[The preparation for dowel-cores. Part 1. Convergency of three root canals of the mandibular first molar].

117 Mandibular first molars were radiographed mesiodistally and buccolingually and the convergency of three root canals was assessed on the supposition that every root canal should be prepared upto the two-thirds depth of canals.The results were as follows:1. From the viewpoint of the design of dowel-cores, there were three significant angles that is, angles between the mesial wall of the mesiolingual canal and the distal wall of the distal canal, between the buccal wall of the mesiobuccal canal and the lingual wall of the mesiolingual canal and between the lingual wall of the mesiobuccal canal and the buccal wall of the mesiolingual canal, means of 16.4°, -3.3° and 0.2° respectively.2. The parallelism between the mesiolingual and the distal canals was easily attained in 12.8% and the one between the mesiolingual and the mesiobuccal canals in 64.8% of all mandibular first molars, because of no or very little undercut in itself.3. Three root canals were easily prepared in parallel but in 2.7-6.8% of all teeth. In the others, therefore, every root canal should be widely prepared for parallelism or only one main root canal could be utilized for retention or two-piece dowel-cores could be fabricated.