Biomaterials + Biomechanics Flashcards
What is the stress equation
Stress = force/area
Property of the object (same force can produce different stress in different objects)
What is the strain equation
Strain = change in length or height / original length or height
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
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
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?
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
Stress strain curve:
What is fatigue
Fail below the ultimate strength 2/2 numerous loading cycles
Shading of curve between yield point and ultimate strength
Stress strain curve:
What is stiffness / depends on what type of deformation
What is strength / depends on what type of deformation
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
Stress strain curve:
Define ductility
Define toughness
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
Define:
Viscoelastic
Creep
How do you prevent creep
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
Define hysteresis
Viscoelastic material dissipates energy bet loading/unloading cycles
“Shock absorption”
Define:
Isotropic
Anisotropic
Isotropic = mat’l behaves same regardless of force
Ie: metal, woven bone, golf ball
Anisotropic = behaves diff depending on force
Ie: cortical bone, cartilage
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?
Elastic region only!
Youngs modulus
Ceramic Cost Could Cement Polly to Cancel her Telephone Carrier
Ceramic
Co
S
T
Co
Cement
Polly
Cancel
Te
L
Car
Ceramic
Co - Co-Cr-Mo alloy
Stainless steel
Titanium
Cortical bone
Cement = PMMA
Polly = polyethylene
Cancel = cancellous bone
Tendon
Ligament
Cartilage
What metals make up:
Stainless steel
Titanium
Co-Cr
SS = iron, chromium, nickel….
T = titanium, aluminum, vanadium
CoCr = cobalt, chrome, nickel….
Pros/cons stainless steel
STIFF - cemented stems
Cons: stress + crevice/pitting corrosion (because contains iron!!)
Pros/cons CoCr
Pros:
- Strong
- Corrosion resistant
Cons: wayyyyyy stiffer than bone
Pros/cons titanium
Pros:
- Stiffness = bone (aka more flexible), press fit stems
Cons:
- Poor wear
- Most adherent for bacteria
Corrosion:
Fretting
Galvanic
Fretting = micro motion of stable implants (THA head-neck junction)
Galvanic = current between dissimilar metals (SS + CoCr)
How do we sterilize UHMWPE
What do we have to do after this step
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
Why do we vacuum mix cement
Makes less porous
Increases strength
What are suture anchors made of
How does the body break them down
Lactic acid + polymer
Polymer breaks down -> inflam response -> eaten by macrophages + PMN
Lactic acid enters Kreb cycle -> CO2 + H2O
What forces is bone strong vs weak with
Strong w/ fast longitudinal load (bc bone is anisotropic)
Strongest in compression
Weakest in sheer (comminution)
What is the toe region of the stress strain curve for ligaments
Very start of the curve before elastic region
“Viscoelastic behavior with nonlinear elasticity”
Crimped fibers straightening - then can jump on the normal curve
Screws:
What is the weakest part
What part determines the stiffness
What part determines the pull out strength
Weakest = neck
Stiffness def by core diameter
Pull out strength det by outer diameter
- Max pull out strength = small core, large outer, fine pitch
Plates:
Stiffness proportional to thickness X power
Nails:
Stiffness proportional to radius X power
Plate = ^3
Thicker plate increase stiffness^3
Nail = ^4
Increase radius = stiffness^4
Ways to make an ex fix stiffer
Get fracture opposed
Use larger diameter pins
More pins per segment
Bring rod closer to bone
Add 2nd rod
What is the role of lubrication in a joint
Decreases coeff of friction between surfaces
Why do you use a cane in the hand opposite the affected hip
Drops joint reaction forces in the hip by 60%
How does the tibia move through knee ROM
Knee extends, tibia ER
Knee flex, tibia IR
Screw home mechanism
What is the instant center of motion/rotation for:
Ankle
Wrist
Ankle = talus
Wrist = capitate
Define crevice corrosion
The micro-isolation of O2, leading to disruption of passivation
