Apicoectomy is a procedure done to save an infected endodontically treated tooth. The reasons for an endodontically infected tooth to get reinfected are:
An X-ray will be taken to confirm the reinfection surrounding the tooth. Then you will be given anaesthesia to numb the tooth and surrounding gums. The flap of gum tissue is opened to see the bone surrounding the tooth and the infected tissue is removed. The end of the tooth is also cut and a small filling is given at the cut root end to seal the root canal. The gum flaps are then stitched back in place and you will be given a course of antibiotics.
You may be asked to get periodic X-rays to be taken to make sure the infected tissue is all taken out and healthy bone is taking its place.