An implantable device that is expandable in vivo includes two end pieces and a spacer configured to be inserted between the two end pieces in vivo. The two end pieces are configured to be inserted simultaneously without the spacer. After the two end pieces have been inserted and positioned as desired, the spacer is inserted between the two end pieces in vivo to maintain a desired amount of separation between the two end pieces. The two end pieces each provide contoured surfaces for engaging and holding the spacer between the end pieces.