Infection starting inside a tooth does not stay there. Where it appears in the face follows anatomy, and the pattern is predictable enough to be clinically informative.
Pressure builds inside a closed chamber
The pulp sits in a rigid chamber with only a small opening at the root tip. Inflammation inside cannot expand outward the way it can in soft tissue.
Pressure rises, and inflammatory products travel through the root opening into the bone surrounding the tip.
The process then continues in bone, where it erodes a path through the marrow spaces toward the nearest surface.
Bone thickness decides which side it exits
The bone plate covering the roots is thinner on the cheek side for most teeth, so drainage commonly emerges in that direction.
Some teeth, particularly certain upper molars and lower molars, have thinner bone toward the tongue or palate, changing where the swelling appears.
Root length and angle matter as well, because they determine where the root tip sits relative to the bone plate.
Muscle attachments determine the space entered
Once infection reaches the surface of the bone, whether it enters the mouth or the face depends on where the covering muscle attaches.
If the root tip lies above the muscle attachment, infection is directed into the deeper tissue plane; if below, it drains into the mouth.
Those tissue planes are continuous with one another, which is how an infection from one tooth can present as swelling well away from it.
Why certain presentations are treated as urgent
Swelling that involves the floor of the mouth, spreads across the midline or lifts the tongue can affect the airway.
Swelling around the eye, difficulty swallowing, restricted mouth opening, fever or a spreading area of redness are recognized signs of a process that is no longer localized.
These features are the reason dental infections are triaged by extent and location rather than by how much pain is reported.
Why the source still has to be addressed
Pain sometimes eases when pressure releases through a drainage path, which can be mistaken for resolution while infection continues.
Antibiotics may control spread but do not remove the source inside the tooth, so definitive treatment addresses the tooth itself.
Facial swelling of dental origin warrants prompt professional care, and features involving breathing, swallowing or the eye are reasons to seek emergency medical attention immediately.