Structure
Synovial joints contain the following structures:
- Synovial cavity: all diarthroses have this characteristic space between the bones that is filled with synovial fluid
- Articular capsule: the fibrous capsule, continuous with the periosteum of articulating bones, surrounds the diarthrosis and unites the articulating bones; the articular capsule consists of two layers - (1) the outer fibrous membrane that may contain ligaments and (2) the inner synovial membrane that secretes the lubricating, shock absorbing, and joint-nourishing synovial fluid; the articular capsule is highly innervated, but avascular (lacking blood and lymph vessels), and receives nutrition from the surrounding blood supply via either diffusion (a slow process) or by convection, a far more efficient process achieved through exercise
- Articular cartilage: the bones of a synovial joint are covered by this layer of hyaline cartilage that lines the epiphyses of joint end of bone with a smooth, slippery surface that does not bind them together; articular cartilage functions to absorb shock and reduce friction during movement
Many, but not all, synovial joints also contain additional structures:
- Articular discs or menisci - the fibrocartilage pads between opposing surfaces in a joint
- Articular fat pads - adipose tissue pads that protect the articular cartilage, as seen in the infrapatellar fat pad in the knee
- Tendons - cords of dense regular connective tissue composed of parallel bundles of collagen fibers
- Accessory ligaments (extracapsular and intracapsular) - the fibers of some fibrous membranes are arranged in parallel bundles of dense regular connective tissue that are highly adapted for resisting strains to prevent extreme movements that may damage the articulation
- Bursae - saclike structures that are situated strategically to alleviate friction in some joints (shoulder and knee) that are filled with fluid that is similar to synovial fluid
The bone surrounding the joint on the proximal side is sometimes called the plafond, especially in the talocrural joint. A damage to this occurs in a Gosselin fracture.
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