Joints

Question 1

Describe the basic structure of a synovial joint and the tissues of which a synovial joint is composed.

Answer 1

Ligaments:
- Surround the joint to provide further structural support

Cartilage:
- Cells (chondrocytes)
- Collagen and other organic matrix components (matrix>cellular matter)
- Water
- Regional variation
- Articular cartilage is located on the end of articulating bones

Menisci:
- Small cartilage discs that are found on the surface of articulating joints (stifle joint)
- Provides support
- Prevents excess motion

Synovial fluid:
- Sits in the area between the neighbouring bones
- Secreted by synovial membrane
- Clear/straw coloured viscous fluid
- Contains hyaluronic acid (lubrication)
- Shock absorption
- Nutrients-waste transportation (no blood supply for this transmission)

Bursae:
- Synovial fluid-filled sac that sits beneath tendons where a tendon may pass across a joint (cushioning)
- Prevents friction during movement

Tendon sheath:
- Contains synovial fluid (like bursae) but wraps around the tendon where they pass over joints
- Can be damaged and leak its fluid, no longer being able to perform its lubricative function

Blood / nerve supply:
- Articular cartilage is avascular (no blood supply)
- Blood vessels supply epiphysis and joint capsule / synovial membrane
- Nerves are located around the joint for: pain, reflex, posture and locomotion
- 4 types of fibre:
– free sensory (pain) fibres - from joint capsule and synovial membrane
– efferent fibres - to blood vessels
- sensory fibres - from blood vessels
- proprioceptive fibres - from joint capsule (signalling level of stretch of tendon)

Question 2

Explain how the structure of tissues within a joint are able to facilitate effective joint function.

Answer 2

Articular (hyaline) cartilage:
- Provides a smooth gliding surface (lubrication)
- Shock absorption (stiff to compression when loads are directly placed on top)
- Load transmission - distributes load and protects underlying bone (no localised pressure)
- No nerves / blood vessels - limits capacity of healing/repair and access to nutrients (no pain signal in cartilage)

Versatility of joint types allows adaptable limb function:

• Flexibility
• Stability
• Direction of movement suitable to gait/lifestyle (flexion-extension, rotation, etc…)
• Joint locking (energetic efficiency)
• Moment arms

Question 3

Classify synovial joints according to their action. Give examples of types of synovial joints.

Answer 3

• Most common
• Most moveable (articulating bones separated by a fluid-filled space)
• Joint space is surrounded by a synovial membrane which is usually strengthened by a fibrous joint capsule to encapsulate the synovial fluid
• Synovial joints are multiaxial (3+ directions) as opposed to uni/bi-axial

Types of joint:
- Plantar joint - allows for dorsi/plantar flexion and can be found between articular processes of neighbouring bones (eg; ankle, vertebrae)
- Pivot joint - allows for rotation about a single axis (eg: proximal radioulnar joint)
- Hinge joint - provides side to side movement (eg: humeroulnar joint)
- Condylar joint - concave/convex grooves that provide a uniaxial movement (eg: femorotibial joint)
- Saddle joint - contains 2 surfaces that allow biaxial movement (eg: carpometacarpal joint)
- Ball and socket joint - multiaxial movement (eg: hip joint)

Other joint types:
- Synsarcosis - joint that has no bone involved (only muscular), allowing for sliding and increased mobility (scapula joining to ribcage)
- Fibrous - joint that is formed of dense fibrous tissue, allowing just enough movement to accommodate to change eg: growth (jaw / suture lines in the skull)
- Cartilaginous - made from fibrocartilage to make intermediate joints where small movement is required (interverbal discs / pubis symphysis)