In order to improve a modular implant part for replacing a part of a natural knee joint with an implant component comprising a shaft extending away from the implant component and a connecting device for connecting the shaft to the implant component such that the shaft is adapted to be fixed to the implant component in a multiplicity of desired positions in a simple manner, it is proposed that the connecting device be formed in such a manner that, in a mounting disposition, the shaft is adapted to be moved into different translatory positions by a translatory movement in a direction transverse or substantially transverse to its longitudinal axis, and that, in an implantation disposition, the shaft is adapted to be fixed immovably to the implant component in one of the different translatory positions. Furthermore, an improved knee joint prosthesis is proposed.