In subjects with agenesis of the second mandibular premolar teeth, the primary molar may be left in situ. The long-term prognosis of a retained primary molar has not been established in any study that included large number of patients. The aim of this investigation was to monitor the survival of retained mandibular second primary molars, from 12–13 years of age to adulthood. The material comprised the radiographs of 99 subjects, 37 males and 62 females, with retained primary molars and agenesis of 149 mandibular second premolars. In subjects with bilateral agenesis, only one randomly selected primary molar was included to ensure that all primary molars were statistically independent units. Three variables were recorded on intraoral radiographs at 11 registration stages starting at 12–13 years of age: the distance between the permanent molar and the premolar abutting the retained primary molar, infraocclusion of the primary molar, and resorption of the primary molar roots. The mean age at the final registration was 24 years 7 months, [standard deviation (SD) 5.3 3 years, range 18.25–45.44 years]. Paired t-tests were used to analyse changes in the variables and intraclass correlation coefficient (ICC) to determine whether all primary molars could be included in the analyses.
During the observation period, only seven of the 99 primary molars were lost due to extensive root resorption, infraocclusion, or caries. Long-term survival may be expected in more than 90 per cent of patients with retained primary molars with agenesis of mandibular second premolars.