A 9-year-old presents with tooth #9 fully erupted while #8 is absent. What is the most probable cause?

Correct answer: D. A supernumerary tooth (mesiodens) obstructing eruption

• A. Simple physiologic delay does not explain a UNILATERAL failure when the contralateral tooth has already fully erupted; contralateral eruption normally occurs within about six months.
• B. Ankylosis of a retained primary incisor could delay eruption, but it would be evident clinically as an infraoccluded primary tooth with a characteristic dull percussion note, and the stem gives no such finding.
• C. AGENESIS of a maxillary CENTRAL incisor is very rare. The commonly missing permanent teeth are third molars, mandibular second premolars and maxillary lateral incisors — the central incisor is one of the least likely teeth to be congenitally absent, so agenesis is a poor first explanation.
• D. CORRECT. A MESIODENS is the most common supernumerary tooth, occurring in the anterior maxillary midline, and OBSTRUCTED ERUPTION of a permanent central incisor is its most common presentation. The classic picture is exactly this: one central incisor erupts on schedule while its contralateral partner does not. The asymmetry is the diagnostic clue. Management is a periapical and occlusal radiograph (or CBCT) to localise it, surgical removal of the supernumerary, and orthodontic traction if the incisor does not then erupt spontaneously.

Note: Rule of thumb: whenever a permanent incisor fails to erupt more than 6 months after its contralateral partner, image the area. The differential is supernumerary tooth, odontoma, retained or ankylosed primary tooth, dilaceration from earlier trauma, ectopic position, and scar tissue over the crypt.