Measuring the Complete-arch Distortion of an Optical Dental Impression

Summary

Here, we present a protocol to measure the degree of distortion at each part of the compete-arch digital impression acquired from an intraoral scanner with 3D-printed metal phantom with standard geometries.

Abstract

Digital workflows have actively been used to produce dental restorations or oral appliances since dentists started to make digital impressions by acquiring 3D images with an intraoral scanner. Because of the nature of scanning the oral cavity in the patient's mouth, the intraoral scanner is a handheld device with a small optical window, stitching together small data to complete the entire image. During the complete-arch impression procedure, a deformation of the impression body can occur and affect the fit of the restoration or appliance. In order to measure these distortions, a master specimen was designed and produced with a metal 3D printer. Designed reference geometries allow setting independent coordinate systems for each impression and measure x, y, and z displacements of the cylinder top circle center where the distortion of the impression can be evaluated. In order to evaluate the reliability of this method, the coordinate values of the cylinder are calculated and compared between the original computer-aided design (CAD) data and the reference data acquired with the industrial scanner. The coordinate differences between the two groups were mostly less than 50 µm, but the deviations were high due to the tolerance of 3D printing in the z coordinates of the obliquely designed cylinder on the molar. However, since the printed model sets a new standard, it does not affect the results of the test evaluation. The reproducibility of the reference scanner is 11.0 ± 1.8 µm. This test method can be used to identify and improve upon the intrinsic problems of an intraoral scanner or to establish a scanning strategy by measuring the degree of distortion at each part of the complete-arch digital impression.

Introduction

In the traditional dental treatment process, a fixed restoration or a removable denture is made on a model made of gypsum and impregnated with a silicone or irreversible hydrocolloid material. Because an indirectly made prosthesis is delivered in the oral cavity, a lot of research has been done to overcome the errors caused by a series of such manufacturing processes^1,2. Recently, a digital method is used to fabricate a prosthesis through the CAD process by manipulating models in the virtual space after acquiring 3D images instead of making impressions³. In the early days, such an optical impression method was used in a limited range such as a dental caries treatment of one or a small number of teeth. However, as the base technology of the 3D scanner was developed, a digital impression for the complete arch is now used for the fabrication of large-scale fixed restorations, removable restorations such as a partial or full denture, orthodontic appliances, and implant surgical guides^4,5,6,7. The accuracy of the digital impression is satisfactory in a short region such as the unilateral arch. However, since the intraoral scanner is a handheld device that completes the entire dentition by stitching together the image obtained through a narrow optical window, the distortion of the model can be seen after completing the U-shaped dental arch. Thus, an appliance of a large range made on this model might not fit well in the patient's mouth and require a lot of adjustment.

Various studies have been reported on the accuracy of the virtual impression body obtained with an intraoral scanner, and there are various research models and measurement methods. Depending on the research subject, it can be divided into clinical research^8,9,10,11,12 for actual patients and in vitro studies^13,14,15,16 conducted in models separately produced for research. Clinical studies have the advantage of being able to evaluate the conditions of an actual clinical setting, but it is difficult to control the variables and increase the number of clinical cases indefinitely. The number of clinical studies is not large because there is a limit to being able to evaluate the desired variables. On the other hand, many in vitro studies that evaluate the basic performance of the intraoral scanner by controlling variables have been reported¹⁷. The research model also includes a partial or complete arch of natural teeth^{18,19,20,21,22} and a fully edentulous jaw with all teeth lost²³, or the case where the dental implant is installed and spaced apart at a certain interval^24,25,26,27, or a form in which the majority of the teeth remain and only a part of a tooth is missing^16,28. However, studies on the distortion of the virtual impression body made by a handheld intraoral scanner have been limited to the qualitative evaluation of deviations through a color map created by superimposing it with reference data and expressed as one numerical value per data. It is difficult to accurately measure the 3D distortion of the complete arch because most studies only examine the localized portion of the dental arch with a nondirectional distance deviation.

In this study, the distortion of the dental arch during optical impression with an intraoral scanner is investigated by using a standard model with a coordinate system. The aim of this study is to provide information on a method for evaluating the accuracy performance of the intraoral scanners which exhibit various characteristics by the difference in optical hardware and processing software.

Protocol

1. Master specimen preparation

1. Model preparation
   1. Remove the artificial teeth (left and right canines, second premolar, and the second molar) on the mandibular complete-arch model with only 1/5 of the cervical portion left.
2. CAD design
   1. Acquire the data of the master specimen with a reference scanner.
   2. Design the cylinders (with a top diameter of 2 mm and a cylinder height of 7 mm) on top of the trimmed six teeth with the reverse engineering software.
   3. Add three reference spheres (3.5 mm in diameter) posterior to the left second molar for the purpose of defining the reference 3D coordinate system from the reverse engineering software.
   4. Locate one sphere on the distal side of the distal and buccal side of the cylinder on the left second molar so that the coordinates of all the cylinders have positive values.
   5. Design the left second molar cylinder so that it is inclined 30° medially and the right second molar cylinder so that it is tilted 30° distally. Set the other cylinders at right angles from the model.
3. Metal 3D printing
   1. Manufacture a phantom model with CoCr alloy by a metal 3D printer to serve as a patient’s dentition (Figure 1).

2. Reference data acquisition and software analysis

1. Scan the phantom with the testing intraoral scanner.
   1. Obtain the reference image by scanning the metal phantom model with the industrial-level model scanner.
2. Establish a coordinate system by extracting points from reference spheres.
   1. Load the reference image to the reverse engineering analysis software to calculate the reference coordinates of each cylinder position.
   2. Extract the sphere by selecting the Ref. geometry | Create | Sphere | Pick boundary points command and picking the four points on the surface of the reference sphere that are furthest apart from each other (Supplemental Figure 1 and Supplemental Figure 2).
   3. Calculate the center of three reference spheres.
   4. Use the Ref. geometry | Create | Plane | Pick points command to connect the centers of three spheres and create a plane (Supplemental Figure 3).
   5. Set the formed plane as XY plane.
   6. Select the Ref. geometry | Create | Plane | Offset plane command to create a tangent plane above the xy plane (Supplemental Figure 4).
   7. Create points where the tangent plane and two lingual spheres meet by choosing the Ref. geometry | Create | Point | Project on ref. plane command (Supplemental Figure 5).
   8. Generate a plane between the points created and the center of the two lingual spheres by using the Ref. geometry | Create | Plane | Pick points command (Supplemental Figure 6).
   9. Measure the distance from this plane to the center of the buccal sphere with the Inspection | Dimension | Linear command (Supplemental Figure 7).
   10. Create a parallel plane that passes through the midpoint of the buccal sphere with the Geometry | Create | Plane | Offset Plane command (Supplemental Figure 8).
   11. Set the formed plane as YZ plane (Supplemental Figure 9).
3. Set the x, y, and z axes.
   1. Set the center of the buccal sphere as the ‘origin’ of the coordinate system.
   2. Set a line parallel to the line connecting the center points of the remaining two spheres while traveling in the forward and backward direction of the model through the origin as the Y-axis.
   3. Set the line on the xy plane that passes the origin and is perpendicular to the y axis as the X-axis.
   4. Use the Ref. geometry | Create | Coordinate | Pick origin & X, Y direction command to create a new coordinate system with the buccal sphere center as the origin (Supplemental Figure 10).
   5. Set the line perpendicular to the xy plane and passing through the origin as the Z-axis (Supplemental Figure 11).
4. Transfer this detail from the scan coordinate system to the newly established coordinate system.
   1. Use the Ref. geometry | Bind to shell command to fix the geometries created during this process on top of the scan data (Supplemental Figure 12).
   2. Execute the Ref. geometry | Transform | Coordinate | Align coordinate command to transit from the basic coordinate system to the newly created coordinate system (Supplemental Figure 13).
   3. In this way, assign a coordinate system to the metal master specimen with reference to the three reference spheres (Supplemental Figure 14).
5. Extract the measurement points from the cylinders at the main area.
   1. Extract the x, y, and z coordinates for the upper circle centers of six cylinders to be analyzed for the distortion of the specified regions by the reverse engineering process.
   2. For this, use the Ref. geometry | Create | Cylinder | Pick boundary points command and specify at least 10 points on the top border of the cylinder and designate the same amount of points on the ellipse that meets the tooth at the bottom of the cylinder (Supplemental Figure 15, Supplemental Figure 16, and Supplemental Figure 17).
   3. Obtain the extracted coordinates of the cylinder top center. Evaluate the 3D deformation at each position by comparing it with the coordinate values of the same cylinder of the digital impression acquired by the intraoral scanner to be evaluated.

Results

The coordinates of each cylinder calculated from the originally designed CAD data and the reference scan image of the 3D-printed metal master specimen scanned by the industrial-level model scanner are shown in Table 1. The difference between the two showed a value of lower than 50 µm, but the z coordinate value of the right second molar cylinder from the 3D-printed master specimen was low. Although the metal phantom was produced from a high-end industrial 3D...

Discussion

Among the studies evaluating the accuracy of the intraoral scanner by evaluating the resultant digital impression body, the most common method is to superimpose the digital impression data on the reference image and calculate the shell-to-shell deviation^{12,13,14,15,20,23}. However, this method is limited to calculating the dev...

Materials

Name	Company	Catalog Number	Comments
EOS CobaltChrome SP2	Electro Oprical Systems	H051601	Powder type metal alloy for 3D printing
Geomagic Verify	3D Systems	2015.2.0	3D inspection software
Prosthetic Restoration Jaw Model	Nissin Dental Products Inc.		Mandibular complete-arch model
Rapidform	Inus technology	RF90600-10004-010000	Reverse engineering software
stereoSCAN R8	AICON 3D Systems GmbH		Industrial-level model scanner