Measuring the Complete-arch Distortion of an Optical Dental Impression

Intraoral scanners introduce arch distortion in data they collect for dental processes. Our coordinate system extraction protocol provides a tool for evaluating intraoral scanners on the market. It is most important to transform from the arbitrary coordinates of the intraoral scan to the newly created coordinate system using the reverse engineering reference sphere.

A technique of the analysis of distortion in the critical part of dental arch in the x, y, and z directions. Which is an improvement over current methods using best-fit alignment. Demonstrating the procedure will be Dan Young Kim, a technician from our laboratory.

Work with the physical model to create the master specimen. For this protocol, prepare a mandibular complete arch model. First remove the canines, the second premolars and the second molars for the study.

Take the model to a reference scanner to acquire the data necessary to generate the computer model. Next upload the master specimen data to a computer-aided design program. Using reverse engineering software, design cylinders on top of the trimmed teeth each with a radius of two millimeters and a height of seven millimeters.

Design the cylinder of the left second molar so that it is inclined 30 degrees medially and the cylinder for the right second molar so it is inclined 30 degrees distally. To define the coordinate system, add three 3.5 millimeter reference spheres posterior to the left second molar cylinder with one sphere on the buccal side. Use this design in a metal 3D printer to manufacture a cobalt chromium alloy phantom model to serve as a patient's dentition.

Return to the industrial model scanner with the metal phantom. Use it to obtain a reference scan of the phantom. The next step is to set up the phantom for scanning with an intraoral scanner, When ready, use the intraoral scanner being tested to scan the phantom.

Next, within CAD software, create a model of the phantom with the reference scan data. To begin determining the reference coordinate select Reference Geometry then Create then Sphere to choose the Pick Boundary Points command. Select four points on the reference sphere that are as far apart from each other as possible.

Click Done to end point selection. The sphere's center will be determined automatically. Repeat this for each sphere.

Next choose Reference Geometry, Create, Plane, and Pick Points in order to connect the centers of the three spheres to define a plane. Label this plane as the XY plane. Then create a tangent plane by going to Reference Geometry, Create, Plane, Offset Plane.

Place this above the XY plane. Once again, choose Reference Geometry, then Create and Plane to reach the Pick Points command and generate a plane between the created points and the center of the lingual spheres. Go under Inspection, then Dimension, and use the Linear command to measure the distance of the plane from the center of the buccal sphere.

With Geometry, followed by Create and Plane, choose Offset Plane to create a parallel plane that passes through the midpoint of the buccal sphere. Label this plane as the YZ plane. The center of the buccal sphere will be the origin of the coordinate system.

Assign a line parallel to the line connecting the remaining two spheres as the Y-axis. Assign the line on the XY plane that passes through the origin and is perpendicular to the Y-axis as the X-axis. Create a new coordinate system with Reference Geometry, Create, Coordinate, and the Pick Origin X, Y Direction command.

Label the line perpendicular to the XY plane and passing through the origin as the Z-axis. Go on to select Reference Geometry and Bind to Shell to fix the geometries created on top of the scan data. Then, under Reference Geometry, Transform, and Coordinate, find the Align Coordinate command to transition to the newly created coordinate system.

Note the coordinate indicator now coincides with one of the buccal spheres. Now, focus on one of the six cylinders in the image. Go to Reference Geometry, Create, Cylinder, and the Pick Boundary Points command.

Specify at least ten points on the top boundary of the cylinder. Specify the same number of points on the ellipse to find where the tooth meets the bottom of the cylinder. Click Done to end point selection.

Obtain the extracted coordinates of the center of the cylinder top for comparison with the coordinate values from the intraoral scanner. Here are the CAD and 3D printed model. The CAD image has labels for the sites used for comparison.

The difference in the coordinates for these sites between the two models demonstrates that the coordinates from the CAD design cannot be used as a reference and an industrial level scanner must be employed. It is most important to transform from the original coordinates of the intraoral scan to the newly created coordinate system using the reverse engineering reference sphere. Since the same type of data is used, this process can be automated through the macrofunctions in the 3D analysis software to repeat each reverse engineering step.

This create an accurate way to analyze arch distortion but resists the biggest weakness of color coordination aber of buccal distances and could be used as an ISO standard or a test method.