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Classification of Joints by Movement

  • There are three classifications based on the amount of movement at the joint: \text{synarthrosis},\ \text{amphiarthrosis},\ \text{diarthrosis}.
  • Synovial joints are called diarthrotic joints or freely movable joints, i.e., they are \text{diarthrotic}.

Structural Characteristics of Synovial Joints

  • Across the gap between two bones in a synovial joint there is articular cartilage (on the ends of the bones). This cartilage is referred to as either articular cartilage or hyaline cartilage; they are the same structure described with slightly different terminology.
  • Articular cartilage is smooth, glossy, and slippery to reduce friction and prevent bone damage during movement.
  • Between the cartilage surfaces is a joint cavity containing synovial fluid.
  • The synovial membrane lines the joint capsule and secretes synovial fluid to lubricate the joint.
  • The outer layer of the joint capsule is the fibrous layer; collectively these structures form the articular (synovial) capsule.
  • A terminology table (mentioned in the lecture) lists key features of synovial joints with short definitions to aid understanding, especially when discussing diseases like osteoarthritis and rheumatoid arthritis.

Anatomy Visualization: The Shoulder Joint (Glenohumeral Joint)

  • The diagram shows two bones forming the shoulder joint with articular cartilage on the ends.
  • The joint capsule includes the fibrous outer layer and the inner synovial membrane; the synovial fluid fills the joint space.
  • The tendon of the biceps muscle and its tendon sheath attach to the bone near the joint.
  • The glenohumeral joint is the shoulder joint, formed where the head of the humerus articulates with the glenoid fossa (glenoid cavity) of the scapula.
  • The illustration highlights the synovial membrane and synovial fluid within the joint capsule.

Bursa and its Role in Synovial Joints

  • The image includes a bursa, which is a small fluid-filled sac that reduces friction between moving structures.
  • The specific example discussed is the subacromial bursa (sub- meaning below; acromial meaning under the acromion).
  • The subacromial bursa lies below the acromion and above the tendon/soft tissues around the shoulder, aiding smooth movement of the joint.
  • In the diagram, you can see the bursa located between the acromion and the underlying tissues, with the bursa rolling and molding during movement to prevent friction.

Subacromial Bursa: Details and Function

  • Location: subacromial region (below the acromion).
  • Purpose: to minimize friction between the acromion and the underlying tendons/soft tissues during shoulder movement.
  • If bursitis occurs (inflammation of the bursa), the bursa swells, movement becomes painful, and range of motion decreases.

Practical Implications of Joint Structure

  • The combination of articular cartilage, synovial membrane, synovial fluid, and the fibrous outer capsule allows smooth, friction-reduced movement of the joint.
  • The presence of a bursa (e.g., subacromial bursa) adds an additional friction-reducing element in joints with substantial movement and tendon sheaths nearby.
  • The structures described are central to understanding common joint diseases:
    • Osteoarthritis (degenerative joint disease) involves wear and tear of joint tissues, including cartilage.
    • Rheumatoid arthritis (autoimmune) affects the synovial membrane and related structures.

Tissue Components and Connections to Disease

  • Key components repeated for emphasis: articular cartilage (hyaline), synovial membrane, synovial fluid, fibrous capsule, articular capsule, bursa, and tendon sheath.
  • The slide references a terminology table to reinforce understanding of these structures and their roles in health and disease.
  • The shoulder example (glenohumeral joint) provides a concrete illustration of how these components work together in a highly mobile joint and where pathology (e.g., bursitis) can arise.

Summary and Takeaways

  • Synovial joints are diarthrotic, freely movable joints with a specialized structure to support smooth movement.
  • Core components: articular cartilage, synovial membrane, synovial fluid, fibrous outer capsule, and the synovial capsule.
  • Extra-articular structures like bursae (e.g., subacromial bursa) and tendon sheaths further reduce friction and protect moving tissues.
  • Anatomical understanding of the glenohumeral joint (shoulder) provides a practical model for how these elements interact and how disease can affect function.
  • These concepts lay the groundwork for exploring diseases like osteoarthritis and rheumatoid arthritis in subsequent lectures.