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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
What 2 factors determine the stiffness of a structural component?
``` Material stiffness (basic property of the material) Geometrical stiffness (to do with shape of cross-sec) ```
26
What is the shear modulus?
G = shear stress / shear strain
27
How is stiffness defined mathematically?
The force required to produce a unit deflection
28
How do Young's Modulus, cross-sec area and length affect stiffness of a structure?
S = EA/L As E increases stiffness increases As A increases stiffness increases As L increases stiffness decreases
29
How is the stiffness of 2 implants compared if they are the same length?
Rigidity
30
Equation for axial rigidity
R = EA
31
Equation for bending rigidity
R = EI
32
What is the second moment of area (I)?
Measure of how resistant something is to bending
33
What is the formula for I in a rectangular cross-sec?
I = bd3 / 12
34
What is the formula for I in a circular cross-sec?
I = πd4 / 64
35
Equation for torsional rigidity
R = GJ
36
What is the polar second moment of area (J)?
A measue of how resistant something is to torsion
37
For circular sections, how are J and I related?
J = 2I
38
What is the ratio of load taken by bone to load taken by stem equivalent to?
The ratio of their rigidities | Lb / Ls = Rb / Rs
39
How is the proportion of load taken by the bone calculated?
Ratio of the rigidity of the bone to the rigidity of the section Lb/Ls = Rb/Rs = Rb/Rb + Rs
40
What is an interference fit?
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
Why does a bolt cause more trauma to tissues than a screw?
Bolt needs access from both sides whereas screw only needs access from one
42
What is the purpose of bone cement?
To fill gaps between a bone and implant so a perfect geometrical match is not required
43
Why is it not practical to use adhesives in implant fixation?
Bones are wet and difficult to access for cleaning and preparation prior to applying an adhesive
44
What is the principle behind biological fixation?
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
Why are metallic bead coating used mostly with titanium prostheses stems?
Titanium is least corrosive and most biocompatible (beading increases surface area so increased risk of corrosion)
46
What is the main mineral consituent of bone?
Hydroxyapatite (HAp)
47
What technique is used to deposit HAp directly onto the metal surface?
Plasma spray coating
48
What have clinical trials of HAp plasma spray coating shown?
Goof short term bonding | After 1-2 years some of the coating disappears - failure
49
What technique can improve the longevity of HAp bonding?
Apply HAp to porous metal coating (e.g. titanium beads)
50
What are biomaterials?
Non-biological materials used within the body (normally to repair or repair body parts that have failed
51
Define corrosion
Progressive unwanted removal of material by an electrochemical process
52
Describe galvanic corrosion
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
In implants when is the corrosion reaction most severe?
When the electrodes are different metals
54
What are the 3 alloys used in orthopaedic implants?
Stainless steel Cobalt chrome Titanium alloys
55
Which metal is the only one used in pure form in implants?
Titanium
56
What provides the good corrosion resistance in the metal alloys and titanium?
Passivation layer
57
What is fretting corrosion?
When there is abrasion of materials in contact which removes the protective metal oxide later allowing corrosion to occur
58
Describe crevice corrosion
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
What areas are particularly prone to crevice corrosion?
Edges of bone plates and between screws and plates
60
What are the two methods for improving corrosion resistance of metals?
Nitric acid immersion | Titanium nitride coating
61
Describe nitric acid immersion
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
The release of which two chemicals is reduced with titanium nitride coating?
Vanadium | Aluminium
63
Which alloy does titanium nitride coating not work on?
Titanium alloys
64
Name 7 biological reactions to implant materials
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
Where do products of corrosion of metallic and polymetric implants appear?
``` Blood Urine Some tissues Storage organs (e.g. liver) Hair and nails ```
66
When does a fibrous layer tend to form between a bone and implant?
When there is micromotion at the interface
67
Why does tissue necrosis occur in the region of bone cement?
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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