MNSR 11 - Friction in Diarthrotic Joints Flashcards
What is friction?
A force exerted by a surface as an object moves across it or makes an effort to move across it.
Lubrication can be used to…
reduce these forces, and biological lubricants include saliva, mucus (organs), tears.
In conservative forces, we see that they…
Do no net work over a closed loop.
Kinetic energy is the same at the beginning and end of the loop.
Do equal work between two points irrespective of its path.
Examples of conservative forces are…
Gravitational force, Spring force, Magnetic force
In dissipative forces, we see that they…
Do net work over a closed loop.
Kinetic energy is lost on returning to the same position each time.
Do more work over longer distances.
Examples of conservative forces are…
Friction, Viscosity, Air resistance
Static friction is…
those that are effective between surfaces that are at rest with respect to each other.
Kinetic friction is…
those that are effective between surfaces that are in relative motion.
If a small force (F) is applied, there will be…
a frictional force fs opposing F.
The point at which the block just starts to move, corresponds to the…
maximum possible static friction fs(max).
fs(max) is…
Independent of the contact area between the object and the surface.
Proportional to the normal or reaction force N (= mg) of the block to the surface.
fs(max) = µsN is…
µs is the coefficient of static friction and depends on the surface smoothness
The coefficient of static friction for rubber on concrete is ~ 1
Once the object is moving, the force required to keep it moving is…
smaller than that required to start it moving.
Since, fk < fs, then…
µk < µs.
When two surfaces which form a joint move relative to each other:
Frictional forces exist which resist the relative motion.
Wear occurs resulting in a progressive loss of material due to mechanical or chemical action.
Diarthrotic (Synovial) Joints contain two common materials…
articular cartilage & synovial fluid - are encased in a watertight fibrous capsule
Diarthrotic (Synovial) Joints are…
Not directly joined
Freely movable
Shoulder, hip, knee, elbow
Articular (Hyaline) Cartilage is a…
wear resistant material with low coefficient of friction approximately 0.001 (compared to 0.04 for Teflon).
75% interstitial fluid, 20% type -2 collagen fibrils, 5% proteoglycans, <1% hyaluronic acid.
Cartilage Biomechanical Properties: Tension is where…
As the stress increases, the fibrils are stretched and the cartilage becomes more stiff - higher Young’s Modulus (Stress/Strain).
The fluid in articular cartilage serves as a…
synovial fluid reserve.
The application of force during joint loading causes an increase in…
interstitial fluid pressure.
This increase in IF pressure causes the fluid to…
flow out of the ECM (weeping lubrication).
As the strain increases…
cartilage permeability decreases and fluid flow is reduced into and out of the cartilage.
Describe synovial fluids:
Resembles egg-white
- Syn (like) + ovia (egg)
Viscous
Contains hyaluronates (lubricating component) & proteoglycans.
About 1 ml of synovial fluid exists in large human joints.
Reduction of friction — synovial fluid lubricates the articulating joints
- Combination of synovial fluid and cartilage are 3 times as slippery as skating on ice.
Shock absorption- distributes pressure evenly across the articular surface.
When the joint is stationary, the fluid is…
reabsorbed, which increases the frictional forces and makes it easier to maintain a fixed position.
In addition, under static conditions synovial fluid is…
thick (viscous) but when stressed it will flow (become thin, less viscous) over time.
Hydrodynamic lubrication is when…
two parallel surfaces are separated by a fluid film.
Elastohydrodynamic lubrication is…
associated with deformable articular cartilage where pressure from fluid film deforms cartilage surface.
Boundary lubrication is when…
Two surfaces are mostly in contact with each other even though a fluid is present.
What is arthritis?
Arthritis is the main disease affecting joints, and the disease has many forms. The most common are osteoarthritis and rheumatoid arthritis.
What is osteoarthritis?
Injury / overuse of joints and general wear and tear.
Weakness of muscles – Weak thigh muscles lead to osteoarthritis knee joint pain.
Age – Above the age of 45
Obesity – Every 1 kilogram puts three extra kilograms of pressure on knees.
Cartilage breaks down and the bones rub together.
The joint then loses shape and alignment.
Bone ends thicken.
Bony spurs form (osteophytes).
Bits of cartilage or bone float in the joint space.
What is Rheumatoid Arthritis?
Autoimmune disease which affects woman much more than men.
Often a symmetrical disease (i.e. it affects both hands or both hips).
Patients suffering from RA can often have periods of remission and exacerbation.
