Mandibular molar teeth and the development of mastication in the miniature pig (Sus scrofa)

The only components of the orofacial complex that are fully developed as soon as they establish function are the occlusal surfaces of teeth. It is usually assumed that the occlusal surface of the molar teeth influences the orientation of the power stroke in spite of the fact that the data supporting this claim are lacking. The purpose of this longitudinal study was therefore to determine whether or not this hypothetical form-function relationship existed during development and whether or not it was related to growth. Serial dorsoventral and lateral radiography, and dorsoventral cineradiography were performed during natural feeding of 5 Hanford miniature pigs (Sus scrofa) of both genders between their 8th and 18th weeks. Sequences of power strokes were analyzed frame by frame and compared between sessions, and related to the position of the fourth primary mandibular molar (dm4). The changes in direction of the power stroke and in position of the dm4 relative to the midline were subsequently related to growth. The results of this study indicate that changes in orientation of the power stroke, though significant only on the balancing side, occur independently of the repositioning working and balancing side mandibular molars, as well as of skeletal growth. The null hypothesis that the mandibular primary molars influence the transverse orientation of the power stroke during growth is therefore refuted.