Double hip replacement, coloured X-ray. The metal prosthetic hip joints are seen running into the thigh bones (femurs) at lower left and right. The metal cups into which the spherical heads of the shafts fit are seen in place in the pelvis. Hip replacements are performed when the hip joint has been eroded by arthritis, or when the femur is badly broken. The latter is common in old women after the menopause, as that marks the cessation of production of the hormone oestrogen, which is necessary for maintaining bone mass. Without it, bones become weaker, and a fall can easily break the head of the femur, as was the case here. Hip replacements can restore mobility in the majority of cases.