Neural symmetry and functional asymmetry of the mandible.

Even in the absence of malformation or skull base asymmetry, the mandible may be physiologically asymmetric and this remains a major challenge in the orthodontic treatment. The mandible is a bone formed by a primary subunit, i.e., the neural part, with different functional secondary subunits, so we suggest that in a normal mandible the asymmetry was caused by the secondary functional subunit and the neural part is nearly symmetric. Eighty-three dry human mandible samples were studied. The measurements of the size of the mandible (corpus, ramus, mandible notch, condylar process, the angle of the mandible) and the neural subunit (the mandibular canal and the position of the mental and mandibular foramina) were measured bilaterally. The left and right sides were compared according to the dental status: 60 dentate and 23 edentulous mandibles. The calculation of the symmetry was based on the paired Student t test, the absolute difference |R-L| and the relative absolute difference |R-L|/|R+L|x100. The mandibular canal and the position of the foramina were symmetric, except for the position of the mandibular foramen in relation to the mandible notch. The symmetry was not modified by the dental status. The total length of the mandible and the length of the ramus were greater on the left side independently from the dental status. The length of the corpus and the mandible angle were symmetric in each group. The mandible notch was always asymmetric and its height was greater in the dentate group. The condylar process was the most asymmetric structure in each group. The primary subunit of the mandible, surrounding the mandibular canal, is a symmetric component of the mandible and is not modified by the dental status. The angle of the mandible between the corpus and ramus is another symmetric parameter that is important for the facial architecture. The ramus and especially the mandibular notch as well as the condylar process are the most asymmetric subunits influenced by the functional matrices.