Clinical, radiographic and histological characteristics of secondary retention of permanent molars.

Secondary retention refers to the cessation of eruption of a tooth after emergence neither due to a physical barrier in the path of eruption nor as a result of an abnormal position. In this study, the clinical and radiographic features of 81 secondary retained permanent molars in a group of 53 patients were evaluated. Retained molars removed for therapeutic reasons (n = 38) were examined histologically to detect any areas of ankylosis. The first molars in the mandible and maxilla were most frequently affected. The mean infraocclusion at the patients' first visit was 4.3 +/- 1.9 mm. After 6 months, infraocclusion increased in adolescents but was stable in adults. Tilting of the adjacent teeth was observed in 39 cases. A sharp, solid percussion sound and a partial absence of the periodontal ligament space on radiographs were noted in less than one-fifth of the affected molars, while histological evaluation revealed that local areas of ankylosis were present in all cases. From the data relating to the 38 molars removed for therapeutic reasons, the sensitivity of the percussion test and radiographic evaluation was found to be 29 and 21 per cent respectively. During a period of 4 years, six new cases of secondary retention were observed in the same population. From this study it is concluded that secondary retention of permanent molars seems to be associated with focal ankylosis and that percussion tests and radiographs are not sufficiently reliable to exclude the presence of ankylotic areas. The presence of ankylotic areas and tendency of infraocclusion to increase in adolescents but to be stable in adults have major implications for therapy.