A medical instrument for surgery includes at least one frame and at least one jointed device. The jointed device includes at least one first joint member, or first link, adapted to connect to at least one portion of the frame and at least one second joint member, or second link. The first joint member is connected by a rotational joint to the second joint member. The medical instrument includes at least a pair of tendons, adapted to move the second joint member with respect to the first joint member. Each of the first joint member and the second joint member includes a main structural body made in a single piece with one or more convex contact surfaces. Each of the convex contact surfaces is a ruled surface formed by straight line portions all parallel to each other and substantially parallel to a joint movement axis.