The prognosis of root canal treatment largely depends on the quality of root canal filling. The shape of each root canal must be properly determined when performing root canal preparation. In the present study, the supernumerary root canal of a mandibular first premolar was examined by the isometric method and X-ray CT to ascertain its morphology before and after root canal preparation and after root canal filling. The results were as follows: 1. In the radiograph taken by the isometric method, the buccal and lingual root canals appeared like the mesial and distal root canals, respectively. 2. There was marked hypocalcification of the root canal wall on the lingual side of the buccal root canal and the buccal side of the lingual root canal. 3. Depending on the height of the orifice of root canals, root canal filling might be difficult to perfirm. These findings suggest that when performing root canal treatment in a supernumerary root canal, it is important to determine the morphology and position of the root canal by radiography, check the height of the origice of the root canal, and pay attention to avoid excessive widening of hypocalcified areas.
