Material and methods

Four IOS systems were assessed in this study: Planmeca Emerald (PE; Planmeca) software v5.9.4, 3Shape TRIOS 3 color model (TR; 3Shape) software v1.17.2.4, iTero Element (IE; Align Technology) software v1.5.0.361, and CEREC Omnicam (CO; Dentsply Sirona) hardware v2.24 and software v4.5.2. A custom reference standard model was fabricated by using the method reported by Mennito el al. Briefly, typodont teeth were prepared and restored by using composite resin (Telio CAD; Ivoclar Vivadent AG) (TC), which has a refractive index (RI) of 1.49. The RI of TC is similar to that of dentin (1.54) and enamel (1.63), thus simulating natural tooth substance. The reference standard model was obtained by using an optical scanner (ATOS III Triple Scan 3D; gom). The ATOS is a noncontact structured blue-light scanner that works by using multiple cameras that record the projection of stripes on an object being measured. For each pixel of the camera sensor’s points, coordinates can be estimated with high precision. For jaw-sized scans, this scanner has a trueness of 3 μm and precision of 2 μm.

Four scan strategies were chosen for this study based on manufacturer-recommended patterns for the scanners. To decrease the risk of operator bias and error, each IOS was tested by an operator who was familiar with and trained on each particular device with a minimum of 3 years of experience. Four experimental scans were obtained from each of the scanners for each of the 4 distinct scan strategies. Scan patterns (SPs) 1 to 4 were based on the IE, CO, PE, and TR operator manuals, respectively. The experimental groups for this study were as follows: SP1, SP2, SP3, and SP4 for CO with a sample size of 4 repeated scans for each pattern for a total of 16 scans. The same SP groups were repeated with EI, PE, and TR. For each scan performed, both scan time and rendering time were recorded. Scan strategies are shown in Figure 2 . One operator (J.L.) was designated as a calibrator and was present during all scanning sessions to ensure that the appropriate scan patterns were used and that the time was recorded accurately. Furthermore, a calibration session demonstrating the appropriate scan patterns was held with all clinicians before the study.

Graphic representation of 4 scan patterns evaluated. Scan technique 1 starts on occlusal surface of most distal molar on patient’s left side and continues to occlusal of contralateral terminal molar. Next sequence captures lingual surfaces back to start molar. Next sequence is facial surfaces from start molar to midline, switching to contralateral molar and back to midline. Lastly, buccal lingual rotations are made starting on ipsilateral canine and terminating on contralateral canine. Scan technique 2 starts on most distal molar on patient’s left side at 45-degree angle and continues at that angle until contralateral first premolar is captured, then returns to starting molar at 90-degree lingual angle. Then, proceed to contralateral canine from occlusal angle. User will rotate to facial at 45-degree angle back to start molar and then back to contralateral canine at 90-degree facial angle. Pattern is repeated with start tooth set to canine on patient’s left side. Scan technique 3 starts on occlusal of patient’s right canine and user rolls facial lingual in half tooth increments until terminal molar is reached on contralateral side. Pattern is then repeated from same start canine to ipsilateral terminal molar. Scan technique 4 starts on patient’s left molar from occlusal and proceeds to ipsilateral canine where user transitions to buccal lingual rotation pattern until contralateral premolar is reached where occlusal scan is made to terminal molar. Then, sequence captures lingual surfaces until starting molar is reached. User then captures facial surfaces starting from original start molar until contralateral molar.

All experimental scans were converted to standard tessellation language (STL) format by the corresponding manufacturers’ recommended conversion method. A comprehensive metrology program (Geomagic Control X; 3D SYSTEMS) was used to compare the reference standard model with the experimental STL models.

Once imported into the software, the models were digitally trimmed along a reference line made on the original solid model by using the software’s trim function ( Fig. 3 ). Once models were trimmed, the reference standard model STL file was imported and trimmed on the opposite side of the original reference line to ensure adequate test model overlap ( Fig. 4 ).

Trimming of reference standard in Geomagic software. A, Untrimmed reference standard. B, Trimmed reference standard, buccal view. C, Trimmed reference standard, occlusal view.

Trimming of sample model best fit alignment to reference standard in Geomagic software. A, Reference standard. B, Experimental IOS scan. C, Aligned scans. IOS, intraoral scanner.

Using Geomagic’s Initial Alignment and Best Fit Alignment functions, the models were overlaid in preparation for 3D comparisons ( Fig. 5 ). The software’s Best Fit Alignment function acts to align the test file and reference file by using an iterative closest point algorithm (ICP). ICP has become one of the most widely adopted methods for aligning digital 3D files. It is a data-driven approach that uses point cloud properties to aid in aligning 3D objects. The algorithm uses 3D correspondences between 2 clouds of points and determines the minimal distance between objects. The software then evaluates the test file, and the points that are the closest are computed on the reference file ( Fig. 6 ).

Demonstration of conversion of discrepancy from best fit alignment into color map. A, Aligned experimental IOS scan and reference standard before metrology. B, after metrology showing color map of differences between the reference standard and the IOS scan. IOS, intraoral scanner.

Standard output from each 3D comparison. For this study, min, max, +avg, and −avg used for statistics.

The software’s 3D compare function allowed for customization of measurements and color mapping of results. A value of 0.5 mm was used to define the upper and lower limits for color mapping ( Fig. 6 ). No reference tolerances were set for this study, and color mapping was chosen to display deviations on the digital models. Models displayed after the 3D compare function showed a range of colors that correlated with potential areas of mismatch between the test and reference models. Areas highlighted in darker blue indicated a negative or inward deviation, whereas areas highlighted in darker red indicated a positive or outward deviation of the test models. Reports were generated for each comparison, and the average, minimum deviation, maximum deviation, and standard deviation values were compiled in a spreadsheet (Excel; Microsoft Corp) ( Fig. 6 ).

Descriptive statistics are presented as means and standard deviations. For each outcome, generalized linear mixed models with scanner, scan pattern, and their interaction in the model were used for trueness, precision, maximum deviation, and scan time. A random intercept was included in the model to account for replicates. Normality assumptions were checked to ensure no transformations were needed. Post hoc pairwise comparisons were made when appropriate by using a Scheffé adjustment (α=.05, except when the Scheffé adjustment was used). All statistical analyses were performed by using a statistical software program (SAS v9.4; SAS Institute Inc).

Results

Scanners were first evaluated overall for trueness, precision, and time averages. This included all 16 scans performed for each IOS device. The scanners were also compared in 2 main ways: scan patterns within each scanner ( Table 1 ) and scanners within each scan pattern ( Table 2 ).

Table 1

Raw data showing scanners compared for each scan pattern

Pattern	Scanner	N	Trueness (Average Micrometer)	Trueness Rank (Within Scanner)	Precision (STD Micrometer)	Precision Rank (Within Scanner)	Scan Time (Minutes)	Scan Time Rank (Within Scanner)
SP1	Element	4	44	1	18	4	3.70	4
	Emerald	4	67	3	11	3	3.33	2
	Omnicam	4	122	4	42	2	3.44	3
	TRIOS	4	49	2	22	1	2.74	1
SP2	Element	4	50	4	17	2	2.16	3
	Emerald	4	49	2	5	1	1.66	2
	Omnicam	4	93	4	30	3	3.20	4
	TRIOS	4	48	1	21	2	1.45	1
SP3	Element	4	46	1	16	2	1.82	1
	Emerald	4	55	3	4	1	2.05	2
	Omnicam	4	138	4	42	4	2.33	4
	TRIOS	4	47	2	23	3	2.05	2
SP4	Element	4	45	1	15	2	1.41	3
	Emerald	4	69	3	10	1	1.32	1
	Omnicam	4	124	4	41	4	1.45	4
	TRIOS	4	46	2	21	3	1.38	2

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