An appendectomy was the surgical removal of the vermiform appendix.
Elias Sandoval underwent an appendectomy some time prior to 2264. The appendix later grew back when Sandoval was exposed to the Omicron spores. (TOS: "This Side of Paradise")
In 2372, The Doctor mentioned that he had performed three appendectomies since his activation the previous year. (VOY: "Lifesigns")