The Structure and Function of Joints Flashcards
What are the three main functions of joints?
- Transmit loads
- Allow movement
- Provide stability
What are the three main classes of joints - with examples of each?
- Fibrous joint (synarthrosis) - immobile (skull sutures, tooth socket)
- Cartilaginous joint (amphiarthrosis) - slightly mobile (intervertebral disc)
- Synovial joint (diarthrosis) - freely mobile (limb joints)
Name 4 types of movement allowed by synovial joints
- 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
What are 5 ways that make joints more stable?
- 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
Name some key features of the synovial joint
- 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
What is the structure of cartilage?
- 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
Why does the synovial lining have lots of little holes in it?
- 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.
What are the two main types of cell in synovial lining?
Macrophages - Type A cell
Synoviocyte, Type B cell.
Why do we need macrophages in synovial lining?
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.
What are the key features of Synovial fluid?
- 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)
What is lubricin?
A glycoprotein that lubricates cartilage under conditions of high load and low velocity
What is hyaluronan?
A gigantic nonsulfated GAG, allowing synovial fluid to become viscous, lubricating the synovial surface under conditions of low load and high velocity
When will synovial fluid enter the joint, and when will it drain?
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.
What happens to synovial fluid in arthritis?
- 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.
What happens to fluid after it has drained from the joints?
Goes to the subsynovium where it goes into lymphatic drainage
