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Impacted teeth occur either because the path for their emergence is blocked, or because they have formed in the wrong position in the mouth. The eruption of teeth into mouth is not fully understood, but it is probably the result of rapid multiplication of the cells which from the tooth root, the pressure of which thrust the tooth through the overlying bone.

In cases of impaction, the problem is not usually one of failure within the developing tooth itself, but either of its position or of its relation to the other teeth nearby. Where there is crowding, that is, inadequate space in the mouth for all the teeth, the last teeth to erupt often have insufficient room, because the space available has all been taken by the teeth which have already emerged. Under these circumstances, the later erupting teeth may grow through, but out of line, or alternatively, they may be unable to emerge fully. When a tooth fails to erupt, either fully or partially, it is described as being impacted.

The cause of the crowding which leads to most impactions is not really understood, but it may arise as the result of the individual inheriting large teeth from one parent and small jaws from the other. It has also been suggested that there may be an evolutionary process in progress leading to a reduction in the size of the human jaw.

Sometimes, teeth are impacted as the result of their being formed in the in the wrong position. For example, upper canine teeth are sometimes too deeply placed, so that when they begin to erupt, they grow towards the alveolar bone [bordering the gum]. It is then not usually possible for them to erupt.

Impactions are comparatively uncommon in the milk teeth [called the deciduous dentition] because, as they are small, crowding is usually less. It is possible for any permanent tooth to become impacted, but impaction due to overcrowding most often affects wisdom teeth, the third molar, canines and premolars [situated in front of molar]. Impactions which are due to tooth development in incorrect positions affect upper canines more commonly than other teeth, and the problem often runs in families, showing that it inherited.