What the word describes
Impaction is a statement about position, not about symptoms. A tooth can be fully impacted and silent for a decade, and a tooth can be fully erupted and causing trouble every few months. The two questions are separate, and conflating them is why patients are sometimes told opposite things by different clinicians.
Partial impaction is the state that produces most of the problems. Where a cusp breaks through the gum but the rest of the crown stays covered, a flap of tissue sits over the tooth with a pocket underneath it that no toothbrush reaches. Bacteria collect there, and pericoronitis — the swelling, soreness and bad taste at the back of the jaw — follows. It tends to recur, because the anatomy that caused it does not change.
Full bony impaction, where the tooth is entirely encased, is less prone to infection but carries a different consideration: the follicle around an unerupted crown can enlarge into a dentigerous cyst, which is usually found on a radiograph before it produces any symptom.
Angle, and why it changes the operation
Mesioangular impaction, where the crown tips forward into the second molar, is the most common pattern in the lower jaw. It is also the pattern that produces decay on the back surface of the second molar — a surface that is difficult to reach with a drill and difficult to restore well, which is how a healthy tooth ends up needing treatment because of its neighbour.
Horizontal impaction, where the tooth lies on its side, generally needs sectioning: the crown is divided from the roots and removed in pieces so the tooth can come out through a smaller opening. Sectioning sounds more invasive and is usually less so, because it removes far less bone than delivering the tooth whole.
Distoangular impaction tips backwards, into the ascending part of the jaw, and is often the more demanding lower removal despite looking straightforward on a film. Vertical impaction sits in a normal orientation but is blocked by bone or by the second molar; it is frequently the simplest of the four.
Depth, the nerve and the sinus
Depth is described in relation to the second molar and to the bone covering the crown. A tooth whose crown sits above the neighbour's widest point is a different operation from one buried below its root line, both in time and in how much bone is removed.
In the lower jaw the constraint is the inferior alveolar nerve, which runs in a canal beneath the roots and supplies feeling to the lip and chin. Where a panoramic radiograph shows the roots crossing the canal, a cone-beam scan is taken to establish whether the two are genuinely in contact, because that changes both the consent conversation and sometimes the plan.
In the upper jaw the neighbouring structure is the maxillary sinus. A root sitting against or into the sinus floor raises the possibility of a small communication at the time of removal, which is managed when it happens and usually closes without further surgery. Both facts are the reason upper and lower third molars are assessed and consented separately rather than as a set of four.
When an impacted tooth is left alone
Not every impaction is a reason to operate. A deeply buried, asymptomatic tooth with no cyst, no decay on the neighbour and no gum pocket can reasonably be reviewed with radiographs at intervals. Saying so is part of the job, and a plan that recommends removing every third molar in every mouth is not a clinical plan.
The arguments in favour of removing an impacted tooth that is currently quiet are that repeat pericoronitis is likely once it has happened twice, that decay on the second molar is difficult to fix, and that surgery is generally more straightforward in the twenties, when roots are less developed and bone is less dense than in later decades.
Where the roots are intimate with the nerve, coronectomy — removing the crown and deliberately leaving the roots in place — is sometimes the more proportionate operation. It is a considered choice rather than a compromise, and it is discussed against the scan.