A medical system is disclosed for treating a defective mitral valve (MV) having an annulus (A). The system has a removable and flexible elongate displacement unit for temporary insertion into a coronary sinus (CS) adjacent the valve, wherein the displacement unit has a delivery state for delivery into said CS, and an activated state to which the displacement unit is temporarily and reversibly transferable from the delivery state. The displacement unit has a proximal reversibly expandable portion, a distal anchoring portion being movable in relation to the proximal expandable portion in a longitudinal direction of the displacement unit to the activated state in which the shape of the annulus is modified to a modified shape (A′；)； and an annuloplasty device for permanent fixation at the mitral valve annulus by annuloplasty of the valve when the modified shape is obtained.
