Joint

The junction between 2 or more bones. Many joints are highly mobile, while others are fixed or allow only a small amount of movement. Joints in the skull are fixed joints firmly secured by fibrous tissue. The bone surfaces of mobile joints are coated with smooth cartilage to reduce friction. The joint is sealed within a tough fibrous capsule lined with synovial membrane (see synovium), which produces a lubricating fluid. Each joint is surrounded by strong ligaments that support it and prevent excessive movement. Movement is controlled by muscles that are attached to bone by tendons on either side of the joint. Most mobile joints have at least one bursa nearby, which cushions a pressure point. There are several types of mobile joint. The hinge joint is the simplest, allowing bending and straightening, as in the fingers. The knee and elbow joints are modified hinge joints that allow some rotation as well. Pivot joints, such as the joint between the 1st and 2nd vertebrae (see vertebra), allow rotation only. Ellipsoidal joints, such as the wrist, allow all types of movement except pivotal. Ball-and-socket joints include the hip and shoulder joints. These allow the widest range of movement (backwards or forwards, sideways, and rotation). Common joint injuries include sprains, damage to the cartilage, torn ligaments, and tearing of the joint capsule. Joint dislocation is usually caused by injury but is occasionally congenital. A less severe injury may cause subluxation (partial dislocation). Rarely, the bone ends are fractured, which may cause bleeding into the joint ( haemarthrosis) or effusion (build-up of fluid in a joint) due to synovitis (inflammation of the joint lining). Joints are commonly affected by arthritis. Bursitis may occur as a result of local irritation or strain.