Consideration of the Mechanisms Involved in Dental Crowding —Comparison of Dentition Growth Changes in Children of Two Primary Schools during Two Different Eras—

Abstract

The aim of this study was to elucidate the mechanisms involved in dental crowding by comparing dentition growth changes in children of two primary schools during two different eras. Dental casts obtained from primary school students aged 6-7 and 11-12 year-olds for the periods of 1975-1982 [Asahi Primary School (APS), Ito City, Shizuoka] and 1997-2002 [Kogasaki Primary School (KPS), Matsudo City, Chiba] were studied. The materials were classified into a group in which mild dental crowding observed (arch length discrepancy (ALD) less than -4.0 mm) at the age of 11-12 year-olds (N-group), and a group in which dental crowding was severe (ALD more than -4.0 mm) (E-group). Dental casts were scanned with a contact type 3D scanner. Arch sizes and inclination of the first molars were measured. Maxillary and mandibular arch lengths of the 6-7 year-olds in KPS were significantly longer than those in APS. There was no significant difference in palatal width and arch width between APS and KPS. Comparison of tooth sizes between the N-group and E-group showed no significant differences in size of all permanent teeth. In APS, maxillary arch length was significantly longer in N-group than in E-group. On the contrary, there were no significant differences in arch width and B-L inclination of mandibular molar between N-group and E-group. In KPS, arch widths of maxilla and mandible in 11-12 year-olds were significantly larger in N-group than in E-group. B-L inclination of mandibular molar in 11-12 year-olds was significantly smaller in N-group than in E-group. In N-group, maxillary arch length of 6-7 year-olds was significantly shorter in APS than in KPS, whereas maxillary arch width of 11-12 year-olds was significantly larger in APS than in KPS. In E-group, both maxillary and mandibular arch lengths of 6-7 year-olds were significantly shorter in APS than in KPS. Palatal width and maxillary arch width of 11-12 year-olds were significantly larger in APS than in KPS. Bucco-lingual inclination of mandibular molars of the 11-12 year-olds in APS was significantly smaller than that in KPS. The dmft rate of 6-7 year-olds was higher in APS than in KPS. The highly progressed caries tooth rate was also higher in APS than in KPS. In conclusion, the background of dental crowding is different between the two eras. In 1980 (APS), the cause was mesial movement of the first molar due to highly progressed caries. On the contrary, in 2000 (KPA), the cause was narrowing of the dental arch due to reduction of palatal growth and lingual inclination of mandibular first molar.