[Morphological study on growth process of primary dentition having distal step of the terminal plane. 1. Growth on the average].

Growth process was analyzed on the primary dentition having bilateral distal step of the terminal plane. Plaster models and cephalometrics of 32 cases were examined. They were taken in the following five developmental periods: Hellman's Dental Ages IIA, eruption of the first molars, eruption of the maxillary central incisors, eruption of the maxillary lateral incisors, and half a year past the eruption of the maxillary lateral incisors. The measurement of plaster models was taken on the mesiodistal crown diameter, the interdental spaces in the primary dentition, the traverse width of the dental arches, and the arch length. Cephalometric analysis was made on 21 linear items together with 6 proportional items, 18 angular items, and coordinates. For a control group, plaster models and cephalometrics of 12 cases having normal occlusion were also analyzed in the same areas as the examined group. These normal cases were chosen according to Kito's criteria of normal occlusion. Comparisons were made between the two groups and the results were as follows: 1. Growth process data obtained from the plaster models 1) There was no significant difference between the mesiodistal crown diameter of the two groups except in the mandibular deciduous central incisors. 2) Concerning the total interdental spaces from the primary canine to the opposite canine, the examined group showed longer spaces in the mandibular dentition (3.03 millimeters on the average) than the control group (1.83 millimeters on the average) (p less than 0.05). 3) Concerning the transverse width of the arches, in the maxillary dentition, the examined group showed a narrower width than the control group through all developmental periods (p less than 0.05), and in the mandibular dentition, the examined group showed wider intercanine width than the control group. However, there was no significant difference between the two groups in the growth process of the transverse width. 4) The examined group showed a longer arch length than the control group in the mandibular dentition in the period of Dental Age IIA (p less than 0.05). 2. Growth process data obtained from cephalometrics 1) Linear growth data in the skeletal pattern (1) The examined group showed a larger dimension in the total cranial base length (N-Ar), the posterior cranial base length (S-Ar), and the posterior facial height (S-Go) than the control group through all developmental periods (p less than 0.05). However, there was no significant difference between the two groups in the quantitative growth.(ABSTRACT TRUNCATED AT 400 WORDS)