Synovial Joint

Arthritis is a general term for a group of disorders that cause inflamed. swollen, and painful joints. Joints are found wherever two or more bones or cartilage and bone meet. There are many different kinds of joints in the body. They can be categorized according to structure or function. Among the structure variations are:

>   Fibrous joints.

>   Cartilaginous joints, and

>    Synovial joints

Joints can also be classified according to their ability to provide movement immovable joints are called synarthroses, slightly movable joints are called amphiarthroses, and freely movable joints are called diarthroses. Diarthroses are the most common joints in the body and they are the primary type of joint found in the extremities. Diarthrodial joints are often referred to as synovial joints because they contain a synovial membrane and synovial fluid. Arthritis occurs primarily in freely movable, synovial joints.

Fibrous joints, such as those that are found between the separate bony plates that make up the skull are fastened tightly together by a thin layer of fibrous connective tissue. In general, these joints allow little, if any movement and are therefore synarthroses.

The bones of cartilaginous joints are connected by cartilage. One example is the joint are connected by cartilage. One example is the joint between the public bones of the pelvic girdle. These joints allow limited movement, and are, therefore, amphiarthrotic.

The synovial, or diarthrotic, joint is of greatest interest in the study of arthritis. These joints are freely movable, and are much more complex than fibrous and cartilaginous joints.

Among the joints most commonly affected by osteoarthritis and rheumatoid arthritis are the joints in the fingers, toes, hips knees, wrists, and ankles. The proximal and distal interphalangeal joints of the fingers are hinge joints, while the metacarpophalangeal joints are modified hinge joints or condyloid joints. Hinge joints permit movements in different planes.

The hip and shoulder joints are ball and socket joints. This type of joint consists of a bone with a globular or slightly egg-shaped head that articulates with the cup-shaped cavity of another bone. This arrangement permits movement in a wide variety of planes, and includes movement around a central point or axis.

The knee joint is the largest and most complex of the synovial joints. It includes the upper leg bone the lower leg bone (the tibia), and the kneecap (patella). Although the knee is sometimes considered a modified hinge joint, the connection between the femur and tibia more closely resembles a condyloid joint, while the atriculation of the femur and patella is a gliding joint. In addition, this joint contains two crescent-shaped, fibro-cartilaginous menisci that project into the space between the femur and tibia. The menisci contribute to the stability of the joint and help distribute the body weight that is applied to it. The knee also contains bursae that act as cushions and facilitate the movement of tendons over bony surfaces.