A cephalometric study of orofacial structures during secondary palate closure in the rat

Abstract

The relationships between orofacial structures prior to and subsequent to secondary palate closure were evaluated in midsagittal sections of 30 foetal rats. Efforts were made to differentiate between the effects of shelf elevation and the effects of growth of individual orofacial components. In the day before palatal closure, the growth rates (percentage increase per hour) of the tongue and Meckel's cartilage were greater than those of the mandible and the maxilla. The tongue, Meckel's cartilage and the mandible grew in a downward and forward direction similar to the angulation of the incline formed by the nasal septum and the medial palatal process of the primary palate. Space for growth of the lower face was provided by maxillary growth, opening of the cranial base, and by extension of the head during the day before palatal closure. Palatal closure took place when the length of Meckel's cartilage exceeded the length of the oronasal cavity measured to the medial palatal process. During palatal closure, the medial palatal process increased in infero-posterior prominence, the cross-section of Meckel's cartilage and the anterior portion of the mandible rotated forward, counter-clockwise, head facing right, and the vertical distance of Meckel's cartilage from the primary palate increased. Anterior tongue protrusion was present during the day of palatal closure but the degree of protrusion relative to the mandible did not change when the shelves elevated. In the day after palatal closure, mandibular growth became more horizontally directed and the rotation of Meckel's cartilage and the anterior portion of the mandible continued. The rate of growth of the mandible was similar to its rate before palatal closure. Growth of the tongue, Meckel's cartilage, the mandible and the medial palatal process appeared to be important factors in creating the environment conducive to secondary palatal closure. Rotation of the anterior portion of the mandible and Meckel's cartilage beginning at the time of palatal closure may produce a change in position of the mid-portion of the mandible, allowing for tongue and palatine shelf exchange.