In a normal hip joint, the rounded top of the thigh bone (femur) fits into a cup-shaped socket in the pelvis called the acetabulum. This type of joint is called a ball-and-socket joint. When the top of the femur moves out of its normal position in the socket, the hip is said to be disclocated.
A hip can become dislocated during many kinds of accidents, including falls from high places and motorcycle or car accidents. When this injury occurs in a head-on car crash, it is often nicknamed a "dashboard dislocation," because it happens when the knee strikes the dashboard.
A traumatic hip dislocation is a medical emergency and needs to be treated immediately, ideally within 6 hours. That is because the injury stops blood from reaching the top of the femur, depriving the bone of its vital oxygen supply. Unless the dislocated hip is replaced in its socket promptly and normal circulation is restored within the hip joint, the top part of the femur can be permanently damaged. This permanent damage is called avascular necrosis.
Because traumatic hip dislocations often occur during serious high-impact accidents, up to 50% of patients also have a fractured bone in some part of the body, particularly in the upper part of the femur.