Accuracy in Dental Medicine, A New Way to Measure Trueness and Precision

Summary

Accuracy is a major demand in dental medicine. To verify accuracy, reference scanners are needed. This article presents a new reference scanner with an adjusted scanning method to acquire a broad variety of dental morphologies with high trueness and precision.

Abstract

Reference scanners are used in dental medicine to verify a lot of procedures. The main interest is to verify impression methods as they serve as a base for dental restorations. The current limitation of many reference scanners is the lack of accuracy scanning large objects like full dental arches, or the limited possibility to assess detailed tooth surfaces. A new reference scanner, based on focus variation scanning technique, was evaluated with regards to highest local and general accuracy. A specific scanning protocol was tested to scan original tooth surface from dental impressions. Also, different model materials were verified. The results showed a high scanning accuracy of the reference scanner with a mean deviation of 5.3 ± 1.1 µm for trueness and 1.6 ± 0.6 µm for precision in case of full arch scans. Current dental impression methods showed much higher deviations (trueness: 20.4 ± 2.2 µm, precision: 12.5 ± 2.5 µm) than the internal scanning accuracy of the reference scanner. Smaller objects like single tooth surface can be scanned with an even higher accuracy, enabling the system to assess erosive and abrasive tooth surface loss. The reference scanner can be used to measure differences for a lot of dental research fields. The different magnification levels combined with a high local and general accuracy can be used to assess changes of single teeth or restorations up to full arch changes.

Introduction

Accuracy is a major interest in many fields in dental medicine. Replacing dental hard tissue needs an exact fitting prosthesis to ensure proper function and prevent further destroying the remaining tooth structure^1,2. Fixed partial dentures and total prosthesis are especially critical for exact fitting at supporting structures like prepared teeth or implants³. This is why a highly accurate reproduction is needed, especially in the field of dental impressions and dental laboratory workflow. However, other fields of dental treatment also benefit from a true and precise metrical outcome, to verify treatment success and evaluate new treatment strategies, e.g. soft and hard tissue augmentation, erosion and abrasion monitoring, periodontal treatments, and orthodontic treatments^4,5. In many of these fields, current validation procedures are linear distance measurements with calipers or microscopes^6,7. These methods are limited to only few measuring points and limited information of three-dimensional (3D) changes of the testing area. Newer measuring methods include the optical or radiographic capturing of the entire surface of the test object^8,9. Here, the entire surface or volume is measured and displayed as a 3D object on the computer screen. Linear measurements are possible, as well as superimpositions of models from different scan times. With this superimposition, an evaluation of the surface changes at every scan point is possible. This enables the possibility of monitoring a specific area or displaying deformations in all three coordinate axes. Also, volumetric changes can be measured¹⁰. The limiting point with these new methods is the accuracy of the scanner, used to capture the test object. None of the changes within the accuracy of the reference scanner can be divided into changes of the test object or scan errors. Scan accuracy is often a value given by the manufacturer derived from scanning small, calibrated objects¹¹. This minimal scan error is different when scanning large objects like a dental arch. Accuracy consists of trueness and precision. Trueness is the deviation of the scanned object from its real geometry. Precision is the deviation between repeated scans (ISO 5725-1). In this study, a new optical reference scanner, based on the focus variation scanning technique, was introduced to scan specimens from single tooth up to full arch models with highest accuracy. This reference scanner was used as a base for several studies, comparing dental impression accuracy from conventional and digital techniques^12–14 and for actual projects concerning dental occlusion and abrasion of dental materials. The aim of this study was to provide basic information of the accuracy of the reference scanner and some possibilities to use this device in the field of dental research.

Protocol

1. Specimen Preparation/Base

1. Apply a flat base around the specimen. Place the specimen on the scanning table. Orientate the occlusal surface to the horizontal plane (Figure 1).

2. Software Analysis

1. Start the program and then the Laboratory Measurement Module (Figure 2)
2. Position the specimen at the center of the scanning table.
3. Choose the correct magnification lens. For large objects, like full arch scans, use the 5X objective.
4. Move the scan optic by using the 3D mouse until the surface of the specimen is displayed on the live view window (Figure 3).
5. Use sensor control to set Exposure and Contrast to achieve optimal scanning parameters. For metal surfaces, use an exposure between 400 and 800 µsec and a contrast between 0.3 and 0.8 (Figure 4).
6. Check image quality by checking the button "Show Image Quality in Live Preview".
7. Set the correct parameters in the section "Measurement Control". The measurement type is "3D Dataset"; ImageField Type is "General ImageField" (Figure 5).
8. Click on "New ImageField".
9. Define the scan volume. The software needs the bounding coordinates of the measure volume.
   1. Move the specimen to the highest and lowest scan level and click on "Add Position" at both points. The "Z Range" value shows the actual height of the scan volume (Figure 6).
   2. Move the specimen to the X-Y boundings of the scan volume. Click on "Add Position" to define X-axis and Y-axis lengths. The dimension of the scan volume is shown at the "Information" tab and should exceed the dimensions of the specimen by 1 cm in the X-axis and Y-axis.
10. Check the "Points" number. The software is capable of scanning 100 million surface points in one "ImageField" scan. The actual number of points exceeds this limit. "Decimate" the measure point's number by a "Lateral Downsampling" of the point size.
    1. Click on "Advanced Settings" (Figure 7).
    2. Move the "Lateral Downsampling" slider to the right until the "Points" number is reduced below 100 M (Figure 7). Downsampling reduces the lateral resolution of the surface points, resulting in larger pixel size of the scanned object. To ensure optimal scan results, the vertical resolution is increased.
11. Click on "Start Measurement". This will start the "Preview Mode". The software performs a prescan with the selected X- and Y-axis dimensions.
12. After the prescan is completed, select the region of interest. This helps to reduce file size and scanning time (Figure 8).
13. Select all parts of the prescan except the specimen and two measure fields around the specimen, containing the flat base (Figure 9).
14. Click on "Start" to begin the scan.
15. Control the scan in the "ImageViewer" using the mouse and press the left mouse button (Figure 10).
16. Close the window and click on "View Pseudo Color Only".
17. Click on "Settings" and "Pseudo Coloring", and choose "Repeatability".
18. Set the "Max." value to 0.2 and click on "Apply Range".
19. Control the repeatability, it should be equal for areas with the same incline and material. In particular, the base around the specimen should display a homogeneous repeatability (Figure 11).
20. Click on "Database" and save the scan to the appropriate folder.
21. Export the scan to different file formats, if needed. Click on "File/Export/3D Data As/…." The protocol can be paused at that point and continued later.

3. Difference Analysis

1. Use the "3D-Editor" to cut away the base. This area will not be used for the difference analysis.
2. To compare and analyze two scans, start the "Difference Measurement" software (Figure 12).
3. Choose the second model to compare.
4. Click on "Automatic Rough Alignment" to perform a first match of the models.
5. Click on "Manual Adjustment" and align the models with rotating and moving until they are in the same orientation (Figure 13).
6. Click on "Automatic Alignment" and "Apply" to start the best fit algorithm for the optimal model matching.
7. Click on "Differences" to view the difference map of the two matched models.
8. Select a proper color range to display the deviations (Figure 14). Save the visual deviation pattern as a screenshot for analysis.

Click on "Statistics" to display the statistical values of the differences. Choose 1 µm class size and save histogram data to a text file for statistical comparison (Figure 15).

Results

Figure 16A shows the scan of a gypsum specimen. To verify optimal scan parameters, control the repeatability for the material. A section of the specimen is scanned with different contrast and brightness settings and the repeatability is checked after each scan to find the optimal scan settings. Metal surfaces show the best results with a contrast lower 1.0 and gypsum or resin materials with a higher contrast. Figure 16B shows the repeatability without optimal scanning parameters after th...

Discussion

Accuracy is a basic demand in dental medicine. The reference scanner is capable of scanning small and large objects with high trueness and precision. With the optimal scanning method, even individual morphological detailed dental surfaces can be scanned with a high resolution and repeatability. With the different magnification levels of the scanner, macro and micro morphological structures can be acquired. It is possible to scan a variety of model materials.

The evaluation of the reference sca...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Reference model			individual non-precious metal model, derived from a patient impression
Araldit repair	Huntsmen Advanced Material, Basel, Switzerland		used for making the base of the reference model
CamBase	Dentona, Dortmund, Germany		Type IV dental ston for pouring conventional impressions
Identium	Kettenbach, Eschenburg, Germany		Vinylsiloxanether impression material for conventional impression
inEOS model holder	Sirona Dental Systems, Bensheim, Germany		used for fixing stone models at the reference scanner
Accutrans	Coltene Whaledent, Altstätten, Switzerland		used for making the base of thestone models
President putty	Coltene Whaledent, Altstätten, Switzerland		mix with accutrans for betterstability of the base
Alicona Infinite Focus	Alicona Imaging, Graz, Austria		Reference scanner