Stent graft delivery system

A system for treating an aneurysm comprises an elongate flexible shaft and an expandable member. An expandable scaffold is disposed over the expandable member and may be expanded from a collapsed configuration to an expanded configuration. A double-walled filling structure is disposed over the scaffold and has an outer wall and an inner wall. The filling structure is adapted to be filled with a hardenable fluid filing medium so that the outer wall conforms to an inside surface of the aneurysm and the inner wall forms a substantially tubular lumen to provide a path for blood flow. In the expanded configuration the scaffold engages the inner wall of the filling structure. A tether is releasably coupled with the filling structure and the flexible shaft thereby constraining axial movement of the structures relative to each other.