Inclusion Criteria:  Patient ≥ 18 years of age at time of On-X AAP implant  Patient understands and has signed the Informed Consent Form:  Prior to implant of On-X AAP or Post implant (US only) or Patient died prior to enrollment (US only) Patient is willing and able to participate in follow-up  Exclusion Criteria:  Patient unable to tolerate anticoagulation therapy Patient with active endocarditis at the time of his / her On-X AAP implant Patient participates / participated in another clinical investigation which may interfere / may have interfered with the effectiveness of anticoagulation therapy Patient has anomalous anatomy or medical, surgical, psychological or social history or conditions that, in the investigator's opinion, could limit the subject's ability to participate in the clinical investigation or to comply with follow-up requirements of the clinical investigation results Patients in which On-X AAP is implanted in combination with AMDS or FET Patient's life expectancy is less than 3 years in the opinion of the Investigator