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Millers > Biomaterials + Biomechanics > Flashcards

Biomaterials + Biomechanics Flashcards

1
Q

What is the stress equation

A

Stress = force/area
Property of the object (same force can produce different stress in different objects)

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2
Q

What is the strain equation

A

Strain = change in length or height / original length or height

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3
Q

Stress strain curve:
What is the x vs y axis of the graph?
What is the yield point
What is the linear portion of the graph before the yield point
What do we call the slope of the linear portion

A

x = strain, y = stress
Yield point = first peak on graph, stress is directly proportional to strain until the yield pt where permanent deformation occurs
The linear area before the yield point = elastic zone
- If stress removed, strain is recovered
Slope = y/x = stress/strain = Young’s modulus
- Straighter the line = steeper slope = stiffer material

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4
Q

Stress strain curve:
What is the region of the curve after the yield point called?
What is the peak of this second curve called?
What is necking?
What is the last point on the graph called?

A

Plastic region/deformation
2nd peak = ultimate strength = max stress point on graph
Necking = the downturn in the graph after ultimate strength point
- One part of the material sees an increase in stress - narrows here - reduce cross sectional area
- Overall decreased stress
Last pt on graph = fracture point = material failure

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5
Q

Stress strain curve:
What is fatigue

A

Fail below the ultimate strength 2/2 numerous loading cycles
Shading of curve between yield point and ultimate strength

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6
Q

Stress strain curve:
What is stiffness / depends on what type of deformation
What is strength / depends on what type of deformation

A

Stiffness = resistance to shape change = Young’s modulus
- Elastic deformation
- Vertical line / steep slope = stiffer
- Horizontal line = flexible

Strength = load to ultimate strength
- How high is the ultimate strength pt on the Y axis
- Plastic deformation

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7
Q

Stress strain curve:
Define ductility
Define toughness

A

Ductility = difference between yield and frx pts
- Small difference = brittle (glass)
- Big difference = ductile (copper wire)

Toughness = ability to absorb energy = area under curve
- Aka how far the curve stretches to the right
- Wide curve = ductile
- Narrow curve = brittle

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8
Q

Define:
Viscoelastic
Creep
How do you prevent creep

A

Viscoelastic - mech prop change with external force
- Stronger/stiff/tougher when loaded over short time

Creep - apply constant stress over long time, slowly increase strain
- Plastic deformation below the yield point
- Cause of failure under loads significantly below ultimate strength

Prevent creep w/ stress relaxation
- Hold stain constant (aka keep at certain length)
- Stress decreases over time

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9
Q

Define hysteresis

A

Viscoelastic material dissipates energy bet loading/unloading cycles
“Shock absorption”

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10
Q

Define:
Isotropic
Anisotropic

A

Isotropic = mat’l behaves same regardless of force
Ie: metal, woven bone, golf ball

Anisotropic = behaves diff depending on force
Ie: cortical bone, cartilage

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11
Q

When they show you the stress strain curve on the test and have you identify which material is which - what part are they showing you?

A

Elastic region only!
Youngs modulus

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12
Q

Ceramic Cost Could Cement Polly to Cancel her Telephone Carrier

Ceramic
Co
S
T
Co
Cement
Polly
Cancel
Te
L
Car

A

Ceramic
Co - Co-Cr-Mo alloy
Stainless steel
Titanium
Cortical bone
Cement = PMMA
Polly = polyethylene
Cancel = cancellous bone
Tendon
Ligament
Cartilage

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13
Q

What metals make up:
Stainless steel
Titanium
Co-Cr

A

SS = iron, chromium, nickel….

T = titanium, aluminum, vanadium

CoCr = cobalt, chrome, nickel….

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14
Q

Pros/cons stainless steel

A

STIFF - cemented stems
Cons: stress + crevice/pitting corrosion (because contains iron!!)

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15
Q

Pros/cons CoCr

A

Pros:
- Strong
- Corrosion resistant

Cons: wayyyyyy stiffer than bone

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16
Q

Pros/cons titanium

A

Pros:
- Stiffness = bone (aka more flexible), press fit stems

Cons:
- Poor wear
- Most adherent for bacteria

17
Q

Corrosion:
Fretting
Galvanic

A

Fretting = micro motion of stable implants (THA head-neck junction)

Galvanic = current between dissimilar metals (SS + CoCr)

18
Q

How do we sterilize UHMWPE
What do we have to do after this step

A

Sterilize via radiation
- O2 rich environment = old technique bc creates free radicals
- Inert gas
1 Increase cross linking = improves wear
2 BUT makes weaker

Then do something to remove any lingering O2 free radicles from either sterilization technique:
- Remelt - most effect bc happens just above melting point
- Anneal - heat to below melting point

19
Q

Why do we vacuum mix cement

A

Makes less porous
Increases strength

20
Q

What are suture anchors made of
How does the body break them down

A

Lactic acid + polymer

Polymer breaks down -> inflam response -> eaten by macrophages + PMN
Lactic acid enters Kreb cycle -> CO2 + H2O

21
Q

What forces is bone strong vs weak with

A

Strong w/ fast longitudinal load (bc bone is anisotropic)
Strongest in compression

Weakest in sheer (comminution)

22
Q

What is the toe region of the stress strain curve for ligaments

A

Very start of the curve before elastic region
“Viscoelastic behavior with nonlinear elasticity”
Crimped fibers straightening - then can jump on the normal curve

23
Q

Screws:
What is the weakest part
What part determines the stiffness
What part determines the pull out strength

A

Weakest = neck
Stiffness def by core diameter
Pull out strength det by outer diameter
- Max pull out strength = small core, large outer, fine pitch

24
Q

Plates:
Stiffness proportional to thickness X power

Nails:
Stiffness proportional to radius X power

A

Plate = ^3
Thicker plate increase stiffness^3

Nail = ^4
Increase radius = stiffness^4

25
Q

Ways to make an ex fix stiffer

A

Get fracture opposed
Use larger diameter pins
More pins per segment
Bring rod closer to bone
Add 2nd rod

26
Q

What is the role of lubrication in a joint

A

Decreases coeff of friction between surfaces

27
Q

Why do you use a cane in the hand opposite the affected hip

A

Drops joint reaction forces in the hip by 60%

28
Q

How does the tibia move through knee ROM

A

Knee extends, tibia ER
Knee flex, tibia IR
Screw home mechanism

29
Q

What is the instant center of motion/rotation for:
Ankle
Wrist

A

Ankle = talus
Wrist = capitate

30
Q

Define crevice corrosion

A

The micro-isolation of O2, leading to disruption of passivation

Millers (29 decks)

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