PHYSIOLOGICAL TOOTH MIGRATION AND ITS SIGNIFICANCE FOR THE DEVELOPMENT OF OCCLUSION

III. THE BIOGENESIS OF THE SUCCESSIONAL DENTITION

¹ Division of Dental Medicine, College of Dentistry, and The George Williams Hooper Foundation for Medical Research, University of California, San Francisco, Calif.

A comparative and biometrical study of serial casts of 60 cases before, during, and after eruption of the permanent incisors demonstrated the following findings:

1. Expansion of the dental arches in the anterior region, to accommodate the larger successional incisors into proper alignment was brought about by a lateral and frontal alveolar growth during the time of the eruption of these teeth.

2. The mean increase in intercanine width was greater in the upper arches than in the lower ones, and again greater in previously closed upper or lower deciduous arches than in previously spaced ones.

3. In the mandibular arches the strongest impulse of lateral growth was noted during the eruption of the second incisors, in the maxillary arches during the eruption of the first incisors.

4. An occasional "secondary" spacing of the upper deciduous anteriors occurred when the still undeveloped maxillary arch was somewhat widened upon eruption of the permanent lower first incisors.

5. Spaced deciduous arches generally produced favorable alignment of the permanent incisors while about 40 per cent of the arches without spaces produced crowded anteriors.

6. The average amount of forward extension of the arches showed no difference between previously spaced and closed arches but was 1.0 mm. greater in the upper arches than in the lower ones.

7. According to the tooth measurements the difference between the maxillary and mandibular forward extension could not be credited to a greater increase in thickness of the crowns of the upper permanent incisors over that of the lower ones. It was interpreted as an expression of an evolutionary tendency toward a lessened forward growth of the mandibular alveolar process.