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BMSc - Applied Ortho Semester 2 > Implant technology - unit 3 deck 3 > Flashcards

Implant technology - unit 3 deck 3 Flashcards

1
Q

how does the young’s modulus of the HDP component affect the contact stress?

(higher youngs modulus = stiffer)

A

higher the young’s modulus of HDP the greater the contact stress

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2
Q

List four design features of a tibial component that affect the magnitude of the stresses on the underlying bone.

A
  • thickness of HDP component
  • whether or not HDP has a metal backing plate
  • whether tibial component has a stem
  • stiffness of a material
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3
Q

State one advantage of a modular tibial component that can take different thicknesses of tibial insert.

A

Ligament tension can be set correctly be selecting the right thickness of insert

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4
Q

State one advantage and one possible disadvantage of using a metal backed tibial component.

A

adv

    • provides a more even loading distribution on the underlyning bone

disadv

    • may cause excessively high stresses on the medial bone if the knee is unevenly loaded as the metal plate is stiff
    • thus does not spread out a high contact load as the less stiff HDP
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5
Q

Explain how a tibial peg can reduce the tendency for a tibial plateau to sink medially.

A

The peg can help resist the medial load place on the tibial component

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6
Q

what are the headings for the priniciple features of knee prostheses

A
  • femoral component shape
  • tibial surface shape
  • methods of anchorage of components
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7
Q

how does the femoral component shape of the prostheses vary from the shape of the normal knee

A
  • left and right side of the prostheses are symmetrical
  • in a normal knee, the medial condyle is larger than the lateral condyle
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8
Q

what is the main disadvantage of trying to copy the knee’s natural asymmetry

A

doubles the size of the required inventory of components required to carry out knee joint replacement

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9
Q

what does the anterior part of the femoral component curvature accommodate

A

the movement of the patella during flexion and extension

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10
Q

what largely determines the degree of constraint of a knee prosthesis

A

the surface shape design of the tibial component

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11
Q

what shape of tibial component is favoured and why

A
  • partially constrained shapes, used in total condylar prostheses
  • they provide the required degree of functional movement and do not suffer greatly from loosening due to overstressing and limit the range of sliding motion to help reduce wear
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12
Q

what tibial component shape is needed if the PCL is retained

A

flattish surface profile

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13
Q

what tibial component shape is required if the PCL is not retained

A

surface shape which is dished in all directions

(and a posterior stabalised design mechanism is used to compensate for lack of PCL)

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14
Q

what does the posterior stabilised design mechanism need to be able to do in knee prostheses

A
    • prevent posterior femoral subluxation of the femur over the tibia
    • cause the femur to “roll back” as it flexes
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15
Q

what is an example of a knee prostheses which has a posterior stabilised design mechanism

A

“cam” shaped designs

  • employ more of a gradual curve on the tibial component
  • produces a smoother transition to the roll back position in flexion
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16
Q

if a knee joint replacement is properly aligned and the ligaments balanced, how should the femoral and tibial component be loaded and therefore what should provide good anchorage to these components?

A
  • the femoral and tibial components should be maintained in compression throughtout the range of joint motion
  • [if tibial component is loaded evenly]

therefore use of PMMA bone cement should provide good anchorage

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17
Q

what will happen to the tibial component if only one condyle is loaded and what type of moment causes this ?

A

due to a lateral turning moment, the other side will tend to lift off and give rise to tensile stress in any bonding material between tibial tray and underlying bone

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18
Q

what can cause uneven loading in the tibial component

A

imbalance in the ligaments will result in uneven loading and greater stresses

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19
Q

In cemented total condylar knee prostheses how is the femoral component of the prosthesis attached to the bone ?

A

The shape of the femoral component means that it is well anchored to the bone, assisted by a peg or other projection

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20
Q

what can be added to the undersurface of the tibial component to give additional rotatory control

A

projections built into the undersurface

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21
Q

when may a cementless knee bone protheses used? (this isn’t to say they are always used for these cases)

A

PCL-retaining knee protheses

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22
Q

In cementless knee prosthesis what fixation methods is required for the tibial and femoral component ?

A
  • The tibial component requires a screw fixation or a stem
  • The femoral component tends not to require more than a press fit and the use of pegs
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23
Q

What is the difference between a standard total condylar prosthesis and a posterior stabilised prosthesis?

A

posterior stabilised version has a stability device to substitute for the lost PCL.

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24
Q

What features of a knee prosthesis provide rotational stability?

A

Projections on the undersurface of the tibial or femoral components, the stem (if present), screws (if present).

25
Q

What additional fixation methods are required for cementless knee prostheses?

A

Tibial component

    • requires screw fixation to a stem

Femoral Component

    • press fit
    • use of pegs
26
Q

what part of the knee can be a principal source of pain in OA in the knee and how can this be solved in a TKR?

A

The patello-femoral joint

In a TKR the patellar bearing surface of the femur is replaced and therefore may be regarded as a form of hemi-arthroplasty (because the patella surface isn’t getting replaced but its bearing surface on the femur is)

27
Q

Can pain relief be total in TKR where the patella surface is not resurfaced?

A

Yes

28
Q

what is the biomechanical function of the patella

A

provides a better leverage for patellar tendon, so flexion movement can be provided by lower patellar force than w/out

thus, causes lower joint reaction force, so component wear and loading are reduced

29
Q

what needs to be taken in to account when designing a replacement patella to prevent stress failure?

A
  • reaction force of the patella against the femur can be as high as 4-5 times body weight
  • fractures of replacement patella are not common but can occur in more active patients
30
Q

how is the femoral component of a knee replacement designed to accomodate the patella?

A

anterior part is grooved in the frontal plane to better accommodate the patella and to encourage patellar tracking

31
Q

what is the a replacement patellar bearing surface made from and what therefore needs to be considered?

A

HDP
==> so excessive wear and delamination needs to be taken into account

32
Q

what can affect the wear rate of a HDP replacement patella ?

A
  1. shape of the contact surface of the patella - conforming shapes that match the femur wear less than convex (non-conforming) shapes
  2. wear also worse in metal backed patellas - as HDP is insufficiently thick to distribute the loads and is prone to higher contact stresses than the less rigid all-HDP component
33
Q

Is the natural patella always replaced in knee replacements ?

A

No - it can be used if its surface is in good condition

34
Q

some studies have shown that patella replacements have what benefits

A
  • patients complained of less pain
  • also found the knee to be stronger in flexion for demanding activities such as stair climbing
35
Q

why in some TKR the patella made thinner ?

A

To achieve closure of the surgical wound after TKR. The use of an HDP patellar replacement component can help this as it is thinner than a patella created by placing a surface replacement on the patella bone.

36
Q

how might RA and OA differ in regards to the whether to replace the patella?

A
  1. RA preferable to replace all joint surfaces due to involvement of whole joint
  2. OA, patella resurface may be appropriate for some stiff OA knees with deformed patellae
37
Q

How does the absence of a patella affect the knee extension force applied in the quadriceps?

A

The patella tendon lever arm is reduced, requiring a greater quadriceps force (M = Fd). This adds to the patello-femoral joint reaction force.

38
Q

Why is a metal backed patella replacement generally undesirable?

A

The HDP is too thin to prevent high stresses in the material, which can result in extremely fast wear.

39
Q

What do meniscal bearing prostheses consist of ?

A

a metal femoral component, a metal tibial component and an HDP meniscus

40
Q

What are the key design features of a meniscal bearing and why

A
  • Has a large contact area (to reduce contact stresses)
  • But a low degree of constraint (to avoid high stresses, other than compression, during load transfer).
41
Q

how does the meniscus in a meniscal bearing prostheses move during flexion/extension and long axis rotation/lateral movement

A

extension

  • moves forward

flexion

    • moves backwards

long axis rotation and lateral movement

    • meniscus slides at the lower bearing surface
42
Q

what is the principle disadvantage of meniscal designs

A

increased technical difficulty in achieving ligamentous balance and overall alignment w/out risking dislocation of the moving bearing

43
Q

How has the design of menical bearing prostheses improved?

A
  • Improved by providing a larger meniscal bearing surface and a smaller gap between the two menisci.
  • Newer designs reduce the risk of dislocation by limiting sliding motion of the menisci through the use of stops
44
Q

what does a hemi-arthroplasty consist of in the knee

A

replacing only one side of the tibio-femoral joint - either medial or lateral side

45
Q

when is a hemi-athroplasty done

A

as an alternative to osteotomy in younger patients (40/50s) with moderate functional requirements who have painful and deformed joints that is not severe enough to warrant TKR but too advanced disease process to permit osteotomy

46
Q

what are both osteotomy and hemi-athroplasty surgeries designed to do

A

restore joint aligment to normal so to balance the forces on the medial and lateral sides of the joint

47
Q

what is the design of a hemi-athroplasty

A

1) femoral component

    • must be broad enough to cap damaged condyle
    • small pegs or lugs are used to limit loss of bone stock in case of need of revision op

2) two lugs are required to counter any tendency to rotate
3) unicompartmental meniscal bearings may be used but require removal of more bone in order to fit

4) HDP thickness of at least 8mm to prevent excessive wear

48
Q

TKR fail through wear, loosening and infection - why are revision knee surgeries difficult

A

same problem as seen in hip revision surgeries
- loss of bone stock for anchoring the revision prosthesis

problems trying to achieve ligamentous balance especially when ligaments may have been destroyed in loosening process

49
Q

what is the design of revision knee prostheses

A

Their bearing surface has a high degree of constraint built into the tibio-femoral bearing - this can be provided by having high peg (central peg) projection above tibial plateau which is partially captured by a central groove in the femoral component

50
Q

why are linked hinged designs unsatisfactory

A

their inability to accommodate axially generated torque w/out putting great strain on the bone prosthesis interface

51
Q

what is the function of the central peg in revision knee prostheses

A

secondary constraint - fail safe mechanism - to offer stability only if the joint is loaded heavily either medially or laterally

52
Q

if there is considerbale bone loss, what is needed in knee revision prostheses

A

modular components which are flat or wedge shaped that can be added to prosthesis to fill the gaps and allow prosthesis to rest on the bone

- called augmentation blocks

53
Q

what can be done for knee revision prostheses if there is gross bone loss

A
  • intramedullary stems may be needed to get some degree of anchorage
  • stems need to be fixed initimately within the long bone medullae of the femur and tibia
54
Q

what does anchorage depend on

A

converting the axial loads into hoop stresses at the stem-bone interface

55
Q

Name one advantage and one disadvantage of using meniscal bearing knee prostheses

A

Advantage
- lowers contact stresses.

Disadvantages
- requires removal of more bone, more difficult operation to perform.

56
Q

Why are two pegs commonly used on unicompartmental surface replacement?

A

To resist the tendency to rotate under torsional loading.

57
Q

What is the function of an augmentation block?

A

To fill in gaps between the prosthesis and bone, frequently required during revision knee replacements in which there is bone loss

58
Q

what is the main problem with knee replacements

A

HDP wear particles limit the knee replacement longevity

59
Q

why is HDP fatigue and wear more of a problem in the knee than in the hip

A

smaller bearing surface contact area so stresses in the material are higher

BMSc - Applied Ortho Semester 2 (25 decks)

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