Ankylosis of permanent first molars: genetics or environment? A case report of a discordant twin pair.

The rare condition of secondary retention has been reported in the literature as being of genetic origin, with some authors suggesting an autosomal dominant pattern. We report the unusual case of two monozygotic biamniotic, bichorionic male twins, who were discordant for permanent first molar secondary retention, involving ankylosis. Twin A showed normal occlusion and eruption patterns, whereas Twin B displayed a left open bite, in relation with a totally submerged primary second molar leading to retention of the underlying premolar (35), and severe infraocclusion of the adjacent permanent molar (36). After orthodontic failure to close the open bite, ankylosis of 36 was confirmed, whereas 26 became severely infraoccluded.The mother had a history of bilateral molar ankylosis and presented reduced posterior alveolar height. Discordance in this twin pair demonstrates that environmental influences, in addition to epigenetic and local factors, may play a role in secondary retention, which is difficult to diagnose and challenging to treat.