A sterile, self-expanding, manual surgical device designed to separate the edges of a left atrium wall incision (atriotomy) so that the mitral valve can be easily accessed during surgery. The device is typically made of a flat polymeric sheet that is bent by the surgeon to form a conical cylinder; the opposing ends are locked together with a sliding mechanism that allows the device to be held together with forceps or a clamp as a compressed cylinder for introduction to the site where it expands once in situ. It is available in a variety of sizes. This is a single-use device.