Accuracy of a Digital Impression System Based on ...

Accuracy of a Digital Impression System Based on Parallel Confocal Laser Technology for Implants with Consideration of Operator Experience and Implant Angulation and Depth Beatriz Giménez, DDS1/Mutlu Özcan, DDS, Dr Med Dent, PhD2/ Francisco Martínez-Rus, DDS, PhD3/Guillermo Pradíes, DDS, PhD4 Purpose: To evaluate the accuracy of a digital impression system based on parallel confocal red laser technology, taking into consideration clinical parameters such as operator experience and angulation and depth of implants. Materials and Methods: A maxillary master model with six implants (located bilaterally in the second molar, second premolar, and lateral incisor positions) was fitted with six polyether ether ketone scan bodies. One second premolar implant was placed with 30 degrees of mesial angulation; the opposite implant was positioned with 30 degrees of distal angulation. The lateral incisor implants were placed 2 or 4 mm subgingivally. Two experienced and two inexperienced operators performed intraoral scanning. Five different interimplant distances were then measured. The files obtained from the scans were imported with reverseengineering software. Measurements were then made with a coordinate measurement machine, with values from the master model used as reference values. The deviations from the actual values were then calculated. The differences between experienced and inexperienced operators and the effects of different implant angulations and depths were compared statistically. Results: Overall, operator 3 obtained significantly less accurate results. The angulated implants did not significantly influence accuracy compared to the parallel implants. Differences were found in the amount of error in the different quadrants. The second scanned quadrant had significantly worse results than the first scanned quadrant. Impressions of the implants placed at the tissue level were less accurate than implants placed 2 and 4 mm subgingivally. Conclusions: The operator affected the accuracy of measurements, but the performance of the operator was not necessarily dependent on experience. Angulated implants did not decrease the accuracy of the digital impression system tested. The scanned distance affected the predictability of the accuracy of the scanner, and the error increased with the increased length of the scanned section. Int J Oral Maxillofac Implants 2014;29:853–862. doi: 10.11607/jomi.3343 Key words: accuracy, dental implant, digital impression, implant angulation, implant depth, intraoral scanner

I

mpression making is a critical step in the accurate recording of the three-dimensional intraoral

1Research

Fellow, Department of Buccofacial Prostheses, Faculty of Odontology, University Complutense of Madrid, Madrid, Spain. 2Professor and Head of Dental Materials Unit, University of Zürich, Center for Dental and Oral Medicine, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Zurich, Switzerland. 3Associate Professor, Department of Buccofacial Prostheses, Faculty of Odontology, University Complutense of Madrid, Madrid, Spain. 4Professor and Associate Dean, Department of Buccofacial Prostheses, Faculty of Odontology, University Complutense of Madrid, Madrid, Spain. Correspondence to: Dr Beatriz Giménez González, Departamento de Estomatología I, Prótesis Bucofacial, Plaza Ramón y Cajal s/n, Ciudad Universitaria, 28040 Madrid, Spain. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.

relationships between implants, teeth, and adjacent structures.1,2 Inaccuracies that occur during impression making may certainly lead to laboratory errors, resulting in misfit, particularly in fixed implant prostheses.1–3 Obtaining absolute passive fit of the prosthetic framework on implants has been reported to be nearly impossible.4 A fit is “passive” when the restoration does not create static loads within the prosthetic system or in the surrounding bone tissue.1,5 However, because multiple steps are involved in processing and manufacturing implant-supported prostheses, a certain amount of misfit seems to be unavoidable.5,6 It is commonly accepted that prosthesis misfit tends to increase the incidence of mechanical complications.1,2 In addition, marginal discrepancies caused by misfit might increase plaque accumulation, affecting the soft and/or hard tissues around the implants.7,8 Although the relationship between misfit and biologic complications remains controversial, it is generally accepted that an optimal fit of the prosthesis is beneficial for The International Journal of Oral & Maxillofacial Implants 853

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the long-term success of the restorations.2,9 Thus, the importance of fabricating a restoration with high accuracy is vital. Multiple dental implants are generally connected by splinting to withstand lateral forces and torque.10,11 Splinting the implants may improve the distribution of the masticatory loads, reduce mechanical complications,12 decrease stress in the peri-implant tissues,13,14 and result in a need for fewer implants, eventually decreasing the total financial cost of implant therapy.15 Despite these benefits, splinting the implants involves several challenges, since excellent accuracy is a prerequisite to achieve proper fit of the subsequent prosthesis.16 Several studies of the implant impression process have reported that working casts fail to replicate the original location of the implants exactly.17–19 Since the early 1990s, in vitro studies on different impression techniques (eg, indirect technique with closed tray, direct technique with open tray, direct technique splinted with acrylic resin) have presented remarkably nonhomogenous results.20 The accuracy of the impression depends on many factors, including the impression technique, type of impression material, implant angulation(s), and the type of implant connection.21,22 Nevertheless, even if the impressions are free of defects, accuracy could still be lost during casting or following displacement of the implant components. The latter could involve displacement of each impression coping on the fitting surface of each implant across the machining tolerance range, displacement of each impression coping depending on the impression technique and the material used, displacement of implant analogs on the fitting surface of each impression coping owing to the machining tolerance range, and displacement of each analog in the definitive cast as a result of dimensional changes in the dental stone.22 Moreover, lack of parallelism of the implants and of uniform depths of placement may also influence the accuracy of impressions.2,21–24 The angulation of the implants may increase the likelihood of dislodgement of the impression material and its subsequent distortion during removal of the impression tray from the mouth.23 Consequently, for deeply placed implants, a smaller portion of the coping can be impressed, which may diminish the stability of the impression coping in the impression material, thereby affecting the accuracy.23,24 Digital impression procedures may be an approach to improve the accuracy of implant-supported restorations, as by their nature, these prostheses eliminate the error-prone conventional impression and gypsum model casting and warrant a high degree of standardization.25 Beginning in the early 1980s, digital impressions and fabrication of ceramic restorations using computer-aided design/computer-assisted manufacture (CAD/CAM) technology were offered as an

alternative to conventional restorative procedures.26 Since then, various optical impression systems have been developed with which direct impressions could be made in the oral cavity. The most commonly used systems are Cerec AC (Sirona), Lava Chairside Oral Scanner (Lava COS, 3M ESPE), E4D Dentist (D4D Technologies), and iTero (Cadent). Digital impressions play an important role in the development of digital dental technology because they are the first step toward a fully digital prosthetic fabrication protocol.27 Digital impressions improve patient acceptance, reduce possible distortion of impression materials, allow for visualization of the preparation three-dimensionally prior to fabrication, decrease potential cost, and increase efficacy.28 On the other hand, optical impression methods require better soft tissue management and a dry working field, and they certainly involve a learning curve. In the implant field, digital impressions would allow virtual assessments of the implant prosthetic space, depth of the restoration interface, and the emergence profile configuration before proceeding with the laboratory steps.29 In a study performed in second-year dental students with no experience in impression making, digital impression making was reported to be a more efficient method than conventional impressions for single implants.29 The number of studies on the accuracy of digital impression methods is increasing, but implant-related factors have not yet been evaluated systematically.25,30–40 Thus, there is little knowledge about the accuracy and repeatability of intraoral digital impression systems for dental implants, including implant-related factors and other clinical aspects, such as the experience of the operator. The objectives of this study, therefore, were to evaluate the accuracy and repeatability of a digital impression system based on parallel confocal laser technology using a six-implant model with implants placed at different angulations and depths. The following null hypotheses were tested: (1) operator factor, (2) angulation of implants, and (3) depth of implants would not affect the accuracy of the digital impressions.

MATERIALS AND METHODS Master Model

Six implants (Certain, 4.1 × 11 mm, Biomet 3i) were placed in the areas of the lateral incisors (12 and 22), second premolars (15 and 25), and second molars (17 and 27) in an edentulous resin model of the maxilla (Frasaco) (Fig 1). The Certain implant was chosen because of its favorable design, which features an internal connection with two flat surfaces that allows measurements to be made in the best possible way using a coordinate measuring machine (CMM). An

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a Fig 1 Resin master model with six implants in the areas of the second molars, second premolars, and lateral incisors; the second premolar implants were placed at an angle. The depth of implant no. 22 (left lateral incisor) was 2 mm, and implant no. 12 (right lateral incisor) was placed 4 mm subgingivally. A removable soft tissue model allowed proper measurements with a CMM and allowed the implants to be placed at different depths relative to the gingiva.

b Figs 2a and 2b (a) The PEEK scan body (height: 8 mm) that was used to make high-accuracy measurements; (b) view of the complete model with the removable artificial gingiva and the six scan bodies.

external-connection or conical-connection implant would result in less accurate “true” (reference) values because of the difficulty in measuring their geometries. The implants were placed in the model with the following angulations and depths: (1) implants 17 and 27 (first molars), 0 degrees of angulation, 0 mm depth (gingival margin level); (2) implant 15 (right second premolar), 30 degrees of distal angulation, 0 mm depth; (3) implant 25 (left second premolar), 30 degrees of mesial angulation, 0 mm depth; (4) implant 12 (left lateral incisor), 0 degrees of angulation, 4 mm depth; and (5) implant 22 (right lateral incisor), 0 degrees of angulation, 2 mm depth. Placements were accomplished with the help of a micro-milling machine (Cendres & Metaux). The soft tissue was simulated with silicone (Vestogum, 3M ESPE) to enable accurate measurements of the heads of the implants with the CMM. Six high-precision scan bodies were manufactured from polyether ether ketone (PEEK) (Createch Medical). The height of the scan bodies was 8 mm (Figs 2a and 2b).

Coordinate Measuring Machine

An independent laboratory that specialized in extremely accurate design and fabrication of CAD/CAM structures (Createch Medical) made the measurements of the master model and assessed the accuracy of the intraoral scanner (Lava Chairside Oral Scanner-COS). An industrial three-dimensional CMM (Crista Apex, Mitutoyo) was used to measure the master model to obtain the actual data regarding the three-dimensional

positions of the implants. The accuracy of the CMM was certified by the National Entity of Accreditation with a maximum permissible error for length measurement of 1.9 ± 3 µm/1,000 µm according to the appropriate standard published by the International Organization for Standardization (ISO 10360-2).41 A high-accuracy touch signal probe with a 1-mm ruby sphere was used to measure the points of the heads of the implants to locate them in the x-, y-, and z-axes of the space. The plane of the implant platform was measured to establish the orientation of the implant. The circumference of the implant was also measured to precisely locate its center. The coordinates x, y, and z gave the locations for each of these two figures. This procedure was performed three times. A mean of the three measurements performed with the CMM was used as the final location of each implant and served as a reference or “true” value.

Impression Procedures

The software (version 4.5.0.151) that accompanied the digital scanner iTero (Cadent) (Fig 3) was used to capture all data. The iTero system is based on parallel confocal laser technology. In this technology a red laser beam is projected onto an object through a beam splitter. The reflected beam is guided through a focal filter so that only the image that lies within the focal point of the lens is projected onto the sensor. Because the focal distance is known, the distance from the scanned part to the lens is known as well (focal distance). To scan the entire object, the lens is moved up The International Journal of Oral & Maxillofacial Implants 855


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17

27

25

15

22

12

Fig 3 The iTero System based on parallel confocal red laser technology.

Fig 4 Distances used to assess deviation errors (27–25, 27–22, 27–12, 27–15, and 27–17); measurements were made from the center points of each implant.

and down, each time projecting a part of the object onto the sensor. This technology requires no powder, and the camera can be seated on the teeth for better stabilization. Four operators participated in the study. Two of the operators (operators 1 and 2) had at least 2 years of experience with the digital impression systems (more than 100 scans each). The other two operators (operators 3 and 4) were dentists who had no experience with the digital scanner tested or any other intraoral scanner. The scanning technique was explained to the inexperienced operators. To calibrate operators 3 and 4 before the impressions for the study were made, each made three impressions of the implant study model, and one of the experienced operators supervised. Each operator made five full-arch impressions, and measurements were made between the left second molar and the left second premolar, left lateral, right lateral, right second premolar, and right second molar (Fig 4). The first step of the impression procedure was to complete a dental chart that included the positions of the scan bodies and the edentulous spaces. With this information, the system determined the images that would be required and prompted the user to capture each one of them in a specific order. The initial quadrant that is requested by the scanner in this particular model was the second quadrant. The starting point was therefore the left second molar implant (no. 27), followed by no. 25 and no. 22. The scanner asked the user to capture three different images of each scan body (occlusal, buccal, and lingual), and additional scans were made until the complete scan body was registered and no missing data were detected visually by the operator in the virtual model shown on the screen of the device. In posterior areas, the edentulous spaces were captured

until the second quadrant was completed. The same procedure was followed for the first quadrant, starting with the left second premolar implant (no. 17). After both quadrants were completed, overlapping of the first and second quadrants was carried out by the scanner in the anterior area of the maxilla (Figs 5a to 5d). Finally, the stereolithographic (STL) files obtained from the impressions were sent to the laboratory for assessment of the accuracy of the scanner.

Assessment of Accuracy

All the data from the CMM and the iTero system were imported with industrial reverse-engineering software (Rapidform) that could read the STL files. The distances and angles between the center points of the implants were used to evaluate the accuracy of the intraoral scanner. To locate the center point of each implant for the iTero system, the STL file and the original design of the scan bodies (the CAD used to manufacture the scan bodies) were imported into the reverse-engineering software. The cylinders of the STL data captured by the scanner were isolated and matched one by one with the original CAD designs of the scan bodies. The center line of the cylinder was determined, and consequently the center point of the implant was established. The linear distance from the center point of implant no. 27 was considered as the reference point for measurements, following the “zero method” described elsewhere, and the center points of the remaining implants were measured in the CMM data.42 Subsequently, the same procedure was performed for the data obtained from the iTero system. Next, each distance between implants obtained with the iTero system was compared with its corresponding distance captured on the CMM. The angulations were


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Figs 5a to 5d Digital impression sequence.

Fig 5a A dental chart was completed before each impression was made. The chart determines the images that the system will require for each specific case.

Fig 5b The system guides the operator through the scanning sequence, starting with the scan body in the left second molar position (no. 27).

measured between the fit plane of implant no. 27 and each fit plane of the other implants measured by the CMM. The same angulation measurement procedure was repeated with the geometries obtained from the iTero system. The measurements were not divided into x-, y-, and z-axis components because the iTero, the CMM, and the CAD cylinders use different coordinate systems. The data sets of the measurements obtained from digitization were composed of points that were located in a common coordinate system. Each point was defined in x, y, and z coordinates. Concurrently, the points described the surface of the digitized object partially. Each single data set produced by independent measurements received its own coordinate system.31 If the data had been broken down into points on the x, y, and z axes, errors would have been introduced and the data would not be useful for reliability analyses.33

Statistical Analysis

The data were analyzed using statistical software (Minitab Release 14, Minitab). The measured distances

Fig 5c Occlusal, buccal, and lingual scans are requested by the scanner. More scans were added until the entire scan body was registered. The red arrows indicate the number of images taken to register one scan body.

Fig 5d After both quadrants have been registered (scan bodies and edentulous spaces of each quadrant), the system automatically performs an anterior overlap of the quadrants.

(in microns) between the implants obtained with the iTero system were compared with the distances between the implants of the “true data” measured with the CMM. The homogeneity of the data for implant distances, operator, and experience were assessed with the Anderson-Darling and Levene tests (α = .05). The The International Journal of Oral & Maxillofacial Implants 857


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Table 1 Distance Deviations Between iTero Scanner Measurements and CMM Measurements (“True” Values)

Table 2 Differences in Deviations and 95% Bonferroni Confidence Intervals for Each Implant Distance

No. of Mean Implant distance measurements deviation (µm)

SD

Implant distance

No. of measurements

SD

Lower

Upper 42.644

20

–14.3

25.6

27–25

20

25.594

17.961

27–22

20

–16.2

34.6

27–22

20

34.569

24.259

57.598

27–12

20

–27.9

61.6

27–12

20

61.643

43.258

102.709

27–15

20

–23.1

148.0

27–15

20

147.957

103.829

246.523

27–17

20

–32.0

216.1

27–17

20

216.052

151.615

359.958

Overall deviation for implant distance (µm)

27–25

300 200 100 0 –100 –200 –300 –400 –500 –600 Implant 2 3 4 5 6 2 3 4 5 6 order 1 2 Experienced

2 3 4 5 6 3

2 3 4 5 6 4

Inexperienced

Fig 6 Amount of error and deviation of the iTero scanner depending on the experience of the operator and implant distance. Operators 1 and 2 were experienced, and operators 3 and 4 were inexperienced.

differences between experienced and inexperienced operators, implant angulation, and implant depth were compared using the two-sample t test and oneway analysis of variance (ANOVA).

RESULTS The data were normally distributed with respect to the independent variable of implant distance (P = .989). The amounts of error in the digital impression measurements, compared with the CMM measurements (“true” measurements) for the implant distances, were calculated with one-way ANOVA; the results are shown in Table 1. The Levene test for equal variances showed a significant difference for variances among implants (P = .000). Bonferroni confidence intervals (95%) for standard deviations showing the difference for the implant distances are presented in Table 2.

The influence of the operator in the amount of error observed for implant distance measurements was evaluated using one-way ANOVA and considering the variables “implant deviation” versus “operator.” A significant difference was found between the operators (P = .049). The deviation was significantly different for operator 3, one of the inexperienced operators, with 80 µm of difference and double that for the standard deviation (Fig 6). However, operator experience did not significantly affect the amount of error in the measurement of implant distances for the factor “deviation” (P = .073, two-sample t test). Implant angulation (P = .777) and implant depth (P = .953) did not significantly affect the deviations in the distance measurements. However, large deviations in error were found for implants placed at 0 mm of depth (Table 3). Contrary to expectations, the largest variations were observed for implants placed with 0 mm depth. The larger the visible distance of the scan body, the lower the error was in the best-fit process. Provided that the scanning process in the impression was executed in the posterior quadrants and overlapped in the anterior area, significant differences were not found for the mean values (P = .546), although the standard deviations were five times greater. While the first quadrant showed a mean error of –28 (± 153) µm, the second quadrant, being scanned first, presented a mean error of –15 (± 30) µm (P = .000, two-sample t test) (Fig 7). After all quadrants had been analyzed, it was noted that the error increased with the increase in “stitching” (best fit of the images). Because every picture was overlapped with the image anterior to it, the greater the distance from the first image captured by the intraoral scanner, the larger the error (Fig 8).

DISCUSSION To the best of the authors’ knowledge, this is the first study to analyze the influence of different clinical factors, such as operator experience, implant angulation, and implant depth, on the accuracy of an intraoral


Table 3 Errors in Implant Distance with Different Implant Depths Implant No. of Mean depth measurements (µm)

SD

Lower

Upper

0 mm

60

–23.1 149.485 122.318 190.924

2 mm

20

–16.2

34.569

24.829

55.344

4 mm

20

–27.9

61.643

44.276

98.690

Overall deviation from “true” measurements

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500

Quadrant 2

Quadrant 1 UCL = 431

250 0

X = –28

–250 LCL = –486

–500 1

10

20

30

40

50

60

70

80

90 100

Samples

Fig 7 Amount of error in implant distances obtained by the iTero scanner, shown by quadrant. Each black point on the graph represents the deviation created by the scanner for every implant distance (27–25, 27–22, 27–12, 27–15, 27–17). The deviation of the distance was calculated by subtracting the CMM distance from the STL distance. When the STL file values obtained with the iTero scanner were close to the “true” measurements (CMM), the results were considered close to 0 (green horizontal line). The closer to the green line, the smaller the error obtained by the scanner. The first scanned quadrant gave better results, whereas the second scanned quadrant showed larger errors and deviations. UCL = upper confidence limit; LCL = lower confidence limit; X = mean. Overall deviation in implant distance (μm)

scanner based on parallel confocal red laser technology. Based on the results of this study, the null hypotheses that operator experience, depth of implants, and implant angulation would not affect the accuracy of the digital impressions could be rejected. In this study, the experienced operators had more than 2 years of training in digital dentistry, and both of them had made more than 100 impressions with intraoral devices other than that tested in this study. In principle, experience is a difficult parameter to measure. The exact number of digital impressions that the experienced operators had made prior to this study could not be identified. However, both were known to have been using such techniques on a daily basis. It was interesting to note that there was no difference in the results with regard to experience, but the individual operators did vary. The inexperienced operators scored different results, in that one inexperienced operator (operator 3) obtained significantly less accurate results than the remaining operators, but operator 4, who was also inexperienced, did not. The iTero system guides the user by pointing to the image that is required next. When the image made is not well understood or is overlapped by the system, the operator is able to see this on the screen and repeat the scan. Although the parameter of time was not included in the scope of this study, the inexperienced operators spent more than twice as much time scanning one model compared to the experienced ones. This was simply a result of the number of repetitions of images that were required. It could be deduced that experience might not be a factor, but the expertise of each individual user is definitely crucial to the accuracy of the digital impressions. Furthermore, the iTero camera tip is one of the largest among the available digital impression systems in the dental market and is larger in width than in height. Because the system does not require the use of powder and allows the camera to be set upon the tooth surface for stability, the technique is easier to learn in fully dentate arches. When scanning intraorally, scanning vertical surfaces (mesial and distal sites of teeth or scan bodies) is more difficult. To scan these surfaces properly, the camera tip must be

a

adr

Qu

2 nt

)

-25

(27

a

adr

Qu

2 nt

)

-22

(27

)

-12

(27

a

adr

Qu

1 nt

1 nt

a

adr

Qu

)

-15

(27

500

)

-17

(27

a

adr

Qu

1 nt

UCL = 637

250 0

X = –32

–250 –500 LCL = –701

–750 1

10

20

30

40

50

60

70

80

90 100

Cumulative error (μm)

Fig 8 Amount of error observed for each implant distance (27–25, 27–22, 27–12, 27–15, 27–17). The error obtained by the scanner increased with the distance. The scanner functions by overlapping the images that it captures. With larger scanning areas, more “stitching together” of images is required. The error therefore increased cumulative with the stitching process. UCL = upper confidence limit; LCL = lower confidence limit; X = mean.

rotated, since the camera is wider in the vertical axis. This makes image capturing very complicated, especially in posterior areas because of the lack of space, and eventually the whole process becomes uncomfortable for the patient. The accuracy of digital impressions was not significantly affected by the angulation of the implants. The angle chosen in this study was 30 degrees, which would be considered very pronounced. Several previous studies tested the accuracy of silicone impressions, The International Journal of Oral & Maxillofacial Implants 859


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and the angulations of implants were reported to be between 5 and 20 degrees.23,24 In a conventional impression, the elastic recovery of the impression material may compensate for any undercuts. However, this may cause distortion in the impression as it yields to plastic deformation of the impression material. In this digital impression system, an angulation of 30 degrees did not affect the accuracy of the scanner significantly. The results with regard to implant depth were the opposite of what was expected. The largest variations were seen for implants placed at 0 mm depth, which is the maximum visible height of the scan body. In general, measurement experts accept that the larger the visible distance, the lower the error in the best-fit process. The scanning process was executed by quadrant, beginning in the posterior areas, and images were connected in the anterior area. The differences between quadrants indicated large differences in the standard deviations. For this reason, the authors looked for other possible factors that would yield this result. In fact, the intraoral scanners lack fixed references. Thus, what it uses as a reference is the first image made by the scanner. All subsequent images are “stitched” to the previous one by a best-fit algorithm that represents the best possible overlap of both images. Each overlap has an inherent error; as a consequence, the final error should increase with every stitching process. Hence, it can be anticipated that the longer the distance, and the more stitching processes completed, the larger the error. It could be interpreted from the results that restorations made from short captured sections have a higher probability of success, and greater length of the sections increases the probability of decreased fit of the restoration. This could explain why the indications of the intraoral scanners in general are limited to “short length” restorations. Similarly, the manufacturer of the scanner used in this study limits its indication to three- to four-unit fixed dental prostheses. Nonetheless, there is a need for studies with different specifications regarding the accuracy needed for each type of restoration. The results of the present study obtained using iTero vary from those seen in a previous study36 in which a mandibular stone model was measured; teeth and three scan bodies were alternated. The authors obtained a mean deviation of 70.5 ± 56.3 µm for the first distance (between scan bodies 1 and 2) and 61.1 ± 53.9 µm for the second distance (between scan bodies 1 and 3). The errors in their study were remarkably smaller than in the present study; this could be attributed to the model design and the presence of teeth versus the edentulous space scanned in this study, especially since the latter had fewer reference points for the scanner and the operator. Another possible reason could be the model material, namely, the dental stone;

because it is a matte substance, it is easier to capture images. The results are not comparable with other studies of digital impression accuracy33,35,39,40,42 because of an important difference in methodology. To compare the digital impressions in these studies, a general overlap of the data (best-fit algorithm) was used. The best fit overlaps two point clouds in the best possible way to create an average of the errors and does not display the actual divergences sufficiently, as explained previously.43 The present study used the “zero method,”41 considering the center point of implant no. 27 as the origin, and obtained the linear and angular deviations between implants without averaging the overlap of the best fit. A study of accuracy of conventional impressions that used a similar methodology21 described four types of displacement of the implant components that occur from impression making to the master cast with two different impression techniques (with splint, without splint). The fabrication tolerance of the impression copings, displacement of the impression coping within the impression material as a result of the contraction of the material, fabrication tolerance of the implant replica, and displacement of the implant replica within the dental stone following expansion of the stone were responsible for the errors. Such errors were noted to be 98.5 ± 29.9 µm for nonsplinted impression copings and 99.3 ± 28.28 µm for splinted copings. It could be stated that the present system is still not as accurate as conventional standards for implant-supported multiple-unit reconstructions. The present study was based on the principles of metrology. Implants were used because they have a known geometry that is measurable with a very high accuracy by a CMM. This produces results that are closest to the actual measurements. This study has provided the amount of error or deviation of the scanner in measuring different specific distances within a complete arch. However, it is difficult to extrapolate these results to all prosthetic indications, since there is a lack of specifications in dentistry that correlate with the specific accuracy needed for implant- or toothsupported restorations. It may be assumed that, in the presence of subgingival margins, the use of retraction cords on natural teeth may lead to problems in recognizing the reference point; this needs to be elaborated in future studies. The dental prosthesis market is changing remarkably toward CAD/CAM applications with the purpose of automating the entire prosthesis manufacturing process. Implementing technology to fully digitize the prosthesis workflow has numerous benefits.25,28,30,34,36–40 Nevertheless, with respect to recent developments in the field of intraoral scanners and the lack of sufficient clinical experience and independent


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studies, information is often delivered to dentists from the manufacturers. The information that companies are allowed to give to dentists is very limited because of confidentiality concerns. This situation makes the choice for the dentist very challenging. Therefore, it is necessary to carry out more studies with these digital impression systems to understand their accuracy and the factors affecting their performance during potential use. Additionally, the use of a methodology that allows for correct evaluation of current and future systems is crucial.

CONCLUSION Based on the results of this study, the operator influenced the accuracy of the digital impression system, but the performance of the operator was not necessarily dependent on experience. Angulated implants did not decrease the accuracy of the digital impression system tested. Impressions of implants placed at 0 mm depth were less accurate than implants placed at 2 or 4 mm subgingivally. This was not caused by the depth of the implant itself but the distance from the first implant captured and the number of overlaps made by the system. Overall, the accuracy of impressions decreased with the increased length of the scanned section.

ACKNOWLEDGMENTS The authors would like to acknowledge Createch Medical for fabricating the scan bodies, Mikel Gomez Picaza for helping with the measurements and counselling on high-accuracy methods, Alberto Alvarez for his knowledge of intraoral scanners, and Adrián Hernandez for providing technical expertise. The authors have no support or funding to report and declare that no competing interests exist.

REFERENCES 1. Wee AG, Aquilino SA, Schneider RL. Strategies to achieve fit in implant prosthodontics: A review of the literature. Int J Prosthodont 1993;2:75–82. 2. Lee H, Ercoli C, Funkenbusch PD, Feng C. Effect of subgingival depth of implant placement on the dimensional accuracy of the implant impression: An in vitro study. J Prosthet Dent 2008;99:107–113. 3. Jemt T. In vivo measurements of precision of fit involving implantsupported prostheses in the edentulous jaw. Int J Oral Maxillofac Implants 1996;11:151–158. 4. Wenneberg A, Albrektsson T. Current challenges in successful rehabilitation with oral implants. J Oral Rehabil 2010;38:286–294. 5. Heckmann SM, Karl M, Wichmann MG, Winter W, Graef F, Taylor TD. Cement fixation and screw retention: Parameters of passive fit. An in vitro study of the three-unit implant supported fixed partial dentures. Clin Oral Implants Res 2004;15:466–473.

6. Del Corso M, Abà G, Vazquez L, Dargaud J, Ehrenfest DMD. Optical three-dimensional scanning acquisition of the position of osseointegrated implants: An in vitro study to determine method accuracy and operational feasibility. Clin Implant Dent Relat Res 2009;11:214–221. 7. Aglietta M, Siciliano VI, Zwahlen M, et al. A systematic review of the survival and complication rates of implant supported fixed dental prostheses with cantilever extensions after an observation period of at least 5 years. Clin Oral Implants Res 2009;20:441–451. 8. Winter W, Mohrle S, Hdst S, Karl M. Bone loading caused by different types of misfits of implant-supported fixed dental prostheses: A three-dimensional finite element analysis based on experimental results. Clin Oral Implants Res 2010;25:947–952. 9. Jemt T, Linden B, Lekholm U. Failures and complications in 127 consecutively placed fixed partial prostheses supported by Brånemark implants: From prosthetic treatment to first annual checkup. Int J Oral Maxillofac Implants 1992;7:40–44. 10. Taylor TK, Agar JR, Vogiatzi T. Implant prosthodontics: Current perspective and future direction. Int J Oral Maxillofac Implants 2000;15:66–75. 11. Ono S, Yamaguchi S, Kusumoto N, Nakano T, Sohmura T, Yatani H. Optical impression method to measure three-dimensional position and orientation of dental implants using an optical tracker. Clin Oral Implants Res 2012;19:1–6. 12. Guichet DL, Yoshinobu D, Caputo AA. Effect of splinting and interproximal contact tightness on load transfer by implant restorations J Prosthet Dent 2012;87:528–535. 13. Winter W, Dickinson AJ, Wichmann MG. Different bone loading patterns due to fixation of three-unit and five-unit implant prostheses. Aust Dent 2007;52:47–54. 14. Bergkvist G, Sahlholm S, Nilner K, Lindh C. Implant-supported fixed prostheses in the edentulous maxilla. A 2-year clinical and radiological follow-up of treatment with non-submerged ITI implants. Clin Oral Implants Res 2012;15:351–359. 15. Akca K, Akkocaoglum M, Cömert A, Tekdemir I, Cehreli MC. Bone strains around immediately loaded implants supporting mandibular overdentures in human cadavers. Int J Oral Maxillofac Implants 2007;22:101–109. 16. Jivraj S, Chee W. Treatment planning of implants in posterior quadrants. Br Dent J 2006;201:13–23. 17. Abduo J, Bennani V, Waddell N, Lyons K, Swain M. Assessing the fit of implant fixed prostheses: A critical review. Int J Oral Maxillofac Implants 2010;25:506–515. 18. Eliasson A, Örtorp A. The accuracy of an implant impression technique using digitally coded healing abutments. Clin Implant Dent and Rel Res 2011;14:30–38. 19. Papaspyridakos P, Benic GI, Hogsett VL, White GS, Lal K, Gallucci GO. Accuracy of implant casts generated with splinted and nonsplinted impression techniques for edentulous patients: An optical scanning study. Clin Oral Implants Res 2011;23:676–681. 20. Lee H, So JS, Hochstedler JL, Ercoli C. The accuracy of implant impressions: A systematic review. J Prosthet Dent 2008;100:285–291. 21. Kim S, Nicholls JI, Han CH, Lee KW. Displacement of implant components from impressions to definitive casts. Int J Oral Maxillofac Implants 2006;21:747–755. 22. Choi JH, Lim YJ, Yim SH, Kim CW. Evaluation of the accuracy of implant-level impression techniques for internal-connection implant prostheses in parallel and divergent models. Int J Oral Maxillofac Implants 2007;22:761–768. 23. Sorrentino R, Gherlone EF, Calesini G, Zarone F. Effect of implant angulation, connection length, and impression material on the dimensional accuracy of implant impressions: An in vitro comparative study. Clin Implant Dent Relat Res 2010;12:63–76. 24. Jan HK, Kim S, Shim JS, Lee KW, Moon HK. Accuracy of impressions for internal-connection implant prosthesis with various divergent angles. Int J Oral Maxillofac Implants 2011;26:1011–1015. 25. Howell KJ, McGlumphy EA, Drago C, Knapik G. Comparison of the accuracy of Biomet 3i Encode Robocast Technology and conventional implant impression techniques. Int J Oral Maxillofac Implants 2013;28:228–240.

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Giménez et al

26. Jo SH, Kim S, Shim JS, Lee KW, Moon HK. Effect of impression coping and implant angulation on the accuracy of implant impressions: An in vitro study. J Adv Prosthodont 2010;2:128–133. 27. Mörmann WH. The evolution of the CEREC system. J Am Dent Assoc 2006;137:7–13. 28. Lee SJ, Gallucci GO. Digital vs. conventional implant impressions: Efficiency outcomes. Clin Oral Implants Res 2012;22:111–115. 29. Christensen GJ. Impressions are changing. J Am Dent Assoc 2009;140:1301–1304. 30. Luthardt RG, Loos R, Quaas S. Accuracy of intraoral data acquisition in comparison to the conventional impression. Int J Comput Dent 2005;26:283–294. 31. Henkel GL. A comparison of fixed prostheses generated from conventional vs digitally scanned dental impressions. Compend Contin Educ Dent 2007;28:422–431. 32. Garg AK. Cadent iTero´s digital system for dental impressions: The end of trays and putty? Dent Implant Update 2008;19:1–4. 33. Mehl A, Ender A, Mörmann W, Attin T. Accuracy testing of a new intraoral 3D camera. Int J Comput Dent 2009;12:11–28. 34. Syrek A, Reich G, Ranftl D, Klein C, Cerny B, Brodesser J. Clinical evaluation of all-ceramic crowns fabricated from intraoral digital impressions based on the principle of active wavefront sampling. J Dent 2010;38:553–559. 35. Ender A, Melh A. Full arch scans: Conventional versus digital impressions—An in vitro study. Int J Comput Dent 2011;14:11–21. 36. Van der Meer WJ, Andriessen FS, Wismeijer D, Ren Y. Application of intra-oral dental scanners in the digital workflow of implantology. PLoS ONE 2012;7(8):e43312. Epub 2012 Aug 22.

37. Brawek PK, Wolfart S, Endres L, Kirsten A, Reich S. The clinical accuracy of single crowns exclusively fabricated by digital workflow—The comparison of two systems. Clin Oral Investig 2013 Dec;17(9):2119–2125. 38. Kim SY, Kim MJ, Han JS, Yeo IS, Lim YJ, Kwon HB. Accuracy of dies captured by an intraoral digital impression system using parallel confocal imaging. Int J Prosthodont 2013;26:161–163. 39. Ender A, Mehl A. Influence of scanning strategies on the accuracy of digital intraoral scanning systems. Int J Comput Dent 2013;16:11–21. 40. Ender A, Mehl A. Accuracy of complete-arch dental impressions: A new method of measuring trueness and precision. J Prosthet Dent 2013;109:121–128. 41. International Organization for Standardization. 10360-2: Geometrical Product Specifications (GPS). Acceptance and Reverification tests for Coordinated Measuring Machines (CMMs). Part 2: CMMs Used for Measuring Linear Dimensions. Geneva: International Organization for Standardization, 2009. 42. Jemt T, Hjalmarsson L. In vitro measurements of precision of fit of implant-supported frameworks. A comparison between “virtual” and “physical” assessments of fit using two different techniques of measurements. Clin Implant Dent Relat Res 2011;14:175–182. 43. Güth JF, Keul C, Stimmelmayr M, Beuer F, Edelhoff D. Accuracy of digital models obtained by direct and indirect data capturing. Clin Oral Invest 2012 May;17(4):1201–1208.
