S3: The Structure and Function of Joints Flashcards
3 main classes of joints and describe them
- Fibrous joints/Synarthrosis. These are immobile e.g. skull sutures
- Cartilaginous joints/Amphiarthrosis. These are slightly mobile e.g. intervertebral disc.
- Synovial joint/Diarthrosis. These joints are freely mobile e.g. limb joints. It is the synovial cavity and fluid that allows movement. The lining layer of cartilage prevents grinding and synovial lining provides blood supply and nutrients.
What is sciatica and herniated disc/slip disk?
The intervertebral disk is between two vertebrae/ The annulus fibrosus surrounds the nucleus pulposus. Sometimes, the annulus fibrosus ruptures and liquid can leak (slip disc) which causes swelling and can push on the spinal cord or in the lumbar region it pushes on the cauda equina. Sciatica is when the sciatic nerve is irritated.
List and explain the type of movements allowed by synovial joints
- Planar joints - sliding e.g. intertarsal joint in foot
- Simple hinge joint e.g. humero-ulnar
- Pivot joints - rotation e.g. the atlas vertebrae allows head to turn and it sites on axis
- Saddle joints e.g. base of thumb
- Complex hinge - sliding and rotation e.g. knee
- Ball and socket e.g. hip
What are the 5 ways to achieve stability of joint (role of intra and extra articular structures)?
- Needs to be congruity (matching shapes of bone ends).
- A fibrous capsule provides more stability.
- Intra articular ligaments e.g. cruciate ligaments of knee allow smooth forward and backwards movements.
- Packing improves congruity. Fat pads provide stability (infrapatellar of knee), menisci (semilunar cartilage) also does.
- Muscles acting across the joint improve stability drastically - especially for the shoulder joint which is easily dislocated due to poor congruity and a slack capsule.
Describe the key features of a synovial joint
- Articular cartilage lines the bone surface decreasing friction between movements. This cartilage is avascular and aneural so moving out joints is not painful.
- Blood, lymphatic drainage and nerve supply is by diffusion through synovial lining. The lining produces synovial fluid which allows the joint to move more freely and also provides nutrients to the cartilage.
- This is all contained in a fibrous capsule which prevents the joint from leaking and collapsing.
- Stretch receptors supply our joint which enable us to be aware of movement of our limbs (proprioception).
- Nociceptors are present to signal arthritic pain
Why does articular cartilage take a long time to repair itself?
It is avascular and aneural. Its nutrients come from the synovial fluid, as well as taking waste products away (lymphatics take lymph away running with the blood vessels) so it does not regenerate quickly.
Describe molecular structure of cartilage and physiology of cartilage
- The main chemical that gives cartilage its properties is GAG (glycosamino-glycan chain). These are sugar molecules that exist within cartilage e.g. chondroitin sulphate, keratin sulphate.
- The GAG side chains come off a central core protein. This forms a very large structure which is the large proteoglycan ‘aggrecan’.
- GAG + Core Protein = Proteoglycan
- Hyaluronic acid/Hyaluronan tether large amounts of aggrecan together
- Aggrecan produces a huge osmotic pressure that draws water into cartilage inflating it (gel swelling pressure).
- There is also type I collagen fibrils (parallel which enable the cartilage to weight bear.
What is the only cell type in cartilage?
The chondrocyte. It sits and produces all the molecules needed to produce cartilage such as collagen, proteoglycans and hyaluronan. These cells have a low turn over rate.
Describe Physiology and Pathophysiology of Synovial Fluid/Synovial Lining
- The nutrients in the synovial fluid are pushed by diffusion into the cartilage.
- The synovial fluid receives its nutrients from synovial capillaries which are found inside the synovium.
- The synovial capillaries are very superficial and fenestrated which allows the production of synovial water (by ultrafiltration) and to stock it with nutrients for avascular cartilage.
- Type B synoviocytes also help secrete hyaluronon and lubricin which are useful.
- There are sometimes other cells in the synovial lining such as macrophages.
- The synovium on the outside has the interstitial matrix and there is contact between the interstitial matrix and synovial fluid which provides a fluid drainage pathway.
What is lubricin?
Lubricin, is a glycoprotein that lubricates cartilage under conditions of high load and low velocity (boundary lubrication).
Why is hyaluran important in joints?
- Hyaluronan is a gigantic nonsulphated GAG, it makes the synovial fluid very viscous. It lubricates the synovial fluid surface and cartilage under conditions of low load and high velocity (hydrodynamic lubrication). It also binds aggrecan molecules together in cartilage. Hyaluronan allows more fluid to be drawn in.
- Hyaluronan is very important because it buffers fluid loss from joints i.e. fluid drainage rate (on increase in pressure) in absence of hyaluronan is MUCH HIGHER than when hyaluronan is present (reduced fluid loss on increase in pressure).
How is synovial fluid formed and drained?
Synovial fluid is an ultrafiltrate of plasma generated by fenestrated capillaries just below the synovial surface. The electrolyte and plasma protein content of synovial fluid is similar to other interstitial fluids. Pressure in capillary usually higher so synovial fluid is formed when pressure in the joint is low, this is in extension.
However on flexion, pressure in the joint is increased, which drives fluid out of the joint into the lymphatic drainage.
What is arthritis?
Arthritis is inflammation of a joint, this is characterised by pain, swelling and loss of function.
Categories of arthritis
- Acute (could be caused by infection or injury)
- Chronic (immune-mediated, degenerative e.g. osteoarthritis, other).
- Monoarticular (1 joint)
- Polyarticular (>1 Joint).
2 main types of arthritis
Osteoarthritis and Inflammatory arthritis
