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BMSc - Applied Ortho Semester 2 > Implant technology - unit 3 deck 1 > Flashcards

Implant technology - unit 3 deck 1 Flashcards

1
Q

what are the main design difficulty for knee prostheses

A
  • needs to have an acceptable replication of the motion of the natural joint
  • sufficient stability without being so rigidly constratined in its motion that it results in high stresses at the bone-implant interfaces under lateral and twisting loads
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2
Q

what is the most successful knee protheses design to date

A

“total condylar” design

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3
Q

Define what a hemi-arthroplasty is

A

Joint replacement in which only one of the joint surfaces is replaced

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4
Q

Unlike the hip joint the knee joints shape contributes little to its stability - what does the knee joint rely on to keep it stable ?

A
  1. Ligaments
  2. The posterior joint capsule
  3. Good musculature

These soft tissues act together to hold the knee in place throughout its range of motion.

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5
Q

A hip joint replacement may be designed with a well constrained shape - a ball and socket - which does not rely on ligaments to maintain stability.

What must knee joint replacement to achieve good kinematic function and mechanical stability?

A

The ligaments

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6
Q

What do the collateral and cruciate ligaments act together to prevent ?

A

subluxation (partial or complete dislocation)

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7
Q

What are the main stabalising roles of the ligaments of the knee ?

A
  • Anterior cruciate ligament (ACL): resists posterior subluxation of the femur
  • Posterior cruciate ligament (PCL): resists anterior subluxation of the femur
  • Lateral collateral ligament (LCL): resists adduction of the joint
  • Medial collateral ligament (MCL): resists abduction of the joint

All the ligaments act together to limit distraction (pulling apart) of the knee and long axis rotation of the joint

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8
Q

What is the function of the posterior capsule (a band of tendinous material) in stabalising the knee ?

A

Resists hyperextension

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9
Q

what are the ACL and PCL named in relation too

A

their attachment to the tibia

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10
Q

what would happen if there was no ACL or no PCL

A
  • no ACL = femur can slide backwards over tibia
  • no PCL = femur can slide excessively forward
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11
Q

What is it important for a surgeon to correct during a knee replacement surgery?

A

Any ligament imbalances and looseness

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12
Q

if ligaments are damaged or removed during knee replacements surgery, the resulsting loss of stability must be compensated by what ?

A

The design of the prosthesis

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13
Q

Define subluxation of a joint

A

partial or complete dislocation of a joint

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14
Q

What type of knee subluxation does the anterior cruciate ligament prevent?

A

anterior subluxation of the tibia

[posterior subluxation of the femur = same thing]

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15
Q

Which ligament resists knee joint adduction?

A

LCL

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16
Q

How does the knee joint rotate ?

A

It rotates with a varying centre of rotation

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17
Q

What together determines the pattern of rotation of the knee and therefore must be considered in knee replacement designs ?

A
  1. Shape of the joint surfaces
  2. Constraining role of the ligaments
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18
Q

During motion of the knee joint, how do the ligaments move ?

A

They move isometrically (they keep the same length as they move and do not lengthen or shorten)

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19
Q

what happens to the axis of rotation in the knee as it flexes?

A

Its axis of rotation changes and moves posteriorly as the knee rotates

[the point of surface contact of the femur and tibia also moves posteriorly]

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20
Q

Why is the centre of rotation in the knee joint known as the instantaneous centre of rotation?

A

because it changes at every instant of knee motion

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21
Q

what is the shape of the tibia plateau?

A
  • medial compartment is slightly concave [lower at the centre than at the edges]
  • lateral compartment is convex
22
Q

Describe the surface shape of the femoral condyles

A

Over the 140 degrees, or so, of flexion the femoral condyles in contact are nearly circular. They do get flatter in the part that is in contact with the tibia as the knee reaches full extension

23
Q

what is the motion at the knee joint as it flexes and extends

A
  • knee extends - tibia rotates externally
  • knee flexes - tibia rotates internally

[at full extension rotation is restricted by interlocking femoral and tibial condyles]

24
Q

what is the name of the mechanism that desribes the movement of the knee

A

screw home mechanism

25
Q

what does the four bar linkage cruciate mechanism do

A

Constrains the motion of the femur on the tibia so that there is a combination of rolling and sliding motion as the knee rotates

26
Q

if the radius of the posterior part of the femoral condule is 22mm and the knee flexion is 140 degrees - calculate the lenght of the arc

A

s = [2 x pie x radius] x 140/360

radius in this case would be 22mm

==> s = 54mm

27
Q

Why is it clear that rolling and sliding motion occurs as the knee rotates ?

A

The radius of the condyle in fact increases anteriorly so the total rolling distance would in reality need to be much more than 54 mm in order to rotate 140 degrees. (referring to the example)

It is clear, then, that knee motion during flexion/extension involves sliding due to rotation as well as rolling ) for the range of angular motion to be accommodated with just 20 mm of translation of the instantaneous centre of rotation.

28
Q

what does the limit to rolling distance in the knee prevent and what is it provided by ?

A

Provided by the cruciate ligaments and posterior joint capsule

controls the position of the most posterior point of the centre of rotation so enabling the knee to flex fully without rolling up against the posterior capsule and in turn roll off the tibia

29
Q

How does the position of the instantaneous centre of rotation change as the knee moves from extension to flexion?

A

Moves posteriorly by upwards of 10mm and distally by a few mm

30
Q

what force is the knee joint normally under and why is the magnitude greater than that of the body weight?

A

Compression

  • The forces much higher than the body weight due to the combined effect of these gravitational forces, the contracting forces of the muscle and the balancing loads of the ligaments
  • joint force ranges from 2 to 6 times body weight under normal daily activity
31
Q

since the knee is under mainly compressive load, what does the mean for designs of prostheses

A

that cement is a good option as it is very effective in compression

32
Q

The knee joint is not always loaded in compression, what are the 3 main components of the ground reaction forces loading the knee joint during gait?

A
  1. Vertical (contributing to compression)
  2. Fore-aft (anterior-posterior in x direction)
  3. Medial-lateral (horizontal)
33
Q

during gait what compressive forces are seen at the knee? (obv this is during stance phase of gait same goes for the other following flashcards on this bit)

A

Vertical component of the GRF which reaches a max of 1 x Body weight is transmitted to the knee and adds to the compressive force due to the action of the quadriceps acting via the patellar ligament which generates a max of 3 x BW

==> resultant compressive joint reaction force of about 4 x BW

34
Q

up to what percentage of BW does the fore-aft GRF load the knee during gait and what counters this loading ?

A

up to 20% BW is transmitted to the joint

  • The cruciate ligaments are required to balance this component of the joint reaction force
  • (as think the fore-aft force acts in the ant.-post. direction so cruciates counter this and cause it acts in this direction it will act to shear femur over the tibia)

The forward component of this force acts to push the femur forwards over the tibia - the PCL prevents this

35
Q

What is the effect of the horizontal component of the GRF loading the knee during gait and what is the size of this force ?

A
  • It acts medially (so lateral to medial) which generates a turning moment on the knee which must be balanced by the muscles and ligaments
  • It is 5% BW
36
Q

The horizontal component of GRF acting on the knee increases with more strenuous activities, describe this increase affects the loading on the knee as you move from activities such as walking to more strenuous ones and the role the muscles (particularly quads and hammys play)

A
  1. Medially (horizontal) acting GRF causes a adduction moment the load distribution shifts such that the greater the horizontal force component the greater the load transferred from the lateral compartment to the medial compartment of the joint.
  2. When the magnitude is low such as those during gait, the quadriceps muscle, acting via the patellar tendon ligament, can pull the joint together hard enough to keep both condylar surfaces in contact with the tibial plateau (pic A and B)
  3. As the horizontal force increases, in activities more strenuous than normal walking, it becomes necessary to use the hamstrings as well, increasing the joint reaction force. Eventually, as the load increases, the muscles do not have the strength to maintain contact at both condylar surfaces, the lateral side loses contact and all the load is taken by the medial condyle (pic C)
  4. The stability of the joint then relies on the LCL, which is required to balance the turning moment due to the sideways acting force.
37
Q

what implications does the high loads acting on the medial compartment of the knee have on joint replacement designs

A

the tibial component needs to be able to transfer high medial compartment loads on its upper surface to the underlying bone without causing high compressive stresses which could cause the bone to fail

38
Q

what would be required if the collateral ligaments where absent or cannot be retained during knee replacement surgery

A

the replacement joint would be required to provide all the lateral stability ==> linked prosthesis, such as a hinge, would be required

39
Q

what other additional forces is the knee required to resist

A

axially generated torques which try to twist the knee axially and, if excessive can cause a meniscus to tear

[stability once again relies on ligaments, replacement joint would have to do the same]

40
Q

why the joint reaction force at the knee increases as the sideways horizontal component of the ground reaction force increases.

A

as this force increases, a greater patella tendon force and a greater hamstring force are required to balance its effects which adds to the joint reaction force

41
Q

What adverse effect could a high contact force in the medial compartment of the knee have on a joint replacement?

A

cause high local stresses medially which could cause the underlying cancellous bone to fail

42
Q

State the general criteria for knee prostheses

A
    • Be tolerated within the human body with no short term and little long term risk of adverse toxic effects such as carcinogenesis
    • Achieve its aim of relieving pain and restoring the activities of daily living.
    • Last a reasonable length of time which ideally should extend beyond the expected life span of the individual patient without the need for revision
    • Be insertable by a competent surgeon of average ability such that a predictable outcome can reasonably be guaranteed.
    • cost-effective
43
Q

what are most commerically avaliable knee replacements made of

A
  • femoral component - cobalt chrome
  • tibial component - HDP
44
Q

What is the 10 year survival of knee prostheses ?

A

90-95%

45
Q

what is the minimum functional kinematic requirements of a knee replacement?

A
  1. It should fully extend to 180° at which point the patient should be able to stand without the need for muscular effort by the quadriceps ==> collateral ligaments and posterior capsule must be intact to enable the screw-home mechanism or designed with an alternate stabalising mechanism
  2. should flex to 90 degrees [allows person to walk up/down stairs]
  3. should permit slight axial rotation as the knee extends to maintain natural ligamet tension throughout flexion and extension
46
Q

what is essential for the surgeon to do in a knee replacement

[apart from balance the ligaments]

A

essential that the two bearing surfaces are cut parallel

  • means the tibial surface is maintained at right angles to the tibial shaft, parallel with the ground when weight bearing
  • femoral cut will have to be at an angle to compensate for the natural angulation of the femur relative to the tibia [cut needs to be 6 or 7 degrees relative to axis of femur]
47
Q

What must be dissected off (not removed) to ensure the knee replacement can extend fully?

A

The posterior capsule of the back of the femur

48
Q

What is the importance of balancing the collateral ligaments in tension during a knee replacement ?

A

so that the bony cuts are parallel when the bones are stretched apart by the new joint and there is no tendency of the joint to open more medially than laterally or for the ligaments to become lax

49
Q

what is the easiest method to balance the collateral ligaments

A

to lengthen tightened ligaments to match slack ones via soft tissue dissection

50
Q

what is the controversy of the cost of knee replacements compared to hips

A

they cost on average 5 times as much as hips

BMSc - Applied Ortho Semester 2 (25 decks)

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