Lecture Notes: Synovial Joints

Overview of Joint Types

• Fibrous Joints: Previously discussed.
• Cartilaginous Joints: Previously discussed.
• Synovial Joints:
  • Characterized by bones separated by a fluid-filled cavity.
  • Contains synovial fluid within a joint cavity.
  • Lined by a synovial membrane.
  • Freely movable, classified as diarthrotic.
  • Includes most joints in limbs (shoulder, hip, knee, elbow, fingers).

Structure of Synovial Joints

• Articular Cartilage:

  • Covers the ends of bones at joints.
  • Made of hyaline cartilage to prevent crushing and provide cushioning.

• Joint (Synovial) Cavity:

  • Small, fluid-filled space containing synovial fluid.
  • Acts as lubricant and nutrient for cartilage.

• Articular Capsule:

  • Encases synovial cavity, composed of two layers:
    • External Fibrous Layer: Dense irregular connective tissue.
    • Internal Synovial Membrane: Loose connective tissue producing synovial fluid by filtering blood plasma.

• Synovial Fluid:

  • Viscous, slippery fluid composed mainly of water and hyaluronic acid.
  • Lubricates and nourishes articular cartilage.
  • Facilitates nourishment through compression and relaxation cycles.

• Ligaments:

  • Capsular Ligaments: Part of the fibrous layer.
  • Extracapsular Ligaments: Support joints externally.
  • Intracapsular Ligaments: Located within the joint cavity (e.g., anterior cruciate ligament in the knee).

• Additional Features:

  • Fatty Pads: Provide cushioning.
  • Articular Discs/Menisci: Rings of fibrocartilage, reduce wear and tear, stabilize joints (e.g., in the knee).

Synovial Tissues

• Nerves and Blood Vessels:

  • Damage to tissues can cause pain.
  • Monitor joint position and stretch.
  • Capillary beds nourish synovial membrane by filtering plasma.

• Associated Structures:

  • Bursae: Fluid-filled sacs reducing friction between tissues.
  • Tendon Sheaths: Elongated bursae wrapping around tendons, especially in areas of high friction like the wrist.

Stability of Synovial Joints

• Three Key Factors:
  1. Shape of Articular Surface: Minor role, important in ball-and-socket joints.
  2. Supporting Ligaments: Provide additional strength, limited role due to flexibility limits.
  3. Muscle Tone: Most important, constant low-level tension helps maintain joint alignment.

Joint Types and Characteristics

• Fibrous Joints: Immovable (e.g., cranial sutures).
• Cartilaginous Joints: Slightly movable (e.g., intervertebral discs, pubic symphysis).
• Synovial Joints: Freely movable (e.g., appendages, shoulder).

Examples of Joints

• Atlanto-occipital Joint: Synovial, diarthrotic.
• Intervertebral Discs: Cartilaginous, amphiarthrotic.
• Sternoclavicular Joint: Synovial, allows significant movement.
• Pubic Symphysis: Cartilaginous, provides cushioning during movement.

Summary:

• Synovial joints are crucial for movement and include various structural and functional components designed to facilitate motion and reduce wear. Understanding their anatomy and associated tissues helps appreciate their role in mobility and stability.