Assessment of 3D superimposition of digital model in extraction case based on morphological study of palatal rugae

Abstract

This study was composed of two experiments. The purposes of the two investigations were (1) to analyze the morphological characteristic of palatal ruga as landmarks for superimposition of dental casts and (2) to evaluate the accuracy of digital superimposition in the moderate and maximum retraction group 343 patients (114 males, 299 females, mean age 25.6 years) who had digital model were chosen to analyze the number and distribution of palatal ruga as well as the position of the third primary ruga. 40 participants( 9 males, 31 females, mean age 21.4 years) treated with extraction of bilateral maxillary first premolars were divided into two groups, a moderate retraction group (<7 mm) and a maximum retraction group (≥7.0 mm), based on the amount of maxillary incisor. Displacement of the central incisors was measured by the cephalometric superimposition and the pro-posed superimposition technique (third ruga-palate-superimposition method, forth ruga-palate-superimposition) in two groups. Paired t-test and correlation analysis were performed to determine whether a significant difference exist between the two measuring techniques. We acquired following conclusions.

1. Approximately 43.5% have three primary palatal rugae, and 36.1% have four.

2. When there is an additional primary or secondary ruga posterior to the 3rdprimary palatal ruga, the anterior and posterior limits of the 3rdprimary palatal ruga are located more anteriorly compared to those in the individuals without an additional posterior ruga (P< 0.05).

3. In the moderate retraction group, the displacement of the central incisors measured using the third and forth ruga-palate-superimposition and that measured with cephalometric super imposition did not show much difference, while the maximum retraction group had significantly different results depending on the two methods.(P<0.05)

4. In the maximum retraction group, the mean difference in tooth movement along the X-axis between the lateral cephalometric radiographs and the 3rd palatal ruga superimposition was 0.7 mm, and that with the 4th palatal ruga superimposition was 0.2 mm, with the 3rd palatal ruga superimposition showing a significantly larger difference (P< 0.05).

Based on the above results, it may be concluded that the number of the palatal rugae vary greatly among the population, and that this number affects the anteroposterior position of the 3rdprimary palatal ruga. When assessing the tooth movements in the patients with a moderate retraction, the 3D superimposition methods proposed in this study may be clinically accurate and reliable as cephalometric superimposition for assessing orthodontic tooth movement in extraction case. However, when the amount of the incisor movement was over the 7mm, the 3rd palatal ruga superimposition method was imprecise. In such cases, the superimposition should be performed using the palatal rugae located posterior to the 2nd premolar as the reference points, taking into account the number and position of the rugae in each patient.