What common tooth fails to exfoliate in cases of ankylosis?

In cases of ankylosis, the primary second molar is commonly the tooth that fails to exfoliate. Ankylosis occurs when the primary tooth's roots fuse with the surrounding alveolar bone, making it difficult, if not impossible, for the tooth to loosen and fall out as would normally happen during the natural process of exfoliation.

The primary second molar has a significant role in the dental arch and is often subjected to more stress due to its positioning and the forces exerted during chewing. This makes it more susceptible to ankylosis compared to other primary teeth, which generally have shorter roots and a different position in the dental arch.

In contrast, primary canines, incisors, and first molars usually have a more independent pattern of exfoliation and are less likely to experience ankylosis to the same degree. The primary canine and primary incisors typically exhibit a smoother natural transition to allow for the eruption of permanent successors, while the primary first molar, though important, does not have the same frequency of ankylosis issues as the primary second molar.

Therefore, understanding the anatomical and functional characteristics of these teeth helps clarify why the primary second molar is particularly affected in cases of ankylosis.