NVT ALLEGRA TAVI System TF in Failing Surgical Aortic Bioprosthesis

Inclusion Criteria: * ≥18 years * Symptomatic degeneration of aortic bioprosthesis showing echocardiographically mean aortic gradient \>40mmHg or peak jet velocity \>4.0m/s and AVA\<1.0cm2 OR symptomatic patients with severe bioprosthetic valve insufficiency. * High risk for redo surgery defined by STS ≥10% / EuroScore II ≥7% OR as assessed by the heart team * Has signed the Patient Informed Consent Form * Willing and able to comply with requirements of the study, including all follow-up visits * Female patients of childbearing potential have a negative pregnancy test ≤7 days before the procedure and are willing to use a reliable method of birth control for the duration of study participation Exclusion Criteria: * Low position of the coronary ostia, especially in combination with shallow sinuses * Echocardiographic evidence of intracardiac mass, thrombus or vegetation * Significant aortic disease such as severe obstructive calcification or marked tortuosity or kinking which will preclude a safe advancement of the ALLEGRA TAVI System TF * Iliofemoral vessel conditions such as severe obstructive calcification, severe tortuosity or kinking that would preclude safe placement of an 18 Fr introducer sheath or make endovascular access to the aortic valve impossible * Severe mitral insufficiency * Internal diameter of the bioprosthesis is ≤16 mm or \>28 mm * Patient prosthesis mismatch (EOAi ≤0.65 cm2/m2) as the underlying cause of the poor valve function and need for re-intervention * Non-valvular stenosis as the underlying cause of the poor valve function and need for re-intervention * Failing pre-existing prosthetic heart valve or prosthetic ring in any other position than aortic * Partially detached leaflets that in the aortic position may obstruct a coronary ostium. * Existence of aortic conduit, aortic arch replacement, stentless bioprosthesis and autologous valve replacement * Paravalvular leak of the failing surgical bioprosthesis (between failing surgical bioprosthesis and native annulus) * LVEF \<20% * Evidence of active endocarditis or other acute infections * End stage renal disease requiring chronic dialysis or creatinine clearance \<20 ml/min or serum creatinine \>3.0 mg/dl (264 µmol/l) * Known hypersensitivity to contrast media, which cannot be adequately pre-medicated or contraindication to anticoagulant or anti-platelet medication or to Nitinol alloy or to bovine tissue * Evidence of an acute myocardial infarction within the past 30 days * Cerebral vascular accident (TIA, Stroke) within past 6 months (≤180 days) * Evidence of active peptic ulcer or upper gastrointestinal bleeding within past 90 days * Untreated clinically significant coronary artery disease requiring revascularization * Hemodynamic instability (e.g. cardiogenic shock) requiring inotropic support or mechanical heart assistance (e.g. VAD, IABP) * Uncontrolled (therapy resistant) atrial fibrillation * Need for emergency surgery for any reason * Life expectancy ≤ 12 months due to other medical illness * Currently participating in another investigational drug or device study