A false presence of bifid mandibular canals in panoramic radiographs

Regarding the research presented on actual bifid mandibular canals in the article published in the October 2011 issue of DMFR,1 the authors must be congratulated on work well presented. However, it is important to bear in mind that a radiograph is a two-dimensional image of a three-dimensional object. Thus, other anatomic structures in the region under discussion must also be considered and included when hypothesizing, even though they may not be in focus. The authors should have included the mylohyoid anatomy in their discussion. The mylohyoid groove commences in the region of the inferior margin of the mandibular foramen, runs downwards and forwards and fades away in the submandibular salivary gland fossa.2 Arensburg and Nathan3 found that in approximately 16% of cases, the mylohyoid groove is partially or totally converted into a bony canal by ossification of the covering membrane; the opening of the canal being seen on a mandible, proximal to the mandibular foramen, while a second opening is present at the distal end of the canal. So in these cases the authors were indeed correct about the presence of the bifid canal, but the second canal could have been the mylohyoid canal. However, the authors did not discuss the anatomy of the mylohyoid structures. It is further noted that fairly deep and/or wide mylohyoid grooves were often not observed on a panoramic radiograph, whereas, at other times, relatively shallow mylohyoid grooves were observed radiologically.2 Thus, the radiological presence of the mylohyoid groove is no indication of its size or depth. The mylohyoid groove may then give the impression, on a panoramic radiograph, that there is a bifid mandibular canal, the subject of the title of the presentation. Further, it was shown that the mylohyoid groove sometimes commences from within the mandibular canal.4 Nevertheless, there are two canals (the mandibular and the mylohyoid) for a short distance. If one is discussing bifid mandibular canals, one may wish to discuss the double foramina exiting the mandible. Kerr et al5 stated that multiple mental foramina do occur. In addition, it should be noted,6,7 particularly when performing surgery in the region of the mental foramen, that the mandibular nerve may have a short extra-osseous course, exiting the mandible at the mental foramen and re-entering via the mandibular incisive foramen. Thus, for a short course there is no intra-osseous course of the mandibular nerve.6 What I do find rather confusing is that the title of the article infers panoramic radiology but most of the article discusses cone beam CT. I hope that the authors will include the above facts in their future research.