Infections at the root apex can occur for various reasons, and they always occur in teeth where the nerve is no longer vital. The most common reason is untreated and deep tooth decay. Often, infections at the root apex occur due to uneven pressure on that tooth during chewing, which is called occlusal trauma. Any other trauma to the tooth can also lead to nerve death and subsequent infection. Inadequate prior nerve treatment (endodontic therapy) can result in an infection at the root apex.
These infections can be acute and manifest as tooth pain on touch or pressure, sometimes with spontaneous pain without pressure. The area of the mucosa in the projection of the root apex is red and sensitive to touch with a finger or tongue. Swelling or fistula (an opening from which pus can drain) may occur.
If the infection at the root apex is chronic, it can go unnoticed without any symptoms, and patients often do not know that it exists and can only be detected accidentally during a dental examination through a dental x-ray analysis.
In both situations, the tooth can darken, which can reliably signal that the nerve in the tooth is no longer vital and that an infection at the root apex may occur.