Coronectomy is a medical process used to prevent damage to the alveolar nerve during extraction of the wisdom teeth. The process is used when removal of the whole tooth would damage the nerve, as sometimes the root and nerve are intertwined, and removing the tooth could cause temporary or permanent numbness to that side of the face.
The clinical crown can often be removed leaving the root, thus preventing damage to the associated nerve complex.
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Coronectomy Wikipedia(Text) CC BY-SA