Three-dimensional analysis of mandibular growth and tooth eruption

Abstract

Normal and abnormal jaw growth and tooth eruption are topics of great importance for several dental and medical disciplines. Thus far, clinical studies on these topics have used two-dimensional (2D) radiographic techniques. The purpose of the present study was to analyse normal mandibular growth and tooth eruption in three dimensions based on computer tomography (CT) scans, extending the principles of mandibular growth analysis proposed by Björk in 1969 from two to three dimensions. As longitudinal CT data from normal children are not available (for ethical reasons), CT data from children with Apert syndrome were employed, because it has been shown that the mandible in Apert syndrome is unaffected by the malformation, and these children often have several craniofacial CT scans performed during childhood for planning of cranial and midface surgery and for follow-up after surgery. A total of 49 datasets from ten children with Apert syndrome were available for study. The number of datasets from each individual ranged from three to seven. The first CT scan in each of the ten series was carried out before 1 year of age, and the ages for the 49 scans ranged from 1 week to 14.5 years. The mandible and the teeth were segmented and iso-surfaces generated. Landmarks were placed on the surface of the mandible, along the mandibular canals, the inner contour of the cortical plate at the lower border of the symphysis menti, and on the teeth. Superimposition of the mandibles in the longitudinal series was performed using the symphysis menti and the mandibular canals as suggested by Björk. The study supported the findings of stability of the symphysis menti and the mandibular canals as seen in profile view previously reported by Björk & Skieller in 1983. However, the mandibular canals were, actually, relocated laterally during growth. Furthermore, the position of tooth buds remained relatively stable inside the jaw until root formation started. Eruption paths of canines and premolars were vertical, whereas molars erupted in a lingual direction. The 3D method would seem to offer new insight into jaw growth and tooth eruption, but further studies are needed.

Fig. 1

Two-dimensional mandibular growth analysis from 4 to 21 years of age in a normal individual, based on cephalometric radiographs in the lateral projection and superimposed on metallic implants inserted into the jaw. Note the surface remodelling and the eruption of the teeth. (Modified from Björk & Skieller, 1983.)

Fig. 10

Patient no. 2. Oblique view. Three-dimensional superimposition of transparent mandibles at four different ages (0, 1, 7 and 10 years). Mental foramina, and segmented teeth (posterior to the incisors) from four different ages (0, 1, 7 and 10 years) are aligned automatically on the symphysis menti and the mandibular canals. Only the teeth and the mental foramen on the right side are illustrated. Tooth colour code: purple = permanent third molar, blue = permanent second molar, red = permanent first molar, and green = permanent premolars and the permanent canine. The mental foramen = blue. The symphysis menti = yellow. The mandibular canals = pink.

Fig. 11

Patient no. 2. Frontal view. Transparent mandible at age 10 years. Mental foramina, and segmented teeth (posterior to the incisors) from four different ages (0, 1, 7 and 10 years) are aligned automatically on the symphysis menti and the mandibular canals. Only the teeth and the mental foramen on the right side are illustrated. Lines indicate the eruption paths of the individual teeth. For colour coding see Fig. 10.

Fig. 12

Patient no. 2. Posterior view. For colour coding see Fig. 10.

Fig. 13

Patient no. 2. Lateral view. Lines indicate the eruption paths of the individual teeth. For colour coding see Fig. 10.

Fig. 14

Patient no. 2. Oblique view. For colour coding see Fig. 10.

Fig. 2

Anatomical reference structures: (1) the anterior contour of the chin; (2) the inner contour of the cortical plate at the lower border of the symphysis menti; (3) any distinct trabecular structure in the symphysis menti; (4) the contour of the mandibular canal; (5) the lower contour of mineralized tooth germs before root development begins. (Modified from Björk & Skieller, 1983.)

Fig. 3

Ideal head position in the cephalometer (below), and potential errors in head position (above) which will result in various types of distortion in the radiographs.

Fig. 4

Ten patients with Apert syndrome. Indication of age at time of CT scans.

Fig. 5

Skeletal landmarks illustrated in the lateral and frontal aspects.

Fig. 6

Dental landmarks illustrated on a mandibular left canine, second left premolar and first left molar. The top and bottom landmarks on the molar are not shown. (Modified from Carlsen, 1987.)

Fig. 7

Histogram distribution of the mean difference, , between the first and the second digitization (N = 25).

Fig. 8

Patient no. 2. Frontal view. Superimposed computed plots showing the symphysis menti and the mandibular canals at four different ages (0, 1, 7 and 10 years), as indicated in the figure.

Fig. 9

Patient no. 2. Lateral view. Superimposed computed plots showing the symphysis menti and the mandibular canals at four different ages (0, 1, 7 and 10 years).