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As is well known, the human mandible develops from the first branchial arches, and it ossifies before birth as separate left and right hemimandibles that meet ventrally at the mandibular symphysis (symphysis menti). Overviews of the embryology of the mandible are provided in Arey (1965), Corliss (1976), Scheuer and Black (2000), and most textbooks on mammalian embryology. At birth (Fig. 1), the mandibular midline is patent, though this suture normally fuses and is obliterated during the first year of life (Fig. 2). After symphyseal fusion, the mandible is rigid, and masticatory forces from the working side are transmitted through the chin to the balancing side. The need for resistance to torsion has been cited as a cause of development of the uniquely human chin (e.g., Sicher, 1947; DuBrul and Sicher, 1953; Schwartz and Tattersal, 2000), which occurs principally in adolescence (Ricketts, 1972). Rarely, the mandibular symphysis fails to fuse, and the present case report describes such an anomaly in a 4-year-old girl who also exhibits congenital absence of a primary incisor in combination with ankyloglossia. Persistent patency of the symphysis menti is an easily observed condition in skeletal material, so this report may be of interest to skeletal biologists. Moreover, it may stimulate readers to share similar findings.

Case Report: Patent Mandibular Symphysis with Congenital Absence and Ankyloglossia