ?
• excessive loads
• inferior biomaterials
• Some Factors
• heredity
• chemical changes
• altered joint mechanics (ACL- laxity)
• obesity

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Articular Cartilage Use Disuse

• Disuse
• atrophy
• reduction of synthesis
• ? PG
• ? fibrillation
• ? mechanical properties
• deforms rapidly
• Changes are reversible

Control
Biological properties
Lack
Nonstrenuous
Strenuous
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Tendon Ligament

• Ultimate tensile stress of tendon considerably
  high (50-100 MPa)
• Viscoelastic behaviors
• creep, stress-relaxation
• strain rate sensitivity, different from bone
• fast strain rate ligament injuries, slow rate
  (avulsion fracture)
• Partial failure
• Geometry

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Tendon Ligament

• Age
• before maturity more viscous compliant
• maturity ? stiffness modulus of elasticity
• After middle age ? viscosity, less compliant,
  weak insertions (avulsion fractures)

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Tendon Ligament

• Sensitive to training and disuse
• Hypertrophy increase in size and mechanical
  strength
• Exercise can produce increases up to 20 in
  ligament strength
• Increase in number of collagen fibrils and
  cross-sectional area of tendons, collagen
  synthesis
• Disuse
• deterioration of both mechanical and biochemical
  properties
• ? strength, GAG, water, collagen synthesis, mass

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Skeletal Muscle

• Force production
• twitch
• tetanus
• depends on cross-bridges
• rate force sarcomeres in series
• High power output

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Skeletal Muscle strains

• Tears
• bone tendon junctions
• muscle belly
• myotendinous junction
• Contracting muscles required more force and
  energy to reach failure

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Skeletal Muscle Adaptation

• muscle fibers set at birth ?
• Muscle length associated with addition of
  sarcomeres at myotendinous junction
• Muscle adaptations in children ? strength no
  increase in size (neural factors)
• Maximal strength 20-30 years
• Plateau age 50 with a decline
• Loss of strength ? fibers, fast twitch
• Gender women 75 total cross-sectional are
• Same relative strength

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Skeletal Muscle Adaptation

• Hypertrophy vs. Hyperplasia
• Neurological components
• Specific demands
• strength vs. endurance
• Atrophy
• immobilization
• bed rest
• sedentary life
• weightlessness
• Changes in fiber size
• lower protein synthesis
• increase degradation
• Slow twitch fibers more affected