It is an object of the present invention a medical instrument (60, 160, 260, 360) for surgery comprising at least one frame (57) and at least one jointed device (70, 170, 270), wherein said jointed device (70, 170, 270) comprises at least one first joint member (71), or first link (71), adapted to connect to at least one portion of said frame (57) and at least one second joint member (72), or second link (72)； wherein said first joint member (71) is connected by means of a rotational joint (171) to said second joint member (72)； and wherein said medical instrument (60, 60, 260, 360) further comprising at least a pair of tendons (90, 190), adapted to move said second joint member (72) with respect to said first joint member (71), pulling it； and wherein each of said first joint member (71) and said second joint member (72) comprises a main structural body comprising in a single piece one or more convex contact surfaces (40, 80, 86, 40, 180), and wherein each of said convex contact surfaces (40, 80, 86, 140, 180) is a ruled surface formed by a plurality of straight line portions all parallel to each other and substantially parallel to a joint movement axis (P-P, Y-Y).
