Coronectomy is the removal of the crown of a tooth, When applied to a lower wisdom tooth or any lower un-erupted posterior tooth, it is a measure adopted to avoid
damage to the Inferior Alveolar Nerve (IAN) (the nerve that supplies feeling / sensation to the lip and chin) when the X-ray has suggested an intimate relationship between the roots of the lower wisdom tooth and the IAN and the tooth still needed to be removed.

The expectation after removing the top of the tooth is that the roots will remain in place and eventually cover with bone.

Roots encased in bone can remain buried in the jaw for years and rarely cause problems.