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Accuracy of 3-dimensional printed dental models reconstructed from digital intraoral impressions

      Highlights

      • Alginate and intraoral digital impressions of 30 patients were taken.
      • Linear tooth and arch measurements were evaluated for accuracy.
      • Digital light precessing (DLP) and polyjet printing produced 3D models that were clinically acceptable.
      • DLP and polyjet printers are viable options for clinical applications.
      • DLP and polyjet printers may serve as a replacement for stone models.

      Introduction

      A rapidly advancing digital technology in orthodontics is 3-dimensional (3D) modeling and printing, prompting a transition from a more traditional clinical workflow toward an almost exclusively digital format. There is limited literature on the accuracy of the 3D printed dental models. The aim of this study was to assess the accuracy of 2 types of 3D printing techniques.

      Methods

      Digital and alginate impressions of the oral environment were collected from 30 patients. Subsequently, digital impressions were used to print 3D models using digital light processing (DLP) and polyjet printing techniques, and alginate impressions were poured up in stone. Measurements for the 3 model types (digital, DLP, and polyjet) were compared with the stone models. Tooth measurements (first molar to first molar) included mesiodistal (crown width) and incisal/occlusal-gingival (crown height). Arch measurements included arch depth and intercanine and intermolar widths. Intraobserver reliability of the repeated measurement error was assessed using intraclass correlation coefficients.

      Results

      The intraclass correlation coefficients were high for all recorded measurements, indicating that all measurements on all model types were highly reproducible. There were high degrees of agreement between all sets of models and all measurements, with the exception of the crown height measurements between the stone and DLP models, where the mean difference was statistically significant.

      Conclusions

      Both the DLP and polyjet printers produced clinically acceptable models and should be considered viable options for clinical application.
      The evolution of digital technologies in orthodontics has influenced the traditional clinical workflow toward an almost exclusively digital format.
      • Akyalcin S.
      • Cozad B.E.
      • English J.D.
      • Colville C.D.
      • Laman S.
      Diagnostic accuracy of impression-free digital models.
      • Shastry S.
      • Park J.H.
      Evaluation of the use of digital study models in postgraduate orthodontic programs in the United States and Canada.
      One of the most rapidly advancing digital technologies is 3-dimensional (3D) modeling that uses scanned data or digital impressions to create digital models.
      • Lane C.
      • Harrell W.
      Completing the 3-dimensional picture.
      • Kravitz N.D.
      • Groth C.
      • Jones P.E.
      • Graham J.W.
      • Redmond W.R.
      Intraoral digital scanners.
      • Fleming P.S.
      • Marinho V.
      • Johal A.
      Orthodontic measurements on digital study models compared with plaster models: a systematic review.
      • Van Noort R.
      The future of dental devices is digital.
      These digital impressions can be acquired either indirectly from a patient's models or impressions using a desktop or intraoral scanner, or directly from the patient's oral cavity using an intraoral scanner.
      The authors of most previous scanner studies have evaluated the accuracy of digital models produced from indirectly acquired digital impressions.
      • Akyalcin S.
      • Cozad B.E.
      • English J.D.
      • Colville C.D.
      • Laman S.
      Diagnostic accuracy of impression-free digital models.
      • Fleming P.S.
      • Marinho V.
      • Johal A.
      Orthodontic measurements on digital study models compared with plaster models: a systematic review.
      • Sousa M.V.
      • Vasconcelos E.C.
      • Janson G.
      • Garib D.
      • Pinzan A.
      Accuracy and reproducibility of 3-dimensional digital model measurements.
      • Wiranto M.G.
      • Engelbrecht W.P.
      • Nolthenius H.E.
      • van der Meer W.J.
      • Ren Y.
      Validity, reliability, and reproducibility of linear measurements on digital models obtained from intraoral and cone-beam computed tomography scans of alginate impressions.
      • Jacob H.B.
      • Wyatt G.D.
      • Buschang P.H.
      Reliability and validity of intraoral and extraoral scanners.
      • Dalstra M.
      • Melsen B.
      From alginate impressions to digital virtual models: accuracy and reproducibility.
      However, the current digital workflow in orthodontics predominantly relies on digital impressions directly acquired from the oral environment with an intraoral scanner.
      • Camardella L.T.
      • de Vasconcellos Vilella O.
      • Breuning H.
      Accuracy of printed dental models made with 2 prototype technologies and different designs of model bases.
      • Flügge T.V.
      • Schlager S.
      • Nelson K.
      • Nahles S.
      • Metzger M.C.
      Precision of intraoral digital dental impressions with iTero and extraoral digitization with the iTero and a model scanner.
      This inconsistency between research and practice demonstrates a need to test the accuracy of the latter, as has been done for the former.
      The advantages of digital models include no physical storage requirement; instant accessibility; ability to do digital diagnostic or treatment simulations; positive patient perceptions; ability to immediately send to an outside laboratory; no risk of breakage, wear, degradation, or loss; and an overall improved continuity of care; in addition, digital models are less labor intensive.
      • Fleming P.S.
      • Marinho V.
      • Johal A.
      Orthodontic measurements on digital study models compared with plaster models: a systematic review.
      • Rheude B.
      • Sadowsky P.L.
      • Ferriera A.
      • Jacobson A.
      An evaluation of the use of digital study models in orthodontic diagnosis and treatment planning.
      • Hassan W.N.
      • Yusoff Y.
      • Mardi N.A.
      Comparison of reconstructed rapid prototyping models produced by 3-dimensional printing and conventional stone models with different degrees of crowding.
      • McGuinness N.J.
      • Stephens C.D.
      Storage of orthodontic study models in hospital units in the UK.
      Although the advantages are numerous, digital models come with at least 1 important disadvantage. Without a physical model, treatment planning for complex cases in a teaching environment can be challenging, and a physical model is still required for appliance fabrication.
      • Shastry S.
      • Park J.H.
      Evaluation of the use of digital study models in postgraduate orthodontic programs in the United States and Canada.
      • Hazeveld A.
      • Slater J.J.
      • Ren Y.
      Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques.
      Rapid prototyping provides a solution to this limitation by producing a physical replica of a digital model. In orthodontics, the most common rapid prototyping technique is 3D printing.
      • Camardella L.T.
      • de Vasconcellos Vilella O.
      • Breuning H.
      Accuracy of printed dental models made with 2 prototype technologies and different designs of model bases.
      • Hazeveld A.
      • Slater J.J.
      • Ren Y.
      Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques.
      Two of the most common 3D techniques in the marketplace today are digital light processing (DLP) printing and polyjet printing.
      • Keating A.P.
      • Knox J.
      • Bibb R.
      • Zhurov A.I.
      A comparison of plaster, digital and reconstructed study model accuracy.
      Both techniques use additive manufacturing to fabricate a 3D model, layer by layer, based on a digital model.
      • Liu Q.
      • Leu M.C.
      • Schmitt S.M.
      Rapid prototyping in dentistry: technology and application.
      • Chua C.K.
      • Leong K.F.
      • Lim C.S.
      Rapid prototyping: principles and applications 2nd edition (with companion CD-ROM).
      • Carmer S.
      The free beginner's guide.
      DLP printing uses a conventional light source such as an arc lamp, liquid crystal display panel, or projection source to cure the surface layer of a vat of photo-polymerizing resin in a specific orientation based on a digital model. Polyjet printing uses jet heads that spray or jet the resin in the desired areas. As the jet heads make subsequent passes, each sprayed layer is cured using an ultraviolet light source. In both printing techniques, the initial layer of resin is cured onto a build platform or a build plate, with each subsequent layer cured directly to the previous layer of cured resin in the z-axis to create a 3D object. The build plate lowers at a predetermined increment, which determines the thickness of each layer of resin that is cured.
      • Carmer S.
      The free beginner's guide.
      • Zyzalo J.R.
      Masked projection stereolithography: improvement of the Limaye model for curing single layer medium sized part.
      The process of producing a 3D printed model from a digital impression is summarized in Figure 1.
      Figure thumbnail gr1
      Fig 1The process of producing a 3D printed model.
      There is limited literature on the accuracy of 3D printed dental models. Most previous studies have concluded that 3D printed models are reasonably accurate and may be suitable for clinical use.
      • Hazeveld A.
      • Slater J.J.
      • Ren Y.
      Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques.
      • Cuperus A.M.
      • Harms M.C.
      • Rangel F.A.
      • Bronkhorst E.M.
      • Schols J.G.
      • Breuning K.H.
      Dental models made with an intraoral scanner: a validation study.
      • Saleh W.K.
      • Ariffin E.
      • Sherriff M.
      • Bister D.
      Accuracy and reproducibility of linear measurements of resin, plaster, digital and printed study-models.
      However, these studies have had limited sample sizes, did not measure arch dimensions, and used digital impressions that were not recorded directly from the patient. The authors of a recent study investigated the accuracy of 3D printed models produced from digital models acquired from the oral environment and measured multiple arch dimensions.
      • Camardella L.T.
      • de Vasconcellos Vilella O.
      • Breuning H.
      Accuracy of printed dental models made with 2 prototype technologies and different designs of model bases.
      However, they used a limited sample size and did not compare the printed models with stone models, which have been established as the gold standard in previous studies.
      • Hazeveld A.
      • Slater J.J.
      • Ren Y.
      Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques.
      • Keating A.P.
      • Knox J.
      • Bibb R.
      • Zhurov A.I.
      A comparison of plaster, digital and reconstructed study model accuracy.
      • Cuperus A.M.
      • Harms M.C.
      • Rangel F.A.
      • Bronkhorst E.M.
      • Schols J.G.
      • Breuning K.H.
      Dental models made with an intraoral scanner: a validation study.
      • Saleh W.K.
      • Ariffin E.
      • Sherriff M.
      • Bister D.
      Accuracy and reproducibility of linear measurements of resin, plaster, digital and printed study-models.
      The accuracy of a 3D printer must be confirmed through research for it to be mainstreamed into the orthodontic specialty. Therefore, this study was designed to assess the accuracy of 3D digital models and printed models in the context of a commonly used digital workflow in orthodontic practice.

      Material and methods

      Ethical approval for this study was obtained from the Institutional Review Board for the Protection of Human Subjects at the University of Oklahoma (Institutional Review Board number 7132; reference number 655134) in Oklahoma City.
      A power analysis was done based on a similar precursor study.
      • Hazeveld A.
      • Slater J.J.
      • Ren Y.
      Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques.
      A total of 22 patients were required for this study to oftain an 80% probability of generating 95% confidence intervals with margins of error less than 0.1 mm for tooth measurements and 0.25 mm for arch dimension measurements.
      Thirty patients were randomly selected from a pool of retention patients who received treatment at the University of Oklahoma Graduate Orthodontic Clinic. A patient was considered in retention if the appliances had been removed 6 to 18 months before records collection for this study. The inclusion criteria were (1) orthodontic treatment at the Graduate Orthodontic Clinic, (2) complete comprehensive treatment, and (3) all permanent teeth, first molar to first molar, in both dental arches.
      Both the digital and alginate impressions were taken at the 1-year retention appointment for all patients. Digital impressions were recorded using a chairside intraoral scanner (Omnicam; Dentsply Sirona, York, Pa). Jeltrate fast-set alginate (Sirona Dentsply) was used to make impressions with stock impression trays. The alginate was mixed using Cadco Alginator II electric mixer (Patterson Dental, St Paul, Minn). Impressions were poured with white ISO type 3 orthodontic model stone (Whip Mix, Louisville, Ky) and mixed using a Vac-U-Mixer (Whip Mix). All aspects of record collection followed the manufacturers' recommendations. Alginate impressions were poured with dental stone within 24 hours.
      All digital models generated with the intraoral scanner were converted to .STL file format using additional 3D imaging software (Dolphin Imaging and Management Solutions, Chatsworth, Calif). The .STL files were cleaned and prepared for 3D printing using Netfabb additive manufacturing and design software (Autodesk, San Rafael, Calif). The prepared .STL file for each patient was printed using two 3D printing techniques: DLP (Juell 3D Flash OC; Park Dental Research, New York, NY) and polyjet (Objet Eden 260VS; Statasys, Eden Prairie, Minn). Models were printed successively from their respective printers. The workflow for collecting patient records and producing study models is summarized in Figure 2. All 3D printed models were printed as solid models with a horseshoe-shaped base and in the horizontal position. Figure 3 illustrates both 3D printed model types.
      Figure thumbnail gr3
      Fig 3Three-dimensional printed model types: A, polyjet; B, DLP.
      A total of 240 maxillary and mandibular arches were measured with 60 arches from each of the 4 model types (stone, digital, DLP, and polyjet). All measurements of the physical models were obtained using a calibrated digital caliper (Orthopli, Philadelphia, Pa) and measured to the nearest 0.01 mm directly from the models, with the exception of arch depth, which was measured using the digital caliper from a 1:1 scale photocopy of the occlusal view. Measurements of the digital models were all obtained using Dolphin Imaging software. For all teeth, first molar to first molar, in both arches, the tooth measurements included (1) the mesiodistal widths from point contact to point contact (crown width) and (2) the incisal/occlusal-gingival heights from gingival zenith to cusp tip or incisal edge (crown height). For both arches, arch dimension measurements included (1) intercanine width from cusp tip to cusp tip, (2) intermolar width from mesiolingual cusp tip to mesiolingual cusp tip, and (3) arch depth from the midline of the central incisors to a perpendicular line crossing the mesial contact of the first molars. Crown height measurements for the digital models were not possible because of the inability of the software to replicate the same measurement technique used on the physical models. All other digital measurements were collected using the same measurement technique as on the physical models. Three calibrated researchers measured and recorded the data, with 1 researcher measuring either crown width, crown height, or arch dimensions for all models.

      Statistical analysis

      Bland-Altman plots
      • Bland J.M.
      • Altman D.G.
      Measuring agreement in method comparison studies.
      were used to evaluate the agreement of the crown width, crown height, intercanine, intermolar, and arch depth measurements between the stone, digital, DLP, and polyjet models. Table I lists the comparisons and what each evaluated. Ten percent of each model type was measured a second time with a 2-week interval between measurements. Intraclass correlation coefficients (ICC) were used to assess the intraobserver reliability of the repeated measurement errors.
      Table IComparisons of models to evaluate accuracy
      ComparisonEvaluation
      Stone and digitalClinical accuracy of digital impression
      Digital and DLPAccuracy of the DLP printing process
      Digital and polyjetAccuracy of the polyjet printing process
      Stone and DLPClinical accuracy of DLP printer
      Stone and polyjetClinical accuracy of polyjet printer

      Results

      A total of 5760 tooth measurements were recorded from 240 maxillary and mandibular arches. An additional 720 arch dimension measurements were recorded from the same sets of arches, for a total of 6480 measurements. The ICC values were high for all recorded measurements based on repeated measurement error, indicating that all measurements on all model types were highly reproducible (Table II).
      Table IIRepeated measurement mean errors (mm)
      ModelCrown widthCrown heightIntercanine widthIntermolar widthArch depth
      Stone0.030.040.070.080.06
      Digital0.04NA
      NA indicates that repeated measurement mean errors were not calculated due to the inability to replicate the same measuring technique digitally as was done physically with a digital caliper.
      0.070.080.04
      DLP0.030.050.070.070.06
      Polyjet0.020.070.080.080.06
      NA indicates that repeated measurement mean errors were not calculated due to the inability to replicate the same measuring technique digitally as was done physically with a digital caliper.
      The Bland-Altman plots
      • Bland J.M.
      • Altman D.G.
      Measuring agreement in method comparison studies.
      demonstrated high agreement between (1) stone and digital models for crown width, intercanine, intermolar, and arch depth measurements; (2) stone and polyjet models for all measurements; and (3) digital and polyjet models for crown width, intercanine, intermolar, and arch depth measurements (Table III). The limits of agreement are given in Table III. In addition, the Bland-Altman plots showed high agreement between stone and DLP models and between digital and DLP models for crown width, intercanine, intermolar, and arch depth measurements. However, the mean difference was statistically significant for crown height measurements between stone and DLP models. The mean crown height measurements of the DLP models were significantly lower than the mean crown height measurements for stone models by 0.29 mm, suggesting that the DLP printed models underestimated the true crown height dimensions.
      Table IIIAgreement between the different model measurements (mm)
      MeasurementMean difference (mm)Limits of agreement (mm)
      Stone (1) vs digital (2)
       Crown width0.06–0.27 to 0.38
       Crown heightNA
      NA indicates that agreement between model measurement assessments were not possible due to the inability to replicate the same measuring technique digitally as was done physically with a digital caliper.
      NA
      NA indicates that agreement between model measurement assessments were not possible due to the inability to replicate the same measuring technique digitally as was done physically with a digital caliper.
       Intercanine0.03–0.41 to 0.47
       Intermolar0.09–0.54 to 0.71
       Arch depth0.11–0.41 to 0.63
      Digital (1) vs DLP (2)
       Crown width0.01–0.30 to 0.32
       Crown heightNA
      NA indicates that agreement between model measurement assessments were not possible due to the inability to replicate the same measuring technique digitally as was done physically with a digital caliper.
      NA
      NA indicates that agreement between model measurement assessments were not possible due to the inability to replicate the same measuring technique digitally as was done physically with a digital caliper.
       Intercanine–0.09–0.64 to 0.46
       Intermolar–0.15–0.72 to 0.41
       Arch depth0.02–0.48 to 0.52
      Digital (1) vs polyjet (2)
       Crown width–0.01–0.36 to 0.35
       Crown heightNA
      NA indicates that agreement between model measurement assessments were not possible due to the inability to replicate the same measuring technique digitally as was done physically with a digital caliper.
      NA
      NA indicates that agreement between model measurement assessments were not possible due to the inability to replicate the same measuring technique digitally as was done physically with a digital caliper.
       Intercanine–0.06–0.55 to 0.42
       Intermolar–0.10–0.63 to 0.44
       Arch depth0.01–0.49 to 0.50
      Stone (1) vs DLP (2)
       Crown width0.06–0.20 to 0.43
       Crown height–0.29–1.04 to 0.46
       Intercanine–0.06–0.61 to 0.49
       Intermolar–0.07–0.61 to 0.47
       Arch depth0.13–0.37 to 0.63
      Stone (1) vs polyjet (2)
       Crown width0.05–0.33 to 0.43
       Crown height0.08–0.57 to 0.73
       Intercanine–0.04–0.48 to 0.41
       Intermolar–0.01–0.51 to 0.49
       Arch depth0.11–0.32 to 0.55
      The mean difference equals the (2) measurements minus the (1) measurements where (1) represents the gold standard. Thus, a positive value represents on average that the (2) measurements are overestimated compared with the (1) measurements. Conversely, a negative mean difference represents on average that the (2) measurements are underestimated compared with the (1) measurements.
      NA indicates that agreement between model measurement assessments were not possible due to the inability to replicate the same measuring technique digitally as was done physically with a digital caliper.

      Discussion

      The accuracy of two 3D printing techniques commonly used in orthodontics was assessed in this study. A unique aspect of this research was that the 3D printed models were produced from digital impressions acquired directly from the oral environment and then compared with stone models. The importance of this comparison was the ability to evaluate the entire digital workflow from directly acquiring a digital impression from the oral environment to producing a 3D printed model of it. To our knowledge, no previous studies have made such a comparison that would evaluate the accuracy of this technology the way it is currently being clinically implemented.
      Only 1 study has evaluated the application of using digital impressions to produce 3D printed models that were directly acquired from the patient.
      • Camardella L.T.
      • de Vasconcellos Vilella O.
      • Breuning H.
      Accuracy of printed dental models made with 2 prototype technologies and different designs of model bases.
      However, the method did not include stone models. In our opinion, including stone and digital model comparisons provides a more complete picture when testing for accuracy. Taking digital impressions from the oral environment without stone models, while lacking data on the accuracy of the scanner under clinical conditions, would make it difficult to evaluate whether any errors in the accuracy of the 3D printing technique were due to variables not otherwise controlled, such as saliva, tongue, lips, cheeks, or movement. The authors of the only study that has assessed the accuracy of an intraoral scanner under clinical conditions suggested that oral conditions do affect the accuracy.
      • Flügge T.V.
      • Schlager S.
      • Nelson K.
      • Nahles S.
      • Metzger M.C.
      Precision of intraoral digital dental impressions with iTero and extraoral digitization with the iTero and a model scanner.
      In addition to comparing 3D printed models produced from two 3D printed techniques with stone models, we evaluated the accuracy between the digital models and the 3D printed models. This comparison solely assessed how accurate the 3D printing techniques were by eliminating variables introduced during acquisition of the digital impressions. All previous studies investigating the accuracy of 3D printed dental models have expressed the need for studies with larger sample sizes. We used a sample 3 times larger than the next largest study. With the exception of 2 previous studies, all failed to assess the accuracy of arch dimension measurements and focused only on measurements specifically related to the teeth.
      • Camardella L.T.
      • de Vasconcellos Vilella O.
      • Breuning H.
      Accuracy of printed dental models made with 2 prototype technologies and different designs of model bases.
      • Hassan W.N.
      • Yusoff Y.
      • Mardi N.A.
      Comparison of reconstructed rapid prototyping models produced by 3-dimensional printing and conventional stone models with different degrees of crowding.
      Arch dimension measurements may not be critical if the sole purpose of a model is to be used for Bolton analysis, as previous studies have done. However, to maximize the use of a 3D printed model, arch dimension measurements must also be evaluated because these measurements are critical if a 3D printed model will be used to produce an appliance or aligner, as many orthodontists are already doing in practice.
      Both 3D printing techniques used in this study, DLP and polyjet, have been documented to provide excellent accuracy and surface finish.
      • Chua C.K.
      • Leong K.F.
      • Lim C.S.
      Rapid prototyping: principles and applications 2nd edition (with companion CD-ROM).
      • Carmer S.
      The free beginner's guide.
      • Zyzalo J.R.
      Masked projection stereolithography: improvement of the Limaye model for curing single layer medium sized part.
      A number of previous studies have shown that the polyjet printing technique has produced more accurate models compared with other printing methods.
      • Camardella L.T.
      • de Vasconcellos Vilella O.
      • Breuning H.
      Accuracy of printed dental models made with 2 prototype technologies and different designs of model bases.
      • Hazeveld A.
      • Slater J.J.
      • Ren Y.
      Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques.
      In this study, the DLP and polyjet printed models showed a similar mean difference and had similar limits of agreement when compared with a stone model for all recorded measurements, with the exception of the crown height measurement (z-axis). The agreement between polyjet and stone models was high for this measurement, whereas the agreement between DLP and stone models was low, with a mean difference of 0.29 mm and limits of agreement from –1.04 to 0.46 mm.
      The DLP printer used here prints to a minimum thickness of 50 μm. The polyjet printer used here prints to a minimum thickness of 16 μm. Both printers were set to their most accurate settings. Accuracy and surface finish of a 3D printer are limited by the thickness of the layers that are successively added in the z-axis, with thicker layers leading to greater inaccuracy.
      • Keating A.P.
      • Knox J.
      • Bibb R.
      • Zhurov A.I.
      A comparison of plaster, digital and reconstructed study model accuracy.
      • Chua C.K.
      • Leong K.F.
      • Lim C.S.
      Rapid prototyping: principles and applications 2nd edition (with companion CD-ROM).
      • Zyzalo J.R.
      Masked projection stereolithography: improvement of the Limaye model for curing single layer medium sized part.
      The variation in thickness of the layers that build the model in the z-axis may explain the variations found in the crown height measurements in this study. It would be expected that the polyjet printer, which prints thinner layers in the z-axis, ought to produce more accurate models. The variation in the accuracy of crown height measurements between the DLP and polyjet printed models when compared with stone models could also partially be explained by the fact that the polyjet printing technique does not require postcuring once the model is fully printed. This may lead to greater stability during printing.
      • Carmer S.
      The free beginner's guide.
      DLP printed models require postcuring, which can lead to additional shrinking of the resin. The shrinkage related to postcuring has the greatest impact on the z-axis.
      • Keating A.P.
      • Knox J.
      • Bibb R.
      • Zhurov A.I.
      A comparison of plaster, digital and reconstructed study model accuracy.
      Few studies have defined or established a range of error that is considered to be clinically acceptable. Of those, 0.20 to 0.50 mm has been considered an acceptable range for clinical accuracy.
      • Hassan W.N.
      • Yusoff Y.
      • Mardi N.A.
      Comparison of reconstructed rapid prototyping models produced by 3-dimensional printing and conventional stone models with different degrees of crowding.
      • Hazeveld A.
      • Slater J.J.
      • Ren Y.
      Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques.
      • Schirmer U.R.
      • Wiltshire W.A.
      Manual and computer-aided space analysis: a comparative study.
      • Hirogaki Y.
      • Sohmura T.
      • Satoh H.
      • Takahashi J.
      • Takada K.
      Complete 3-D reconstruction of dental cast shape using perceptual grouping.
      • Halazonetis D.J.
      Acquisition of 3-dimensional shapes from images.
      • Bell A.
      • Ayoub A.F.
      • Siebert P.
      Assessment of the accuracy of a three-dimensional imaging system for archiving dental study models.
      The mean difference of 0.29 mm between the DLP and stone models, although statistically significant, is in the range of clinically acceptable norms for 5 of the 6 studies that have discussed this topic and fell only 0.09 mm outside the strictest recommended value to be considered clinically accurate. Given the recommendations of previous studies and the accuracy produced by the 3D printing techniques used here, both the DLP and polyjet printers produced clinically acceptable models and could be considered viable options for clinical applications.
      Crown height measurements for digital models were not possible because of the inability to replicate the same measuring technique digitally as was done physically with a digital caliper. In a future study, it would be ideal to compare the crown height measurements of the digital model with both stone and printed models. For this study, we assumed that the statistically significant mean differences for the crown height measurements between the DLP and stone models were a result of inaccuracy in the DLP printing technique rather than the digital impression. This assumption was made, because both the DLP and polyjet printed models were produced from the same initial digital impression, and the mean differences for the crown height measurements between the polyjet and stone models had high agreement.
      The intraoral scanner used in this study produced digital models that demonstrated high agreement compared with stone models. We concluded that the digital models are clinically acceptable and could be considered viable options for clinical applications. It would be useful for future studies to assess the accuracy of other scanners in the orthodontic marketplace in clinical settings. The accuracy of the digital models in this study suggested that digital impressions acquired directly from the oral environment could serve as a gold standard to compare the accuracy of 3D printing techniques.
      Other considerations for future studies could focus on the accuracy of 3D printing models in vertical orientations, allowing for additional models to be printed at a given time, testing hollow formats that require less resin in the printing process, and testing in high-speed settings that print with thicker layers in the z-axis.
      In this study, we assessed (1) the accuracy of an intraoral scanner under clinical conditions, (2) the accuracy of two 3D printed techniques, and (3) the accuracy of the combination of (1) and (2), which assessed the clinical application of acquiring a digital impression under clinical conditions and 3D printing a physical model.

      Conclusions

      • 1.
        Both the DLP and polyjet printers produced clinically acceptable models with high accuracy that may allow them to serve as a replacement for stone models and to be considered viable options for clinical applications.
      • 2.
        The polyjet printer produced models with clinically acceptable accuracy for all recorded measurements. Compared with the DLP printer, the polyjet produced models with closer accuracy to the stone models.
      • 3.
        The mean difference between crown height measurements for the DLP and stone models was statistically significant in this study, but the mean differences were within a clinically acceptable range.

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