FUSION of two teeth involves the union of which tissues?

Correct answer: A. Enamel and dentin (with or without pulp)

• A. CORRECT. FUSION is the union of two separate tooth germs during development, and because it occurs before or during hard tissue formation the union involves DENTIN — with the enamel of the two crowns continuous across the join, and the pulp chambers either separate or confluent depending on the stage at which fusion occurred. The defining requirement is dentinal confluence. The result is one fewer tooth in the arch than expected when counting.
• B. Union of enamel alone is not a recognised entity — enamel is acellular and cannot form a union independently of the underlying dentin.
• C. Dentin and cementum without enamel involvement does not describe fusion; the crowns of fused teeth are joined through enamel and dentin.
• D. Union by CEMENTUM ONLY, occurring after root formation is complete, defines CONCRESCENCE — a distinct entity affecting maxillary molars.

Note: The counting rule that separates fusion from gemination: count the teeth in the arch, treating the anomalous tooth as ONE. If the count is NORMAL, it is GEMINATION (one germ tried to split). If the count is one tooth SHORT, it is FUSION (two germs joined). Ambiguity arises when fusion involves a supernumerary tooth, which gives a normal count.