An implantable device configured to withstand forces imparted thereto by anatomical structure at a deployment site. The implantable device includes an outer portion and inner portion. The outer portion is positioned between the inner portion and the walls of the anatomical deployment site. The outer portion may enclose the inner portion. The outer portion may be a compliant balloon compressible in response to radially-inwardly directed anatomical forces exerted thereon. The inner portion may include one or more portions configured to retain the implantable device at the deployment site and to resist migration of the implantable device. The inner portion may be movable with respect to the outer portion. The inner portion may include more than one portions, such portions being movable with respect to each other. A shaft may extend through the device, such as to facilitate inflation of the outer portion and/or positioning of the inner portion.