In a human hip replacement process, the steps include providing a selected size generally longitudinally extending stem and laterally angled neck, the stem received endwise into a recess in an elongated femur, there being a ball on the neck, the neck and stem having a non-circular tongue and groove interfit connection to resist relative rotary displacement therebetween, the connection defining a generally longitudinal axis, fitting the ball in a socket on a hip bone, to pivot in the socket, the selection of stem and neck, and tongue and groove connection between the size selected stem and neck, enabling accurate pivoting of the ball. A set of different size balls and neck units are enabled for selective choice, as during surgery.