Basic Science Flashcards
1
Q
Load
A
Force that acts on a body
2
Q
Stress
A
Force over area (N/m2) - intensity of a force
3
Q
Strain
A
Change in length/original length - measure of deformation (%)
4
Q
Elastic deformation
A
Reversible change in shape to a material due to a load (compressive/tensile). Material returns to original shape when load is removed. Material obeys Hooke’s law (stress and strain have linear relationship). Stiffer materials have more elastic deformation, not plastic; less give.
5
Q
Plastic deformation
A
Irreversible change in shape to a material due to a load. Material doesn’t return to original shape when load is removed.
6
Q
Toughness
A
Amount of energy a material can absorb before failure. Equivalent to area under stress/strain curve (J/m3)
7
Q
Hooke’s Law
A
Stress is linearly proportional to strain until a certain limit (proportional limit). Materials in the elastic zone obey this law.
8
Q
Ultimate tensile strength
A
Load to failure
9
Q
Strain hardening
A
Sharp increase in stress strain curve at end of plastic zone, prior to ultimate strength. Need much greater stress to create change in length/deformation/strain.
10
Q
Necking
A
Gradual decrease in cross sectional area until a material fails
11
Q
Stiffness
A
Ability of a material to resist deformation.Higher Young’s modulus. Stiffer material will tolerate far more load before changing length than less stiff material.
12
Q
Stress shielding
A
Modulus mismatch between materials with different Young’s Modulus values. Stiffer material bears more load.
13
Q
Young’s Modulus of Elasticity (E) = stress/strain
A
Ratio of stress to strain. Ability of a material to resist deformation under tension. Greater E = stiffer.
14
Q
Viscoelasticity
A
Material possesses both solid and fluid properties (elastic and viscous, respectively). Stress/strain relationship is dependent on duration of applied load and rate at which load is applied (time and rate dependent).
15
Q
Creep
A
INCREASING deformation over time under a CONSTANT load
16
Q
Stress relaxation
A
REDUCING stress over time under a constant load e.g. preparing a femur for uncemented stem, cycling ACL graft, Ponseti casting.
17
Q
Hysteresis
A
Different loading and unloading curves of a stress/strain curve. Energy is lost to internal friction (usually heat). e.g. breathing in vs out
18
Q
Rate-dependent strain
A
Different stiffness dependent on rate of applied load e.g punching vs pushing finger into plasticine, spasticity in CP patients/Parkinson’s disease.
19
Q
Force
A
A mechanical load that causes external (acceleration) and internal (strain) effects
20
Q
Normal force
A
Perpendicular to the surface
21
Q
Tangential force
A
Parallel to the surface
22
Q
Compressive force
A
Shrinks a body in the direction of a force
23
Q
Tensile force
A
Elongates a body
24
Q
Brittle
A
Linear stress/strain relationship until point of failure. Elastic deformation only.
25
Ductile
Undergoes large amounts of plastic deformation
26
Isotropic
Possesses same mechanical properties in all directions (e.g. golf ball)
27
Anisotropic
Possesses different mechanical properties depending on the direction of applied load (e.g. bone is stronger with axial load rather than radial load).
28
Homogenous
Uniform structure/composition throughout
29
Heterogenous
Non-uniform structure/composition throughout
30
Load/deformation curve
Similar to stress/strain curve. Slope in the elastic range is the rigidity of structure rather than stiffness
31
Bending rigidity of rectangular structure
Proportional to the base multiplied by height, cubed. (bh)3/12
32
Bending rigidity of cylindrical structure
Proportional to the fourth power of the radius.
For solid cylinders = (pi x r)4/64.
For hollow cylinders = [(Outer radius4 - inner radius4) x pi]/4
33
Second moment of area
Change in stiffness of a beam with a change in cross sectional area. Bending rigidity of cylinder.
34
Bending stiffness of a plate
Proportional to the third power of its thickness
35
Second moment of inertia
Bending rigidity of a rectangle
36
Polar moment of inertia
Bending stiffness of a nail
37
Surface hardness
Ability of a material to resist localized plastic deformation by mechanical pressure or abrasion.
38
Wear
Progressive loss of bearing substance from a material secondary to mechanical or chemical action.
39
Modes of wear
Mode 1 - between two bearing surfaces
Mode 2 - between one bearing and a non-bearing surface
Mode 3 - third body wear between two bearing surfaces
Mode 4 - between two non-bearing surfaces
40
Types of wear
Abrasive wear
Adhesive wear
Fatigue
41
Abrasive wear
Two body wear - occurs when a softer material comes into contact with a significantly harder material. Asperities in the harder material may plough into the softer material creating grooves.
Third body wear - occurs when extraneous material enters the interfascial region and becomes imbedded in the softer material, scratching the harder material.
42
Two body wear
Occurs when a softer material comes into contact with a significantly harder material. Asperities in the harder material may plough into the softer material creating grooves.
43
Third body wear
Occurs when extraneous material enters the interfascial region and becomes imbedded in the softer material, scratching the harder material.
44
Adhesive wear
Occurs when a junction is formed between two opposing surfaces. If this junction is stronger than the intermolecular bonds between the surface materials, fragments of the weaker material may be sheared off and adhere to the stronger material.
45
Fatigue wear
Delamination. A form of failure that occurs in materials subject to dynamic stresses. Failure can occur at stress considerably lower than the yield strength for a load.
46
Fatigue life
Number of cycles needed to cause failure at a specified stress level. Taken from S-n plot.
47
Endurance limit
Stress level below whicha specimen will withstand cyclic stress indefinitely with exhibiting fatigue failure. Arbitrarily set at 10 million cycles.
48
Volumetric wear
Volume of material detached from softer bearing surface as a result of wear (mm3/year)
49
Linear wear
Loss of height of bearing surface (mm/year)
50
Consequences of wear & wear particles (6 answers)
1. Synovitis
2. Aseptic loosening/osteolysis
3. Immune reaction
4. Systemic distribution
5. Increased friction of joint
6. Misalignment of joint/catastrophic failure
51
Friction
Resistance to sliding motion between two bodies in contact
52
Asperities
Projections from a surface of a material. The taller and more numerous the asperities, the rougher a surface is.
53
Mean surface roughness
Average height of asperities on a material. Expressed as Ra value.
54
Viscosity
Measure of internal friction of a fluid. Resistance to flow.
55
Rheology
Science of deformation and flow of matter
56
Shear
Rate of deformation of a fluid when subjected to a mechanical shearing stress
57
Shear stress
An applied force per unit area needed to produce deformation in a fluid
58
Newtonian fluid
Fluid that obeys Newton's law of viscosity. Shear stress is proportional to shear rate.
59
Non-Newtonian fluid
The shear stress does not vary in the same proportion or direction with changes in the shear rate (honey dripping off a spoon, synovial fluid in RA)
60
Pseudoplastic
A non-Newtonian fluid whose viscosity decreases as the applied shear rate increases. Also termed shear thinning (e.g. paints, emulsions, synovial fluid)
61
Dilatant
A non-Newtonian fluid whose viscosity increases as the applied shear rate increases. Also termed shear thickening.
62
Thixotropic
Pseudo-plastic flow that is time dependent. When sheared at a constant rate, viscosity gradually decreases (e.g. synovial fluid, grease, heavy paints)
63
Rheopexy
Opposite of thixotropic behaviour. Fluid's viscosity increases with time as it is sheared at a constant rate.
64
Types of lubrication
Fluid-film lubrication
Boundary lubricationFluid
65
Fluid-film lubrication
Surfaces are separated by a fluid film that fully supports the applied load prevent direct contact between the surfaces. The minimum thickness of fluid film must exceed the surface roughness of a bearing surface in order to prevent contact between asperities.
66
Boundary lubrication
Bearing surfaces are in contact but separated by a boundary lubricant of molecular thickness, which prevents excessive bearing friction and wear. The load is carried by the surface asperities, rather than by the lubricant.
67
Types of fluid-film lubrication
Hydrodynamic lubrication
Elastohydrodynamic lubrication
Micro-elastohydrodynamic lubrication (MEHD)
Squeeze-film lubrication
Boosted lubrication
Weeping lubrication
68
Wettability
Relative affinity of a lubricant for a material. Measured by the angle of contact at the edge of a drop of lubricant. A larger angle = less wettability. More hydrophilic materials and more wettable.
69
Factors determining lubrication
1. Magnitude & direction of loading
2. Geometry of the bearing surface/surface roughness
3. Material properties of the surface e.g. wettability
4. Velocity at which the bearing operates
5. Viscosity of the lubricant
70
Parts of limb bud (3 parts)
1. Apical ectodermal ridge
2. Progress Zone
3. Zone of Polarising Activity
71
Apical ectodermal ridge
Controls proximodistal growth under influence of FGF-2, 4, 8 & 10.
72
Progress Zone
Controls dorsoventral growth under influence of Wnt-7a and En-1. Maintained by AER.
73
Zone of Polarising Activity
Controls radio-ulnar growth under influence of Shh protein. Little finger is made where Shh is highest.
74
Congenital Limb Deformity Classification (7 types)
Originally described by Swanson
1. Failure of formation (transverse or longitudinal, pre or post axial)
2. Failure of differentiation
3. Duplication
4. Overgrowth
5. Undergrowth
6. Amniotic band syndrome
7. Other generalised skeletal abnormalities
75
Physis
Structure consisting of highly ordered chondrocytes within an extracellular matrix in line with the longitudinal axis of the bone
76
Zones of physis (5 layers)
1. Resting
2. Proliferative
3. Hypertrophic (maturation, death, zone of provisional calcification
4. Primary spongiosa
5. Secondary spongiosa
77
Bone graft types
Autograft
Allograft
Xenograft
Synthetic graft
78
Properties of bone graft
Osteoconductive (scaffolding)
Osteoinductive (biology)
Osteogenic (bone precursor)
79
80
Fatigue failure
Failure of a material with repetitive loading at stress levels below the ultimate tensile strength
81
Notch sensitivity
Extent to which the sensitivity of a material to fracture is increased by the presence of a surface inhomogeneity e.g. cracks, scratches. Ductile materials have less notch sensitivity than brittle ones.
82
Corrosion
Unwanted dissolution of a metal in a solution resulting in its continued degradation
83
Types of corrosion
Generalised or localized
84
Galvanic corrosion
Occurs when two dissimilar metals are electrically coupled together forcing the more active alloy to become anionic and the more noble alloy to become cathodic. If corrosion occurs, it will affect the more active metal in an accelerated way.
85
Types of localised corrosion
1. Pitting corrosion
2. Crevice corrosion
3. Fretting corrosion
4. Stress corrosion (fatigue)
5. Intergranular corrosion
6. Intragranular corrosion (leaching)
7. Inclusion corrosion
86
Theatre Design:
Four Zones of Operating Theatre
1. Outer zone (theatre reception and beyond)
2. Clean zone (from reception up to theatre doors)
3. Aseptic zone (beyond red line, between clean corridor and dirty corridor)
4. Disposal zone (sluice etc)
87
Theatre Design:
Ideal theatre temperature
24-26 degrees Celcius as patients are at risk of hypothermia from paralysis, cool IV fluids and large exposed wounds. Ideal temperatures for surgeons are 19-20 degrees so patient has a microclimate created by airflow mattresses. If temperatures too high than working time for PMMA is significantly reduced.
88
Theatre Design:
Relative humidity for theatre environment
40-60%. High humidity reduces the working time of PMMA.
89
Theatre Design:
Illumination in theatre
High quality artificial illumination without shadows is required. Should be capable of minimum 40,000 lux at the incision site. Should be easily adjustable by the surgical team. Satellite lights create heat and subsequent convection currents which can affect airflow systems.
90
Theatre Design:
Sources of contamination (four)
1. Direct contamination by theatre personnel
2. Instruments/equipment
3. Airborne contamination
4. The patient
91
Theatre Design:
Air cleanliness
In ultra-clean laminar flow theatres there should be less than 20 CFU/m3 at the periphery of the enclosure and less than 10 CFU/m3 at the centre.
CFU = Colony Forming Units
92
Theatre Design:
Types of Air Ventilation
Plenum
Laminar flow
93
Theatre Design:
Plenum air flow
Pressure inside theatre is greater than outside to drive clean air down from wall or ceiling diffusers and let out via vents just above floor level. Air passes out around doors so doors opening should be minimised as 2m3 air transfers and creates turbulence. Standard positive pressure ventilated theatres should deliver around 15-25 air changes per hour.
94
Theatre Design:
Laminar air flow
Entire body of air within a designated space moving with uniform velocity in a single direction along parallel flow lines. Laminar flow theatres deliver 300-500 air changes/hour.
95
Theatre Design:
Three types of laminar air flow
1. Horizontal laminar flow
2. Vertical laminar flow
3. Exponential flow (Howorth enclosures)
96
Theatre Design:
Purpose of ventilation in theatre
To reduce airborne contamination, ventilation systems must provide a clean source of air and produce positive-pressure to displace contaminated air away from operation site and to prevent bacteria entry from contaminated sites.
97
Theatre Design:
Process of air ventilation in theatre
Air is drawn down at roof level and drawn through series of fans and filters to remove bacteria-carrying particles. Air is humidified and warmed or cooled. HEPA filters are commonly employed.
98
Theatre Design:
HEPA filtration
High-efficiency Particulate Air. HEPA filters are capable of filtering particles of 0.5micrometres in size with 99.97% efficiency.
99
Theatre Design:
Pros & Cons of horizontal laminar air flow
Pros: Easy to retrofit into existing theatres
Cons: Difficult to place staff/equipment and patient
Increased infection risk with TKR vs THR (Salvati et al 1982)
100
Theatre Design:
Pros & Cons of vertical laminar air flow
Pros: enclosures from the ceiling encasing the HEPA filters to within 2m of the floor direct air to ground
Cons: entrainment of the flow can be caused by personnel moving on the periphery
Flow is broken around obstructions such as operating lights but quickly forms again
101
Skin preparation:
Iodophors
Iodine complexed with solubilising agent such as povidone resulting in free iodine release when in solution. They work by disrupting bacterial proteins and DNA
102
Skin preparation:
Pros & cons of iodophors
Pros: potent, broad-spectrum and rapid-acting bactericidal agents
Active against viruses, spores and fungi
Cons: Inactivated by blood, faeces and pus
