A system for fixing spinal vertebrae utilizes a spinal rod that has varying properties along its length such that a lower stiffness portion is placed in the cranial or caudal end positions of the spinal rod to address potential adjacent level failure. The spinal rod includes a core member formed of a first material having a first modulus of elasticity and a sheath member formed of a second material having a second modulus of elasticity different from the first modulus of elasticity. The core member includes a first portion having a uniform outer diameter and a second portion extending from the first portion. At least a portion of the second portion has an outer diameter smaller than the uniform outer diameter of the first portion. The sheath member has a lumen extending therethrough that is sized for slidably receiving the second portion of the core member therein.
