An implant for insertion between vertebral bodies in a patient includes a flexible tether that is coupled between fasteners attached to the vertebral bodies. An energy storing device may be coupled to the tether to convert extension forces between the vertebral bodies into potential energy. The energy storing device may reduce defects at the interface between the fasteners and the vertebral bodies. Various embodiments are provided, including energy storing devices implemented as springs, leaf springs, coiled wire, and corrugated shapes. The energy storing device may be preloaded between the vertebral bodies to further reduce shock from sudden extensions. The energy storing device may be secured to the tether before or after the tether is secured to the fasteners. Further, the energy storing device may be sized to allow the tether and energy storing device to pass laterally through a fastener opening.