The dry skulls of 160 Indian infants were used. The period of the mandibular growth was divided into six stages, from the pre-eruption stage of the primary teeth through the eruption stage of the central and lateral permanent incisors, and the mode of the mandibular growth was compared among these stages. The following results were obtained.
With an increase in the dental age, the values increased for all of following parameters, the minimal breadth, height, and vertical height of mandibular ramus with respect to the mandibular ramus, the mandibular length and the length of the mandibular body, both representing the length of the mandible, the condylar breadth, the maximal breadth of the condyle, the angular breadth of the mandible and the anterior mandibular breadth, which were all related to the growth rate of the mandibular width. These mandibular parameters showed maximal growth at the eruption stage of the primary teeth in particular. The second largest growth was seen after the second primary molar had completely erupted or at the start of eruption of the permanent teeth. The growth was slow at the eruption stage of the central and lateral permanent incisors. Exceptionally, the maximal sagittal breadth of the condyle showed no changes in the period from the pre-eruption stage of the primary teeth through the stage of the completed primary dentition. It started to show a significant increase only at the beginning of the eruption stage of the permanent teeth. Although the mandibular notch showed a transient decrease in both its width and height at the eruption stage of the primary teeth, it subsequently increased with an increase in the dental age. The mandibular notch growth increased at the eruption stage of the second primary molar, reaching its maximum at the beginning of the eruption stage of the permanent teeth. The angles of both the mandible and the mandibular ramus decreased with an increase in the dental age, and the decreases were largest at the beginning of the eruption stage of the permanent teeth. The antegonial notch continued to show a gradual decrease after completion of the primary dentition. It was assumed that the growth of the mandible occurred primarily in the body of the mandible, from which the growth was induced in the mandibular ramus and condyle, and that the spurt of the growth of the mandibular body was correlated with eruption of the teeth. It was suggested that, in addition to the factors discussed above, an increase in intramembranous ossification of the mandible itself is involved in securing the available arch space for eruption of the posterior molars.