A method for measuring the accuracy deviation of an intraoral scanner

Abstract

The present invention belongs to the field of oral cavity, and specifically relates to a method for measuring the accuracy deviation of an intraoral scanner, which specifically includes the following steps: (1) making a scanning model, (2) acquiring scanning data, and (3) a measuring method. The present invention establishes a method for measuring the accuracy deviation of an intraoral scanner, and applies the method to evaluate the data splicing accuracy of four intraoral scanners. The results show that scanners with different principles and algorithms also have differences in their data splicing accuracy, and the influence of scanning strategies on scanning accuracy is related to the differences.

Description

Technical Field

[0001] The invention belongs to the oral field, and in particular relates to a method for measuring the accuracy deviation of an intraoral scanner. Background Art

[0002] Intraoral scanning technology has been widely used in the fields of orthodontics, oral implantology, and dental prosthodontics. During use, it was found that the factors that affect the accuracy of intraoral scanning are mainly: (1) Factors of the scanner itself; factors of the scanner itself include instrument measurement accuracy, scanning resolution, structure of the scanning head, etc.; in addition, the measurement accuracy will also change significantly with the change of the scanner's operating temperature, and recalibration is required at this time. (2) Factors of the object being measured; factors of the object being measured include surface roughness, material, reflectivity, texture, color, shape, etc. of the surface being measured. (3) Accidental factors; accidental factors include liquid on the surface of the tissue, ambient light during scanning, and aerosol generated during scanning. (4) Data splicing factors; each scanner has different data splicing accuracies due to the different scanning technologies and algorithms it uses, and common data splicing factors include scanning strategies, scanning ranges, etc.

[0003] The four common intraoral scanners in the field of oral restoration each have their own unique principles. TRIOS is a more advanced intraoral digital impression system developed by Denmark's 3Shape in recent years. Its imaging principle is ultra-fast optical cutting technology and confocal microscopy technology, and it has a very high image capture speed. When scanning, the system can infer the position of the confocal plane that makes each pixel position in focus by the change of pixel value over time, thereby realizing the conversion of a two-dimensional image into a three-dimensional model. TRIOS3Shape relies entirely on the extraction of the photo contour to pick up the point cloud, so regular calibration is required to ensure scanning accuracy and color accuracy. CEREC's previous generation Bluecam intraoral scanner uses active triangulation imaging technology and confocal microscopy technology, but requires the surface of the scanned object to produce a good diffuse reflection effect. Therefore, Bluecam needs to be properly isolated from moisture and sprayed with powder before scanning. The current Omnicam intraoral scanner has been improved on the basis of the Bluecam monochromatic light source. Its core is the use of continuous stereo photography technology, which can simultaneously project light beams of different wavelengths, calculate the height information of the projected surface in real time, and does not require powder spraying, which greatly improves the scanning speed and achieves a scanning speed similar to that of dynamic photography. CS3600 uses structured light imaging technology and laser triangulation technology. It forms a triangle between the image planes of two cameras (or the image planes of a single camera at different positions) and the object being measured. Through algorithms, multiple grating fringe images with different distributions (obtained based on triangulation) are reconstructed into a 3D model (STL), and true color information is merged into the 3D model surface (PLY); through the software level, the Data Communication Module (DCM) format file is exported as a pure color digital model or a true color 3D model. Different from the photo imaging of TRIOS, Planmeca Emerald uses grating imaging based on optical coherence tomography and confocal microscopy technology. The grating has active light and dark stripes as a reference, so the device only needs color calibration and no other calibration.

[0004] There are many studies on the comparison of the accuracy differences of different intraoral scanners. CAOsnes et al. compared the scanning accuracy of 6 intraoral scanners. The results showed that Aadva, 3ShapeTrios, CERECOmniCam and TDS are more suitable for clinical use (Osnes CA, Wu JH, Venezia P, et al. Full arch precision of six intraoral scanners in vitro[J]. J Prosthodont Res, 2020, 64(1): 6-11.). The study by Yasmine Ashraf et al. showed that there was no significant difference in the scanning accuracy of 3Shape Trios (35.70±14.12μm) and medit i500 (44.31±11.41μm) (P>0.05), but compared with OmniCam (57.83±22.14μm), the scanning accuracy of the former two was significantly higher (Ashraf Y, Sabet A, Hamdy A, et al. Influence of preparation type and tooth geometry on the accuracy of different intraoral scanners[J]. J Prosthodont, 2020, 29(9): 800-804.).Hasan et al. used the Trios intraoral scanner representing confocal microscopy technology to evaluate the accuracy of intraoral scanning strategies under four different intraoral environmental conditions, but did not compare it with intraoral scanners based on other principles (Camc1H, Salmanpour F. Effect of saliva isolation and intraoral light levels on performance of intraoral scanners[J]. Am J Orthod Dentofacial Orthopedics, 2020, 158(5): 759-766.), while Sakura et al. evaluated the accuracy difference between active triangulation intraoral scanners (CerecOmniCam) and confocal laser intraoral scanners (3Shape Trios), which was not comprehensive enough compared with the current types of intraoral scanners (Shimizu S, Shinya A, Kuroda S, et al. The accuracy of the CAD systemusing intraoral and extraoral scanners for designing of fixed dental prostheses[J]. Dent MaterJ, 2017, 36(4): 402-407.).

[0005] The above studies all show that intraoral scanners based on different principles have different scanning accuracies. In addition to the differences in image acquisition principles, the different data stitching processing algorithms of the scanners are also an important reason. Different scanning strategies have different effects on the scanning accuracy of different scanners, which should also be related to the accuracy of the scanner's data stitching. However, no research has been seen on the accuracy of intraoral scanner data stitching. Therefore, it is very important to study the differences in the accuracy of data stitching between different intraoral scanners, which can provide a reference for the application of clinical intraoral digital impression technology. Summary of the invention

[0006] The object of the present invention is to provide a method for measuring the accuracy deviation of an intraoral scanner.

[0007] The implementation process of the present invention is as follows:

[0008] A method for measuring the accuracy deviation of an intraoral scanner comprises the following steps:

[0009] (1) Preparation of scanning model

[0010] Select a standard mandibular dentition super-hard plaster model, and use a dental technician's special grinder and ball drill to prepare 1-17 series of circular concave marks at low speed on the buccal and lingual surfaces of each tooth position of the plaster model as the measurement points of the later model; then place a 20mm*90mm double-layer red wax sheet on the occlusal surface of teeth 47-36, and fix it with a dental technician's special electric wax spoon, which is the scanning model;

[0011] (2) Obtaining Scan Data

[0012] The same operator used different intraoral scanners to scan the scanned model of the occlusal fixed red wax high-grade plate. The scanning path was lingual side of tooth 37-occlusal surface of tooth 37-buccal side of tooth 37-buccal side of tooth 36-labial side of anterior tooth-buccal side of tooth 47-lingual side of tooth 47-lingual side of anterior tooth-lingual side of tooth 35. In the process of scanning teeth 36-47, the red wax high-grade plate of the occlusal surface was not crossed. The scanning start point and end point were not connected, that is, the lingual surface of tooth 36 was not scanned. The scanning data was exported in STL format file.

[0013] The same model is placed in a model scanner for scanning, and the scanned data is also exported in STL format;

[0014] (3) Measurement method

[0015] The scanned model data was imported into Geomagic wrap 2017, and the digital model obtained by the model scanner was used as the reference model to fix its position. The model obtained by the intraoral scanner in the software was set as a floating model. The concave on the buccal surface of teeth 37, 36, 35, and 33 was used as the landmark point. The floating model and the reference model were preliminarily aligned using the "manual registration" function. The crown of tooth 37 in the floating model and the reference model was selected using the "select by angle" function, and the "global registration" function was used for automatic alignment. After the alignment was completed, the concave landmarks were selected one by one using the "select by angle" function. By determining the "center of mass", a measurement landmark point was generated at each concave. The three-dimensional coordinates of all the landmark points were output, and the distances between each measurement landmark point on the reference model and the corresponding landmark points of each group of experimental models were calculated to analyze the accuracy deviation of different intraoral scanners.

[0016] Furthermore, in step (1), the diameter of the circular concave mark is 1.3 mm to 1.5 mm.

[0017] Further, in step (1), the specific distribution of each landmark point is as follows: points 1 and 2 are located on the lingual side and buccal side of tooth 37 respectively; point 3 is located on the buccal side of tooth 36; points 4 and 17 are located on the buccal side and lingual side of tooth 35 respectively; points 5 and 16 are located on the buccal side and lingual side of tooth 33 respectively; points 6 and 15 are located on the buccal side and lingual side of tooth 31 respectively; points 7 and 14 are located on the buccal side and lingual side of tooth 42 respectively; points 8 and 13 are located on the buccal side and lingual side of tooth 44 respectively; points 9 and 12 are located on the buccal side and lingual side of tooth 46 respectively; points 10 and 11 are located on the buccal side and lingual side of tooth 47 respectively.

[0018] Furthermore, in step (2), the operator is an operator who has received formal training and has clinical experience.

[0019] Furthermore, in step (2), each intraoral scanner repeats the scan 10 times.

[0020] Furthermore, in step (2) and step (3), the model scanner is a Ceramill map 600 model scanner.

[0021] Positive effects of the present invention: The present invention establishes a method for measuring the accuracy deviation of an intraoral scanner, and applies the method to evaluate the data stitching accuracy of four intraoral scanners. The results show that scanners with different principles and algorithms have different data stitching accuracy, and the impact of the scanning strategy on the scanning accuracy is related to the difference. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A superhard plaster model of the standard mandibular dentition was prepared to create a rounded concave landmark and fix the red wax stopper;

[0023] Figure 2 is the scanning path diagram, where: 1 is the starting point and 2 is the end point;

[0024] Figure 3 Align for "register";

[0025] Figure 4 Create and measure maps for feature points;

[0026] Figure 5 It is the distance deviation diagram between each landmark point of the four intraoral scanners and the corresponding landmark point on the reference model;

[0027] Figure 6 Comparison of deviations of the starting points (37 lingual side) of the four scanners;

[0028] Figure 7 It is the comparison of the deviation of the end point (35 lingual side) of the four scanners;

[0029] Figure 8 Data acquisition schematics for different scanning strategies. DETAILED DESCRIPTION

[0030] The present invention will be further described below in conjunction with the embodiments.

[0031] In an embodiment of the present invention, a standard mandibular dentition superhard gypsum model was selected as an experimental object, and four commonly used intraoral scanners were used to scan and obtain a digital model. The data obtained by the model scanner was used as a control. The accuracy of the splicing of the data from the four intraoral scanners was compared by measuring the distance deviation between each landmark point in the model obtained by the intraoral scanner and the corresponding landmark point in the control model.

[0032] 1 Materials

[0033] 1.1 Scanning model and preparing materials

[0034] Standard mandibular dentition superhard plaster model; red wax sheet; electric wax spoon for dental technicians (Renfert); grinder for dental technicians (NSK Ertimet XL); diamond ball drill (Φ=1.4mm).

[0035] 1.2 Scanner Brand

[0036] Four intraoral scanners: 3Shape Trios (Copenhagen, Denmark); (SironaDental System GmbH, Bensheim, Germany); CS (Carestream, Rochester, NY, USA); Planmeca Emerald SW 4.6 (Planmeca, Helsinki, Finland).

[0037] Model scanner: Ceramill map600 (Amann Girrbach, Germany).

[0038] 1.3 Computers and Software

[0039] DELL Precision M4800 workstation (system: Windows 7, CPU: Intel Core i7-4900MQ, memory: 16G, hard disk: 1TB5400 to SATA interface hybrid hard disk, graphics card: NVIDIA Quadro K2100M). Software is Geomagic Wrap 2017 (3D Systems, USA) and GraphPad Prism 9.0 statistical analysis software (GraphPad Software, USA).

[0040] 2 Methods

[0041] 2.1 Preparation of scanning model

[0042] A standard superhard plaster model of mandibular dentition was selected, and a series of circular concave marks (about 1.4 mm in diameter) from No. 1 to No. 17 were prepared at low speed on the buccal and lingual surfaces of each tooth position of the plaster model using a dental technician-specific grinder and a ball drill as measurement points for the later model; the specific distribution of each mark point is as follows: No. 1 and No. 2 are located on the lingual and buccal sides of tooth 37, respectively; No. 3 is located on the buccal side of tooth 36; No. 4 and No. 17 are located on the buccal and lingual sides of tooth 35, respectively; No. 5 and No. 16 are located on the buccal and lingual sides of tooth 33, respectively; No. 6 and No. 15 are located on the buccal and lingual sides of tooth 31, respectively; No. 7 and No. 14 are located on the buccal and lingual sides of tooth 42, respectively; No. 8 and No. 13 are located on the buccal and lingual sides of tooth 44, respectively; No. 9 and No. 12 are located on the buccal and lingual sides of tooth 46, respectively; No. 10 and No. 11 are located on the buccal and lingual sides of tooth 47, respectively (see Figure 1 ). Then place a 20mm*90mm double-layer red wax sheet on the occlusal surface of teeth 47-36, and fix it with a special electric wax spoon for dental technicians to obtain a scanning model.

[0043] 2.2 Scanning data acquisition

[0044] The same operator (who has undergone formal training and has clinical experience) used four intraoral scanners to scan the scanned model of the occlusal surface fixed red wax high-grade plate. Each scanner repeated the scan 10 times, and the scanning path was lingual side of tooth 37-occlusal surface of tooth 37-buccal side of tooth 37-buccal side of tooth 36-lip side of anterior tooth-buccal side of tooth 47-lingual side of tooth 47-lingual side of anterior tooth-lingual side of tooth 35. In the process of scanning tooth 36-tooth 47, the occlusal surface was not crossed, and the scanning start and end points were not connected, that is, the lingual surface of tooth 36 was not scanned ( Figure 2 ). The scanned data is exported as an STL file.

[0045] The same model was placed in the Ceramill map 600 model scanner for scanning, and the scanning data was also exported in STL format.

[0046] 2.3 Measurement method

[0047] The scanned model data was imported into Geomagic wrap 2017, and the digital model obtained by the model scanner Ceramill map 600 was used as the reference model to fix its position. The model obtained by the intraoral scanner in the software was set as a floating model; the concave on the buccal surface of teeth 37, 36, 35, and 33 was used as the landmark point, and the floating model and the reference model were preliminarily aligned using the "manual registration" function. Then, the crown of tooth 37 in the floating model and the reference model was selected using the "select by angle" function, and the "global registration" function was used for automatic alignment. After the alignment was completed, the concave landmarks were selected one by one using the "select by angle" function, and a measurement landmark point was generated at each concave by determining the "center of mass". The three-dimensional coordinates of all the landmark points were output, and the distance between each measurement landmark point on the reference model and the corresponding landmark point of each group of experimental models was calculated to analyze the accuracy deviation of each scanner ( Figure 3 , 4 ).

[0048] 2.4 Statistical analysis

[0049] The data of each group were imported into GraphPad Prism 9.0 statistical analysis software, and the statistical differences of the distance deviations between the landmarks of the four intraoral scanners and the corresponding landmarks on the reference model were compared and analyzed. The Kolmogorov-Smirnov test was used to test whether the data of each group conformed to the normal distribution (P>0.1), and the Brown-Forsythe test was used to compare the variance homogeneity of the data of each group (P>0.1). For data that conformed to the normal distribution and had homogeneous variance, one-way ANOVA test was used to calculate the statistical difference of the data (P<0.05); for data that conformed to the normal distribution but had unequal variance, Dunnett's T3 test was used to analyze the statistical difference of the data (P<0.05); for data that did not conform to the normal distribution, Kruskal-Wallis test was used to analyze the statistical difference of the data (P<0.05).

[0050] 3 Results

[0051] From the starting point to the end point, the distance deviation between each landmark point of the four scanners and the corresponding landmark point on the reference model gradually increased, among which CS3600 increased the least, followed by Trios2 and Emerald, and Omnicam increased the most (Table 1, Figure 5 ).

[0052] Table 1 Distance deviation between the landmarks of the four intraoral scanners and the corresponding landmarks on the reference model (mm)

[0053]

[0054]

[0055] from Figure 5 It was also observed that the distance deviation changes of different scanners during the scanning process were different. The distance deviation of each landmark of CS3600 changed slightly. The distance deviation of Emerald began to increase quickly, but gradually stabilized after scanning from the buccal side to the lingual side. The deviation of Trios2 and Omnicam increased when scanning to the opposite side, and then stabilized. After the lingual side of tooth 44, the distance deviation began to increase again.

[0056] Comparative analysis of the data of the starting point and the end point on the experimental model and the reference model showed that: for the starting point, the average distance deviation of the four scanners had no significant difference (P>0.05). For the end point, the deviation of CS3600 was smaller and significantly different from the other three scanners (P<0.05), while there was no significant difference between the other three scanners (P>0.05). Figure 6 , Figure 7 ); among them, the average value and maximum value measured by Omnicam are the largest, which are 1.983mm and 3.226mm; the average value and maximum value measured by CS3600 are the smallest, which are 0.637mm and 1.215mm. The rest are shown in Table 2.

[0057] Table 2 Distance deviation of the scanning end points of four intraoral scanners

[0058]

[0059] The starting point and problem to be solved by the present invention is that the ability to register and splice scanned images is crucial for scanners, especially for the accuracy of full dental arch scanning. Figure 8, A is the order of data acquisition during S-shaped scanning, and B is the order of data acquisition during circular scanning. In both scanning processes, the scanned data was not only linearly registered and stitched as shown by the blue arrow in the same scanning path during post-processing, but also registered and stitched with the adjacent data. For scanners, the ability of linear registration and stitching in scanning image processing is the basis, especially for the scanning accuracy of the entire dental arch. However, the ability of linear registration and stitching of the scanner cannot be directly observed in both scanning methods A and B; only by using scanning method C, that is, the intraoral scanner avoids the stitching of buccal and lingual data during the scanning process, can the ability of linear registration and stitching of the scanner be truly observed. To achieve this goal, we added a 20mm high and 90mm long red wax baffle to the occlusal surface of the standard mandibular dentition plaster model. The 20mm baffle height can block the light of the scanning head and prevent the scanning light from crossing the occlusal surface. The 90mm baffle length can cover the incisal edge and occlusal surface of the dental arch. Therefore, when scanning the lingual side, the data stitching will not be affected by the buccal data. 37 The occlusal surface is not covered with a baffle, which is mainly used to provide a reference area for the automatic registration of the experimental model and the reference model; in addition, it can also be compared whether there is a significant difference in the distance deviation of the buccal and lingual surfaces after the starting end passes the occlusal surface. The series of circular concave marks prepared on the buccal and lingual surfaces of different tooth positions are used to establish corresponding measurement mark points, so that the deviation between the experimental model and the reference model can be accurately measured, and it is also convenient for the preliminary registration of the experimental model and the reference model.

[0060] The results of the present invention show that the accuracy deviation caused by registration and stitching increases with the increase of scanning range in each intraoral scanner; among the four intraoral scanners, the increase of CS3600 is the smallest; at the starting mark point, there is no significant difference in the distance deviation between the four intraoral scanners and the corresponding mark point of the reference model, while at the terminal mark point, the average values ​​of the scanning distance deviation of the four intraoral scanners Omnicam, CS3600, Trios2, and Emerald are 1.983mm, 0.637mm, 1.452mm, and 1.683mm, respectively. By comparing the starting point and the terminal point, it can be found that CS3600 has the smallest accuracy deviation, and the structured light imaging and laser triangulation measurement principles it represents show the best linear registration and stitching capabilities under the measurement conditions of this study. Although the distance deviation of Emerald gradually stabilized later, it began to increase significantly very early, indicating that its linear stitching ability may be poor.

[0061] This paper establishes a method for measuring the accuracy deviation of intraoral scanners, more specifically a method for measuring the accuracy deviation of data splicing of intraoral scanners, and uses this method to evaluate the data splicing accuracy of four intraoral scanners. The results show that there are differences in the data splicing accuracy of intraoral scanners with different principles and algorithms, and the influence of scanning strategies on scanning accuracy is related to the differences. In the clinical application of Emerald scanners, the S-shaped cross scanning strategy should be used as much as possible.

[0062] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and the specific implementation of the present invention cannot be limited to these descriptions. For ordinary technicians in the field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A method for measuring the accuracy deviation of an intraoral scanner, characterized in that: The steps include: (1) Preparation of scanning model A standard mandibular dentition superhard plaster model was selected, and a series of circular concave marks No. 1 to No. 17 were prepared at a low speed on the buccal and lingual surfaces of each tooth position of the plaster model using a dental technician-specific grinder and a ball drill as the measurement points of the later model; then a 20mm*90mm double-layer red wax sheet was placed on the occlusal surface of teeth 47-36 and fixed with a dental technician-specific electric wax spoon, which was the scanning model; the specific distribution of the mark points was as follows: points 1 and 2 were located on the lingual surface of teeth 37 and 36, respectively. Point 3 is located on the buccal side and the lingual side of tooth 37; Point 3 is located on the buccal side of tooth 36; Points 4 and 17 are located on the buccal side and the lingual side of tooth 35 respectively; Points 5 and 16 are located on the buccal side and the lingual side of tooth 33 respectively; Points 6 and 15 are located on the buccal side and the lingual side of tooth 31 respectively; Points 7 and 14 are located on the buccal side and the lingual side of tooth 42 respectively; Points 8 and 13 are located on the buccal side and the lingual side of tooth 44 respectively; Points 9 and 12 are located on the buccal side and the lingual side of tooth 46 respectively; Points 10 and 11 are located on the buccal side and the lingual side of tooth 47 respectively; (2) Obtaining Scan Data The same operator used different intraoral scanners to scan the scanned model of the occlusal fixed red wax high-grade plate. The scanning path was lingual side of tooth 37-occlusal surface of tooth 37-buccal side of tooth 37-buccal side of tooth 36-labial side of anterior tooth-buccal side of tooth 47-lingual side of tooth 47-lingual side of anterior tooth-lingual side of tooth 35. In the process of scanning teeth 36-47, the red wax high-grade plate of the occlusal surface was not crossed. The scanning start point and end point were not connected, that is, the lingual surface of tooth 36 was not scanned. The scanning data was exported in STL format file. The same model is placed in a model scanner for scanning, and the scanned data is also exported in STL format; (3) Measurement method The scanned model data was imported into Geomagic wrap 2017, and the digital model obtained by the model scanner was used as the reference model to fix its position. The model obtained by the intraoral scanner in the software was set as a floating model. The concave on the buccal surface of teeth 37, 36, 35, and 33 was used as the landmark point. The floating model and the reference model were preliminarily registered using the "manual registration" function. The crown of tooth 37 in the floating model and the reference model was selected using the "select by angle" function, and the "global registration" function was used for automatic registration. After the registration was completed, the concave landmarks were selected one by one using the "select by angle" function. By determining the "center of mass", a measurement landmark point was generated at each concave. The three-dimensional coordinates of all the landmark points were output, and the distances between each measurement landmark point on the reference model and the corresponding landmark points of each group of experimental models were calculated to analyze the accuracy deviation of different intraoral scanners.

2. The method for measuring the accuracy deviation of an intraoral scanner according to claim 1, characterized in that: In step (1), the diameter of the circular concave mark is 1.3 mm to 1.5 mm.

3. The method for measuring the accuracy deviation of an intraoral scanner according to claim 1, characterized in that: In step (2), the operator is an operator who has received formal training and has clinical experience.

4. The method for measuring the accuracy deviation of an intraoral scanner according to claim 1, characterized in that: In step (2), each intraoral scanner repeated the scan 10 times.

5. The method for measuring the accuracy deviation of an intraoral scanner according to claim 1, characterized in that: In step (2) and step (3), the model scanner is a Ceramill map 600 model scanner.

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