The Human Body

Hip

Hip, one of two projecting portions on opposite sides of the body between the lowest ribs and the thigh. The hip is made up of the hipbone, or innominate bone (a part of the pelvis), and of the skin, fat, muscles, and membranes overlying it.

The hip is a ball-and-socket joint; the ball is the spherical head of the thighbone, or femur, and the socket is a region on the side of the hipbone known as the acetabulum (Latin, “vinegar cup”). Friction between the bones of the hip joint is reduced by a coating of cartilage and by a lubricating agent known as synovial fluid. The center of gravity of the human body is located behind the hip joint, tending to throw the body backward. With the aid of various muscles from the pelvis and thigh, the femur is able to rotate in the acetabulum and move in any direction, the extent of its movement being limited only by certain supporting ligaments.

The hip and hip joint are subject to a variety of disorders and injuries. Congenital disorders of the hip are occasionally seen in children. Among the most important of these is congenital dislocation of the hip, where the end of the femur, which normally fits into the acetabulum, is not properly developed and does not lie in its proper relationship to the acetabulum. Injuries to the hip joint are common. In athletic injuries involving severe trauma, the head of the femur may be torn out of its normal position in the acetabulum by the force of the injury, causing dislocation. In older people, injuries, even relatively minor ones, may cause a fracture of the neck of the femur, the small portion that lies just below the head. In addition, the hip joint is subject to tuberculosis and to a variety of inflammations and degenerative changes in arthritis. Hip joints severely damaged by falls or arthritis are now often removed and replaced with an artificial hip joint that allows nearly normal activity in most persons.