Fundoplication is a surgical procedure used to treat stomach acid reflux. During fundoplication, the top part of your stomach — called the fundus — is folded and sewn around the lower esophageal sphincter, a muscular valve at the bottom of your esophagus. This reinforces the lower esophageal sphincter, making it less likely that acid will back up into the esophagus. If you have a hiatal hernia, it can be repaired at the same time. Your surgeon may also tighten the hiatus, the opening in your diaphragm through which the esophagus passes.