Evaluating 3rd molar impactions

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The inferior alveolar nerve canal is partially surrounded by cribriform bone which is full of small holes to allow vessels and nerves to pass from the neurovascular bundle to the teeth and surrounding bone. Many textbooks incorrectly state that the canal is made of cortical bone. Cribriform bone is intermediate in strength between cortical and trabecular bone which makes it vulnerable to crushing during surgical removal of third molars. Evaluation of the proximity of third molar roots to the canal is essential in treatment planning to avoid surgery if the roots are close to the canal.

The riskiest situation is where the root is touching the bundle with no intervening cribriform canal tube. The CBCT image A below shows a circular ring of canal some distance away from the root and this has a low risk of crushing.

The CBCT image in D shows a black arrow pointing to a white circular ring of canal surrounding the bundle and the root touching the canal. This has a higher risk of damage than A but still low at 1%.

The last image F shows a black arrow pointing to a concavity on the lingual cortex and no bone between the root apices and the bundle which is the dark oblong region between the roots and the lingual plate. The risk of paresthesia is 12%

CBCT can be invaluable in selected cases for risk assessment of third molar removal.

JADA 2010;141(3):271-278.

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