A flexible connection unit for use in a spinal fixation device is provided. The flexible connection unit includes an elongated member, such as a vertebral rod, having a first end portion and a second end portion opposite each other with a transition portion extending therebetween. The first end portion of the vertebral rod comprises an oval shape and the second end portion contains an oval or non-oval shape. The major diameter of the oval shaped first end is parallel to the sagittal plane of a patient's body and provides the higher stiffness for fusion to resist flexion in the sagittal plane. The second end portion of the vertebral rod is perpendicular to the sagittal end of a patient's body and provides the lower stiffness for fusion.