Anatomy of a Joint

Joints are the areas of the body where two or more bones meet. Most joints are mobile, allowing the bones to move. Joints consist of the following:

• Bones. The framework of your body, bones are the primary structures that support the connecting tissue. For example, the knee joint consists of three bones ? the femur (thighbone), tibia (shin bone) and patella (kneecap). 
• Tendons. Tendons (a type of tough connective tissue) on each side of a joint that attach to the muscles that control movement of the joint.
• Ligaments. Strong ligaments (tough, elastic bands of connective tissue) surround the joint to give support and limit the joint's movement. 
• Cartilage. A type of tissue that covers the surface of a bone at a joint. Cartilage helps reduce the friction of movement within a joint.
• Meniscus. This is a curved part of cartilage in the knees and other joints.
• Synovial membrane. A tissue called the synovial membrane lines the joint and seals it into a joint capsule. The synovial membrane secretes synovial fluid (a clear, sticky fluid) around the joint to lubricate it.
• ?Bursas. Fluid-filled sacs, called bursas, between bones, ligaments, or other adjacent structures help cushion the friction in a joint.
• Synovial fluid. A clear, sticky fluid secreted by the synovial membrane.

There are many types of joints, including joints that do not move in adults, such as the suture joints in the skull. Joints that do not move are called fixed. Other joints may move a little, such as the vertebrae. Examples of mobile joints include the following:

• Ball-and-socket joints. Ball-and-socket joints, such as the shoulder and hip joints, allow backward, forward, sideways, and rotating movements.
• Hinge joints. Hinge joints, such as in the fingers, knees, elbows, and toes, allow only bending and straightening movements.
• Pivot joints. Pivot joints, such as the neck joints, allow limited rotating movements.
• Ellipsoidal joints. Ellipsoidal joints, such as the wrist joint, allow all types of movement except pivotal movements