Biomechanics unit 2 - biomech of skeletal muscle tissue deck 4 Flashcards
What is the most common protein in the human body?
Collagen
What is articular cartilage made up of?
An organic matrix of non-cellular material, interspersed with cells and fluid.
What is the organic matrix of non-cellular material in articular cartilage primarily made up of?
Collagen
In articular cartilage describe the structure of collagen
It is structured into strong, fine collagen fibrils
What % of the dry and wet weight of articular cartilage is collagen?
- 50-80% of the dry weight
- 10-20% of the wet weight
What are the collagen fibrils enmeshed (entangled) in, in articular cartilage ?
A concentrated solution of proteoglycans
What part of articular cartilage has the highest concentration of proteoglycans and what has the lowest concentration ?
- Highest concentration - The middle zone
- Lowest concentration - the deep layers adjacent to the bone
What % of the wet weight of articular cartilage is made up of proteoglycans ?
3-10%
What are the sparsely distributed cells in articular cartilage called?
Chondrocytes
What % of the articular cartilages tissue volume does chondrocytes account for ?
<10%
What part of the articular cartilage is the highest concentration of chondrocytes found ?
The deeper layers adjacent to the bone
What is the function of chondrocytes ?
They manufacture, secrete and maintain the organic matrix of articular cartilage
What is the interstitial fluid found in articular cartilage ?
It is the fluid that occupies the spaces in the matrix, it is mainly made up of water. (the white spaces)
What % of the wet and dry weight of articular cartilage does the interstitial fluid make up?
- 68-85% of wet weight
- 0% of dry weight
What 3 zones can the articular cartilage be split up into?
- Superficial tangential zone
- Middle zone
- Deep zone
Structurally, articular cartilage can be divided into three zones according to what?
The orientation and density of chondrocytes and the orientation of the collagen fibres.
Describe the orientation and appearance of chrondrocytes and collagen fibrils in the superficial tangential zone of articular cartilage
- The collagen fibrils are tightly woven into sheets arranged parallel to the articular surface
- The chondrocytes are oblong with their longitudinal axes aligned parallel to the articular surface.
Describe the orientation and appearance of chrondrocytes and collagen fibrils in the middle zone of articular cartilage
- The collagen fibrils are arranged more randomly but still broadly parallel to the articular surface. They are less densely packed to accommodate the high concentration of proteoglycans
- The chondrocytes are circular and randomly distributed.
Describe the orientation and appearance of chrondrocytes and collagen fibrils in the deep zone of articular cartilage
- The collagen fibrils are arranged in larger fibre bundles that are anchored in the underlying bone tissue (fibre bundles perpendiculat to articular surface), thereby attaching the articular cartilage to the bone.
- The chondrocytes are arranged in loose columns aligned perpendicular to the line dividing the articular cartilage and the underlying bone.
What is below the deep zone of articular cartilage ?
A thin layer of calcified cartilage which gradually merges into the underlying subchondral bone.
What is the name given to the interface between the articular cartilage and calcified cartilage beneath it?
The tidemark
Is the thickness of articular cartilage the same in all joints?
No it varies e.g. in the knee joint average is approx 2.3 mm over the lateral condyle and 3.6 mm in the patellar groove.
What is the mechanical behaviour of articular cartilage ?
Viscoelastic
What is viscoelastic behaviour dependent on ?
It means that the response of the material is time dependent, meaning response varies according to the length of time that a load is applied and the rate at which a load is applied.
When a load is removed from a viscoelastic material what will happen?
It will return to its original size and shape, like an elastic material
(Note this response is not immediate.)
What are the 2 other characteristics of viscoelastic materials and therefore articular cartilage?
Creep and stress relaxation.
When does creep behaviour of a viscoelastic material occur ?
It occurs when a viscoelastic material is subjected to a constant load.
Describe creep behaviour of a viscoelastic material
When the load is first applied the material will deform quite rapidly. Followed by a slowly (creeping) increasing deformation.
(pic shows articular cartilage under a constant compressive load over a period of time)
Explain why creep behaviour occurs in articular cartilage ( / viscoelastic material)
Consider its composition and structure (i.e. high composition of intersistial fluid)
During the initial rapid deformation (between point A and B), fluid is rapidly forced out of the articular cartilage. As the amount of fluid remaining in the articular cartilage diminishes, so the rate of expellation decreases until at equilibrium (point C) fluid flow ceases completely and the applied load is borne entirely by the solid matrix.
How long does it typically take to reach equilibrium in terms of creep behaviour for normal articular cartilage ?
4-16 hrs
When equilibrium is reached when articular cartilage has a load applied to it, how much of the fluid remains in the cartilage?
The majority of fluid still remains in the articular cartilage e.g. a high stress of 1.0 MPa approximately 50% of the total fluid content will remain.
When does stress relaxation occur in a viscoelastic material, such as articular cartilage?
When the material is kept at a constant deformation.
Describe what stress relaxation is
- This is where the stress is reduced over time as the material is maintained at a constant strain (deformation).
- In other words if a material is deformed to a certain degree the load required to maintain that deformation decreases with time.
Pic shows the stress in articular cartilage as it is maintained at a constant deformation over a period of time.
Explain why stress relaxation occurs in articular cartilage ( / viscoelastic material)
Consider its composition and structure.
During the initial deformation (between points A and B) interstitial fluid is forced out as the surface layers are compacted. Because of the large frictional drag associated with the flow of fluid through the solid matrix, large loads are needed to compress the tissue.
During the stress relaxation phase (from point B onwards) the stress required to maintain the deformation is reduced as fluid is no longer being forced out and the fluid within the tissue is redistributed from the least compacted deeper layers to the most compacted surface layers within the articular cartilage.
What gives articular cartilage that gives it its ability to cushion the high loads that occur between the bones in joints, effectively smoothing out any high peaks thus reducing the potential damage that they may cause?
Its viscoelastic behaviour
Define what is meant by the coefficients of friction?
This is a measure of the maximum friction force between 2 surfaces
What is the difference in coefficient of friction in synovial joints compared to artificial joints
Coefficient of friction in synovial joints is very low - 0.02 compared to artificial joints which is 0.03 to 0.1
What does the very low coefficient of friction of synovial joints allow ?
Allows the articulating surfaces to move freely over one another with the minimum amount of wear.
What is the clinical relevance of the higher coefficient of friction of artificial joints compared to synovial joints?
It prevents them from moving as freely as healthy synovial joints and also means that they wear out more quickly than is sometimes desirable
What 2 things are responsible for the very low coefficient of friction in synovial joints?
The combined action of articular cartilage and synovial fluid
How does synovial fluid reduce friction in a joint?
It lubricates the surface of the articular cartilage, reducing the contact between the two surfaces and reducing the amount of friction and wear.
What are the 2 most important factors which determine the way in which lubrication is brought about in a joint ?
- The magnitude of the load on the joint pressing the two surfaces together
- The length of time that this load is maintained.
What are the 3 main types of lubrication?
- Elastohydrodynamic lubrication
- Boosted lubrication
- Boundary lubrication.
Think - ‘‘EBB’’
For lubricating a joint does only one of the types of lubrication mechanisms act at a time?
At any one moment in time one of these types of lubrication mechanisms may be dominant but the others will also be present to a lesser extent.
Give a reason for variation in the most dominant mechanisms of lubrication in a joint?
The loading in different parts of the joint will also vary
