Abstract: A device for digitizing a center of rotation of a hip joint implant component with respect to a bone element in computer-assisted surgery. The device comprises a detectable member trackable for position and orientation by a computer-assisted surgery system. A body is connected to the detectable member in a known geometry. The body has a coupling portion adapted to be coupled to the hip joint implant component in a predetermined configuration. The center of rotation of the hip joint implant component is calculable in the predetermined configuration as a function of the known geometry and of the position and orientation of the detectable member.