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AY BMSc Lectures > Fracture Mechanics (Rigidity of Constructs) > Flashcards

Fracture Mechanics (Rigidity of Constructs) Flashcards

1
Q

What is strain?

A

Change in length/original length

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2
Q

In bone healing, what happens if strain is <2%?

A

Promotes primary bone healing

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3
Q

In bone healing, what happens if strain is >2% but <10%?

A

Promotes healing by callus

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4
Q

In bone healing, what happens if strain is >10%?

A

No healing (pseudoarthoses)

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5
Q

Why does failure occur if the plate is too flexible or weak/

A

Breakage

Loss of position (bending)

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6
Q

What does failure occur if the plate is too thick and strong?

A 
Non-union or delayed union 
Late fracture (either with the plate on or after removal of the plate)
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7
Q

What properties describe ressistance to breakage?

A

Ultimate strength

Fatigue strength

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8
Q

What property describes resistance to bending failure?

A

Yield point

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9
Q

What property describes resistance to failure of bone-implant interface?

A

Pull-out strength

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10
Q

What factors influence the mechanical properties of the device?

A

Stress/strain (and fatigue) characteristics of the material from which it is made
The dimensions of the device
The distribution of the material within the device

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11
Q

What is approx the Youngs modulus of cortical bone, titanium and SS?

A

Cortical bone = 20 GPa
Titanium = 100 GPa
SS = 200 GPa

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12
Q

What is a moment?

A

A force applied through a lever (moment) arm

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13
Q

What is the magnitude of deformation of a beam under load dependent on?

A

Length
Width
Thickness

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14
Q

How is the resistance of a simple plate to deformation related ro its thickness?

A

Proportional to the 3rd power of its thickness (“height”)

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15
Q

How is the strength of a simple beam related to its thickness?

A

Proportional to the 2nd power of the thickness

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16
Q

How do compression and tension forces within a beam change with the distance from the neutral axis?

A

Compression and tension forces increase with distane with distance from the neutral axis

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17
Q

What is the second moment of area?

A

Measure of a structures resistance to bending (the distribution of amterial around the centroidal axis)

18
Q

How is the second moment of area calculated for a simple beam with a rectangular cross section?

A

I = bh3 / 12

19
Q

What are the units for second moment of area?

A

mm4

20
Q

How is the second moment of area calculated for a cylindrical shape?

A

I = (1/4) πr4

21
Q

What is the purpose of locking screws?

A

To change the load sharing/load bearing relationship between the bone and the IM nail

22
Q

How do shear forces vary with distance from the neutral axis in torsion?

A

Increase with distance from the neutral axis

23
Q

What does the polar moment of area describe?

A

The distribution of maerial around the neutral axis (resistace to torson)

24
Q

What is resistance to torsion dependent on?

A

Shear modulus

J = polar moment of area

25
Q

How are I and J proportional to the radius?

A

Both proportional to the 4th power of the radius

I = (1/4) πr4
J = (1/2) πr4
26
Q

For any given mass of material which is stifer hollow or solid rod?

A

Hollow

27
Q

For any given external diameter which is stiffer hollow or solid tube?

A

Solid

28
Q

What is the strenght of a nail proprtional to?

A

Volume of material

Proprtional to 3rd power of radius

29
Q

Which material is particularly prone to notching?

A

Titanium

30
Q

What is the resistance of a nail to shear dependent on?

A

Cross-sec area

31
Q

What is the working length of a device?

A

Total unsupported length (easier to bend something that is long than short)

32
Q

How does the total movement (strain) at the fracture site change with working length?

A

As working length increases the total movement (strain) at the fracture site also increases

33
Q

What is the working length for torsion?

A

Distance between locking points for locked unreamed nail

34
Q

How does the working length for bending change as the fracture heals?

A

Working length decreases as fracture heals

35
Q

How is the torsional stiffness across a fracture proportional to the working lenght?

A

Torsional stiffness across fracture inversely proportional to working length

36
Q

What are the risks with a long working length?

A

Increases the risk of fatigue failure

37
Q

How does strain change with working length>

A

Strain increases with working length

38
Q

How are stress and working length related?

A

Stress is shared along the working length of the plate

39
Q

What affects the second moment of area of an ext fix?

A

The configuration of the fixator (e.g. uniaxial, V-frame etc)
The dimensions of the pins and the rods

40
Q

In fine wire what replaces second moment of area as the critical factor?

A

Tension

AY BMSc Lectures (8 decks)

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