Biomechanics unit 2 Flashcards
(117 cards)
1
Q
4 principle types of tissue
A
Epithelial, connective, muscle and nervous tissue
2
Q
4 types of connective tissue
A
Bone tissue, articular cartilage, tendon, ligament
3
Q
4 types of bone size/shape
A
long bones, short bones, flat bones and irregular bones
4
Q
What are bones composed of
A
Osteophytes, non cellular component, inorganic component
5
Q
What does non cellular component of bone consist of
A
v strong collagen fibres embedded in a jelly like matrix called ground substance
6
Q
What does inorganic component of bone consist of
A
Calcium phosphate crystals deposited in the matrix- gives bone hardness and rigidity
7
Q
Two types of bone tissue
A
compact bone
cancellous bone
8
Q
Compact bone
A
Forms outer layer of bones and has a dense structure- also called cortical bone
9
Q
cancellous bone
A
Forms inner part of short flat and irregular bones. In long bones it lines the inner surface and makes up the greater part of the metaphases and epiphyses
10
Q
Cancellous bone structure
A
Mesh like structure- spongy bone- spaces between the mesh contain red bone marrow
11
Q
Structure of compact bone
A
Haversian system (basic structural unit). These are arranged longitudinally in columns of about 200 micrometers diameter.
12
Q
Haversian system
A
Bone tissue arranged in layers (lamellae) forming cylinders around a central canal. Small central channel, haversian canal, contains blood vessels and nerve fibres. Small cavities between lamellae called lacunae - that contain osteocytes
13
Q
What links osteocytes to haversian canal and other lacunae
A
minute channels called canaliculi- along which nutrients are carried from the blood vessels
14
Q
What interconnects the layers of lamellae within the haversian system
A
Collagen fibres.
15
Q
What is the weakest part of the bones microstructure
A
haversian canal surrounded by a cement like ground substance, probably because it contains no collagen fibres
16
Q
Basic structural unit in cancellous bone
A
Trabecula
17
Q
How are trabecular arranged
A
In a latticework of branching sheets and columns
18
Q
What do trabecullae consist of
A
layers of lamellae with lacunae containing osteocytes connected by canaliculi
19
Q
main difference between trabecular and haversian system
A
Trabeculae dont contain haversian canals- they are not needed because blood vessels pass through the marrow filled spaces between the latticework of trabecular, supplying nutrients to the osteocytes through the canaliculi
20
Q
Why does compact bone need haversian canals
A
they contain blood vessels which are needed to supply the bone tissue with nutrients
21
Q
Tension
A
Load acting to stretch a material
22
Q
Compression
A
load acting to compress a material
23
Q
Stress
A
force per cross sectional area (force/area)
24
Q
Units for stress
A
N/m2 (Pa)
25
Strain
change in length/ original length
26
Units for strain
No units
27
Stress strain curve for cortical bone
stress increases with increasing strain - as the bone is increasingly deformed, it becomes harder to deform it further
28
Elastic region of stress strain curve
Linear- stress directly proportional to strain
29
Elastic behaviour
Provided bone specimen is not deformed beyond its yield point, it will return to its original size and shape once the load is removed
30
Yield stress and yield strain
amount of stress and strain at the yield point
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yield point
division between the elastic and plastic regions of stress strain curve
32
Plastic region
curve not linear. For a small increase in stress, the bone deforms a lot
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What happens when bone is stressed beyond its yield point
it is permanently deformed- plastic behaviour
34
Ultimate stress and strain
Stress and strain at the point at which the bone fractures
35
Youngs modulus eq
stress/strain
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young modulus units
Pa
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Small young modulus
only a small amount of stress to produce a large amount of strain
38
Shear loading
two forces acting in opposite directions- tend to cause layers within the materials to slip or shear
39
What load is cortical bone weakest in
Shear
40
Bending loading
Loads applied to a structure that tend to cause the structure to bend
41
Two types of bending
Cantilever bending and 3 point bending
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Cantilever bending
e.g. diving board. one end fixed and load applied to other end- causing it to bend
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3 point bending
3 forces applied to the object- see saw
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Neutral axis
Between the two sides of a structure being bent- along which no deformation occurs
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Neutral axis of femur
shape of femur causes it to be bent when it is loaded vertically- neutral axis runs approx along centre of the femur
46
Bone stronger under compression or tension
compression
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Where does bone tend to fracture
the elongated side- under tensile stress
48
e.g. of bending fracture
boot top fracture- skiing
49
Torsional loading
bone twisted about longitudinal axis- often occur when one end of the bone is fixed and the other end is twisted
50
What is most distorted in torsional load
No deformation along neutral axis, outer surface of the bar/bone is most distorted
51
How are long bones designed to resist torsional loads
Hollow with strong cortical bone forming the outer layer- maximises strength to weight ratio
52
Why do torsional fractures of tibia often occur distally
distal cross sectional area is smaller than proximal. Although the amount of bone tissue is the same, the distal part is less able to resist torsional loads
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Combined loading
Presence of more than one type of loading
54
Muscle contraction affect on loading
Muscles can contract to alter the stress distribution within a bone- produce a compressive load on the bone to eliminate any tensile loads as bones are stronger in compression than tension
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Wolffs law
bone is laid down where needed and resorbed where not needed
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How to bones respond to increased levels of stress e.g. during exercise
They lay down more collagen fibres and mineral salts to strengthen the bone
57
Bone atrophy
inactivity leads to resorption of bone tissue
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Stress shielding
During fracture healing- the plate will carry most of the load on the limb- if the plate isn't removed soon after the fracture has healed, the bone will weaken as unstressed bone tissue is resorbed
59
Bone hypertrophy
Increase in bone tissue. At points in the bone where screws are inserted, bone will strengthen as the bone tissue will be carrying a greater load than normal
60
Fatigue fractures
fracture resulting from the repeated application of a load that is smaller than the ultimate strength
61
What are fatigue fractures also known as
stress fractures or march fractures
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Small load
greater no. of repetitions required to produce a fatigue fracture
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Why is freq of repetitions important for fatigue fractures
Bone can remodel. If time between repetitions is long enough, bone can repair itself
64
How do childrens bones differ from adults
Contain a greater proportion of collagen
65
Why are greenstick fractures more common in children
Their bones are more flexible- due to increased collagen
66
what is a greenstick fracture
incomplete fracture- one side of the bone is bent and the other side is buckled
67
What kind of loads cause greenstick fractures
Excessive bending or torsional loads
68
What happens to bones between ages of 35-40yrs
Bone tissue begins to be lost as resorption exceeds formation. Some thinning of compact bone and a larger reduction in the amount of cancellous bone due to the thinning of longitudinal trabeculae and the resorption of some transverse trabeculae
69
Result of bone age related changes
bones that are slightly weaker but significantly more brittle
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Types of cartilage
hyaline, elastic, fibrocartilage
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Hyaline cartilage
covers articular surfaces of bones in synovial joints and forms tip of nose
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elastic cartilage
external ear, epiglottis
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fibrocartilage
symphysis pubis and the intervertebral discs
74
articular cartilage
form of hyaline cartilage that is found on the articulating ends of bones in synovial joints
75
How is articular cartilage adapted
adapted to withstand very large loads, it cushions bone while at the same time providing a smooth lubricated bearing surface with minimal wear
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Articular cartilage appearance
glassy smooth glistening blueish white.
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Articular cartilage properties
shock absorbing, allows it to distribute loads evenly over a large surface area, thus reducing contact stress
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Composition of articular cartilage
Organic matrix of non cellular material, interspersed with cells and fluid.
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Articular cartilage organic matrix of non cellular material composition
mainly made up of collagen- structured into strong, fine collagen fibrils. Collagen fibrils are enmeshed in a concentrated solution of proteoglycans.
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What do proteoglycans do
these large protein based molecules are important contributors to the mechanical properties of articular cartilage and make uo 3-10% of the wet weight of articular cartilage.
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where are proteoglycans most concentrated
the middle portion of articular cartilage
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Chondrocytes
sparsely distributed in articular cartilage - less than 10% of the tissues volume. Densely packed in the deeper layers adjacent to the bone
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what do chondrocytes do?
manufacture, secrete and maintain the organic matrix
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What is interstitial fluid of articular cartilage made up of
mostly water
85
3 zones of articular cartilage
superficial tangential, middle and deep
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Arrangement of collagen fibres in superficial tangential zone
tightly woven into sheets arranged parallel to the articular surface and the chondrocytes are oblong with their longitudinal axes aligned parallel to the articular surface
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arrangement of collagen fibres in middle zone
more randomly but still parallel to the articular surface. less densely packed to accommodate the hugh conc of proteoglycans and the chondrocytes are circular and randomly distributed
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collagen and chondrocyte arrangement in deep zone
collagen arranged in larger fibre bundles that are anchored in the underlying bone tissue, thereby attaching articular cartilage to the bone. Chondrocytes arranged in loose columns aligned perpendicular to the line dividing articular cartilage and underlying bone
89
Whats below the deep zone of articular cartilage
thin layer of calcified cartilage which gradually merges into the subchondral bone
90
what is the interface between the articular cartilage and calcified cartilage beneath called
tidemark
91
what is the role of the chondrocytes in articular cartilage
they manufacture, secrete and maintain the organic matrix
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Mechanical behaviour of articular cartilage
viscoelastic
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what is viscoelastic behaviour dependent on
time- response of the material varies according to the length of time that a load is applied and the rate at which a load is applied
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what happens to viscoelastic material once load is removed
it returns to its original size and shape- however the response is not immediate
95
creep
occurs when viscoelastic material is subjected to a constant load. when the load is first applied the material will deform rapidly, followed by a slow (creeping) increasing deformation
96
explanation of creep behaviour of articular cartilage
initial rapid deformation- fluid rapidly forced out. as the amount of fluid decreases, the rate of expellation decreases until it reaches equilibrium
97
stress relaxation
stress is reduced over time as the material is maintained at a constant strain
98
3 main types of lubrication
elastohydrodynamic lubrication
boosted lubrication
boundary lubrication
99
elastohydrodynamic lubrication
2 surfaces, one of which is deformable, lubricated by a film of fluid as they move relative to each other
100
do surfaces touch in elastohydrodynamic lubrication
no, film completely separates them
101
2 ways in which 2 surfaces can move relative to each other
slide over each other (hydrodynamic), move closer together (squeeze film)
102
hydrodynamic lubrication
two surfaces slide over each other forming a wedge of fluid. Pressure is generated as the surfaces slide- motion drags viscous lubricant into the narrowing gap between the surfaces.
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squeeze film lubrication
2 surfaces are forced together. viscous lubricant will not instantaneously be squeezed out from the gap between the two surfaces
104
what happens if high loads are maintained in squeeze film lubrication
lubricant will eventually be depleted and the two surfaces will be in contact
105
what happens if one/both of the surfaces in squeeze film is relatively soft
deformation- will increase the area over which the load is distributed. this occurs in synovial joints
106
boosted lubrication
articular cartilage only permeable to molecules below a certain size. As the size of the gap between articular surfaces decreases, resistance to sideways flow of the lubricant eventually becomes greater than the resistance of flow go the small molecules into the articular cartilage. These small molecules include water that makes up the solvent component of synovial fluid. Water moves into articular cartilage leaving a thick viscose gel behind- enriched lubricant
107
When does boundary lubrication occur
if film of fluid between two surfaces isn't thick enough to keep surfaces apart
108
what is boundary lubrication
lubricant molecules attach themselves chemically to the surfaces, creating a boundary layer
109
Boundary lubricant molecules in synovial joints
lubricin
110
What do tendons connect
muscle to bone
111
what do ligaments connect
bone to bone
112
composition of tendons and ligaments
dense fibrous connective tissue, fibroblasts embedded in mainly collagen fibre matrix
113
how are collagen fibres arranged in tendons
in parallel - as they need to withstand large loads in one direction only
114
collagen fibres in ligaments
not arranged completely in parallel, need to withstand large load in one direction and smaller loads in other directions
115
where are collagen fibres branched and interwoven
cruciate ligaments of knee
116
mechanical properties of tendons and ligaments
viscoelastic, are able to withstand large tensile forces and are very flexible
117
Approx how far can the ACL be elongated before rupturing completely
7 mm
