The lingual surface of the mandible's body is commonly indicated as presenting the submandibular and sublingual fossae, which are separated by the mylohyoid line. The mylohyoid line attaches to the mylohyoid muscle (MM). Less attention has been paid to the 'mylohyoid boutonnières', which allow the 'sublingual buttons' to pass through the mylohyoid muscle in the submandibular space. The cone-beam computed tomography files of patients were routinely examined for anatomical studies. Two cases were found with unexpected morphologies of the mandible's body - the mylohyoid lines were incomplete anteriorly, and herniated sublingual tissue determined an additional fossa inferior to that line in the premolar region. That fossa was termed the 'accessory submandibular fossa'. It determined on coronal slices a 'sand watch' contour of the mandible's body. With such a peculiar morphology, the mandible is more prone to fracture. Moreover, when inserting endosseous implants, the procedure should be carefully personalised in such rare cases.
Keywords: cone-beam computed tomography; endosseous implants; mandible; mental foramen; premolar teeth; sublingual fossa.