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Biomechanics > Lecture 3 - Tissue Mechanics > Flashcards

Lecture 3 - Tissue Mechanics Flashcards

1
Q

What is tissue mechanics?

A

Study of mechanical behaviour or properties of the tissues of the body

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2
Q

Divisions of tissue mechanics

A
  • Hard tissues e.g. bone and cartilage

- Soft tissue e.g. muscle, tendon, ligament, skin and nervous tissue

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3
Q

What does tissue mechanics help us to do?

A
  • Predict thresholds/mechanisms of injury
  • Predict effects of disease
  • Investigate mechanics of structural disorders
  • Develop appropriate finite element models and tissue engineered constructs
  • Develop realistic surgical simulations
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4
Q

Components of a mechanical test system

A
  • Sample source, preparation and coupling
  • Environmental conditions
  • Sensor systems
  • Test protocol including sample preconditioning
  • Equipment
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5
Q

How to choose test protocol?

A
  • Examine the orientation, direction and magnitude of the applied force e.g. tension, compression, bending, torsion, shear
  • Rate of load application
  • Preconditioning
  • Boundary conditions
  • Is cyclic loading or rest periods necessary?
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6
Q

Types of samples

A
  • Live humans
  • Human cadavers
  • Animal models
  • Computer models
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7
Q

Considerations of selected samples

A
  • In vivo vs in vitro
  • Degree of dissections
  • Age
  • Time post mortem
  • Health or diseased
  • Shape, preparation or mounting of sample
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8
Q

Environmental conditions

A
  • Perfusion
  • Hydration
  • Temperature
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9
Q

What is hierarchical multiscale modelling?

A

Involves carrying out modelling at multiple stages in biological life e.g. atomic, molecular, cellular, tissue, organ of organ system level

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10
Q

What is viscoelasticity?

A
  • Resist shear flow and strain linearly with time when a stress is applied
  • Exhibit both viscous and elastic characteristics when undergoing deformation
  • Materials exhibit a time delay in returning the material to original shape with some energy loss
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11
Q

Properties of elastic materials

A

Strain when stretched and quickly return to original state once stress is removed

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12
Q

Conditions affecting viscoelasticity

A

Strain rate, time and temperature

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13
Q

Overview of elastin

A
  • Consists of long, flexible molecules

- Cross linked to form 3D networks

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14
Q

Properties of elastin

A
  • Slight differences in the loading and unloading cycles

- For strains up to 60% remains fairly linear

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15
Q

Modulus of elastin

A

0.4MPa

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16
Q

Overview of collagen

A
  • 3 stranded helix protein
  • At least 20 different forms
  • Main constituent of tendons, ligaments and most membranes
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17
Q

Properties of collagen

A
  • Non linear and viscoelastic
  • When held at a constant strain, the load relaxes over time
  • Key influencer of collagen properties is the extent of cross linking
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18
Q

Overview of cortical bone

A
  • Made with cylindrical osteons/haversian systems with a network of veins and arteries
  • Strength in bending and torsion e.g. in the middle of long bones
  • High stiffness
  • Fracture point at strain >2%
  • Withstands greater stress
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19
Q

Overview of bone materials

A
  • Made of HA and collagen
  • HA is a strong, stiff material which gives bone rigidity
  • Collagen fibers are more elastic and give bone its toughness, also prevents brittle cracking
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20
Q

Effect of osteocytes on bone properties

A
  • Bone contains 13,000 osteocytes per cubic mm
  • Form an interconnected network through dendrites
  • Communicate with each other and bone surface lining local cells
  • Measure strains from fluid flow through the bone matrix caused by tissue deformation
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21
Q

What is Wolff’s law?

A
  • Bone remodels depending on the loading environment
  • Bone which has no loading over time decreases in density and strength
  • Occurs through the action of osteoblasts and osteoclasts which deposit and remove bone respectively
  • Static strains do not lead to adaptive remodelling
  • High frequency impact loading induces a greater adaptive remodelling response than low frequency loading
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22
Q

Anisotropy of bone

A
  • Bone is quite anisotropic due to its composite structure
    properties vary with age, sex, location and strain rate
  • Strongest in compression, followed by tension and shear
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23
Q

Implications of bone design on stress

A
  • Compression tends to bend the bones on one side and stretch on the other
  • Stresses are greatest at external surfaces at the epiphyses
  • Stronger and denser compact bone is
  • Medullary cavity which experiences no stress therefore can store things and lighten the bone
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24
Q

Modulus of HA

A

Tension 165GPA

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25
Q

Modulus of collagen

A

Tension 1.2GPa

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26
Q

Modulus of bone

A

Tension 18GPA

  • Bone usually experiences only small strains in normal physiology
  • Fairly linear elastic within these small deformations
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27
Q

Hierarchy of bones

A
  • At each size scale, the structure of bones influences its susceptibility to freature
  • Smaller levels affect intrinsic toughness
  • Higher levels impacting extrinsic toughness
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28
Q

Factors affecting material biomechanical properties e.g. ultimate stress, modulus and toughness

A

Mineralisation, microdamage and the organic matrix

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29
Q

Factors affecting structural biomechanical properties e.g. ultimate load, stiffness and energy to fracture

A

Bone mass, material biomechanical properties and geometry/architecture

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30
Q

Overview of cancellous bone

A
  • Lattice like framework
  • Withstands greater strain
  • Strength in compression
  • Young’s modulus much greater in longitudinal direction
  • Lower stiffness
  • Fracture point at strain >75%
31
Q

Elastic modulus of cortical bone

A

Tension 11-19GPa

Compression 15-20 GPa

32
Q

Elastic modulus of cancellous bone

A

Tension 0.2-5GPa

Compression 0.1-3GPa

33
Q

Ultimate stress of cortical bone

A

Tension 107-146MPa
Compression 156-212MPa
Shear 73-82MPa

34
Q

Ultimate stress of cancellous bone

A

Tension 3-20MPa
Compression 1.5-50MPa
Shear 6.6+/-1.6MPa

35
Q

Why are bones curved?

A
  • When loaded in the longitudinal direction, the bone will deform in a predictable direction
  • Will bend more when the load is increased
  • Curved bones have enhanced ability to differentiate between loading conditions and then alter the osteogenic stimulus according to the magnitude of strain
36
Q

Why can children’s bones bend before they break?

A
  • Children’s bones are more porous than adults
  • Have lower osteoid density than adults
  • Haversian canals occupy a larger space in bone of a child
37
Q

Overview of cartilage

A
  • Articular cartilages lines the surfaces of most joints
  • Hydrated tissue which has complex structure
  • Chrondrocytes are found in superficial, transitional and deep zones
38
Q

What makes up a cartilage matrix?

A
  • Made up of cells and collagenous fibres in a fluid matrix

- Mostly water with proteoglycans, whoe interaction with fibres gives rise to complex nonlinear mechanical behaviour

39
Q

General mechanical properties of cartilage

A
  • Under high rates of loading the cartilage is stiff and protects the bone from harmful high frequency forces
  • Under low rates of loading it is not stiff and passes the load onto the bone tissue (causing strain to bone tissue)
40
Q

Specific mechanical properties of cartilage

A
  • Cartilage/synovial fluid lubrication very efficient

- Coefficient of 0.02

41
Q

Factors affecting stiffness of cartilage

A
  • Deformation of the collagen matrix and flow through it
  • Concentrations of proteoglycans
  • Health
  • Loading rate
  • Direction and location of sample
42
Q

Overview of ligaments

A
  • Bind joints together
  • Provide strength and stability
  • Carry only tensile loads
  • Poor blood supply so do not heal well when damaged
  • Blood vessels are at periphery
43
Q

What are ligaments comprised of?

A

Collagen and elastin fibres with interspersed fibroblasts

44
Q

Mechanical properties of ligaments

A
  • Also hydrated tissues meaning behaviour is also controlled by this
  • Nonlinear viscoelastic
  • Exhibiting J-shaped stress strain response and creep response
45
Q

Overview of tendons

A
  • Carry tensile forces from muscle to bone
  • Carry compressive forces when wrapped around bone
  • Structurally similar to ligaments and muscle fibers
  • Often insert over a region via muscle
46
Q

Important note about tendon, ligament and muscle properties

A

Often reported in terms of force vs length rather than stress v strain

47
Q

Mechanical properties of tendons

A

Typical J-shaped curve with almost linear relationship after toe region

48
Q

Ultimate stress of tendons

A

80-120MPa

49
Q

Failure of tendons

A

Strains of 8-10%

50
Q

Overview of muscle

A

Skeletal (direct voluntary), smooth and cardiac (involuntary)

51
Q

Active properties of skeletal muscle

A

Relationship between muscle length and force generating ability

52
Q

Passive properties of skeletal muscle

A

Relaxed force length relationship

53
Q

What is the relationship between muscle length and force?

A
  • As length increases AND decreases, fewer binding sites available to produce forces
  • If cell is stretched so myosin and actin no longer overlap, no longer generate force
  • If cell is shortened so thin filaments overlap, Z discs contact thick filaments and no more shortening can occur
54
Q

What is the relationship between binding sites and force?

A

The greater number of binding sites means more force

55
Q

What is the resting length of a muscle?

A

Length the muscle returns to when unloaded

56
Q

Overview of skin

A
  • Constructed of layers of collagen fibre networks

- Anisotropic

57
Q

Mechanical properties of skin

A
  • Nonlinear stress strain curve
58
Q

What occurs in skin during wound healing?

A

Strength of skin changes and scar tissue is much stiffer due to dense collagen fibres

59
Q

What is optical elastography?

A

Using a piezoelectric transducer to evaluate mechanical properties, with the potential to improve differentiation of tissue pathologies

60
Q

Overview of nervous tissue

A
  • Peripheral nerves are bundles of parallel nerve fibres
  • Soft hydrated tissues with collagen fibres and high water conten
  • Once again, mechanical behaviour influenced by this flow
61
Q

What is myelin?

A

Protein coating wrapped around the nerve fibre (electrical insulation)

62
Q

Mechanical properties of nervous tissue (brain)

A
  • Nonlinear and visoelastic

- Highly strain rate dependent

63
Q

What is higher, physical or functional tolerance to deformation?

A
  • Physical tolerance to deformation is higher than functional
  • Relevant during mechanical testing because we are only testing physical tolerance
64
Q

What is stiffer, brain or spinal cord tissue?

A

Spinal cord stiffer than brain tissue due to highly organised longitudinal fibres

65
Q

What is laboratory motion analysis and what is it used for?

A

Used to study tissue mechanics by measuring the kinematics and dynamics of human or animal motion

66
Q

What are common applications of LMA?

A

Gait analysis whether abnormal or normal and sports biomechanics

67
Q

What are the reasons for using LMA?

A

Improving performance or efficiency, clinical diagnosis or suggestions for therapeutic or surgical interventions

68
Q

What methods for data acquisition are there?

A

Video, 3D optical or analogue

69
Q

What does video acquisition involve?

A

Points are digitised manually or by attaching reflective markers and digitised automatically

70
Q

What does 3D acquisition involve?

A
  • Marker only system that uses 2-12 cameras and IR lights to collect 3D coordinates
  • Cameras may be optical, IR, x-ray, fluoroescent
  • Records motion of skin mounted markers on a moving person OR rigidly mounted markers on a specimen
  • Software helps to digitise markers positions and calculate joint angle, displacement, strain fields or limb velocities
71
Q

What does analog acquisition involve?

A

Analogue sampling from force platforms, EMG or other devices, and can be integrated with the other two

72
Q

Limitations of LMA

A
  • Skin to bone motion artefact
  • Alternative markers are percutaneous pins but this is invasive and painful if conscious
  • Cannot determine individual muscle forces
73
Q

Applications of body kinematic

A
  • Non invasive joint kinematics
  • Analysis of patella tracking
  • Comparison of pain against normal biomechanics to track disease progression
  • In vivo performance of orthopaedic implants

Biomechanics (3 decks)

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