Medical instrument (60, 160, 260, 360) comprising at least one first, second and third joint member (71, 72, 73), at least two pairs of tendons (90, 190, 191, 192) connected with the same joint member and adapted to move said third member (73) with respect to said second member (72), pulling it; and wherein each of said joint member (71, 72, 73) comprises a main structural body comprising in a single piece one or more convex contact surfaces (40, 80, 86, 140, 180), all said convex contact surfaces (40, 80, 86, 140, 180) defining with their prolongations thereof at least partially a single convex volume for each joint member; and wherein the main portion of each tendon is in contact with said jointed device (70, 170, 270) only on said convex contact surfaces; and wherein at least two tendons of said at least two pairs of tendons (90, 190, 191, 192) are in contact with a same convex contact surface.