The Structure and Function of Joints

Question 1

What are the three main functions of joints?

Answer 1

• Transmit loads
• Allow movement
• Provide stability

Question 2

What are the three main classes of joints - with examples of each?

Answer 2

• Fibrous joint (synarthrosis) - immobile (skull sutures, tooth socket)
• Cartilaginous joint (amphiarthrosis) - slightly mobile (intervertebral disc)
• Synovial joint (diarthrosis) - freely mobile (limb joints)

Question 3

Name 4 types of movement allowed by synovial joints

Answer 3

• Planar (sliding joints) - intertarsal joints
• Simple hinge - interphalangeal, humero-ulnar (elbow)
• Pivot - atlas on axis
• Saddle - carpo-metacarpal, base of thumb
• Complex hinge - the knee
• Ball and socket - hip, shoulder

Question 4

What are 5 ways that make joints more stable?

Answer 4

• Congruity (matching shapes at bone ends) - head of femur and acetabulum
• Fibrous capsule + thickening into extra-articular ligaments - MCL + LCL
• Intra-articular ligaments - ACL + PCL
• Packing to improve congruity - menisci and fat pads
• Muscles acting across the joint

Question 5

Name some key features of the synovial joint

Answer 5

• Synovium - creates synovial fluid
• Fibrous capsule
• Nociceptors
• Synovial fluid
• Articular carilage (avascular otherwise it would hurt whenever we walked)
• Synovial blood supply and lymph drainage
• Stretch receptors

Question 6

What is the structure of cartilage?

Answer 6

• Glycosamino-glycan (GAG) molecules form long side chains on a core protein
• The chains give a huge osmotic pressure as they attract cations - gives inflation by water
• Lots of these compounds join together to hyaluronic acid
• Chondrocytes are in amongst these aggrecan compnds secreting collagen, proteoglycans and hyaluronan.
• Type II collagen fibrils then hold it altogether

Question 7

Why does the synovial lining have lots of little holes in it?

Answer 7

• Allows synovial fluid through from the synovium - needs a connection between the space and the cartilage
• Articular cartilage is avascular so has to get its nutrients from the synovial fluid.

Question 8

What are the two main types of cell in synovial lining?

Answer 8

Macrophages - Type A cell

Synoviocyte, Type B cell.

Question 9

Why do we need macrophages in synovial lining?

Answer 9

Macrophages digest all of the break down products from wear and tear of the cartilage and then allow it to get back into the lymphatics.

Question 10

What are the key features of Synovial fluid?

Answer 10

• Ultrafiltrate of plasma generated by fenestrated capillaries in synovium
• Has a similar chemistry to other interstitial fluids
• Contains actively secreted molecules lubricin and hyaluronan
• Very small volume of fluid (increases 10-100x in arthritis - joint effusion)

Question 11

What is lubricin?

Answer 11

A glycoprotein that lubricates cartilage under conditions of high load and low velocity

Question 12

What is hyaluronan?

Answer 12

A gigantic nonsulfated GAG, allowing synovial fluid to become viscous, lubricating the synovial surface under conditions of low load and high velocity

Question 13

When will synovial fluid enter the joint, and when will it drain?

Answer 13

On flexion the pressure is above atmospheric, so fluid will be driven out.
On extension, the pressure is subatmospheric and so fluid enters the joint.

Question 14

What happens to synovial fluid in arthritis?

Answer 14

• You get joint effusion, so pressure is above atmospheric even in extension
• inflammatory mediators cause gaps in the endothelium, increasing the permeability to water and proteins. Causes vasodilation of arterioles, increasing capillary pressure.

Question 15

What happens to fluid after it has drained from the joints?

Answer 15

Goes to the subsynovium where it goes into lymphatic drainage

Question 16

What does Hyaluronan do in terms of fluid drainage?

Answer 16

Buffers fluid loss from joint - much less is lost even at high joint pressures
- if there was no hyaluronan, then as joint pressure increases, the drainage of fluid would increase rapidly

Question 17

Briefly describe Rheumatoid arthritis

Answer 17

• inflammation of synovium by inflamamtory mediators - PGE2, bradykinin, histamine, NO, cytokines.
• mediators released by autoimmune mechanism
• Secondary erosion of cartilage by invading hypertrophic synovium (pannus) - releases metalloproteinases
• Hot swelling
• Have disease modifying drugs

Question 18

Briefly describe osteoarthritis

Answer 18

• Wear and tear of articular cartilage
• Fibrillation and erosion
• due to collagen fibril rupture and loss of aggrecan
• associated with ageing, mechanical dysfunction, obesity
• inflammation of synovium

Question 19

What can we do to fibrillation of joints?

Answer 19

• Shave it down to make it smooth

- Put holes into the bone, allowing stem cells to grow out and form a new layer