A morphological study of root resorption of the maxillary first deciduous molars.

The aim of this study was to elucidate the relationship between root resorption of the maxillary first deciduous molars and the developmental state of the successive permanent teeth. Twenty-four specimens of the maxilla from 12 dry skulls were classified into four dental eruption stages based upon their deciduous and permanent dentition. Serial sections prepared from the maxilla and embedded in polyester resin were observed with soft X-ray films. The area from the lowest point of the protruded part of the zygomatic process to the alveolar crest of the maxillary first deciduous molar was divided into 6 areas parallel to the Frankfort plane. Specimens from the upper 1/6, 3/6 and 5/6 areas were used. In addition, the shortest distance from the root resorption surface to the bony crypt was measured. The bony crypt containing the successive permanent tooth germ was located closer to the buccal root among the 3 roots of first deciduous molars but grew lingually with the progress of eruption stage. The distance from the root resorption surface to the bony crypt shortened and resorption actively progressed from the deciduous dentition stage to the first molar, and the central incisor reached the occlusal line in the deciduous dentition stage. Three-dimensional reconstruction to reproduce the relationship between the root and bony crypt revealed an increase in the root resorption surface with the eruption phase and growth of the bony crypt in the direction of root furcation.