Abstract: Prosthesis device intended for replacing a bone joint, more particularly that of the hip, and comprising two parts which replace its active portions, the portion of each of the two parts on which the contact surface between these latter is formed being made of sintered alumina. One of the two parts has a spherical internal face forming a contact surface and an external face which is to be fixed into one of the bones of the joint, contained in a sphere and provided with grooves a part of which is arranged between cones whose apexes are on the axis and/or between cylinders having the same axis of symmetry as the external face. The surface provided for the bone cells to grow again is greater than that which would be provided by a part having grooves between planes perpendicular to the axis.