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While all synovial joints have an articular capsule, articular cavity, and articular cartilage, many of the more complex synovial joints will have accessory structures that play a role in the protection, support, etc. of the joint.
The most common structures are ligaments. Ligaments connect bone to bone and can be either intrinsic (thickening of capsule) or extrinsic (separate from capsule). This differs from tendons, which connect muscle to bone. Both ligaments and tendons (and commonly muscle bellies) play important roles in structural integrity of joints.
Labra, articular discs, and menisci are all composed of predominantly fibrocartilage and have slightly different compositions and functions. Labra are typically associated with ball and socket joints and form a lip around the socket portion to provide a better fit for the ball portion of the joint. Articular discs can be complete or incomplete and can serve to divide an articular cavity into 2 components (as in the temporomandibular [TMJ] joint) or as a shock absorber. Menisci are similar in function to labra, but are typically incomplete.
Bursae are sac-like structures (very similar in composition to a synovial capsule with synovial fluid) that are often associated with synovial joints, particularly in areas of increased friction. Specialized bursae surrounding long tendons are often referred to as tendon sheaths.
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