Introduction
When bileaflet mechanical mitral valves (MMVs) were first introduced in the 1970s, it seemed intuitive to position them in an anatomical orientation in an attempt to imitate nature1. However, in the early 1980s, cases of early valve thrombosis were observed1. Imaging studies suggested that these thrombotic complications were due to a restricted excursion of the posterior occluder and resultant stagnation of blood flow2,3. This phenomenon, coined as the “lazy” leaflet1, was found to be avoided with anti-anatomical implantation1. This historical observation shaped the standard implantation technique, but practice variation in leaflet orientation persists. Some new-generation MMVs, such as the On-X valve (On-X Life Technologies, Austin, Texas), benefit from a rotatable cuff design, but it is unclear if they are susceptible to asymmetric occluder mobility in the anatomical plane. Valve manufacturers typically endorse anti-anatomical implantation, although it is not formulated as a strong recommendation.