Each of the following would be expected in a healthy 17-year-old EXCEPT one. Which is the EXCEPTION?

Correct answer: C. Mamelons on the maxillary central incisors

• A. This IS expected. Third molars typically complete root formation between 18 and 25 years, so open apices at 17 are normal — and third molar development is used in forensic age estimation for exactly this reason.
• B. This IS expected. Root formation of the permanent second molars completes at roughly 14 to 16 years, so fully formed roots at 17 are entirely normal.
• C. CORRECT as the exception. MAMELONS are the three rounded tubercles on the incisal edge of a NEWLY ERUPTED permanent incisor, representing the three developmental lobes of the crown. They are worn away by normal incisal function within a few years of eruption — and permanent incisors erupt at 7 to 9 years, so by 17 they should be long gone. Their PERSISTENCE means the incisors are not making functional contact, which points directly to an ANTERIOR OPEN BITE. In this patient the finding is not merely unexpected, it is diagnostic of the problem.
• D. This IS expected. Cementum is deposited THROUGHOUT life, unlike enamel. Continued apical and lateral apposition means it is thicker at 17 than at 8, and thicker still in old age.

Note: The trap is that mamelons feel like a ‘young person’ finding, and 17 feels young. The relevant age is the ERUPTION age of the tooth, not adolescence generally. Retained mamelons in an adult should always prompt a look at the incisal relationship — open bite, an edge-to-edge relationship, or a missing antagonist.