M01.01.007 Structures of a Synovial Joint

Synovial joints are characterized by a fluid-filled joint cavity within a fibrous capsule, the most common type of joint in the human body. The primary structures include:

1. Articular Capsule
   • Fibrous Layer (Outer): Composed of white fibrous tissue (capsular ligament) that holds the articulating bones together and supports the underlying synovium.
   • Synovial Layer (Inner): Highly vascularized serous connective tissue that absorbs and secretes synovial fluid, mediating nutrient exchange between blood and joint (synovium).
2. Articular Cartilage
   • Covers the articulating surfaces with a thin layer of hyaline cartilage.
   • Functions: Minimizes friction during joint movement and absorbs shock.
3. Synovial Fluid
   • Found within the joint cavity.
   • Functions: Provides lubrication, nutrient distribution, and shock absorption.

Accessory Structures

1. Accessory Ligaments
   • Separate ligaments or parts of the joint capsule are made of dense regular connective tissue, adapted for resisting strain and protecting the joint from extreme movements.
2. Bursae
   • Small sacs lined by synovial membrane and filled with synovial fluid.
   • Function: Reduce friction at key points in a joint, allowing greater freedom of movement and protecting articular surfaces from degeneration.

Neurovascular Structures

1. Innervation
   • Supplied by articular nerves, which transmit proprioceptive (joint position) and nociceptive (pain) sensations.
   • Follows Hilton’s Law: nerves supplying a joint also supply the muscles moving the joint and the skin covering their distal attachments.
2. Vasculature
   • Arterial Supply: Via articular arteries from vessels around the joint, mostly in the synovial membrane, featuring frequent anastomoses to ensure a consistent blood supply.
   • Venous Drainage: Articular veins accompany the arteries within the synovial membrane.

Clinical Relevance: Osteoarthritis

• The most common form of joint inflammation, often resulting from years of heavy use.
• Pathophysiology: Wearing away from articular cartilage and erosion of underlying bone surfaces, leads to decreased cartilage effectiveness, roughened bone edges, and joint pain, stiffness, and discomfort.
• Risk Factors: Commonly affects weight-bearing joints like hips and knees.
• Other Causes: Infection, autoimmune responses (e.g., rheumatoid arthritis), or reactive arthritis.

Bibliography

• TeachMeSeries Ltd (2024). Anatomy and Clinical Relevance of Synovial Joints.
• OpenStax College [CC BY 3.0], via Wikimedia Commons.