['Severe sclerosis of the native aortic annulus can result in a tilted implantation position of mechanical prostheses. In this study, the effects of tilting and rotation on the hemodynamic performance of standard bileaflet valves were assessed in an extracorporeal mock circulatory system.', 'A pulsatile mock circulation driven by a Berlin Heart system was developed. Main physiological components of the human circulation were mimicked. SJM-AHPJ prostheses (21, 23, 25 mm) were mounted in an artificial aortic root containing physiologically oriented coronary ostia. All experiments were performed under constant conditions (stroke volume 60 ml, heart rate 70 bpm, systolic pressure 130 mmHg). Hydrostatic pressures were measured via fluid-filled catheters, transvalvular flow by ultrasonic probes. Data were digitally recorded at 50 Hz. Multiple pressure, volume, energy, and dimension parameters were derived off-line. Each valve was tested in a 0 degrees (untilted) versus 20 degrees (tilted) position at three axial rotation angles (0 degrees, 45 degrees, 90 degrees ). Tilting was performed independent of rotation by elevation of the prosthesis in the non-coronary sinus.', 'In all valves and all rotation angles, tilting resulted in a size-dependent significant increase of mean pressure gradient (range, 28-35% [21 mm valve], 59-96% [23 mm valve], 124-220% [25 mm valve]), valvular resistance (39-51, 84-121, 177-332%), regurgitation volume (84-148, 32-131, 93-118%), and systolic energy loss (113-146, 30-132, 69-213%), as well as a decrease of total stroke volume (2-5, 0-11, 3-10%), effective stroke volume (6-11, 9-14, 14-22%), cardiac output (6-11, 8-14, 13-22%), and effective opening area (16-24, 32-37, 47-57%). The strongest impairment of hemodynamic performance was seen at 90 degrees rotation with reference to total and effective stroke volume, cardiac output, mean pressure gradient, and regurgitation fraction.'] Is the hemodynamic performance of standard bileaflet valves impaired by a tilted implantation position?