Relationship between the eruption of mandibular first permanent molar and growth changes of adjacent structures.

The relationship between the eruption of mandibular first permanent molar and growth changes of the adjacent structures was statistically investigated on 16 children during the active eruption of the tooth. Semiannual records of 45-degree oblique cephoalgrams and dental casts were used as materials. The factor analysis with orthogonal rotation was applied and the pattern of associations was revealed by the following six factors: Factor 1. Proximity factor. With larger mesiodistal tooth diameter, the tooth tends to be more mesially positioned. Factor 2. Eruption factor. The tooth erupts rapidly, nearing the distal surface of mandibular second deciduous molar more quickly and simultaneously widening the space posterior to the dental arch more rapidly. Factor 3. Root development factor. As the root develops faster, the interdental space tends to close more rapidly. Factor 4. Eruption rate factor. As the tooth nears the occlusal plane, the rate of eruptive movement decreases. Factor 5. Tooth uprighting factor. With more amount of space posterior to the dental arch, the subsequent uprighting of the long axis of the tooth can be expected. Factor 6. Tooth inclination factor. Inclination of the long axis of the tooth crown is relatively independent from the variables examined.