The mental symphysis is the fibrocartilaginous midline junction between the two mandibular halves at the chin, which fuses completely by the second year of life in humans. Internally, the superior and inferior mental spines (genial tubercles) project posteriorly from the symphysis, providing attachment for the genioglossus (superior) and geniohyoid (inferior) muscles. The mental protuberance is the external chin prominence.
The mental symphysis is the donor site for chin osteotomies (genioplasty) and for symphyseal bone grafts used in small alveolar reconstructions. Symphyseal parasymphyseal fractures from direct chin trauma are classified with subcondylar fractures as bilateral condylar fractures producing the classic anterior open bite. Genial tubercle advancement osteotomy (GOSA) for obstructive sleep apnoea advances the genioglossus and geniohyoid attachment anteriorly to increase posterior airspace.
Direct impact to the chin in falls and assaults produces symphyseal or parasymphyseal mandibular fracture; parasymphyseal fractures extending from the mental foramen to the canine tooth may damage the inferior alveolar nerve in the mandibular canal, and bilateral body or parasymphyseal fractures with subcondylar fractures produce unstable mandibular segment requiring ORIF.
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