A prosthetic femoral joint comprising a part-spherical cup adapted for location in an actabulum and having a part-spherical inner bearing surface to receive a part-spherical ball head which can be attached to a stem for location in a femur, and in which the part-spherical inner bearing surface of the cup extends around an angle of more than 180 ° and a portion thereof adjacent an entry mouth is formed with a substantially flat face which is at a radius from the centre of the hemispherical inner bearing surface which is less than the radius of the remainder, and in which the ball head has a co-operating substantially flat face on its part-spherical surface on which is provided means to receive and retain the stem with which it is to be used, and which prior to attachment to said stem allows it to be located in the cup and rotated so that it is retained thereon, the dimensions and configuration of the part- spherical inner bearing surface and the part-spherical bearing surface on the ball head being arranged to cause a movement of translation of the head during rotation to displace the head so that there is a crescent shaped retention area on each opposed side thereof.
