What is the difference between fusion and concrescence? Between twinning and gemination?
Fusion is a more complete process than concrescence and involves fusion of the entire length of two teeth (enamel, dentin, and cementum) to form one large tooth, with one less tooth in the arch, or fusion of the root only (dentin and cementum), with the maintenance of two clinical crowns. Concrescence involves fusion of cementum only.
Twinning is more complete than gemination and results in the formation of two separate teeth from one tooth bud (one extra tooth in the arch). In gemination, separation is attempted, but the two teeth share the same root canal.
Source: Dental Secrets, Elsevier, 2015