A device (10,10′,20,20′) for digitizing a center of rotation of a hip joint implant component (A,F) with respect to a bone element in computer-assisted surgery. The device (10,10′,20,20′) comprises a detectable member (12,22) trackable for position and orientation by a computer-assisted surgery system (30). A body (11,21) is connected to the detectable member (12,22) in a known geometry. The body (11,21) has a coupling portion (14,14′,24,25) adapted to be coupled to the hip joint implant component (A,F) in a predetermined configuration. The center of rotation of the hip joint implant component (A,F) is calculable in the predetermined configuration as a function of the known geometry and of the position and orientation of the detectable member (12,22).