What characterizes a synovial (diarthroses) joint?

• A. A joint cavity filled with synovial fluid
• B. Joint surfaces separated by fibrous tissue only
• C. No joint cavity
• D. Cartilage fused to bone

Answer: (A)

The defining feature of a synovial (diarthrosis) joint is a joint cavity filled with synovial fluid, enclosed by an articular capsule. This cavity keeps the articulating bone ends separated and is lubricated by the synovial fluid, which reduces friction and nourishes the avascular articular cartilage that covers the joint surfaces. The capsule typically consists of an outer fibrous layer and an inner synovial membrane, and the joint may include ligaments, menisci, or discs to enhance stability and congruence.

Joints described as lacking a cavity or being joined purely by fibrous tissue, or bones fused by cartilage, do not have the synovial cavity and are not freely movable diarthroses.