Physiology of Joints

Question 1

3 types of joints?

Answer 1

1. Synovial (AKA diarthrosis)
2. Fibrous (AKA synarthrosis)
3. Cartilaginous (AKA amphiarthrosis)

Question 2

What are fibrous joints?

Answer 2

Bones are united by fibrous tissue and so no movement is allowed, e.g: bones of the skull in adults

Question 3

What are cartilaginous joints?

Answer 3

Bones are united by cartilage and allow limited movement, e.g: intervertebral discs, pubic symphisis, part of the sacroiliac joint and the costochondral joints

Question 4

What are synovial joints?

Answer 4

Bones are separated by a cavity, filled with synovial fluid, and united by a fibrous capsule and other extra-articular structures, e.g: ligaments, tendons and bursae

Question 5

Describe the synovial membrane

Answer 5

Lines the inner aspect of the fibrous capsule; it is a vascular connective tissue, with capillary networks and lymphatics

Fibroblasts (synovial cells) produce synovial fluid

Question 6

2 types of synovial joints?

Answer 6

Simple synovial joint (one pair of articular surfaces), e.g: metacarpophalangeal joint

Compound synovial joint (more than one pair of articular surfaces), e.g: elbow joint

Question 7

Functions of joints?

Answer 7

Provide structural support and allow purposeful motion of the MSK system

Question 8

3 roles of joints during purposeful motion?

Answer 8

Stress distribution

Confer stability

Joint lubrication

Question 9

How do joints confer stability?

Answer 9

Shape of the articular component, e.g: the hip joint

Ligaments provide a second major stabilising force

Synovial fluid acts as an adhesive seal, freely permitting sliding motion between cartilaginous surfaces

Question 10

What provides joint lubrication?

Answer 10

Cartilage interstitial fluid

Synovium (the synovial membrane) is a polymer of disaccharides and is lubricated with hyaluronic acid (mucin)

Synovium-derived lubrcin (glycoprotein)

Question 11

Functions of synovial fluid?

Answer 11

• Lubricates joint
• Facilitates joint movements, reducing friction
• Helps minimise wear-and-tear via lubrication
• Aids in nutrition of articular cartilage
• Supplies chondrocytes with O₂ and nutrients and removes CO₂ and waste products

Question 12

The synovial fluid not a static pool. How?

Answer 12

Continuously replenished and absorbed by the synovial membrane

Question 13

Thickness of synovial fluid?

Answer 13

High viscosity due to hyaluronic acid (mucin), which is produced by the synovial cells

Viscosity varies with joint movement but this is impaired in a diseased joint

Question 14

Constituents of the synovial fluid?

Answer 14

• Hyaluronic acid (mucin)
• Uric acid - derived from dialysis of blood plasma
• Few cells (mainly mononuclear leukocytes)

Question 15

How do the viscosity and elasticity of the synovial fluid vary?

Answer 15

Rapid movement:

Decreased viscosity

Increased elasticity

Question 16

Appearance of synovial fluid in different scenarios?

Answer 16

Normal: viscous, clear and colourless

Red: traumatic synovial tap and in haemorrhagic arthritis

Question 17

Leukocytes in synovial fluid?

Answer 17

< 200 WBC/mm3

Of this, <25/mm3 comprises polymorph cells

Question 18

When does WBC increase in synovial fluid?

Answer 18

Inflammatory and septic arthritis

Question 19

Compare normal, inflammatory and septic synovial fluid in the following aspects:

Viscosity

Colour

Clarity

Total WBC

PMN leukocytes ?

Answer 19

Question 20

Describe the synovial fluid in each of the test tubes

Answer 20

A: normal synovial fluid that is viscous and clear

B: from joint with mild synovial inflammation

C: from joint with mild synovial inflammation; blood stain is caused by trauma

D: from a severely inflamed joint; thin and opaque due to a very high polymorph count

Question 21

Main functions of articular cartilage?

Answer 21

• Provides a low friction lubricated gliding surface to help prevent wear-and-tear of joints
• Distributes contact pressure to subchondral bone

Question 22

How are the mechanical properties of cartilage determined?

Answer 22

Composition of cartilage ECM and the interaction between the fluid and solid phase plays a role in determining mechanical properties of cartilage

Question 23

Name the different zones of the articular cartilage?

Answer 23

Question 24

How do the different zones of articular cartilage vary?

Answer 24

Organisation of collagen fibres

Relative content of cartilage components

Question 25

Type of cartilage that is normally articular?

Answer 25

Hyaline (elastic and sponge-like)

Question 26

Components of hyaline cartilage?

Answer 26

Extracellular matrix (ECM): * Water (70%) - maintain resiliency of tissue and contribute to nutrition and lubrication * Collagen (20%) - mainly type II; contributes to the elastic behaviour of cartilage, tensile stiffness and strength * Proteoglycans (10%) - provides compressive properties assoc. with load-bearing

Question 27

Variation in water content of articular cartilage?

Answer 27

Uneven distribution (most is near the articular surface) Cartilage water content decreases with AGE

Question 28

Types of proteoglycan in articular cartilage?

Answer 28

Mainly GAGs, e.g: chondroitin sulphate Composition changes with age, e.g: chondroitin decreases with age

Question 29

Formation and composition of the ECM of articular cartilage?

Answer 29

Synthesised, organised and degraded and maintained by chrondrocytes; usually, constitutes \<2% of the total volume \>98% is ECM

Question 30

Rate of ECM degradation compared to rate of replacement?

Answer 30

In normal joints, rate of degradation does not exceed rate of replacement In joint disease, rate of degradation exceeds rate of synthesis

Question 31

How is ECM degraded?

Answer 31

Using metalloproteinase proteolytic enzymes, e.g: collagenase and stromelysin

Question 32

How can the mechanical properties of cartilage be changed?

Answer 32

Change relative amounts of water, cartilage, proteoglycan

Question 33

Catabolic factors of cartilage matrix turnover?

Answer 33

Stimulate proteolytic enzymes and inhibit proteoglycan synthesis: * TNFα * IL-1

Question 34

Anabolic factors of cartilage matrix turnover?

Answer 34

Stimulate proteoglycan synthesis and counteract effect of IL-1: * TGF-β * Insulin-like growth factor (IGF-1)

Question 35

Markers of cartilage degradation?

Answer 35

Serum and synovial keratin sulphate, e.g: with age and in osteoarthritis Type II collagen in synovial fluid - useful for evaluating cartilage erosion, e.g: osteoarthritis and rheumatoid arthritis

Question 36

Describe osteoarthritis

Answer 36

Cartilage and synovial composition and function deteriorate with age and repeated wear and tear

Question 37

Describe rheumatoid arthritis

Answer 37

Synovial cell proliferation and inflammation

Question 38

Describe gouty arthritis

Answer 38

Deposition of salt crystals of uric acid

Question 39

Describe pseudogout

Answer 39

Deposition of rhomboid shaped calcium pyrophosphate crystals

Question 40

Describe soft tissue rheumatism

Answer 40

Injury and inflammation to peri-articular structures, e.g. injury to the tendon causes tendonitis

Question 41

Radiological appearance of osteoarthritis?

Answer 41

Subchrondral sclerosis Subchondral cyst Early osteocyte formation ...these may be seen