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)
2
Q
Bones are composed of
A
- Calcium hydroxyapatite
- Water
- Collage
3
Q
Calcium hydroxyapatite content of bone
A
- Calcium carbonate and calcium phosphate
- 60-70% of all minerals
4
Q
Water content of bone
A
- 25-30%
5
Q
Role of collagen in bone structure
A
- Provides flexibility
- Provides strength
- Loss of collagen with agin
6
Q
Wolff’s Law
A
- Changes in the form and function of a bone are followed by changes in its internal structure
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
8
Q
Bone adapts to its macro and microarchitecture
A
- Prevents fragility
- Prevents fracture
9
Q
Bone changes its shape
A
- Absorbs compression energy
10
Q
Bone is light in weight
A
- Allows for rapid movement
11
Q
Bone is a dynamic structure
A
- Porosity can change:
- Aging
- Osteoporosis
- Adaptive response
- Bone adapts to its mechanical environment
12
Q
Criteria for “ideal” bone
A
- Resist mechanical loads
- Resist torsional loads
- Permit movement
- Provides a source of calcium
13
Q
Bones meet criteria for ideal via
A
- Bone mass
- Geometry
- Tissue material composition
14
Q
External forces applies perpendicular to bone
A
- Axial load (along the axis)
- Compression
- Tension
15
Q
External forces applied parallel to bone
A
- Shearing or torsional
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
17
Q
Bending of bone occurs when
A
- Compressive axial load occurs eccentrically
18
Q
Two types of biomechanical forces on bone
A
- Stresses or loads
- Strains
19
Q
Stresses or loads
A
- Force applied to the outside of a structure
- Ground reactive force
- Body weight borne by the foot
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
21
Q
Mechanical forces on bone
A
- Compression
- Tension
- Shearing/Torsional
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
23
Q
Pressure
A
- Pressure = F/area
- Measured in pascals
24
Q
Tension stress
A
- The force in matter that resists being pulled apart or stretched
25
Tendo Achilles rupture
- "Watershed” area
- ~3-6 cm superior to insertion
- Reduction in both actual number and mean relative volume of vessels
26
Shearing forces
- Sliding forces
| - With walking during contact, forces against the foot parallel to the walking surface
27
Torsional forces
- Rotational or twisting forces
| - Ankle fracture from inversion ankle sprain
28
Bending forces
- A combination of compression and tension forces
- Greenstick fractures in pediatric patient
- Butterfly fracture
29
Compression strain (deformation)
- Shortening or “squashing”
30
Tension strain (deformation)
- Elongation or “stretching”
31
Shear strain (deformation)
- Displacement or delamination
32
Tissues react in accordance with Newton's Third Law
- When a force is exerted it causes an equal and opposite force on the substance acted upon
33
Tissue reaction to stress is dependent upon
- Innate structural characteristics of bone that resist external loads
34
Regarding joint stability in general
- The more mobile, the less stable
| - The more stabile, the less mobile
35
Predominant collagen of bone
- Type I collagen
36
Factors affecting the stiffness needed for lever systems to work
- Nonhomogeneous
- Anisotropic
- Viscoelastic
- Brittle
37
Nonhomoceneity macroscopically
- Dense outer cortex carries the load
| - Loosely arranges network of cancellous bone
38
Loosely arranged network of cancellous bone
- Distributes the load evenly to the cortex
- Decreases tension and shear to the cortex
- Equalizes compression forces
39
Cortical bone accounts for
- ~ 80% of all skeletal bone
40
Role of cortical bone
- Provides strength
| - Contributes primarily to the mechanical role of bone
41
Cortical bone stress endurance
- Can sustain greater stress but less strain before failure
- It is “stiffer”
- Will fail is strain >2%
42
Trabecular bone accounts for
- ~ 20% of skeletal bone
43
Trabecular bone characteristics
- Greater capacity to store energy
| - Can accept strains up to 75% before failure
44
Trabecular bone is comprised of
- Highly irregular (anisotropic) trabeculae
45
Trabecular bone in the foot
- The lesser tarsals
| - Second cuneiform has the greatest mechanical advantage for resisting dorsal compression
46
Commonalities of cortical and cancellous bone
- Similar molecular composition
| - Both have extracellular matrix with mineralized and non-mineralized parts
47
Both cortical and cancellous bone strength is determined by
- Calcium concentration (compressive strength)
| - Collagenous proteins (tensile strength)
48
Long bone function
- Serve as levers
49
Short bone functions
- Small, cube shaped
- Large articular surface
- Good for shock absorption
50
Flat bone function
- Provides protection
51
Irregular bone function
- A variety of purposes
| - Example: maxilla
52
Sesamoid bone function
- Embedded in tendon
| - Provide improved mechanical advantage
53
Long bones are designed to
- Carry loads but remain light
54
Long bones load
- Loaded predominantly in bending
| - Periosteal radius provides structural rigidity
55
Long bone structure
- Thin layer of cancellous bone on inner diaphyseal wall
| - Femoral head is plate-like trabeculae
56
Long bone characteristics
- 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
Long bone cartilage
- Hyaline cartilage
| - Shock absorption and protection
58
Broad ends of trabecular bone (long bones)
- Distribute force better
59
Long bone joint surface
- Absorb stress load beneath joint surface
| - Transmit that load into the cortex of long bone
60
Flat bone characteristics
- Anterior and posterior surfaces are compact
| - Provides strength
61
Flat bone center
- Center consists of marrow-containing cancellous bone
| - Red blood cell formation
62
Irregular bone characteristics
- Possess “spring” action
- Primarily cancellous bone
- Thin layer of compact cortical bone
- Vertebrae (rod-like trabeculae)
63
Sesamoid bone characteristics
- Possess special angulations and curvatures
| - They resist compression, tension and torsion
64
Sesamoid bone shape is determined by
- Mechanical loading and modeling during growth
65
Nonhomogeneity microscopically
- Lacunae of osteocytes
| - Haversian canal system
66
Haversian canal system
- Direction determines resistance to compression
- More resistant in direction parallel to system
- Less resistant in direction perpendicular to system
67
Benefits of nonhomogeneity in the calcaneous
- Most dense cancellous bone
- Neutral (silent) triangle
- Dense cortical bone
68
Cancellous bone of calcaneous is most dense
- Immediately inferior to the posterior facet
| - Just proximal to the anterior calcaneal cuboid joint
69
The neutral (silent) triangle
- Below the lateral talar process
| - Sparse trabeculae and osteons
70
Dense cortical bone of calcaneous
- Roof of the neutral triangle
| - Provides strength during gait
71
Anisotropy
- Having properties that differ according to the
| direction of measurement
72
Different resistance of stresses (anisotropy)
- Compressive stress: very resistant
- Tension stress: moderately resistant
- Shear stress: least resistant
73
Torsional forces on bone
- 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
Different stresses of bending bone
- Unequal stresses
- Tension forces: convex side
- Compressive forces: concave side
- Neutral Axis: point of no stress
75
When a bone bends on the concave side
- Compression
| - Decreases end to end distance
76
When a bone bends on the conves side
- Tension
| - Increases end to end distance
77
Neutral axis
- No strain when bending bone
| - The compressive forces equal the tension forces
78
Bone is weaker when placed under tension
- It will fail on the tension side
79
As tension forces increase with a bending force
- On tension side, crack appears
- Neutral axis shifts
- More bone on tension side
- Crack propagates
80
As tension forces increase with high energy trauma
- Many cracks are formed as energy accumulates quickly
| - This bone will shatter
81
As tension forces increase with low energy trauma
- Simple fractures without fragmentation
82
Pilon fracture
- Talus is driven into the tibial plafond
| - Impaction with comminution
83
Pilon fracture mechanism
- Axial
- Lower Energy: skiing
- High Energy: MVA, fall from height
84
The neutral axis of bone
- 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
Stress (definition)
- Force applied to a material
- Usually measured in Newtons
- Application of stress (force) may result in deformation
86
Deformation (strain) in different materials
- An elastic material: full recovery
| - Viscoelastic material: creep
87
Strain is measured as
- A % of the original dimension of the object
88
Stress-strain curve
- The relationship of the amount of stress applied to the % of deformation
89
When a force is applied,
- 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
Young’s modulus of elasticity
- The slope of this line in the elastic region
| - It is always linear
91
Elastic region
- 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
Plastic region
- Area under the stress-strain curve beyond the yield point
93
Yield point
- 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
Failure or fracture point
- Material comes physically apart
- Fracture will be easily visible
- Remaining stored potential energy is released suddenly
95
Consequences of failure or fracture
- Ruptured vessels
- Marked inflammation
- Pain
96
Viscoelasticity of bone
- Elastic properties
| - Viscous properties
97
Viscous properties of bone
- Totally non-elastic stress-strain curve
- Flows according to the density of the substance
- Remains deformed after a force acts upon them
98
Young’s modulus changes with the speed the force is applied
- Stress applied very quickly = increased Young's modulus
| - Slowly applied stress = decreased Young's modulus
99
Regardless of viscoelasticity
- Bone will still fracture at the same percent of strain
100
Viscoelasticity with high speed sports
- Allows bones to withstand higher forces
| - Increases Young's modulus
101
If the high speed force causes the bone to fracture
- More stored potential energy
- Greater dissipation of energy into surrounding tissue
- Greater soft tissue damage
102
Creep
- 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
Creep is time dependent strain (deformation)
- Stress is constant
- Strain accumulates as a result of long term stress
- High levels of stress below the yield point
104
General characteristics of creep
- Constant load (stress)
- Gradual deformation over time
- Complete recovery over time
105
Stress relaxation
- 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
Deformation is maintained with decreasing stress (load) over time. With release of this stress,
- There is an immediate elastic response followed by a gradual return to its original shape
107
Hysteresis
- Loss of energy in a loading/unloading cycle
| - Reflects the dissipation of mechanical energy
108
Hysteresis occurs due to
- Time delay in returning to its original shape
- Purely elastic materials do not dissipate heat
- Bone is not purely elastic
109
Brittleness
- A measure of the length of the plastic portion of the stress-strain curve compared to the elastic portion
110
Brittle (short plastic region)
- Can endure a limited amount of energy loss and deformation
| - Bone is brittle
111
Ductile (long plastic region)
- Capable of withstanding a greater amount of deformation
| - Metals are ductile
112
Brittleness in bone is determined by
- Ratio of hydroxyapetite to collagen
113
Collagen is ductile
- High ratio of protein to mineral: less “brittle” bones (children)
- Low ratio of protein to mineral: “brittle” bones (elderly)
114
Highly mineralized bone
- Stiff and brittle
- Area under the plastic phase of the curve is much less
- Less energy required to cause fracture
115
Relationship between bone stiffness and ultimate strain
- Inverse relationship
