Joints, Synovial Fluid and Cartilage

Question 1

Name types of joints.

Answer 1

Fibrous: are very strong (a lot of tightly packed collagen fibres), hard to pull them apart. Not much movements at these joints.

Cartilaginous: mostly cartilage rather than collagen. More mobile than fibrous but overall movement is restrictive.

Synovial: are the most mobile joints. The skeletal elements are separated by a cavity.

There is no cavity and the components are held together by connective tissue (i.e. solid joints) - fibrous and cartilaginous).

Question 2

Give some examples of fibrous joints.

Answer 2

Cranial sutures (between skull bones)

Periodontal ligament ( attach a tooth to the alveolar bone within which it sits)

Interosseous membrane (between bones in forearm, and legs)

ligamentum flavum (connects adjacent vertebral laminae)

Question 3

Name types of cartilaginous joints and give examples of each.

Answer 3

1. Primary c. joint (aka synchondrosis): only hyaline cartilage. E.g. growth plates between ossification centres in long bones.
2. Secondary c. joint (aka symphysis): hyaline and fibrocartilage. Usually occur in the midline. E.g. manubriosternal joint (between the manubrium and the sternum), intervertebral discs, and the pubic symphysis.

Question 4

List features of synovial joints.

Answer 4

Common features: Fibrous capsule, synovial membrane, articular cartilage (hyaline), synovial cavity (fluid filled).

May also contain:

• articular discs usually made of fibrocartilage. They absorb compression forces, adjust to changes in the contours of joint surfaces during movements, and increase the range of movements that can occur at joints.
• ligaments
• bursae: closed sacs of synovial membrane filled with fluid. They reduce the friction of one structure moving over the other. Bursae often intervene between structures, such as tendons and bone, tendons and joints, or skin and bone.

Question 5

Name types of synovial joints and give examples of each one.

Answer 5

1. Plane: allow sliding or gliding movements when one bone moves across the surface of another (e.g. acromioclavicular joint).
2. Hinge: allow movement around one axis that passes transversely through the joint; permit flexion and extension (e.g., elbow [humero-ulnar] joint, knee and ankle joint)
3. Pivot: allow movement around one axis that passes longitudinally along the shaft of the bone; permit rotation (e.g., atlanto-axial joint, radio-ulnar joint)
4. Condylar (ellipsoid): allow movement around two axes that are at right angles to each other; permit flexion, extension, abduction, adduction, and circumduction (limited) (e.g., wrist joint).
5. Saddle: allow movement around two axes that are at right angles to each other; the articular surfaces are saddle shaped; permit flexion, extension, abduction, adduction, and circumduction (e.g., carpometacarpal joint of the thumb).
6. Ball and socket: allow movement around multiple axes; permit flexion, extension, abduction, adduction, circumduction, and rotation (e.g. hip joint, shoulder joint).

Question 6

What determines joint stability?

Answer 6

• Shape of articulating surfaces: e.g. shoulder joint pretty shallow so allows greater movement but also increases risk of dislocation.
• Capsule & ligaments: loose ligaments increases risk of dislocation.
• Muscles

Question 7

What kind of cartilage are meniscus made of?

Answer 7

Fibrocartilage

Question 8

Name the layers and composition of articular cartilage.

Answer 8

• Superficial/tangential layer: flattened chondrocytes that produce collagen and glycoproteins (e.g. lubricin - important for lubrication). Protects deeper layers from shear stresses.
• Transitional layer: round chondrocytes that produce proteoglycans such as aggrecan (binds to water). First line of resistance to compressive forces.
• Deep layer: provides the greatest resistance to compressive forces. Chondrocytes arranged in columnar orientation.

Question 9

Describe how collagen orientation changes from superficial to deep layers of articular cartilage.

Answer 9

Collagen fibres:

• parallel to articular surface in the superficial layer
• oblique in the transitional layer
• orthogonal (right angle) in the deep layer

Question 10

T/F: articular cartilage’s main component is water.

Answer 10

True (>75% = water). This is important as water = incompressible.

Question 11

What are glycoproteins?

Answer 11

proteins to which oligosaccharide chains are attached, i.e. more a protein than a carbohydrate!
e.g. lubrican

Question 12

What are proteoglycans?

Answer 12

proteins that are heavily glycosylated (= a protein core to which one or more GAGs attach), i.e. tend to be more carb than protein!
e.g. aggrecan

Question 13

What are glycosaminoglycans or GAGs?

Answer 13

long unbranched polysaccharides, which are highly polar and thus attract water.
(e.g. hyaluronic acid)

Question 14

What is the average thickness of articular cartilage? Comment on thickness elsewhere as well.

Answer 14

average: 2-3 mm

interphalangeal joint: 1 mm

patella: 5-6 mm. Patella thickness great: as force (pushing it against the knee) acting on it are high during certain movements e.g. squatting.

Question 15

T/F: articular cartilage has rich blood supply.

Answer 15

False. Articular cartilage is avascular, aneural, and alymphatic.

Question 16

How does cartilage receive nutrients and how does it get rid of waste?

Answer 16

Nutrition of the articular cartilage occurs by diffusion from the synovial fluid.

Question 17

What produces synovial fluid?

Answer 17

Synovial fluid is produced by synovial membrane (synovium). Contains synoviocytes that produce the fluid.
Synovial membrane has rich capillary network. It doesn’t have an epithelial lining so direct exchange of oxygen, carbon dioxide & metabolites between blood & synovial fluid.

Question 18

Name types of synoviocytes and state their function.

Answer 18

Type A:

• Look like macrophages
• Remove debris
• Contribute to synovial fluid production

Type B:

• Fibroblast like
• Main producer of synovial fluid

Question 19

What are the functions of synovial fluid?

Answer 19

• Nutrition of cartilage (articular c., menisci/discs)
• Removal of waste products
• Lubrication (from hyaluronic acid & lubricin) which leads to less friction which leads to less wear.

Question 20

Synovial fluid:

1. T/F: is viscous.
2. T/F: present in large volumes in the synovial cavity.
3. T/F: has a rapid turnover.
4. T/F: fluid component from blood plasma.

Answer 20

1. True.
2. False. Small volumes (knee joint: 0.5 ml) present.
3. True. Rapid turnover (2 hours).
4. True

Question 21

Lubrication:

1. How is it achieved at the boundary?
2. Define hydrodynamic.
3. Define weeping.
4. What is the coefficient of friction between joints due to lubrication?
5. T/F: lower the coefficient of friction, the lower the friction.

Answer 21

1. Glycoproteins such as lubricin bind to receptors on articular surfaces to form a thin film.
2. Hydrodynamic is like aquaplaning where the surfaces are kept apart by liquid pressure. Viscosity changes with load and velocity of movement.
3. Weeping (occurs when there is high pressure on the joint) is when fluid that is present in the cartilage is squeezed out into the synovial cavity to increase fluid volume.
4. 0.002-0.02
5. True

Question 22

What is the purpose of bursae?

Answer 22

Fluid-filled pillows that reduce friction around joints.

Question 23

What is bursitis?

Answer 23

Inflammation of the bursae. Very painful.

Question 24

How does ageing affect viscosity of synovial fluid and water content of cartilage?

Answer 24

Viscosity of synovial fluid increases:

• Slower joint movements
• Reduced lubrication

Water content of cartilage decreases
- Reduced shock absorption

As a consequence less protection of articular surfaces & increased risk of damage.