The structure of a synovial joint – the knee joint. (top) The flexed knee joint from the rear: (a) femur (thigh bone); (b) joint surfaces (cartilage) and cavity; (c) tibia (shin bone); (d) and (e) ligaments; (f) fibula. (bottom) Lateral section through the knee: (1) femur; (2) quadriceps muscle with tendon; (3) patella (knee cap); (4) cavity with meniscus; (5) tibia.
A synovial joint, aso called a diathrosis, a type of joint in which the articular surfaces of the bones are covered by a thin layer of hyaline cartilage separated by a joint cavity. This arrangement permits a great degree of freedom of movement.
The cavity of the joint is lined by synovial membrane, which extends from the margins of one articular surface to those of the other. The synovial membrane is protected on the outside by a tough fibrous membrane referred to as the capsule of the joint. The articular surfaces are lubricated by a viscous fluid called synovial fluid. In certain synovial joints, for example in the knee joint, disks or wedges of fibrocartilage are interposed between the articular surfaces of the bones. These are referred to as articular disks.
Fatty pads are found in some synovial joints lying between the synovial membrane and the fibrous capsule or bone. Examples are found in the hip and knee joints.
The degree of movement in a synovial joint is limited by the shape of the bones participating in the joint, the coming together of adjacent anatomical structures (for example, the thigh against the anterior abdominal wall on flexing the hip joint), and the presence if fibrous ligaments uniting the bones. Most ligaments lie outside the joint capsule, but in the knee some important ligaments, the cruciate ligaments, lie within the capsule.
Types of synovial joints
Synovial joints may be classified according to the arrangement of the articular surfaces and the types of movement that are possible, as follows: