MNSR 11 - Friction in Diarthrotic Joints

Question 1

What is friction?

Answer 1

A force exerted by a surface as an object moves across it or makes an effort to move across it.

Question 2

Lubrication can be used to…

Answer 2

reduce these forces, and biological lubricants include saliva, mucus (organs), tears.

Question 3

In conservative forces, we see that they…

Answer 3

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.

Question 4

Examples of conservative forces are…

Answer 4

Gravitational force, Spring force, Magnetic force

Question 5

In dissipative forces, we see that they…

Answer 5

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.

Question 6

Examples of conservative forces are…

Answer 6

Friction, Viscosity, Air resistance

Question 7

Static friction is…

Answer 7

those that are effective between surfaces that are at rest with respect to each other.

Question 8

Kinetic friction is…

Answer 8

those that are effective between surfaces that are in relative motion.

Question 9

If a small force (F) is applied, there will be…

Answer 9

a frictional force fs opposing F.

Question 10

The point at which the block just starts to move, corresponds to the…

Answer 10

maximum possible static friction fs(max).

Question 11

fs(max) is…

Answer 11

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.

Question 12

fs(max) = µsN is…

Answer 12

µs is the coefficient of static friction and depends on the surface smoothness

The coefficient of static friction for rubber on concrete is ~ 1

Question 13

Once the object is moving, the force required to keep it moving is…

Answer 13

smaller than that required to start it moving.

Question 14

Since, fk < fs, then…

Answer 14

µk < µs.

Question 15

When two surfaces which form a joint move relative to each other:

Answer 15

Frictional forces exist which resist the relative motion.

Wear occurs resulting in a progressive loss of material due to mechanical or chemical action.

Question 16

Diarthrotic (Synovial) Joints contain two common materials…

Answer 16

articular cartilage & synovial fluid - are encased in a watertight fibrous capsule

Question 17

Diarthrotic (Synovial) Joints are…

Answer 17

Not directly joined

Freely movable

Shoulder, hip, knee, elbow

Question 18

Articular (Hyaline) Cartilage is a…

Answer 18

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.

Question 19

Cartilage Biomechanical Properties: Tension is where…

Answer 19

As the stress increases, the fibrils are stretched and the cartilage becomes more stiff - higher Young’s Modulus (Stress/Strain).

Question 20

The fluid in articular cartilage serves as a…

Answer 20

synovial fluid reserve.

Question 21

The application of force during joint loading causes an increase in…

Answer 21

interstitial fluid pressure.

Question 22

This increase in IF pressure causes the fluid to…

Answer 22

flow out of the ECM (weeping lubrication).

Question 23

As the strain increases…

Answer 23

cartilage permeability decreases and fluid flow is reduced into and out of the cartilage.

Question 24

Describe synovial fluids:

Answer 24

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.

Question 25

When the joint is stationary, the fluid is…

Answer 25

reabsorbed, which increases the frictional forces and makes it easier to maintain a fixed position.

Question 26

In addition, under static conditions synovial fluid is…

Answer 26

thick (viscous) but when stressed it will flow (become thin, less viscous) over time.

Question 27

Hydrodynamic lubrication is when…

Answer 27

two parallel surfaces are separated by a fluid film.

Question 28

Elastohydrodynamic lubrication is…

Answer 28

associated with deformable articular cartilage where pressure from fluid film deforms cartilage surface.

Question 29

Boundary lubrication is when…

Answer 29

Two surfaces are mostly in contact with each other even though a fluid is present.

Question 30

What is arthritis?

Answer 30

Arthritis is the main disease affecting joints, and the disease has many forms. The most common are osteoarthritis and rheumatoid arthritis.

Question 31

What is osteoarthritis?

Answer 31

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.

Question 32

What is Rheumatoid Arthritis?

Answer 32

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.