The teeth are held firmly in place by strong roots that extend into the jawbone. Molars and premolars tend to have several roots, whereas the front incisors only have a single root. The end or tip of each root is termed the apex. The apex is where the nerves and blood vessels enter the tooth and aids in the delivery of blood to the crown (the part of the tooth you can see in your mouth).
A root canal treatment refers to the cleaning of the canals and the removal of infected and inflamed tissue within the root. When the inflammation or infection persists after the root canal treatment, an apicoectomy may be required. An apicoectomy is essentially the removal of the apex (or root tip), followed by a filling procedure to seal the root from further infection. When left untreated, infected roots can damage other teeth, spread infection, and cause regression of the jawbone.
This video shows you how an apicoectomy can be performed to remove an infection at the tip of a tooth's root.