What common structures are composed of fibrocartilage
- Illiac surface of SI joints
- Menisci of the knee
- annular fibers of the disks
Components of cartilage and function of cartilage
Function:
- Decrease contact stress on articular surfaces by spreading load over larger area
- Allow movement of articular surfaces with minimal wear and friction
Components: -Solid matrix: proteoglycan gel (20-40% weight) Collagen 60% weight -Chondrocytes (over 2% of solid matrix) -Water (60-80% of total weight)
Why it is appropriate that collagen in cartilage is better suited to resist tensile loads rather
than compressive loads.
The superficial zone 40-60% of thickness has the collagen arrangement in planes parallel to the surface
Syringe effect
An increase in stiffness when a load is applied with increased speed.
Involved with high speed loading:
-insufficient time to squeeze water out of tissue
-the cartilage thus is very stiff
-there is little deformation
Aggrecans, how they help to establish the sponge model, the details of how water moves
to protect the joints.
Aggrecans is a component of proteoglycan gel
- aggrecans are composed of chondritin sulfate and keratan sulfate
-Strong repulsive force between anions stiffens the PG gel to resist compression
-cloud of cations are attracted to negative charges
-the cloud of ions osmotically draw water molecules into cartilage
the charge of the PG gel attracts positive ions and sets up an osmotic pressure
- the cartilage stiffens and swells due to the repulsive force of the aggrecans, osmotic pressure forms attracting water
The characteristics of creep as it applies to cartilage.
- under a constant load a viscoelastic material will deform quickly at first then more slowly over time
How the speed of loading affects the material behaviour of cartilage
- High speed loading: syringe effect
- increasing the speed of loading causes the cartilage to appear stiff
Unloading slowly results in immediate restoration 90% original thickness via elastic recoil
Fluid film (hydrodynamic and squeeze) vs boundary lubrication
Fluid film lubrication is a thick film of lubricant separates the surfaces
-occurs in physiologic loading
two types:
Hydrodynamic- non parallel surfaces, tangential movement, creates lifting action
Squeeze film- surfaces are kept apart, perpendicular force, viscosity holds lubricant together, sufficient for high loads for short durations
Boundary Lubrication: important in severe loading a monolayer of synovial fluid that absorbs to surfaces
Interfacial wear vs fatigue wear
Interfacial wear
-From interaction of articular surfaces, by adhesion and abrasion, unlikely in healthy cartilage, wear produces surface defect, fluid leakage, loss of lubrication
Fatigue wear
- occurs with repetitive cyclic loading
- reloading occurs before cartilage has time to reimbibe
- repetitive stress on matrix
Types of factors/occupations/disease that lead to cartilage injury
High repetitive loads Loss of muscle balance Defect that focuses load into one area Joint space haemorrhage Immobility Degeneration
