Physiological Tooth Migration and its significance for the development of occlusion: The Biogenesis of Accessional Dentition

Resident: Roberts
Article title: Physiological Tooth Migration and its significance for the development of occlusion: The Biogenesis of Accessional Dentition.
Journal: From the Division of Dental Medicine, College of Dentistry, and The George Williams Hooper Foundation for Medical Research, UCSF
Pages: 331-337

Background: The “accessional dentition” makes its appearance with the eruption of the first permanent molars which are guided into position by the distal surfaces of the second deciduous molars.

Distal step: described as the mesial portion of the primary mandibular second molar being distal to that of the mesial portion of the primary maxillary second molar.
End to End: described as the mesial portion of the primary mandibular second molar being flush with the primary maxillary second molar.
Mesial step: described as the mesial portion of the primary mandibular second molar being mesial to the mesial portion of the primary maxillary second molar.

Findings: 60 cases were studied using casts before and after the eruption of the six year molars into occlusion.

Through comparative measurements, three different biologic mechanisms of normal occlusal adjustment of the accessional dentition were found:
1. The occurrence of a terminal plane forming a mesial step in the deciduous denture allowed the first permanent molars to erupt directly into proper occlusion without altering the position of the neighboring teeth.
2. The presence of a mandibular primate space and a straight terminal plane was conducive to proper molar occlusion by means of an early mesial shift of the mandibular deciduous molars into the primate space upon eruption of the lower first permanent molar.
3. Closed deciduous arches and a straight terminal plane resulted in a transitory end to end relationship of the first permanent molars. Proper occlusion was effected through a late mesial shift of the mandibular permanant first molars subsequent to the shedding of the deciduous second molars.

Summary: Patients with End to end or mesial step occlusions of the primary dentition have the best chance of obtaining ideal class 1 occlusion in the young permanant dentition. They due this by one of three ways: First, having unobstructed eruption of the 6 year molars into ideal class 1 occlusion. Second, by having permanent mandibular first molars push the deciduous second molar into the primate space until proper class 1 occlusion is achieved. Lastly, if no primate space is available then the 6 year molar slides forward into ideal class 1 occlusion once the deciduous second molar has been exfoliated.