Structure and Function of Joints

Question 1

What are the 3 main classes of joint? How mobile is the joint? Where is it found?

What are the types of Synovial joints?

Answer 1

• 1. Fibrous joints (Synarthrosis); Immobile - e.g. skull sutures, tooth socket
     
     2. Cartilaginous joints (Amphiarthrosis); Slightly mobile - e.g. intervertebral disc
     3. Synovial joint (Diarthrosis); Freely mobile - e.g. limb joints
• • Planar (Sliding) joint e.g. intertarsal foot joints
  • Simple hinge joint e.g. IP finger joints and elbow
  • Pivot (Rotational) joint e.g. rotation of 1st/Atlas vertebra
  • Saddle joint e.g. carpo-metacarpal at thumb
  • Ball and socket joint; MAXIMUM MOBILITY BUT LEAST STABILITY

Question 2

What are the 5 ways to achieve joint stability?

What is Packing?

Answer 2

• • CONGRUITY; matching the shape of bone ends e.g. femoral head and acetabulum
  • Fibrous capsules + its thickening into Extra-articular ligaments e.g. medial and collateral ligaments of the knee
  • Intra-articular ligaments e.g. ACL
  • Muscles across joint; important for shoulder stability due to its poor congruity
• Improving congruity by Menisci and Fat pads

Question 3

Synovial Joint:
What is its structure?

What does the Joint capsule consist of?

Why is it important to keep working the muscles around an Arthritic joint?

Answer 3

• • Articular cartilage (Avascular)
  • Synovial fluid bound by Joint capsule
  • Synovial blood supply and lymphatics
  • Stretch receptors for Proprioception (joint angle, limb position) + Nociceptors
• Synovial lining and Fibrous Capsule
• To prevent muscle wasting; Arthritic effusions stimulate stretch receptors, which causes reflex muscle inhibition.

Question 4

Chemical properties of Cartilage:
What is its structure?

What is Aggrecan?

What are the properties of Aggrecan?

What secretes the Aggrecan in the cartilage?

Answer 4

• Proteoglycans organised and bound within cartilage
• Proteoglycans, which are Glycosamino-Glycan chains (GAG) combined with core protein; is a high density of fixed negative charges
• High osmotic pressure to inflate cartilage with water, Collagen fibrils form a framework to resist swelling and provide structure
• CHONDROCYTES

Question 5

Synovium:
What does it consist of?

What is its function?

What are Immune cells present here?

Answer 5

• Synovial Lining, Synovial Intima, and Sub-synovium (of loose areolar connective tissue); connected to lymphatics that drain excess synovial fluid
• Produces synovial fluid; from superficial, fenestrated synovial capillaries (blood supply to articular cartilage)
• • Type A Synoviocytes; Macrophage-like cells
  • Type B Synoviocytes; Fibroblast-like cells that secrete HYALURONON and LUBRICIN

Question 6

Synovial Fluid:
What is it?

What secretes Lubricin and Hyaluronan?

What is Lubricin? What type of lubrication does it do?

What is Hyaluronan? What type of lubrication does it do?

How does Hyaluronan affect fluid drainage in joints?

Answer 6

• Ultrafiltrate of plasma from fenestrated capillaries; drained by lymphatics within the Sub-synovium
• Type B Synoviocytes
• Glycoprotein that lubricates cartilage under low velocity + high load; BOUNDARY LUBRICATION
• Large GAG, which makes synovial fluid viscous (like egg whites) and lubricates under high velocity + low load; HYDRODYNAMIC LUBRICATION
• Buffers fluid loss rate from joints to prevent a large increase in drainage rate as joint pressure increases

Question 7

What is Joint Effusion?

How does pressure change with the angle of the joint?

How is the joint pressure affected in Arthritic Joint Effusion?

Answer 7

• Abnormally high amount of fluid
• Sub-atmospheric on Extension and it rises above atmospheric on Flexion;
  o fluid moves into joint on extension and moves out on flexion to maintain a correct pressure
• Pressure is above atmospheric even on Extension, so fluid can’t be effective moved out of the joint

Question 8

What are the ways to describe the type of Arthritis?

What is RA? Cause?

What is OA? Cause? What is it associated with?

Answer 8

• Acute/Chronic, 1 Joint (Monoarticular)/ >1 Joint (Polyarticular), and can be caused by infection, injury, immune-mediated etc.
• • Inflammation of the Synovium due to inflammatory mediators e.g. Prostaglandins, Histamine
  • AUTOIMMUNE
  • Secondary erosion of cartilage by invading hypertrophic synovium (Pannus) which releases Metalloproteinases
  • Systemic disease
• • Inflammation of the Synovium
  • Mechanical breakdown of Articular cartilage causing Fibrillation, Denudation, and Erosion
  • Due to COLLAGEN FIBRIL RUPTURE and AGGRECAN LOSS
  • Associated with Ageing, Obesity, Mechanical dysfunction