[Frequency of three-rooted mandibular first molars. Survey by x-ray photographs]

The frequency of three rooted mandibular first molars was surveyed. Subjects were of 1,353 male and female students trained in radiology at Fukuoka Dental College whose average age was 24.3 years old. Dental (complete radiographic survey using 14 films) and panoramic radiographs of their mandibular first molars were taken during a period of 9 years from 1978 to 1986. The results were as follows: 1) The number of the three-rooted mandibular first molars was 440 (18.8%) of 2,331 teeth, including 240 (20.6%) of 1,163 in the right side and 200 (17.1%) of 1,168 in the left. 2) 274 (11.8%) of the three-rooted mandibular first molars were identified by radiographs of the premolar region, 42 (1.8%) by radiographs of the molar region, 124 (5.3%) by radiographs of both the premolar and the molar region. 3) Panoramic radiography identified 70 (15.9%) out of 440 three-rooted mandibular first molars. 4) 136 (12.7%) of 1,070 subjects who had bilateral mandibular first molars had three-roots on both sides 127 (11.9%) of 1,070 had them on one side.