Pericoronitis is inflammation of the soft tissues surrounding the crown of a partially erupted tooth, including the gingiva (gums) and the dental follicle. The soft tissue covering a partially erupted tooth is known as an operculum, an area which can be difficult to access with normal oral hygiene methods. The hyponym operculitis technically refers to inflammation of the operculum alone. Pericoronitis is caused by an accumulation of bacteria and debris beneath the operculum, or by mechanical trauma (e.g. biting the operculum with the opposing tooth). Pericoronitis is often associated with partially erupted and impacted mandibular third molars (lower wisdom teeth), often occurring at the age of wisdom tooth eruption (15-26). Other common causes of similar pain from the third molar region are food impaction causing periodontal pain, pulpitis from dental caries (tooth decay), and acute myofascial pain in temporomandibular joint disorder. Pericoronitis is classified into chronic and acute. Chronic pericoronitis can present with no or only mild symptoms and long remissions between any escalations to acute pericoronitis. Acute pericoronitis is associated with a wide range of symptoms including severe pain, swelling and fever. Sometimes there is an associated pericoronal abscess (an accumulation of pus). This infection can spread to the cheeks, orbits/periorbits, and other parts of the face or neck, and occasionally can lead to airway compromise (e.g. Ludwigs angina) requiring emergency hospital treatment. The treatment of pericoronitis is through pain management and by resolving the inflammation.
In this scenario, pus may spontaneously discharge via an intra-oral sinus located over the mandibular second or first molar, or even the second premolar. Similar causes of pain, some which can occur in conjunction with pericoronitis may include:

Dental caries (tooth decay) of the wisdom tooth and of the distal surface of the second molar is common. Tooth decay may cause pulpitis (toothache) to occur in the same region, and this may cause pulp necrosis and the formation of a periapical abscess associated with either tooth. Food can also become stuck between the wisdom tooth and the tooth in front, termed food packing, and cause acute inflammation in a periodontal pocket when the bacteria become trapped. A periodontal abscess may even form by this mechanism. Pain associated with temporomandibular joint disorder and myofascial pain also often occurs in the same region as pericoronitis. J are easily missed diagnoses in the presence of mild and chronic pericoronitis, and the latter may not be contributing greatly to the individuals pain (see table).It is rare for pericoronitis to occur in association with both lower third molars at the same time, despite the fact that many young people will have both lower wisdom teeth partially erupted. Therefore, bilateral pain from the lower third molar region is unlikely to be caused by pericoronitis and more likely to be muscular in origin. Prevention
Prevention of pericoronitis can be achieved by removing impacted third molars before they erupt into the mouth, or through preemptive operculectomy.