Imp Tech U3 Flashcards
Which ligaments in the knee resist which movements?
ACL; resists posterior subluxation PCL; resists anterior subluxation LCL; resists adduction MCL; resists abduction Posterior capsule (band of tendinous material); resists hyperextension
What is the centre of rotation of the knee called?
The instantaneous centre of rotation (because it changes throughout motion)
Why does the centre of rotation of the knee change?
Medial tibial plateau slightly concave
Lateral tibial plateau slightly convex
Femoral condyles not perfectly circular
Means sliding is required as well as rolling for rotation
Dependent upon cruciate ligaments to hold femur onto tibia for this
What type of forces are experienced through the knee ? Where do these come from?
Compressive
Gravitational forces (1xBW), muscle contraction and ligament balancing loads (3xBW) Total force of 4xBW
How are shear forces exerted on the knee and what resists these?
Loading not directly down during walking, at an angle
Cruciate ligaments balance this
How are lateral forces balanced throughout normal walking?
Medial force is generated during walking - causes turning moment
Quads (via patellar tendon) holds the femur + tibia together so that there is total contact on both sides
When there is an increasing medial force applied, how is it balanced? (in activities more strenuous than normal walking)
- By the quads (via patellar tendon)
- Hamstrings contract to maintain contact
- Muscles not strong enough, all force taken by medial condyle - reliant on LCL to maintain stability
What are constrained, semi-constrained vs unconstrained knee prostheses?
Constrained; linked prosthesis
Unconstrained; surface replacement only
Semi-constrained; mechanism for restraint only in certain degrees of rotation (all modern knee replacements)
If the collateral ligaments are not intact, what kind of prosthesis is used?
A hinge prosthesis
What are the disadvantages of hinge prostheses?
Has no give under lateral or long axis rotational loading
Transmits high shear forces from loading to the interface
Why is a substitute required for the PCL if it is not maintained during knee replacement?
To allow rotation of femur on tibial plateau without sliding too far posteriorly
What are the advantages of a PCL retaining prothesis?
- Allows anterior-posterior stability and proprioceptive activity
- Doesn’t affect normal gait if lost (walking on stairs more stable with PCL though)
What are the disadvantages of a PCL retaining prosthesis?
- Protection of PCL during surgery prevents full dissection of posterior capsule (so may limit full extension)
- Increases tibial surface wear as femoral component encouraged to slide over it
- Limits deformity correction (removal can be preferred to correct deformities)
What are the features of a PCL retaining prosthesis?
Flat tibial plateau (like the normal tibia)
Why is careful determination of the height of the tibial plateau important in PCL retaining prostheses?
Determines how tight the PCL is
- too loose = forward movement of femur on tibia so rolling back no longer works
- too tight = restricted degree of flexion, high contact stresses (compression of 2 surfaces together) and excessive rolling back of femur
Where is HDP wear more of a problem (hip or knee) and why?
Knee
Smaller contact area so stresses in material are higher
What factors can reduce wear in the HDP component?
Reducing the sliding distance of the femoral component
Increased thickness
Metal backing
Pegs
What materials are used to make the femoral and tibial components of a knee joint? Which is more likely to wear first
Femoral; cobalt chrome
Tibial; HDP
HDP is softer so is likely to wear out first
What types of motion are resisted by all knee prostheses? How are these resisted?
Axial motion + lateral motion
Using short stem or interlocking mechanism
What absorbs the load if there is a plane/axis which there is no motion in?
Bone-cement or cement-implant interface
there is no loading in soft tissues
If there is a sudden load where takes the stress and why?
How can this be limited?
Interface experiences large instantaneous stresses in sudden loading because cement/prosthesis materials are much stiffer than biological materials (viscoelastic)
Needs a large contact area to prevent stresses which might loosen the interface
What is the relationship between constraint of the prosthesis and length of stem?
The more constrained the prosthesis is the longer the stem needs to be
What determines the life of a knee prosthesis?
The rate of HDP wear
What is the relationship between diameter of bearing and volume of wear?
Sliding distance (so volume of wear) directly proportional to diameter greater diameter = greater volume of wear
What factors are important in considering wear of HDP?
Volume of wear and depth of wear
What is the relationship between depth of wear and size of bearing?
Large surface contact area = reduced depth of wear
(either from increased diameter or increased length)
Increased length preferable as means decreased volume of wear (in addition to depth of wear)
Why is a long, small diameter bearing surface of prosthesis preferable?
Small diameter = decreased volume of wear (as reduced sliding distance)
Long = reduced depth of wear (increased surface contact area)
In PCL retaining prostheses what types of motion are necessary?
Sliding and rotation (to mimic natural knee joint)
What are the risks of high contact stresses, why and how can the contact stresses in HDP be reduced?
Increased likelihood of fatigue failure due to cyclic loading (variation between tension + compression during loading and unloading)
By having a larger surface area in contact with the femoral condyles (larger condyles)
How do HDP wear vs fatigue affect the life of a knee replacement?
Wear = most likely cause of failure (thinning of the HDP component) Fatigue = second most likely cause of failure (due to stresses - cause subsurface lamination cracking)
What factors influence the interface stresses between the tibial component and the underlying bone?
- Thickness of the HDP component
- Whether it has a metal backing plate
- Presence of pegs
- Stiffness of HDP material
How does thickness of the HDP affect interface stresses?
Thinner = more stresses
Because stresses can’t be evenly distributed
<8mm, recommend using a metal backing plate
How does a metal backing plate affect loading?
Medial tib plateau takes 60% of load when standing, if a metal plate (held in place by a peg) is used it causes even loading onto bone beneath (so less high stress)
What are the disadvantages of a metal backing tray? Are they commonly used?
- Stress under medial bone is still high due to stiffness of metal tray
- Tensile stresses occur between bone and tray laterally (not well tolerated by cement)
- Overall higher tensile stresses than a purely HDP tibial component
Become less common in practice
What is a peg?
A very short stem (30-50mm)
How does a HDP insert onto a metal backing plate help with surgical tensioning?
Can change the thickness of the HDP insert depending upon the level of tension required in the collateral ligaments
Ensures that the ligaments can be balanced (by lengthening)
How does the stiffness of HDP affect the contact stress?
Greater stiffness (E) = greater contact stress (as deforms less)
How does a peg affect the stress in the underlying bone?
HDP peg has little effect on max compressive stress
Metal backing with metal peg reduced stress by 20-40%
(may work by reducing the high contact stresses due to uneven loading)
What shape of femoral component is preferable and why?
Rounded edges
Then if there is lateral motion (rocking) that lifts off one of the condyles, rather than a sharp corner being in contact with the HDP (high contact stress), there is a larger contact area
Are the femoral condyles anatomically accurate in a knee prosthesis?
No
They are the same size (usually the medial condyle is larger)
How should the tibial surface be designed in PCL retaining and PCL excision prostheses?
PCL retaining; flat surface to mimic the natural surface of the tibia
PCL excision; requires posterior stabilisation but not a flat surface
What features are required in the prosthesis if the PCL is excised? How is this achieved?
- Prevent posterior femoral subluxation
- Cause femur to roll back as it flexes
Central locking of the femoral component on the tibia (on a peg etc) with variable radius for roll back
Why is PMMA (bone cement) a good anchor for a knee prosthesis?
Knee components are under compressive stress throughout the ROM (cement strong under compression)
How is lateral motion of a knee prosthesis countered?
Projections on the undersurface of the tibial components (prevents tensile stress in cement if one side lifts up)
How is the femoral component anchored?
Press fit and a peg
Why should a patellar replacement be made purely from HDP?
Metal backed patella are too thin so don’t distribute high stresses
Why is it preferable for a replacement patella to conform to the shape of the femoral condyles?
Limits wear as they slide up and down
What is the reaction force in the patella and why?
4-5x body weight
Act as a lever for the quads tendon so all the force for stabilisation at the knee is exerted through the patella
What are the surfaces on a meniscal bearing? How does it work
Metal femoral surface, metal tibial surface, HDP bearing in the middle
The HDP bearing slides forwards-backwards during flexion-extension to allow knee to allow femur to slide in anatomical movement
What is replaced in a hemi-arthroplasty at the knee? What is the aim of this?
Either medial or lateral side of tibia-femoral joint
To restore normal alignment by balancing joint forces
When is a hemi-arthoplasty useful?
For people in 40-50s when a TKR is not yet advised - can be revised at a later stage
What style of prothesis is used for a revision?
Simple hinge
Why is anchoring a revision difficult and how is this managed?
Loss of bone stock and loss of ligaments
Use augmentation blocks to fill gaps
