Medical apparatus includes an external drive magnet mounted for rotation externally of a subject's body, an implantable driven magnet configured and dimensioned to be implantable in the subject's body and mounted for t rotation by magnetic coupling with the drive magnet and an implantable device mechanically coupled to the driven magnet to be driven thereby. The driven magnet is mountable for rotation in the subject's body about an axis of rotation which is not collinear with the axis of rotation of the external drive magnet such that the magnetic poles of the external drive magnet are magnetically coupled sequentially with the magnetic poles of the implanted driven magnet to rotate the implanted driven magnet, thereby providing flexibility in the location, design and dimensions of the implanted driven magnet, and in the speed and torque ratios between the two magnets.