Medical instrument (60, 160, 260, 360) for surgery comprising at least one frame and at least one jointed device (70, 170, 270), wherein said jointed device (70, 170, 270) comprises: - at least one first joint member, adapted to connect to at least one portion of said frame; - at least one second joint member, wherein said first joint member is connected by means of a first rotational joint to said second joint member, defining a first joint movement axis (P-P); wherein said at least one second joint member is a wrist member comprising at least one junction portion, adapted to form at least one portion of a second rotational joint having a second joint movement axis (Y-Y), not parallel to said first joint movement axis (P-P); and wherein said medical instrument (60, 160, 260, 360) further comprising: at least one end member, wherein said end member is connected to said wrist member by means of said second rotational joint; - at least one pair of tendons (90, 190), adapted to move the at least one end member with respect to said second joint member; - the least one pair of tendons (90, 190), comprising a tendon and an opposite tendon, with second terminations thereof connected in respective tendon termination seats of said end member, so as to determine a gripping or cutting movement; - at least a further pair of tendons (191, 192), adapted to move said second joint member with respect to said first joint member, pulling it; and wherein each of said first joint member and said second joint member comprises a main structural body comprising in a single piece one or more convex contact surfaces (40, 80, 140, 180), and wherein each of said convex contact surfaces (40, 80, 140, 180) is a ruled surface formed by a plurality of straight line generators all parallel to each other and parallel to said first joint movement axis (P-P), each of said convex contact surfaces (40, 80, 140, 180) being a sliding surface suitable for the sliding of at least a portion of a tendon of said pair of tendons (90, 190) over it; and wherein said end member comprises at least one winding surface, which is a ruled surface formed by a plurality of straight line generators all parallel to each other and parallel to said second joint movement axis (Y-Y), said winding surface being adapted to allow the winding thereon of at least a portion of a tendon of said pair of tendons (90, 190) avoiding any sliding of said tendon. - at least one first joint member, adapted to connect to at least one portion of said frame; - at least one second joint member, wherein said first joint member is connected by means of a first rotational joint to said second joint member, defining a first joint movement axis (P-P); wherein said at least one second joint member is a wrist member comprising at least one junction portion, adapted to form at least one portion of a second rotational joint having a second joint movement axis (Y-Y), not parallel to said first joint movement axis (P-P); and wherein said medical instrument (60, 160, 260, 360) further comprising: at least one end member, wherein said end member is connected to said wrist member by means of said second rotational joint; wherein said first joint member is connected by means of a first rotational joint to said second joint member, defining a first joint movement axis (P-P); wherein said at least one second joint member is a wrist member comprising at least one junction portion, adapted to form at least one portion of a second rotational joint having a second joint movement axis (Y-Y), not parallel to said first joint movement axis (P-P); and wherein said medical instrument (60, 160, 260, 360) further comprising: at least one end member, wherein said end member is connected to said wrist member by means of said second rotational joint; - at least one pair of tendons (90, 190), adapted to move the at least one end member with respect to said second joint member; - the least one pair of tendons (90, 190), comprising a tendon and an opposite tendon, with second terminations thereof connected in respective tendon termination seats of said end member, so as to determine a gripping or cutting movement; - at least a further pair of tendons (191, 192), adapted to move said second joint member with respect to said first joint member, pulling it; and wherein each of said first joint member and said second joint member comprises a main structural body comprising in a single piece one or more convex contact surfaces (40, 80, 140, 180), and wherein each of said convex contact surfaces (40, 80, 140, 180) is a ruled surface formed by a plurality of straight line generators all parallel to each other and parallel to said first joint movement axis (P-P), each of said convex contact surfaces (40, 80, 140, 180) being a sliding surface suitable for the sliding of at least a portion of a tendon of said pair of tendons (90, 190) over it; and wherein said end member comprises at least one winding surface, which is a ruled surface formed by a plurality of straight line generators all parallel to each other and parallel to said second joint movement axis (Y-Y), said winding surface being adapted to allow the winding thereon of at least a portion of a tendon of said pair of tendons (90, 190) avoiding any sliding of said tendon.