Material and methods

A sample size calculation was performed using the formula described by Pandis, considering a power test of 90% and α of 0.05, to detect a difference in measurements of 1 mm (SD, 1.16 mm). The sample size calculation (with 10 volunteers) showed that at least 29 volunteers would be needed for the research. This study was approved by the ethical committee of the Medical School of Federal Fluminense University, Niterói, Brazil, on February 1, 2013 (registration number 221.664).

The inclusion criteria were fully erupted permanent dentition (including all maxillary and mandibular permanent first molars); no anomalies in number, size, or shape of the dentition; no accentuated recessions, abrasions, or erosions; no cavities or restorations that could compromise the mesiodistal diameter; and no orthodontic fixed retention.

Thirty volunteers who met the inclusion criteria were selected at the Department of Orthodontics of Federal Fluminense University. Their ages ranged from 21 to 39 years, with a mean of 27 years 9 months. All volunteers were informed about the research procedures and signed informed consent. After a clinical examination, alginate impressions with Hydrogum alginate (Zhermack, Badia Polesine, Rovigo, Italy) were made following the manufacturer’s guidelines. In addition, a bite registration in maximal occlusion was made using number 7 dental wax (Clássico, São Paulo, Brazil). Twenty minutes after impression taking, to complete the alginate cure period, the alginate impressions were disinfected and placed in a closed plastic bag. Within 1 hour after impression taking, the teeth and the alveolar ridges were covered with type IV plaster (Vigodent, Rio de Janeiro, Brazil). The base of the model was poured with white plaster (Mossoró, Rio de Janeiro, Brazil).

Directly after the making of the alginate impression, a PVS impression was taken from each volunteer. These PVS impressions of both arches were taken using the 2-step technique with Futura impression material (Nova DFL, Rio de Janeiro, Brazil). The first impression was made with the heavy putty material, and then the soft putty material was used to record the anatomic details, according to the manufacturer’s guidelines. The regular-viscosity soft putty was used for the maxillary arch and the light-viscosity soft putty for the mandibular arch to allow evaluation of possible accuracy differences between the 2 materials. After both impressions were taken, a bite registration in maximum intercuspation was obtained with the PVS heavy putty ( Fig 1 ).

Maxillary impression (regular viscosity), mandibular impression (light viscosity), and bite registration.

After impression taking, the PVS impressions and the bite registrations were disinfected, stored in plastic bags, and protected from light for subsequent scanning to obtain digital models. The 60 impressions (30 impressions of each arch) and the respective bite registrations were scanned with the R700 scanner using the dedicated impression scanning software. The scanning procedure started with the maxillary impression, followed by the mandibular impression, and then the bite registration was scanned with a device that kept it stable without deforming it. After making the first scan, the scanner automatically detects the areas where the scan has less quality and scans them during the adaptive scanning by turning the table where the impression is and by moving the cameras on the rail. Of course, there are limits in scanning possibilities and time, so it is difficult to scan properly when the areas are not visible because of some undercuts. In some cases, we used a titanium oxide powder on the mandibular incisor area to improve scanning accuracy; the narrower undercuts in this region can create difficulty with the laser incidence of the scanner ( Fig 2 ). For superimposition of the maxillary model and bite registration, 3 points (first molar and incisal regions) on the models were selected along with 3 identical points on the bite registration for the initial alignment. The same procedure was repeated for the mandibular model. The 3Shape software then automatically superimposed both digital models by a best-fit method. Then sagittal, transverse, and vertical adjustments were made to create the virtual maxillary and mandibular bases ( Fig 3 ). The 30 digital model pairs were divided into 3 groups of 10 pairs each, according to the time interval between taking and scanning the PVS impressions. T5 represented an interval of 5 days; T10, 10 days; and T15, 15 days.

A, Application of titanium oxide powder in the mandibular incisor area; B, maxillary impression scanning; C, mandibular impression scanning; D , bite registration scanning.

Defining the occlusion: A, digital maxillary model; B, digital mandibular model; C, digital bite registration; D, defining points between the maxillary model and the bite registration; E, defining points between the mandibular model and the bite registration; F, digital model after interarch adjustment.

Thirty-four clinically relevant parameters for measuring were defined for each pair of dental models, including tooth diameter, transverse distances (maxillary and mandibular intercanine and intermolar distances), and interarch relationship measurements (overbite, overjet) ( Table I ). Three examiners (L.T.C., D.S.A., and another) were properly trained to measure the plaster and digital models before the start of the study. Among the examiners was an undergraduate student of dentistry, a masters degree student of orthodontics, and a doctoral student of dentistry. The plaster models were measured with a digital caliper (Starrett-Itu, São Paulo, Brazil), with an accuracy of 0.01 mm. The digital models were measured using the Ortho Analyzer software with the direct measuring tool ( Fig 4 ). To calculate the method error, the 3 examiners repeated the measurements on 10 randomly selected plaster models and 10 digital models, 15 days after the first measurements.

Measuring tooth diameters on a digital model with the Ortho Analyzer software.

All plaster models of the sample were also scanned with the same scanner (R700) to acquire the respective digital models and enable comparisons with the digital models from PVS impression scanning. They were compared using a superimposition method by Geomagic Qualify software (3D Systems, Rock Hill, SC). The bases of the 2 types of digital models were cut apical to the gingival margin. The models were aligned using the dentition as a reference by the best-fit surface alignment tool. After alignment, the model edges were trimmed with cutting planes to create common borders. Color displacement maps were generated to confirm accurate crown superimpositions and measure differences between the models. Geomagic Qualify software outputs the mean and maximum displacements, the positive and negative average differences, and the standard deviations measured in the color map analysis. These data are obtained by calculation of the distances of points between each digital model superimposition. The limits used in the color map were 0.25 mm ( Fig 5 ). Figures 6 and 7 illustrate the design of the study.

Evaluation of the accuracy by superimposition of the digital models with the Geomagic Qualify software.

Schematic figure showing the procedures used in the study (* ¹ T5, 5 days after impression taking; * ² T10, 10 days after impression taking; * ³ T15, 15 days after impression taking).

Schematic figure showing the comparisons made in the study (* ¹ T5, 5 days after impression taking; * ² T10, 10 days after impression taking; * ³ T15, 15 days after impression taking).

Results

The intraexaminer errors of each examiner had low values, and all examiners had lower error values for the plaster models than for the digital models by PVS impression scanning. Examiner 1 had a mean difference of all parameters of 0.08 mm for the digital models (maximum error in the sum of 12 mandibular teeth was 1.88 mm) and an average difference of −0.01 mm in all parameters of the plaster models (maximum error in the diameter of the right maxillary second premolar of −1.06 mm). Examiner 2 had a mean difference in the error values of 0.05 mm in all parameters of the digital models (maximum error in the sum of 12 mandibular teeth was 0.99 mm), and the average differences in all parameters in the plaster models were −0.01 mm (maximum error in the maxillary intercanine distance of −1.24 mm). Examiner 3 had similar results to those of examiner 2: 0.05 mm in the average differences in all parameters of the digital models (maximum error of 1.06 mm in the sum of 12 mandibular teeth) and −0.01 mm in the plaster model’s mean differences of all parameters (maximum error of 0.21 mm in the sum of 12 mandibular teeth).

The reproducibility analysis showed high ICC values for both plaster model measurements (r = 0.908) and those on digital models (r = 0.857). Transverse measurements had high ICC values for plaster models (r = 0.966) and digital models (r = 0.976). Overbite and overjet also showed high ICC values for plaster models (0.965 and 0.930) and digital models (0.970 and 0.943). In relation to tooth diameter, plaster models had higher ICC values (maxillary teeth, 0.891; mandibular teeth, 0.881) than the digital models (0.827 and 0.800) ( Table II ).

The paired t test was used to compare the differences in measurements by each examiner for the plaster and digital models by PVS impression scanning. Statistically significant differences were found for some measurements. Examiners 1 and 3 had similar results, but examiner 2 had more clinically significant differences. The maxillary and mandibular tooth diameters showed that examiners 1 and 3 had similar measurements, whereas examiner 2 registered lower values for the measurements on the digital models. Regarding overjet, the 3 examiners registered similar measurements with small differences, whereas measurements of overbite had lower values for digital models for all examiners. Measurements on digital models showed lower values for all examiners, with average differences between all parameters of 0.022 mm for examiner 1, 0.537 mm for examiner 2, and 0.166 mm for examiner 3 ( Table III ). The difference in measurements for overbite was clinically significant for almost all of the digital models, with the exception of examiner 1 at T5.

Table III

Paired t tests mean differences among examiners of plaster models vs digital models

Parameter	Examiner 1			Examiner 2			Examiner 3
	Mean (mm)	SD (mm)	P Value	Mean (mm)	SD (mm)	P Value	Mean (mm)	SD (mm)	P Value
Sum maxillary 6	0.083	0.819	0.580	2.317	1.000	0.000	1.171	0.685	0.000
Sum maxillary 12	0.494	1.208	0.033	3.386	1.762	0.000	1.370	1.141	0.000
Sum mandibular 6	−0.159	0.760	0.261	1.448	1.124	0.000	0.361	0.858	0.029
Sum mandibular 12	−0.382	1.214	0.095	2.151	1.833	0.000	0.270	0.963	0.135
Maxillary ICD	0.291	0.497	0.003	0.500	0.593	0.000	0.447	0.484	0.000
Maxillary IMD	−0.320	0.413	0.000	0.067	0.807	0.653	−0.272	0.572	0.014
Mandibular ICD	0.218	0.363	0.003	0.709	0.494	0.000	0.092	0.436	0.254
Mandibular IMD	−0.289	0.723	0.037	0.831	0.699	0.000	0.296	0.633	0.016
Overjet	0.104	0.415	0.178	0.479	0.581	0.000	−0.248	0.455	0.006
Overbite	0.625	0.583	0.000	0.852	0.505	0.000	0.536	0.541	0.000

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