The invention relates to an attachment member (2) and a connecting member (16) for a carpometacarpal thumb joint prosthesis (1) as well as to said thumb joint prosthesis. The attachment member (2) comprises a screw-like part (4) for attachment by screwing into the second metacarpal bone and a locking device (8) for locking the screw-like part to the connecting member (16). The connecting member comprises an elongated, arcuate element (16) which at one end portion (16a) thereof is configured with a hole (14) for connection to the attachment member (2) for the second metacarpal bone and which at the opposite end portion (16b) thereof is configured with a hole (15) for connection to an attachment member (3) for the first metacarpal bone. The joint prosthesis (1) comprises said attachment member (2) for the second metacarpal bone, an attachment member (3) with a screw-like part (5) for attachment by screwing into the first metacarpal bone and with an articulating ball element (6) as well as a corresponding articulating socket element (7), and said connecting member (16).
