a vertebral rod for fixation of a spine , the rod formed of a first material and defining a passage extending at least partially therethrough , the passage at least partially filled with a second material different from the first material to thereby enable the stiffness of the rod to be varied along its length . the first and second materials are either a metallic material or a biocompatible material such as plastic . the rod may be formed of at least three layers of material , with the layers composed of at least two different materials .