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Intro to Biomechanics > Properties of Bone > Flashcards

Properties of Bone Flashcards

1
Q

Functions of the skeleton

A
  • Protect vital soft tissue organs
  • Support and maintain posture
  • Movement (attachment for muscles, act as levers)
  • Mineral storage (calcium and phosphorus)
  • Hematopoiesis (red marrow)
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2
Q

Bones are composed of

A
  • Calcium hydroxyapatite
  • Water
  • Collage
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3
Q

Calcium hydroxyapatite content of bone

A
  • Calcium carbonate and calcium phosphate

- 60-70% of all minerals

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4
Q

Water content of bone

A
  • 25-30%
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5
Q

Role of collagen in bone structure

A
  • Provides flexibility
  • Provides strength
  • Loss of collagen with agin
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6
Q

Wolff’s Law

A
  • Changes in the form and function of a bone are followed by changes in its internal structure
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7
Q

Application of Wolff’s Law

A
  • Bone “adapts” to the load it is placed under
  • Will become stronger with increased load
  • Will become weaker with decreased load
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8
Q

Bone adapts to its macro and microarchitecture

A
  • Prevents fragility

- Prevents fracture

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9
Q

Bone changes its shape

A
  • Absorbs compression energy
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10
Q

Bone is light in weight

A
  • Allows for rapid movement
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11
Q

Bone is a dynamic structure

A
  • Porosity can change:
  • Aging
  • Osteoporosis
  • Adaptive response
  • Bone adapts to its mechanical environment
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12
Q

Criteria for “ideal” bone

A
  • Resist mechanical loads
  • Resist torsional loads
  • Permit movement
  • Provides a source of calcium
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13
Q

Bones meet criteria for ideal via

A
  • Bone mass
  • Geometry
  • Tissue material composition
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14
Q

External forces applies perpendicular to bone

A
  • Axial load (along the axis)
  • Compression
  • Tension
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15
Q

External forces applied parallel to bone

A
  • Shearing or torsional
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16
Q

Axial load effects

A
  • May be applied in compression or tension

- In walking body weight and ground reactive force provide an axial load

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17
Q

Bending of bone occurs when

A
  • Compressive axial load occurs eccentrically
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18
Q

Two types of biomechanical forces on bone

A
  • Stresses or loads

- Strains

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19
Q

Stresses or loads

A
  • Force applied to the outside of a structure
  • Ground reactive force
  • Body weight borne by the foot
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20
Q

Strain

A
  • Reaction of bone when a load is applied
  • Deformation of tissue
  • Bone can undergo 0.3% strain without deforming
  • Beyond 2% fracture will occur
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21
Q

Mechanical forces on bone

A
  • Compression
  • Tension
  • Shearing/Torsional
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22
Q

Compression stress

A
  • A force in matter that resists being pushed together
  • May be observed as Pressure
  • Body weight on the foot bones
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23
Q

Pressure

A
  • Pressure = F/area

- Measured in pascals

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24
Q

Tension stress

A
  • The force in matter that resists being pulled apart or stretched
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25
Q

Tendo Achilles rupture

A
  • “Watershed” area
  • ~3-6 cm superior to insertion
  • Reduction in both actual number and mean relative volume of vessels
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26
Q

Shearing forces

A
  • Sliding forces

- With walking during contact, forces against the foot parallel to the walking surface

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27
Q

Torsional forces

A
  • Rotational or twisting forces

- Ankle fracture from inversion ankle sprain

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28
Q

Bending forces

A
  • A combination of compression and tension forces
  • Greenstick fractures in pediatric patient
  • Butterfly fracture
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29
Q

Compression strain (deformation)

A
  • Shortening or “squashing”
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30
Q

Tension strain (deformation)

A
  • Elongation or “stretching”
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31
Q

Shear strain (deformation)

A
  • Displacement or delamination
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32
Q

Tissues react in accordance with Newton’s Third Law

A
  • When a force is exerted it causes an equal and opposite force on the substance acted upon
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33
Q

Tissue reaction to stress is dependent upon

A
  • Innate structural characteristics of bone that resist external loads
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34
Q

Regarding joint stability in general

A
  • The more mobile, the less stable

- The more stabile, the less mobile

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35
Q

Predominant collagen of bone

A
  • Type I collagen
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36
Q

Factors affecting the stiffness needed for lever systems to work

A
  • Nonhomogeneous
  • Anisotropic
  • Viscoelastic
  • Brittle
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37
Q

Nonhomoceneity macroscopically

A
  • Dense outer cortex carries the load

- Loosely arranges network of cancellous bone

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38
Q

Loosely arranged network of cancellous bone

A
  • Distributes the load evenly to the cortex
  • Decreases tension and shear to the cortex
  • Equalizes compression forces
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39
Q

Cortical bone accounts for

A
  • ~ 80% of all skeletal bone
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40
Q

Role of cortical bone

A
  • Provides strength

- Contributes primarily to the mechanical role of bone

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41
Q

Cortical bone stress endurance

A
  • Can sustain greater stress but less strain before failure
  • It is “stiffer”
  • Will fail is strain >2%
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42
Q

Trabecular bone accounts for

A
  • ~ 20% of skeletal bone
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43
Q

Trabecular bone characteristics

A
  • Greater capacity to store energy

- Can accept strains up to 75% before failure

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44
Q

Trabecular bone is comprised of

A
  • Highly irregular (anisotropic) trabeculae
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45
Q

Trabecular bone in the foot

A
  • The lesser tarsals

- Second cuneiform has the greatest mechanical advantage for resisting dorsal compression

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46
Q

Commonalities of cortical and cancellous bone

A
  • Similar molecular composition

- Both have extracellular matrix with mineralized and non-mineralized parts

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47
Q

Both cortical and cancellous bone strength is determined by

A
  • Calcium concentration (compressive strength)

- Collagenous proteins (tensile strength)

48
Q

Long bone function

A
  • Serve as levers
49
Q

Short bone functions

A
  • Small, cube shaped
  • Large articular surface
  • Good for shock absorption
50
Q

Flat bone function

A
  • Provides protection
51
Q

Irregular bone function

A
  • A variety of purposes

- Example: maxilla

52
Q

Sesamoid bone function

A
  • Embedded in tendon

- Provide improved mechanical advantage

53
Q

Long bones are designed to

A
  • Carry loads but remain light
54
Q

Long bones load

A
  • Loaded predominantly in bending

- Periosteal radius provides structural rigidity

55
Q

Long bone structure

A
  • Thin layer of cancellous bone on inner diaphyseal wall

- Femoral head is plate-like trabeculae

56
Q

Long bone characteristics

A
  • Emphasize rigidity over flexibility
  • Longer than they are wide
  • Possess growth plate on either end
  • Hard, compact outer surface
  • Spongy cancellous marrow containing interior
57
Q

Long bone cartilage

A
  • Hyaline cartilage

- Shock absorption and protection

58
Q

Broad ends of trabecular bone (long bones)

A
  • Distribute force better
59
Q

Long bone joint surface

A
  • Absorb stress load beneath joint surface

- Transmit that load into the cortex of long bone

60
Q

Flat bone characteristics

A
  • Anterior and posterior surfaces are compact

- Provides strength

61
Q

Flat bone center

A
  • Center consists of marrow-containing cancellous bone

- Red blood cell formation

62
Q

Irregular bone characteristics

A
  • Possess “spring” action
  • Primarily cancellous bone
  • Thin layer of compact cortical bone
  • Vertebrae (rod-like trabeculae)
63
Q

Sesamoid bone characteristics

A
  • Possess special angulations and curvatures

- They resist compression, tension and torsion

64
Q

Sesamoid bone shape is determined by

A
  • Mechanical loading and modeling during growth
65
Q

Nonhomogeneity microscopically

A
  • Lacunae of osteocytes

- Haversian canal system

66
Q

Haversian canal system

A
  • Direction determines resistance to compression
  • More resistant in direction parallel to system
  • Less resistant in direction perpendicular to system
67
Q

Benefits of nonhomogeneity in the calcaneous

A
  • Most dense cancellous bone
  • Neutral (silent) triangle
  • Dense cortical bone
68
Q

Cancellous bone of calcaneous is most dense

A
  • Immediately inferior to the posterior facet

- Just proximal to the anterior calcaneal cuboid joint

69
Q

The neutral (silent) triangle

A
  • Below the lateral talar process

- Sparse trabeculae and osteons

70
Q

Dense cortical bone of calcaneous

A
  • Roof of the neutral triangle

- Provides strength during gait

71
Q

Anisotropy

A
  • Having properties that differ according to the

direction of measurement

72
Q

Different resistance of stresses (anisotropy)

A
  • Compressive stress: very resistant
  • Tension stress: moderately resistant
  • Shear stress: least resistant
73
Q

Torsional forces on bone

A
  • One end is twisted clockwise
  • The other end is twisted counter-clockwise
  • This results in a spiral fracture
  • Fracture begins from the bone’s smallest diameter
74
Q

Different stresses of bending bone

A
  • Unequal stresses
  • Tension forces: convex side
  • Compressive forces: concave side
  • Neutral Axis: point of no stress
75
Q

When a bone bends on the concave side

A
  • Compression

- Decreases end to end distance

76
Q

When a bone bends on the conves side

A
  • Tension

- Increases end to end distance

77
Q

Neutral axis

A
  • No strain when bending bone

- The compressive forces equal the tension forces

78
Q

Bone is weaker when placed under tension

A
  • It will fail on the tension side
79
Q

As tension forces increase with a bending force

A
  • On tension side, crack appears
  • Neutral axis shifts
  • More bone on tension side
  • Crack propagates
80
Q

As tension forces increase with high energy trauma

A
  • Many cracks are formed as energy accumulates quickly

- This bone will shatter

81
Q

As tension forces increase with low energy trauma

A
  • Simple fractures without fragmentation
82
Q

Pilon fracture

A
  • Talus is driven into the tibial plafond

- Impaction with comminution

83
Q

Pilon fracture mechanism

A
  • Axial
  • Lower Energy: skiing
  • High Energy: MVA, fall from height
84
Q

The neutral axis of bone

A
  • An imaginary plane inside the bone
  • Compressive forces = tension forces
  • The further mass is from this axis, the more difficult it is to break
85
Q

Stress (definition)

A
  • Force applied to a material
  • Usually measured in Newtons
  • Application of stress (force) may result in deformation
86
Q

Deformation (strain) in different materials

A
  • An elastic material: full recovery

- Viscoelastic material: creep

87
Q

Strain is measured as

A
  • A % of the original dimension of the object
88
Q

Stress-strain curve

A
  • The relationship of the amount of stress applied to the % of deformation
89
Q

When a force is applied,

A
  • Initial strain is elastic
  • When stress is removed, the bone will return to its normal shape
  • This will be graphed as a straight line
90
Q

Young’s modulus of elasticity

A
  • The slope of this line in the elastic region

- It is always linear

91
Q

Elastic region

A
  • Area under the stress-strain curve
  • A measure of potential energy
  • Energy produced from deforming forces is not lost
  • Returned back into the system as kinetic energy when force is removed (conservation of energy)
92
Q

Plastic region

A
  • Area under the stress-strain curve beyond the yield point
93
Q

Yield point

A
  • Stress-strain curve begins to flatten
  • Decreased stress load is required to increase strain
  • Tissue deformation becomes permanent
  • Potential energy is dissipated (usually as heat)
94
Q

Failure or fracture point

A
  • Material comes physically apart
  • Fracture will be easily visible
  • Remaining stored potential energy is released suddenly
95
Q

Consequences of failure or fracture

A
  • Ruptured vessels
  • Marked inflammation
  • Pain
96
Q

Viscoelasticity of bone

A
  • Elastic properties

- Viscous properties

97
Q

Viscous properties of bone

A
  • Totally non-elastic stress-strain curve
  • Flows according to the density of the substance
  • Remains deformed after a force acts upon them
98
Q

Young’s modulus changes with the speed the force is applied

A
  • Stress applied very quickly = increased Young’s modulus

- Slowly applied stress = decreased Young’s modulus

99
Q

Regardless of viscoelasticity

A
  • Bone will still fracture at the same percent of strain
100
Q

Viscoelasticity with high speed sports

A
  • Allows bones to withstand higher forces

- Increases Young’s modulus

101
Q

If the high speed force causes the bone to fracture

A
  • More stored potential energy
  • Greater dissipation of energy into surrounding tissue
  • Greater soft tissue damage
102
Q

Creep

A
  • A constant force applied in the elastic region
  • The longer this force is applied, the greater the strain retained by bone when the force is removed
  • Bone will gradually return to its former shape
103
Q

Creep is time dependent strain (deformation)

A
  • Stress is constant
  • Strain accumulates as a result of long term stress
  • High levels of stress below the yield point
104
Q

General characteristics of creep

A
  • Constant load (stress)
  • Gradual deformation over time
  • Complete recovery over time
105
Q

Stress relaxation

A
  • Progressive decrease in load with time as the deformation of the structure remains constant
  • Strain is constant
  • The stress required to maintain this strain will decrease over time
106
Q

Deformation is maintained with decreasing stress (load) over time. With release of this stress,

A
  • There is an immediate elastic response followed by a gradual return to its original shape
107
Q

Hysteresis

A
  • Loss of energy in a loading/unloading cycle

- Reflects the dissipation of mechanical energy

108
Q

Hysteresis occurs due to

A
  • Time delay in returning to its original shape
  • Purely elastic materials do not dissipate heat
  • Bone is not purely elastic
109
Q

Brittleness

A
  • A measure of the length of the plastic portion of the stress-strain curve compared to the elastic portion
110
Q

Brittle (short plastic region)

A
  • Can endure a limited amount of energy loss and deformation

- Bone is brittle

111
Q

Ductile (long plastic region)

A
  • Capable of withstanding a greater amount of deformation

- Metals are ductile

112
Q

Brittleness in bone is determined by

A
  • Ratio of hydroxyapetite to collagen
113
Q

Collagen is ductile

A
  • High ratio of protein to mineral: less “brittle” bones (children)
  • Low ratio of protein to mineral: “brittle” bones (elderly)
114
Q

Highly mineralized bone

A
  • Stiff and brittle
  • Area under the plastic phase of the curve is much less
  • Less energy required to cause fracture
115
Q

Relationship between bone stiffness and ultimate strain

A
  • Inverse relationship

Intro to Biomechanics (8 decks)

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