Materials and Methods

The sample consisted of dental casts obtained from 10 patients (4 males and 6 females) who were undergoing orthodontic treatment after a diagnosis of maxillary protrusion at the Orthodontic Department, Nagasaki University Hospital of Medicine and Dentistry. Their ages ranged from 15 years 7 months to 27 years (mean = 20 years). Subjects were given informed consent forms, and the research protocol was examined and approved by the Ethical Committee of Nagasaki University Graduate School of Biomedical Sciences. As part of the orthodontic treatment, bilateral maxillary first premolars were extracted, and three miniscrews were implanted onto the palate by an oral surgeon. Two were placed on the palatal slopes bilaterally between the second premolar and the first molar, and another one was placed on the paramedian region of the hard palate. These three miniscrews were ligated with a transpalatal arch by ligature wires to reinforce orthodontic anchorage (Figure 1), and used as landmarks for superimposition.

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Individual trays were fabricated with resin (Tray resin, Shofu Inc, Kyoto, Japan) and impressions for maxillary dental cast were taken using hydrophilic vinyl polysiloxane impression material (Examixfine, GC Corporation, Tokyo, Japan) at every visit after the miniscrews were placed until the extraction spaces were closed. Dental casts were made with improved dental stone (New fujirock, GC Corporation, Tokyo, Japan) and then scanned by a laser surface scanning system (VMD-25, UNISN, Osaka, Japan). The measuring pitch in the x and y directions was 0.25 mm, and the resolution in the z direction was within the range of ±0.05 mm. Based on scanned data, reconstruction and analysis of 3D images was performed using a commercial software (Imageware 9, UGS PLM Solutions, Plano, Tex).

The rugae were identified as the first, second, and third rugae sequentially from anterior to posterior.11 The most medial and lateral ends of each ruga on the right and left sides were marked on 3D images of dental casts and registered as palatal ruga points, as illustrated in Figure 2.

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To evaluate the stability of the palatal ruga points, the 3D images of the serial dental casts were superimposed with best-fitting method on the three miniscrews as stationary landmarks for superimposition using the least-squares method. This superimposition method was referred to as the miniscrew-superimposition method. The displacement of each palatal ruga point from the initial to final dental casts was measured. To evaluate the direction of displacement the coordination system of Cha et al1 was used.

Results

Displacement of the palatal ruga points from the initial to final dental cast images is presented in Table 1. Displacements of the lateral points were larger than those of the medial points for the first, second, and third palatal rugae, respectively. Among the lateral points of the palatal rugae, the largest amount of displacement was observed for the first palatal rugae, followed by the second palatal rugae; the third palatal rugae showed the smallest amount of displacement. The medial points of the palatal rugae also had the same tendency as the lateral points of palatal rugae.

Table 1.  Displacement of the palatal ruga points from the initial to final dental casts (n = 10)^a

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Mean displacement of the medial points of the third rugae was smaller than that of the other ruga points (Table 1). Also, the standard deviation and coefficient of variation of the medial points of the third rugae were the smallest. The direction and amount of displacement of each ruga point in the x, y, and z axes are also shown in Table 1. Interestingly, the rugae points had a tendency to be displaced to the distal, extrusive, and medial direction during the closure of extraction spaces.

The scattergrams presented in Figure 3 show the relationship between the displacement of the rugae points and the central incisor movement. Displacement of the first ruga point was increased as the central incisor moved further. The slope of the regression lines of the medial points of the third ruga was close to zero, which indicates that these points were rarely affected by the central incisor movement.

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Figure 4 represents the 3D superimposing image of the initial and final dental casts of a sample patient. The contour of the anterior region of the palate located adjacent to retracted anterior teeth was displaced distally, as shown in Figure 4B. The contour of the posterior region of the palatal vault was not considerably changed during closure of the extraction spaces, as shown in sagittal and coronal sectional views of the superimposing images (Figure 4B,C).

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Discussion

By using three miniscrews as stationary landmarks for superimposition, we could evaluate the stability of each palatal ruga point during the closure of extraction spaces. The third rugae points, which were located farther from the retracted anterior teeth were more stable than the first and second rugae points. This result supports the earlier study by van der Linden,13 which noted that the closer the ruga point is located to the displaced teeth, the more the ruga point will be affected. In addition, there was a tendency for medial ruga points to be displaced less substantially than lateral ruga points. The medial points of the third palatal rugae were the least influenced from the tooth movement (Table 1).

The amount of tooth movement may affect the stability of the palatal rugae.9 As presented in Figure 3, the displacement of the ruga points increased gradually as the tooth moved farther. The medial points of the third rugae seemed not to be affected, because no increase in the displacement was found in the medial points of the third palatal rugae.

The reproducibility of the method was estimated by measuring two different dental casts taken twice from the same patient instead of repeatedly measuring a single cast. The mean error of the method was 0.29 ± 0.11 mm, which may be due to the dimensional change of impression material, thermal expansion of dental casts, and human error when marking the ruga points on 3D images and performing superimposition on the miniscrews as registration landmarks. The mean displacement of the medial points of the right and left third rugae, which were 0.28 mm and 0.37 mm, respectively, are not considered actual displacement, but error of the method.

Therefore, our results lead us to conclude that the medial points of the third rugae are stable enough to be used as landmarks for superimposition of dental casts. Our findings support the earlier reports1014 that suggested the possibility of using the medial points of the third palatal rugae as reference points for assessing orthodontic tooth movement. The superimposing images using the miniscrew superimposition method also showed that the posterior region of the palatal vault is least affected by tooth movement (Figures 4B,C).

Materials and Methods

The 3D images of serial dental casts for each patient were superimposed without using the three miniscrews as registration landmarks. Instead, the following superimposition landmarks were identified on the 3D images (Figure 5):

• Point A: the midpoint on the line connecting the medial points of the right and left third palatal rugae.

• Surface B: the surface of the palatal vault surrounded by two transverse and two anteroposterior lines. One of the transverse lines is 10 mm away distally from the third palatal rugae, and the other is 5 mm away mesially from the line in contact with the distal surfaces of the bilateral maxillary second molars. On the palatal side, the anteroposterior lines are 10 mm from the lines in contact with the palatal gingival margins of the posterior teeth bilaterally.

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Registered at point A, the surface B of the subsequent dental cast was best-fitted to that of the initial dental cast using the least-square method. This superimposition method was referred to as ruga-palate-superimposition method.

The midpoints of the right central incisor edges were marked on the initial and final dental casts, and superimposition was performed using the miniscrew-superimposition method and the ruga-palate-superimposition method. Displacement of the midpoint of the right central incisor edge was measured, and correlation analysis and paired t-test were performed to determine whether a significant difference existed between the measurements according to the two superimposition methods. The null hypothesis was that the difference between the measurements of the two superimposition methods was zero. P < .01 was considered statistically significant.

Results

Descriptive statistics of the measurements using the miniscrew-superimposition method and the ruga-palate-superimposition method are presented in Table 2. The displacement of the central incisor measured by the ruga-palate-superimposition method was not significantly different from that measured by the miniscrew-superimposition method (P < .01, paired t-test). The Pearson correlation coefficient between the measurements obtained using the two superimposition methods was 0.99.

Table 2.  Displacement of the central incisors measured on the su~ perimposing images by means of the miniscrew-superimposition method and the ruga-palate-superimposition method^a

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Discussion

Based on the experiment 1, which showed that the medial points of the third palatal rugae and the posterior region of the palatal vault are stable throughout the closure of the extraction spaces, we performed a superimposition using these areas as registration landmarks in experiment 2. The displacement of the central incisor measured using the ruga-palate-superimposition method was not significantly different from that measured using the miniscrew-superimposition method (Table 2), which indicated that maxillary dental casts can be superimposed reliably using the ruga-palate-superimposition method. Consequently, the 3D superimposition method can even be applied to patients who are not candidates for using skeletal anchorage such as miniscrews. This superimposition method is available, not only as an additional tool of cephalometric radiographs but also as a principal method for determining progress of treatment using dental casts taken at required intervals during the treatment (thus avoiding unnecessarily exposing the patients to radiation). Furthermore, the 3D evaluation of orthodontic tooth movement, such as the changes in angulation of the posterior teeth and the central incisors, which was not possible with cephalometric superimposition, can now be accomplished using this superimposition method of dental casts.

The 3D images cannot be superimposed only on the right and left medial points of the third rugae as registration landmarks, because more than three registration points are required to establish an individually specific 3D coordinate system for conducting 3D superimposition. To overcome this problem, we used the posterior region of the palatal vault, together with the medial points of the third palatal rugae, to construct landmarks for superimposition; thus, we could successfully superimpose dental casts with high accuracy. The use of the palatal vault area alone to obtain registration landmarks is also considered undesirable. This area has insufficient shape characteristics; as a result, the registration rotates from its true registration position.

The samples in the present study were obtained from patients who were not expected to have an apparent growth. The height and breadth of alveolar processes of children increase with craniofacial growth,15 and the shape or direction of rugae can possibly change.216 Thus, there is a limitation in applying this superimposition method to the dental casts of growing children.

Clinical application

To demonstrate a clinical application of the developed 3D superimposition method, we selected one sample patient, a 24-year-old woman who underwent orthodontic treatment with extraction of maxillary first premolars and in whom no skeletal anchorage, such as miniscrews, was used. Maxillary dental casts taken before and after treatment were superimposed according the ruga-palate-superimposition method. As presented in Figure 6, the displacement of individual teeth during orthodontic treatment could be visualized in three dimensions. The displacement of the central incisors, as measured by the distance between the midpoints of their edges, was 5.9 mm (right), 5.6 mm (left). The displacement of the first molars, as measured by the distance between the mesial contact points, was 3.7 mm (right), 4.4 mm (left). We projected the bilateral maxillary first molars onto the coronal plane (Figure 6C) and calculated the change in angulation. The crown of the right first molar tipped palatally by 1.0°, and that of the left first molar tipped palatally by 3.5° during treatment. The axis of the central incisor projected on the sagittal plane was retroclined by 19° during treatment (Figure 6D).

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