Structure + Function of joints (S3)

Question 1

What are the main functions of joints?

Answer 1

• transmit loads
• allow movement
• provide stability

Question 2

Describe the 3 main classes of joints

Answer 2

• fibrous (synarthrosis) - immobile eg. skull, tooth
• cartilaginous (amphiarthrosis) - slightly mobile eg. intervert disc
• synovial (diarthrosis) - freely mobile eg. limb joints

Question 3

What are the types of movement allowed by synovial joints?

Answer 3

• planar (intertarsal)
• simple hinge (interphalangeal)
• pivot (atlas)
• saddle (carpo-metacarpal)
• complex hinge (knee)
• ball-and-socket (hip)

Question 4

What are the mobility and stability states like within a ball-and-socket joint?

Answer 4

MOST mobility

LEAST stability

so allows lots of movement, but also very easily dislocated/pop out.

Question 5

What are 5 ways/structures to achieve stability?

Answer 5

• congruity - matching shapes of bone ends
• fibrous capsule + its thickenings into extra-articular ligaments eg. medial + collateral ligaments of knee
• intra-articular ligaments eg. cruicate ligaments of knee
• packing - eg. menisci + fat pads in knee
• muscles - esp imp for shoulder stab, easily dislocated due to poor congruity and slack capsule

Question 6

What are some intra-articular structures of the knee?

Answer 6

• semilunar cartilage with lateral meniscus
• semilunar cartilage with medial meniscus
• anterior and posterior cruciate ligaments
• articular cartilage of medial femoral condyle
• articular cartilage of tibial plateau

Question 7

Describe the key features of a synovial joint

Answer 7

• articular cartilage (attached to bones) - avascular
• synovial fluid
• synovium (makes synovial fluid)
• fibrous capsule
• synovial blood supply/drainage (to synovium)

Question 8

Does cartilage have nerve and/or blood supply?

Answer 8

NO - if it did then it would hurt us to move, instead the synovium (lining) has blood supply and lymph drainage to take away any waste product generated at the joint.

Question 9

What important receptors are present outside the fibrous capsule?

Answer 9

Stretch receptors - send signals to brain on changes in joint angles and limb position, termed proprioception.

Also stim of stretch receptors by arthritic effusions causes reflex muscle inhibiton - can lead to muscle wasting. Nociceptors are for injury/arthritic pain.

Question 10

What molecule gives cartilage its high osmotic force/inflation?

Answer 10

Proteoglycan - made up of GAG and core protein, which has a negative charge so draws in water molecules -> high osmotic pressure causing swelling

Question 11

What are some GAG chains that surround the core protein?

Answer 11

Chondroitin sulphate, keratan sulphate, hyaluronan

Question 12

What is the role of chondrocytes?

Answer 12

Secrete collagen, proteoglycans and hyaluronan

Question 13

What characteristics of cartilage balance the swelling tendency?

Answer 13

Hyaluronan - tethers the aggrecan/proteoglycan

Type 2 collagen fibrils hold it together too

Question 14

How are the collagen fibres arranged in relation to the articulating surface of cartilage?

Answer 14

Collagen is parallel to surface because you don’t want the articular surface to get broken down (which happens in arthritis)

Question 15

How does the articular cartilage obtain its nutrients?

Answer 15

From the synovial fluid as the cartilage is avascular, and the synovial fluid gets nutrients from synovial capillaries

Question 16

Why does the endothelial surface of the synovium have fenestrations?

Answer 16

To allow products to travel through easily, in disease the synovial membrane may get enlarged in response to injury

Question 17

What are key features of synovial fluid?

Answer 17

• ultrafiltrate of plasma (generated by fenestrated capillaries just below synovial surface)
• electrolyte and plasma protein content similar to other interstitial fluids
• actively secreted molecules lubricin + hyaluronan added by synoviocytes

Question 18

What is lubricin?

Answer 18

A glycoprotein, lubricates cartilage under conditions of high load and low velocity (boundary lubrication)

Question 19

What is hyaluronan (hi-al-oo-ron’un)?

Answer 19

Gigantic nonsulphated GAG, bigger than viruses (5000 disaccharides), makes synovial fluid very viscous.

Single unbranched chain, highly hydrated, huge molecular domain, highly viscous, hydroynamic lubricant

This all allows more fluid to be retained

Question 20

What is the difference between boundary lubrication and hydrodynamic lubrication?

Answer 20

Boundary lubriciation is done by lubricin where there is a high load and low velocity.

Hydrodynamic lubrication is done by hyaluronan - lubricates cartilage under conditions of low load and high velocity (like oil in car engine)

Question 21

Discuss the volume and pressure of synovial fluid in a normal person

Answer 21

Volume is tiny - 10-100mm, volume increases 10-100 times in arthritis - called a joint effusion.

Pressure varies with joint angle - increases with flexion, decreases with extension. So fluid enters joint in extension and is driven out in flexion.

Question 22

What is the ‘angle of ease’ and when is it important?

Answer 22

The angle of ease is the minimum pressure at a certain angle in an affected joint.

In a normal person the pressure steadily increases upon flexion from extension, but for some special cases (eg. arthritic effusion, haemarthrosis/haemophilia) the pressure curve dramatically increases and makes it harder for the person to flex the joint.

Question 23

When is P_j raised and what buffers it?

Answer 23

Joint pressure (P_j) raised during flexion, hyaluronan buffers this increase to prevent fluid loss from joints

Question 24

What is arthritis?

Answer 24

Inflammation of a joint -> pain, swelling, loss of function

Question 25

What are the causes of acute arthritis?

Answer 25

Infection, injury

Question 26

What are the causes of chronic arthritis?

Answer 26

Immune-mediated, degenerative eg. OA, other

Question 27

What is the difference between rheumatoid arthritis and osteoarthritis?

Answer 27

Rheum - cytokine driven inflammation, pannus formation, systemic disease

Osteo - metabolic changes in joint, evidence of cartilage loss, synovial inflammation, joint space narrowing

Question 28

What is osteoarthritis?

Answer 28

Primarily a mechanical breakdown of articular cartilage, fibrilation and denudation (erosion, sometimes to a bone) due to collagen and fibril rupture + aggrecan loss.

Inflammation of synovium

Associated with ageing, mechanical dysfunction, obesity

Question 29

What is inflammatory arthritis (eg. RA)?

Answer 29

Primarily an inflammation of synovium by mediators such as PGE2, bradykinin, histamine, NO, cytokines.

Mediator release triggered, in case of rheumatoid, by an autoimmune mechanism.

Secondary erosion of cartilage by invading hypertrophic synovium, which releases metalloproteinases.

Question 30

Discuss the physiology of arthritic swelling (joint effusion)

Answer 30

Effusion increases, so fluid moves from the capillaries into the joint, increasing P_j

Question 31

What is the action of inflammatory mediators within joint effusion?

Answer 31

• > endothelial gap formation
• > inc permeability to H2O + plasma protein
• > vasodilation of arterioles
• > inc capillary pressure Pc, hot joint due to inc blood flow