Oral Health Services Focus Group -
Research Highlight 1

3D Dental Arch Crowding-
Space Analysis and Cleft Palate Shape Measurement -
A project by Assoc Prof Kelvin Foong
and Assoc Prof Ong Sim Heng

Orthodontic problems involving the dentition, skeletal structures and facial soft tissues present as morphological and positional abnormalities in a three-dimensional space. For infants born with the cleft lip and palate anomaly, the palatal segments and alveolar arches show a distinct positional and shape deformity. To document the deformity, the dental plaster model serves as the primary record of the patientís dental arch and palate. In clinical practice, dental models are routinely analyzed with two-dimensional methods. However, the evaluation of such complex three-dimensional spatial relationships requires a more sophisticated system capable of accurately registering the plaster model. Thus, the Department of Preventive Dentistry, in collaboration with the Vision and Image Processing Laboratory of the Faculty of Engineering, NUS, have embarked on developmental research to create an accurate and dynamic three-dimensional method of visualizing and measuring orthodontic space problems and cleft palate deformity.

Figure 1: The cyberware surface laser scanner captures the entire surface of the plaster model and the computer translates it into a high-resolution 3D image.

Advanced image acquisition systems such as the surface laser scanner (Figure 1) are capable of rapid data capture. In the current research, surface laser imaging is used to produce a high-resolution accurate three-dimensional image of the dental plaster model. This technology utilizes an optical range finder for precise recording of three-dimensional data in a digitized format easily recognized by contemporary graphics software systems. The acquired 3D image of the dental model may be viewed in any preferred orientation. Computer vision techniques are used to create software algorithms for interactive quantitative measurement and manipulation of the three-dimensional image. Within the last year, the collaborative effort has produced two substantive software systems for analysis of crowding in dental arches and cleft palate shape.

3D Analysis of Crowding in Dental Arches

Figure 2: The graphical user interface permits the user to orientate the 3D image of the model in any preferred viewing angle, and performs crowding analysis based on different positions of the teeth caaording to the different arch forms (coloured parabolas).

Straightening "crooked" teeth in the dental arch requires space. Space is commonly obtained through expansion of the arch or extraction of teeth. The accurate assessment of the extent of crowding in the dental arch is essential as this forms the basis for extraction of permanent teeth in orthodontic treatment. Performing an accurate space analysis on dental models by manually simulating the idealized alignment of teeth from their original positions is a tedious laboratory process. In developing the graphical software (Figure 2), 3D analysis of crowding becomes a less laborious task. It permits accurate measurement of tooth size and dental arch length. The spatial contour of the untreated dental arch may be visualised and manipulated to give an idealized dental arch form. Arch symmetry may be visualized and the extent of asymmetry quantified (Figures 3a and 3b).

Figure 3a: 3D plan view of the dental model.

Figure 3b: Arch forms superimposed over the 3D image, and calculation of arch symmetry is obtained automatically. The horizontal lines permits the user to have a quick visual grasp of the extent of asymmetry.

Subsequently, 3D images of the teeth may be sliced off from the bony base of the dental model and repositioned over the optimal arch form (Figure 4). In essence, teeth may be moved virtually to perform treatment simulations (Figures 5a and 5b) to determine the best approach for correcting the malocclusion - before real teeth are extracted!

Figure 4: The 3D image of each tooth has been separated from the base of the model and aligned over an idealized arch form. This visual arrangement assists the user in deciding on extraction of teeth.

Figure 5a: The position of each tooth prior to treatment is represented by the segmented images.

Figure 5b: The user simulates the correction of the malpositioned tooth by virtual tooth movement.

3D Analysis of Cleft Palate Shape

Figure 6: Unrepaired complete cleft lip and palate deformity shows significant spatial malpositioning of the lip and alveolar segments.

In infants, the spatial relationship between the palatal segments in complex cleft deformities such as the complete unilateral cleft lip and palate anomaly (Figure 6) is critical to its successful surgical correction. An ongoing multi-centre international clinical research (Singapore-Helsinki-Stockholm cleft palate centers) analyzing short-term palatal shape change following primary lip closure in unilateral cleft lip and palate infants utilizes a second graphical software (Figure 7) for 3D visualization and analysis of palate shape. Through a graphical user interface, landmarks on the 3D image of the cleft palate model are first identified, and subsequent measurements of anatomical length, slope of the palate and the size of the palatal defect as well as volume of the palatal space are easily obtained. The contours of the palatal segments (Figure 8) can be automatically generated to give the clinician a visual impression of the extent of arch asymmetry. Changes in the palatal shape following primary lip closure based on these outcome parameters is easily deduced through analyses of these measurements.

Figure 7: The graphical user interface for analyzing cleft palate shape. As in the earlier interface, the 3D image of the cleft palate model may be viewed in any orientation by moving the controls on the bottom left side of the screen. Landmarks on the image are located by clicking the buttons on the screen's right. Measurements in 3D are given automatically when "analyse" button is clicked.

Figure 8: Alveolar arch contours are automatically generated over the major and minor palatal segments. The length of these contours may be measured and the cleft space bounded by these contours is further determined.

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