Abstract: A spinal rod characterized by a time-varying stiffness. The rod comprises a first member and at least one second member that is mechanically coupled to the first member through a time-varying interface. The interface features a binding mechanism that degrades after surgical installation. For instance, the interface may be bioabsorbable and dissolve upon exposure to bodily fluids. In another instance, the second member may be comprised of a bioabsorbable material. In another embodiment, the interface may fail under cyclic loading. In another embodiment, degradation of the bioabsorbable material may be inhibited through the application of a current source. The second member may be disposed within the first member. Alternatively, the first member and the second member may be disposed aside one another. The first member and the second member may be substantially similar in shape.