Abstract
This study was to discover the factors that cause the irregular alignment of the incisors during the period of the; reruption. The materials were plaster models of 20 maxillary arches and 21 mandibular arches. They were classified into three groups depending on occlusion --1) normal occlusion group (10 maxillary and 7 mandibular arches),2) “flaring”group (5 maxillary and 6 mandibular arches), and 3) crowding group (5 maxillary and 8 mandibular arches).
The phenomena observed in this study were the relationships between the alignment of the incisors and the position of erupted permanent incisors, difference of the sum of the mesiodistal crown diameter between four deciduous and four permanent incisors, developmental increase of the intercanine width and physiological interdental spaces in stage of II A Hellman's dental age.
The results are summarized as follows:
1) Of 43 cases with normal initial eruption of the lateral permanent incisors,13 cases were evaluated as having malposition of fully-erupted inciso rs.
2) However, of the 36 cases with abnormal initial eruption of the lateral permanent incisors,2 cases were evaluated as having normal position of the ful ly-erupted incisors.
3) The sum of each mesio-distal crown diameter of the four deciduous incisors, and of the four permanent incisors in the normal occlusion was slightly smaller than the sum in the “flaring group ”, and in the crowding group.
4) The difference of the sum of the mesio-distal crown diameter between deciduous and permanent incisors in normal occlusion was 7.6 mm in the maxillar y arches, and 6.0 mm in the mandibular arches on the average. The difference in the “flaring group ”, and crowding group was larger than that of the normal occlusion.
5) The intercanine width of the maxillary arch in stage II A of Hellman's dental age and the period of the initial eruption of the lateral incisors was larg er in the crowding group, the normal occlusion group, and “the flaring group ”in that order. The intercanine width of the mandibular arch was larger in the crowding, “the flaring group ”, and the normal occlusion in that order.
6) There was no marked difference in the developmental increase of the deciduous intercanine width among the normal occlusion group, “the flaring group ”, and the crowding group. The deciduous intercanine width was 3.3 mm in the maxillary arch and 2.6 mm in the mandibular arch, on the average.
7) The difference of the mesio-distal crown diameter between the deciduous and permanent incisors was not statistically correlative to the devel opmental increase of the deciduous intercanine width.
8) There was statistically a significant difference in the physiological interdental spaces between the crowding group and the normal occlusion grou p in both the maxillary and mandibular arches. The sum of the physiological spaces was larger in the normal occlusion group than the crowding group.
