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Imp Tech U2 Flashcards

1
Q

What are different methods of preventing the stem from sinking into the medullary canal?

A
  • Tapering the stem
  • Using a proximal collar
  • Fixing the bone to the stem (bone ingrowth or adhesion)
  • Using cement strong enough to withstand shear stresses
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2
Q

How can the interface shear stresses be reduced in a hip implant?

A
  • Proximal collar (or other support)
  • Tapering the stem

Both methods convert shear loads into compressive loads

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3
Q

How is compressive joint force transferred to the femur?

A

As shear force

Either directly between the bone and stem or within cement in cemented prosthesis

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4
Q

What happens if the stem-cement-bone or stem-bone bond is not strong enough?

A

The prosthesis will loosen and sink into medullary canal

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5
Q

How is the compressive stress calculated at any point along the stem? Is it the same along the entire length?

A

Compressive stress = compressive load/cross sectional area

Varies along the length of the stem depending upon the load transfer and stress shielding

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6
Q

How is stem fracture avoided?

A
  • Use a large enough cross section to resist stress

- Use a high strength material

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7
Q

How can excessive stress shielding of bone be avoided?

A

Choosing an appropriate rigidity of stem

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8
Q

Why does the femur experience bending stress? Which side is compressed?

A

Direction of joint force is not along the neutral axis of the bone
To balance body, adductor muscle force is 2x BW = significant bending moment

Compression on medial side and tension of the lateral side

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9
Q

How does a stem affect bending loading the in femur?

A

It takes some of the bending load from the bone (as stiffer) and reduces stresses in the bone = stress shielding

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10
Q

Where are the 2 main contact points between the femur and a stem? How does this affect the rotation of the stem?

A
  1. Medial proximal insertion point
  2. Lateral distal point

Counteracts tendency to rotate due to bending action

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11
Q

What is the main likelihood of stem failure?

A

Loosening proximally –> the bending moment at the distal end increases dramatically and failure occurs

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12
Q

What design considerations should be made to ensure the stem does not fail under bending loading?

A
  • Large second moment of area

- Select shape to limit bending moments due to hip force

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13
Q

What are hoop stresses? Where are they greatest?

A 
Circumferential stresses (mourned the outer circumference)
Radial stresses (radiate from the centre out)

Greatest at the most proximal and distal points of bone-stem contact

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14
Q

What type of interface stress does torsional stress cause? What is the consequence of this?

A

Shear stress across the bone-stem interface

Loosening then sinking under compressive loading

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15
Q

How can torsion in the stem be reduced?

A

By choosing abnormally shaped cross sections (ie triangle/rectangular)
Shear loading then becomes compressive stress

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16
Q

How is the acetabulum formed? (bone types)

A

Cancellous bone encased in cortical bone

Cortical bone shell is highly stressed when femoral head presses into it

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17
Q

What occurs if the cortical bone shell of the acetabulum has to be broken during surgery?

A

The cancellous bone has to take the load (not normally stressed)
Replacement head +cup have a smaller diameter = higher stress concentration (less contact area)

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18
Q

What design factors should be considered regarding stress in the acetabulum?

A
  • Ways to maintain the integrity of the subchondral cortical bone
  • Size + conformity of replacement joint surfaces (affect contact area –> so contact stresses)
  • Whether cup should have a metal backing plate
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19
Q

How are cement less implants fixed to bone?

A

Press fit or bone growth into it

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20
Q

What is bone cement formed from?

A

PMMA (monomer power which becomes a polymer on addition of a catalyst)
Remains plastic for insertion then becomes solid

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21
Q

What are the advantages of a cemented prosthesis?

A
  • Surfaces don’t need to be an exact fit

- Allow even stress distribution (cement inserted under high pressure to fill gaps)

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22
Q

What are the problems with cement? (6)

A
  • Exothermic chemical reaction (can destroy nearby body tissues when setting)
  • Small fragments of cement can cause intense inflammatory reactions
  • Leftover monomer can be very toxic (polymer is okay) - needs proper mixing + proportions
  • Cement fragments that fall into joints can increase surface wear
  • Strong under compression but have weak shear or tensile strength
  • A grout rather than an adhesive
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23
Q

Who developed the first successful hip replacement? What specific features of this were successful?

A

Charnley

Smaller femoral head (to reduce loosening from bearing friction)
Bone cement (to help distribute loads between bone + prosthesis)
HDP bearing material (along with metal had a low friction bearing surface)
Specific set of instrumentation to match the prosthesis

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24
Q

Why should micro motion of the prosthesis be kept to a minimum after surgery?

A

To prevent fibrous tissue formation at the interface (prevents good bonding and ingrowth of bone into hydroxyapatite prostheses)

Has no shear or tensile strength

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25
How successful are cementless prostheses?
Not yet shown to be any better than Charnley
26
Why are cements hard to develop?
They are considered a drug so are expensive and time consuming to design an alternative
27
What is the risk of wear particles?
They can cause a biological reaction leading to bone resorption
28
Why should the surface of a cemented prosthesis be roughened and what is the risk associated with this?
To allow better interlocking | IF interlocking fails will cause greater abrasive wear as surfaces slide past each other
29
How is the Exeter hip designed for binding to the bone-stem?
Smooth metal to allow it to sink into the canal and form an interface fit in the cement
30
How is load transferred from the stem to the bone? What is the risk of this?
As shear force | Can break the interlocking grip and cause loosening
31
How does the rigidity of the stem affect stress shielding/load sharing at the interface?
More rigid = more load taken proximally = more stress shielding More flexible = less load taken proximally = less stress shielding
32
How does the stiffness of the stem affect interface shear stresses?
``` Stiffer = less shear stress (but more stress shielding) Flexible = more shear stress ```
33
How/where is load transferred from the stem to the bone? | How does this affect shear stresses?
All the load initially taken by the stem, some transferred proximally (at load transfer region), then load shared in load sharing region, then the rest of the load transferred in distal load transfer region If the material is more elastic = more proximal load sharing, so higher shear stresses (can be enough to cause failure) Needs to be a compromise between keeping stress shielding low, and keeping proximal interface stresses low
34
What is compromised to keep stress shielding low?
Proximal interface shear stress (if material less stiff = less stress shielding, but greater proximal load transfer so higher proximal interface shear stress)
35
What are the features of proposed computer generated stems?
- Tapered at the ends to reduce proximal + distal cement stresses - Higher stresses within the stem (less material as is tapered) but strong enough to take the loads - Less rigid due to narrowing
36
What is the benefit of cement (other than reducing need for precision in cutting the femur)?
Avoids high stress concentrations as there is contact between the stem and bone along the whole stem
37
What are the risks of having too thin cement (<2mm)?
High cement stresses | Bone resorption at proximal end of femur (maybe due to cement debris causing adverse tissue reaction)
38
What are the risks of having cement too thick?
High cement stresses (similarly as if too thin)
39
What are the risks of cement which is too stiff?
Higher proximal and distal cement and interface stresses
40
What are the arguments for use of a collar?
- Allows compressive stress transfer from implant to the bone (reduces stress shielding) - Lowers cement stress in proximal medial region
41
What are the arguments against using a collar?
- Calcar must be cut very accurately to allow collar to sit on substantial part of bone - Collar-calcar contact area acts as a pivot - exposes distal end of stem to high stress concentrations + failure - Risk of debris due to fretting of collar against cement or bone
42
What is the calcar?
Thick cortical part of femoral neck
43
How can the bond to cement be improved? What is the risk of these techniques if there is loosening?
Coating stem in PMMA Roughening stem surface Applying a porous coating Shaping of the stem Risk; rubbing together and debris
44
What are the advantages of the polished stem in the Exeter hip?
Will sink into medullary canal and form an interference fit (free to slide) Places more compressive stress on cement (advantageous as not shear stress)
45
How is the collared hip advantageous in reducing stress in the cement?
Prevents any unnecessary sinkage | Bone-cement interface is well interlocked so no need for stem-cement interface to slip to protect it
46
What is a consequence of no distal contact from cement less stems?
Thigh pain | sometimes plastic sleeves can be used to provide distal contact and reduce thigh pain
47
What are cement less stems coated in and what is the benefits of this?
Coated in hydroxyapatite - helps with bone ingrowth - eliminates metal debris from bone-metal abrasion - allows bone to bond to a larger area of the stem (reducing chances of failure)
48
What factors can prevent bone ingrowth in cementless stems?
- Excessive movement at the bone-stem interface - Fibrous tissue growth seals the stem If ingrowth occurs often breaks down after a few years (cause unclear)
49
Why is it difficult to choose the right size of stem?
- Femoral canal dimensions do not vary in proportion to the size of the femur - Diameter varies with age (as get older, bone things and canal becomes wider)
50
What kind of stress does femoral offset cause and how can this be reduced?
Bending stress | Can be reduce by reducing length of neck, or increasing ankle between axes (reduce lever arm)
51
What are the disadvantages of reducing femoral offset?
Joint reaction force increases which increases joint wear (materials are stronger now so more anatomically correct ones are now used)
52
Why are steel and titanium rarely used for joint surfaces and what is used instead?
Steel - not very corrosion resistant Titanium - poor wear properties Ceramic used - v corrosion resistant, low friction and less wear on HDP
53
Is the shear force at the acetabular interface less if the head is smaller or larger? Why?
Smaller femoral head = less interface shear force | There is less friction force with a smaller head reducing the stress
54
What are the two types of joint wear? Describe both
Adhesive wear; 2 bearing surfaces stick together when pressed together, softer one is torn off by the harder one (can reduce by using a lubricant) Abrasive wear; surfaces aren't perfectly smooth, third body particles can cause wear accelerating the process (using a lubricant means wear particles removed) softer material will wear away first
55
What are the risks of HDP wear particles?
Cause intense inflammatory tissue reactions and loosening | Can migrate from the acetabular cup (where used) to the distal stem of implant
56
How can adhesive wear be reduced?
- Reduce joint loading - Minimise sliding distance (smaller head slides less) - Find alternative materials to reduce wear on HDP
57
What are the disadvantages of a smaller head?
- Depth of wear is greater (ROM is reduced as HDP wears) | - Greater risk of dislocation post-op (due to neck impingement on the edge of the cup)
58
What is the acetabular cup made of?
HDP | Sometimes has a metal backing between HDP and bone interface
59
What are the advantages/disadvantages of a metal backing plate?
Advantages; - Holds plastic in place + reduces creep/distortion - Avoids high contact stress and focal wear on HDP - In theory reduces high stress concentrations by distributing more evenly (less effective in practice) Disadvantages - Can increase head-cup contact pressure (thinner HDP = high contact pressure)
60
What factors of the acetabular cup affect joint wear?
- clearance between cup and femoral head (should be small) - Diameter of femoral head (bigger = less contact pressure) - Radial clearance - difference in radii between head an cup (if HDP thinner/stiffer, should reduce radial clearance to spread load out over greater area)
61
What are the two main causes of cup loosening?
Mechanical over stressing, biological reaction to HDP wear particles
62
What are the advantages and disadvantages of centralisation of the cup?
Advantages; lower load at hip joint Disadvantages; - have to break the strong cortical bone to move centrally - then mounted on softer/weaker bone - deepened cup means impingement more likely between femoral neck/rim of cup - limits motion + increases dislocation risk
63
What is the most common cause of hip replacement failure if there is not technical failure? What is the actual most common cause of failure?
Acetabular wear | Loosening (due to HDP particles)

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