RecruitingDevice studyALLIANCE AVIV: Safety and Effectiveness of the SAPIEN X4 Transcatheter Heart Valve in Failing Aortic Bioprosthetic ValvesThe objective of this study is to establish the safety and effectiveness of the Edwards SAPIEN X4 Transcatheter Heart Valve (THV) in subjects who are at high or greater risk with a failing aortic bioprosthetic valve.