The lower esophageal sphincter (LES) in healthy individuals allows food to pass into the stomach, but prevents gastric fluids from moving into the esophagus except when the patient vomits.  Aspiration is a clinical risk for patients having a malfunctioning LES or for patients having stents placed across the gastroesophageal junction (GEJ) so that an opening is created at the bottom of the esophagus that can lead to acid reflux and aspiration.  Aspiration occurs when the stomach contents travel from the stomach into the lungs.  Aspiration in the lungs can lead to pneumonia or death.  Risk of aspiration in patients having a compromised LES increases when the patient is in a prone position. 
Anti-reflux esophageal prostheses or stents have been developed to treat tumors or strictures in the vicinity of the LES.  Anti-reflux esophageal prosthesis or stent is typically placed in the lower esophagus and through the LES to maintain the patency thereof due to the presence of a cancerous tumor commonly found in the vicinity thereof or to treat benign tumor conditions, such as blockage or strictures. 
A problem with an esophageal prosthesis or stent is that fluid from the stomach flows into the mouth of the patient when in a prone position, increasing the risk of aspiration.  In an attempt to solve the problem, a number of esophageal prostheses or stents utilize a one-way valve such as a duck-bill or reed-type valve in which only food or fluid from the esophagus flows into the stomach in only an antegrade or forward direction.  However, these one-way anti-reflux prostheses or stents present another problem.  When the patient wants to belch or vomit, the patient is prevented from doing so, because the one-way valve prevents backward flow in the retrograde direction.  Such condition is not only painful to the patient, but can also lead to more complicated medical conditions.  Some esophageal prostheses or stents use a sleeve that extends into the stomach to control the flow of fluids.  However, at standard pressure within the stomach, and in the absence of external pressure (i.e. standard temperature and pressure) such as when the patient is in the prone position, the sleeve may not seal to prevent reflux of the stomach contents into the mouth. 
What is needed is a prosthesis that is normally closed to prevent gastric fluids from entering the esophagus even in the absence of external pressure within the stomach (i.e. at standard temperature and pressure), allows food to pass into the stomach and also allows for vomiting and belching when necessary and returns to the closed position.