Abstract
The developmental process and the stability of the occlusal relationship of the permanent first molar were observed longitudinally from the primary dentition to the permanent dentition. The materials were considered of a total of 1318 pairs of serial plaster models which had been taken every three months, ranging from Hellman's Dental Age IIA to IIIC period. Observation was made in 72 cases which were divided into the right and left side of the dentition of 39 Japanese children.
The results are summarized as follows:
1. Of 41 cases which had remained in the mesial step from IIA to the period of the exfoliation of the second primary molar,22 cases finished as a Class I relationship, and 19 cases finished as a Class III relationship in IIIC period.
2. Of 4 cases with the mesial step in IIA period,
3 cases which had changed to the vertical plane in IIC period, and one case which had changed to the distal step in IIC period finished as a Class II relationship in IIIC period.3. The cases with the mesial step in IIA period which would finish as a Class I or Class III relationship in IIIC period kept a Class I relationship before the eruption of permanent second molar. It was, however, after the eruption of the second molar that some of the cases changed the relationship into Class III in IIIC period.
4. Of 16 cases with the vertical plane in IIA period,11 cases which had changed to the mesial step in IIC or early IIIB period finished as a Class I relationship in IIIC period, and 5 cases which had changed to the distal step in IIC period finished as a Class II relationship in IIIC period.
5. The cases which had remained in vertical plane and finished as a Class I relationship in IIIC period had the normal occlusal relationship of the first permanent premolar. The cases which had remained in the vertical plane but finished as a Class II in IIIC period had an unstable occlusal relation ship that the maxillary first premolar occluded distally to the mandibular first premolar.
6. All of 6 cases with the distal step did not change the terminal plane, and finished as a Class II relationship in IIIC period.
