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AY BMSc Hip Replacement > Unit 1 > Flashcards

Unit 1 Flashcards

1
Q

What are the 5 requirements of an implant?

A 
Biocompatibility 
Adequate strength 
Cost effective manufacture 
Practicability of insertion 
Relieve pain and enable daily activities
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2
Q

How does the stiffness of HDP compare to cancellous bone?

A

Similar

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3
Q

What is the main associated problem with implants?

A

Infection

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4
Q

What does anisotrophic mean?

A

Different mechanical properties in different directions

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5
Q

What are the 5 structural factors involved in implant design?

A 
Strength 
Stiffness
Lubrication 
Wear 
Fatigue
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6
Q

What is a composite structure?

A

A structure consisting of more than one material

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7
Q

Why are most bones wider at the ends?

A

To accomodate the shape of the joint

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8
Q

Why do bone ends contain cancellous bone?

A

More porous and less stiff (more flexible) - shock absorbing properties

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9
Q

How are the trabecular lines arranged in cancellous bone?

A

Along directions of greatest stress (depends on loads)

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10
Q

What type of load is the main body of the femur naturally subjected to? Why?

A

Bending - Head of femur at the joint is displaced laterally from the bone shaft

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11
Q

What direction is the tibia naturally loaded?

A

Compressive in the vertical direction

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12
Q

What is the structure of bone directly beneath articular surfaces and why?

A

More dense than cancellous bone - to provide rigid surface for the joint to bear on without causing excessive deformation of the bearing surfaces

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13
Q

Which type of bone mainly makes up the shafts of bones?

A

Compact (cortical) bone

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14
Q

What is Young’s Modulus?

A

Stiffness property of a material (ratio of stress to strain)

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15
Q

Most non-biological structures are isotropic, what does this mean?

A

Mechanical properties are the same no matter which direction the are loaded

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16
Q

When is cortical bone stiffest and strongest?

A

When loaded longitudinally

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17
Q

How does the rate of loading of bone affect its sitffness?

A

The faster it is loaded the stiffer it becomes

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18
Q

How does the strength of bone differ in shear, tensile and compressive loading?

A 
Tensile = 2x shear
Compressive = 3x shear
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19
Q

Why is it not desirable to create a mesh-like structure (imitating cancellous bone) in an implant?

A

Infection (increased surface area)

Not firm enough for attaching or bonding artificial joint

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20
Q

What is stress shielding?

A

When bone is shielded by an implant from taking its full normal load and is resorbed (Wolff’s law)

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21
Q

What 2 effects can load transfer have on a bone-implant interface?

A 
Interface stresses (when bonded) 
Relative movement at interface (when not bonded)
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22
Q

How does the Young’s Modulus of bonded materials affect the shear stresses at their interface?

A

The greater the difference in the Young’s Modulus the greater the shear stress generated (one material is trying to expand more than the other)

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23
Q

How are shear stresses at a bonded interface reduced?

A

Lubrication

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24
Q

What determines the amount of load transferred from bone to implant (or vice versa)?

A

How loads are shared in load sharing region (which depends on the relative stiffness of the materials)

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25
Q

What 2 factors determine the stiffness of a structural component?

A 
Material stiffness (basic property of the material) 
Geometrical stiffness (to do with shape of cross-sec)
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26
Q

What is the shear modulus?

A

G = shear stress / shear strain

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27
Q

How is stiffness defined mathematically?

A

The force required to produce a unit deflection

28
Q

How do Young’s Modulus, cross-sec area and length affect stiffness of a structure?

A

S = EA/L

As E increases stiffness increases
As A increases stiffness increases
As L increases stiffness decreases

29
Q

How is the stiffness of 2 implants compared if they are the same length?

A

Rigidity

30
Q

Equation for axial rigidity

A

R = EA

31
Q

Equation for bending rigidity

A

R = EI

32
Q

What is the second moment of area (I)?

A

Measure of how resistant something is to bending

33
Q

What is the formula for I in a rectangular cross-sec?

A

I = bd3 / 12

34
Q

What is the formula for I in a circular cross-sec?

A

I = πd4 / 64

35
Q

Equation for torsional rigidity

A

R = GJ

36
Q

What is the polar second moment of area (J)?

A

A measue of how resistant something is to torsion

37
Q

For circular sections, how are J and I related?

A

J = 2I

38
Q

What is the ratio of load taken by bone to load taken by stem equivalent to?

A

The ratio of their rigidities

Lb / Ls = Rb / Rs

39
Q

How is the proportion of load taken by the bone calculated?

A

Ratio of the rigidity of the bone to the rigidity of the section
Lb/Ls = Rb/Rs = Rb/Rb + Rs

40
Q

What is an interference fit?

A

When the dimensions of the inner component are slightly larger than those of the outer component and the implant is pressed into the bone to reduce the risk of loosening

41
Q

Why does a bolt cause more trauma to tissues than a screw?

A

Bolt needs access from both sides whereas screw only needs access from one

42
Q

What is the purpose of bone cement?

A

To fill gaps between a bone and implant so a perfect geometrical match is not required

43
Q

Why is it not practical to use adhesives in implant fixation?

A

Bones are wet and difficult to access for cleaning and preparation prior to applying an adhesive

44
Q

What is the principle behind biological fixation?

A

Bone will grow into a porous coating, mesh r roughened area on the surface of an implant forming an interlock between the two materials

45
Q

Why are metallic bead coating used mostly with titanium prostheses stems?

A

Titanium is least corrosive and most biocompatible (beading increases surface area so increased risk of corrosion)

46
Q

What is the main mineral consituent of bone?

A

Hydroxyapatite (HAp)

47
Q

What technique is used to deposit HAp directly onto the metal surface?

A

Plasma spray coating

48
Q

What have clinical trials of HAp plasma spray coating shown?

A

Goof short term bonding

After 1-2 years some of the coating disappears - failure

49
Q

What technique can improve the longevity of HAp bonding?

A

Apply HAp to porous metal coating (e.g. titanium beads)

50
Q

What are biomaterials?

A

Non-biological materials used within the body (normally to repair or repair body parts that have failed

51
Q

Define corrosion

A

Progressive unwanted removal of material by an electrochemical process

52
Q

Describe galvanic corrosion

A

When 2 electrodes are immersed in an electrolyte an electrical current can flow allowing chemical reaction to take place between electrodes and the electrolyte

53
Q

In implants when is the corrosion reaction most severe?

A

When the electrodes are different metals

54
Q

What are the 3 alloys used in orthopaedic implants?

A

Stainless steel
Cobalt chrome
Titanium alloys

55
Q

Which metal is the only one used in pure form in implants?

A

Titanium

56
Q

What provides the good corrosion resistance in the metal alloys and titanium?

A

Passivation layer

57
Q

What is fretting corrosion?

A

When there is abrasion of materials in contact which removes the protective metal oxide later allowing corrosion to occur

58
Q

Describe crevice corrosion

A

Occurs in crevices between implants where body fluid can become trapped and lose its normal supply of oxygen - high conc acids form which corrodes the metals

59
Q

What areas are particularly prone to crevice corrosion?

A

Edges of bone plates and between screws and plates

60
Q

What are the two methods for improving corrosion resistance of metals?

A

Nitric acid immersion

Titanium nitride coating

61
Q

Describe nitric acid immersion

A

Improves natural passivation (oxide) layer - mechanisms unclear but thought to be related to increased amount of chromium in passivation layer which improves corrosion resistance

62
Q

The release of which two chemicals is reduced with titanium nitride coating?

A

Vanadium

Aluminium

63
Q

Which alloy does titanium nitride coating not work on?

A

Titanium alloys

64
Q

Name 7 biological reactions to implant materials

A

Growth of thin fibrous layer
Local infection
Body sensitisation to metals
Inflammation in regions of metal corrosion
Tissue necrosis in the region of bone cement
Immmune reaction to wear particles
Tumours

65
Q

Where do products of corrosion of metallic and polymetric implants appear?

A 
Blood 
Urine 
Some tissues 
Storage organs (e.g. liver) 
Hair and nails
66
Q

When does a fibrous layer tend to form between a bone and implant?

A

When there is micromotion at the interface

67
Q

Why does tissue necrosis occur in the region of bone cement?

A

Cement generates considerable heat as it cures - also tissue damage due to leaching of monomor and reaction to additives present in the cement

AY BMSc Hip Replacement (8 decks)

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