Effect of 3D printing technology and print orientation on the trueness of additively manufactured definitive casts with different tooth preparations.

OBJECTIVES: To evaluate the fabrication trueness of additively manufactured maxillary definitive casts with various tooth preparations fabricated with different 3-dimensional (3D) printers and print orientations. METHODS: A maxillary typodont with tooth preparations for a posterior 3-unit fixed partial denture, lateral incisor crown, central incisor and canine veneers, first premolar and second molar inlays, and a first molar crown was digitized with an industrial scanner. This scan file was used to fabricate definitive casts with a digital light processing (DLP) or stereolithography (SLA) 3D printer in different orientations (0-degree, 30-degree, 45-degree, and 90-degree) (n = 7). All casts were digitized with the same scanner, and the deviations within each preparation site were evaluated. Generalized linear model analysis was used for statistical analysis (α = 0.05). RESULTS: The interaction between the 3D printer and the print orientation affected measured deviations within all preparations (P ≤ 0.001) except for the lateral incisor crown and canine veneer (P ≥ 0.094), which were affected only by the main factors (P < 0.001). DLP-90 mostly led to the highest and DLP-0 mostly resulted in the lowest deviations within posterior tooth preparations (P ≤ 0.014). DLP-30 led to the lowest deviations within the first premolar inlay and DLP-45 led to the lowest deviations within the central incisor veneer preparation (P ≤ 0.045). CONCLUSIONS: Posterior preparations of tested casts had the highest trueness with DLP-0 or DLP-30, while central incisor veneer preparations had the highest trueness with DLP-45. DLP-90 led to the lowest trueness for most of the tooth preparations. CLINICAL SIGNIFICANCE: Definitive casts with tooth preparations fabricated with the tested DLP 3D printer and the print orientation adjusted on tooth preparation may enable well-fitting restorations. However, 90-degree print orientation should be avoided with this 3D printer, as it led to the lowest fabrication trueness.

Evaluating the feasibility of using augmented reality for tooth preparation.

OBJECTIVES: Tooth preparation is complicated because it requires the preparation of an abutment while simultaneously predicting the ideal shape of the tooth. This study aimed to develop and evaluate a system using augmented reality (AR) head-mounted displays (HMDs) that provide dynamic navigation capabilities for tooth preparation. METHODS: The proposed system utilizes optical see-through HMDs to overlay digital information onto the real world and enrich the user's environment. By integrating tracking algorithms and three-dimensional modeling, the system provides real-time visualization and navigation capabilities during tooth preparation by using two different visualization techniques. The experimental setup involved a comprehensive analysis of the distance to the surface and cross-sectional angles between the ideal and prepared teeth using three scenarios: traditional (without AR), overlay (AR-assisted visualization of the ideal prepared tooth), and cross-sectional (AR-assisted visualization with cross-sectional views and angular displays). RESULTS: A user study (N = 24) revealed that the cross-sectional approach was more effective for angle adjustment and reduced the occurrence of over-reduction. Additional questionnaires revealed that the AR-assisted approaches were perceived as less difficult, with the cross-sectional approach excelling in terms of performance. CONCLUSIONS: Visualization and navigation using cross-sectional approaches have the potential to support safer tooth preparation with less overreduction than traditional and overlay approaches do. The angular displays provided by the cross-sectional approach are considered helpful for tooth preparation. CLINICAL SIGNIFICANCE: The AR navigation system can assist dentists during tooth preparation and has the potential to enhance the accuracy and safety of prosthodontic treatment.

Stress distribution on teeth restored with veneers using various incisal preparation designs: A 3D finite element analysis study.

STATEMENT OF PROBLEM: The type of veneer preparation is often chosen according to the patient's tooth structure and occlusion. Taking biomechanics into account in this decision-making process provides the clinician with more technical information on how to improve the clinical longevity of restorations. However, biomechanical analyses of veneer preparation designs are sparse. PURPOSE: The purpose of this 3-dimensional (3D) finite element analysis with microcomputed tomography (µCT) data obtained from realistic models was to assess the influence of different preparations for ceramic and composite resin veneers on restoration and resin layer stress distribution. MATERIAL AND METHODS: Four replicas of a central incisor were printed and prepared for laminate veneers with 4 different incisal edge designs: shoulder (SH), palatal chamfer (PC), palatal chamfer and oblique fracture involving the distal angle (OF-PC), and palatal chamfer involving horizontal incisal fracture (IF-PC). After fabrication and cementation of the veneers, the restored replicas were assessed with µCT, and 3D finite element models were built. A 100-N load was applied on the palatal surface at 60 and 125 degrees relative to the longitudinal axis. Maximum principal stress and stress distribution on the veneers, cement layer, and tooth structure were calculated and analyzed. RESULTS: The SH preparation exhibited better stress distribution than the PC preparation, and the cement layer and the veneer were subjected to lower stress. The IF-PC preparation had better stress distribution than the OF-PC. The shoulder and IF-PC showed higher stress on laminate veneers, but lower stress on the cement layer. Ceramic veneers exhibited lower stress than composite resin veneers. CONCLUSIONS: The different incisal preparations for laminate veneers influenced stress distribution on restorations and on the resin cement layer. The shoulder type preparation showed better stress distribution and the composite resin veneers showed unfavorable results compared with the ceramic veneers.

Automated scoring and augmented reality visualization software program for evaluating tooth preparations.

STATEMENT OF PROBLEM: Tooth preparation is an essential part of prosthetic dentistry; however, traditional evaluation methods involve subjective visual inspection that is prone to examiner variability. PURPOSE: The purpose of this study was to investigate a newly developed automated scoring and augmented reality (ASAR) visualization software program for evaluating tooth preparations. MATERIAL AND METHODS: A total of 122 tooth models (61 anterior and 61 posterior teeth) prepared by dental students were evaluated by using visual assessments that were conducted by students and an expert, and auto assessment that was performed with an ASAR software program by using a 3-dimensional (3D) point-cloud comparison method. The software program offered comprehensive functions, including generating detailed reports for individual test models, producing a simultaneous summary score report for all tested models, creating 3D color-coded deviation maps, and forming augmented reality quick-response (AR-QR) codes for online data storage with AR visualization. The reliability and efficiency of the evaluation methods were measured by comparing tooth preparation assessment scores and evaluation time. The data underwent statistical analysis using the Kruskal-Wallis test, followed by Mann-Whitney U tests for pairwise comparisons adjusted with the Benjamini-Hochberg method (α=.05). RESULTS: Significant differences were found across the evaluation methods and tooth types in terms of preparation scores and evaluation time (P<.001). A significant difference was observed between the auto- and student self-assessment methods (P<.001) in scoring both the anterior and posterior tooth preparations. However, no significant difference was found between the auto- and expert-assessment methods for the anterior (P=.085) or posterior (P=.14) tooth preparation scores. Notably, the auto-assessment method required significantly shorter time than the expert- and self-assessment methods (P<.001) for both tooth types. Additionally, significant differences in evaluation time between the anterior and posterior tooth were observed in both self- and expert-assessment methods (P<.001), whereas the evaluation times for both the tooth types with the auto-assessment method were statistically similar (P=.32). CONCLUSIONS: ASAR-based evaluation is comparable with expert-assessment while exhibiting significantly higher time efficiency. Moreover, AR-QR codes enhance learning and training experiences by facilitating online data storage and AR visualization.

Evaluating the effectiveness of a novel digital evaluation technology on dental preclinical crown preparation training.

OBJECTIVES: To evaluate the assessment scores of a novel digital training program versus traditional training in dental preclinical crown preparation. METHODS: Crown preparations in two consecutive preclinical training sessions were retrospectively collected and assigned to three groups: traditional group (TG), scanning group (SG), and digital evaluation group (DG). Students in the TG (n = 20) were taught by conventional visual grading, while students in the SG (n = 25) received three-dimensional feedback from digitally scanned preparations. All the SG students continued with supplementary digital evaluation and preparations were allocated into the DG (n = 25). Comparison of total scores between groups was investigated using independent samples t-test and paired samples t-test. Mann‒Whitney U-test and Wilcoxon signed-rank test were used to statistically analyze the differences in subdividing categories. The level of significance was p < 0.05. Questionnaires on the digital evaluation procedure were answered by students in DG. RESULTS: The results showed a significant improvement (p < 0.01) in the total scores of DG than those of TG and SG, while there were no statistically significant differences between TG and SG. Scores of surface finish and undercut improved significantly in DG compared to TG and SG. The reduction scores of DG were significantly higher than those of SG. Students' feedback indicated a positive perspective on the implementation of the novel digital evaluation technology. CONCLUSIONS: These findings suggest that digital evaluation technology is useful for preclinical crown preparation training. Attention should also be paid to studying the optimal integration of digital dentistry into traditional dental curricula and its effects on students' learning curves.

Clinical evaluation of posterior zirconia-based and porcelain-fused-to-metal crowns with a vertical preparation technique: an up to 5-year retrospective cohort study.

OBJECTIVE: To evaluate the clinical conditions of single-unit posterior restorations on teeth prepared without finishing line, after 5.6 years of clinical service. MATERIALS AND METHODS: 50 crowns (25 zirconia-based (Zr) and 25 porcelain-fused-to-metal (PFM)) were selected from 34 patients. The restorations were evaluated according to the California Dental Association (CDA) Quality Criteria, and periodontal variables were studied in the abutment teeth compared with the unrestored contralateral teeth. Variables were examined using Mann-Whitney and Pearson´s Chi-Square tests (α = 0.05). The success of the prosthesis was calculated using Kaplan-Meier test. RESULTS: CDA Quality Criteria was considered satisfactory-excellent in all restorations except for one of them, due to chipping on a PFM crown. At, 66 months, the success rates for PFM and Zr crowns were 85.7 % and 100 %, respectively. The plaque index (PI) showed that the restored abutment teeth accumulated significantly less plaque than the control teeth, but the gingival index (GI) was statistically higher in the abutment teeth. In 80 % of cases the probing depth (PB) was ≤3 mm. In addition, in 21 % of the cases, gingival recession was less than 2 mm. The restoration material had a statistically significant effect on GI and PB, with Zr crowns showing less inflammation and less deep pockets than PFM restorations. On the contrary, greater gingival recession was found at the margins of the Zr crowns when compared to the PFM. No statistical differences were found between the two materials in the GI. CONCLUSIONS: Cemented crowns on vertical preparations show good clinical behavior after 5 years. The periodontal parameters (PI, GI, PD) of the Zr restorations are significantly better than those of PFM, with the exception of gingival recession. CLINICAL RELEVANCE: The use of restorations on vertically prepared teeth is a suitable alternative to classical horizontal preparations.

Haptics Vs Typodonts for Crown Preparation in Undergraduate Dental Student Education.

OBJECTIVES: This study aims to compare students' subjective perceptions and objective results by comparing two methods of crown preparation: typodonts and haptics. MATERIALS AND METHODS: Fifty-four second-year students were given instructions on crown preparation for the upper right second premolar. First on typodonts and then with haptics. They were given five minutes to familiarize with the artificial environment and then thirty minutes for the actual preparation. Finally, they completed a questionnaire about their experience. Their preparations were objectively compared by measuring the angle of total occlusal convergence-TOC on the typodonts and with haptics. RESULTS: Students reported that haptics can enhance the learning process and that they would use them for skill training in the future. Overall, their experience was rated as positive. The TOC of teeth prepared with haptics was significantly higher than those prepared with typodonts, but all values were within the acceptable range. CONCLUSION: Although students did not prefer haptics to typodonts, haptics appear to be a powerful tool in the educational process because it can be a complementary option to traditional methods at the preclinical level.

Evaluation of marginal and internal adaptation of veneers generated by the guided prosthetic tooth preparation system.

OBJECTIVE: This in vitro study aimed to assess and contrast the marginal and internal adaptation of all-ceramic prefabricated veneers manufactured via the FirstFit guided tooth preparation system against all-ceramic veneers produced using the chairside Computer-Aided Design/Computer Aided Manufacture (CAD/CAM) system following identical guided preparation protocols. MATERIALS AND METHODS: Two main groups were included, with 16 lithium disilicate veneers per group. Four typodonts were used for the test (FirstFit) and control CAD/CAM groups. Intraoral scans created master casts and preparation guides. Guides performed preparations on typodont teeth (two central incisors and two lateral incisors). Prepared teeth were scanned (CEREC Omnicam) to design and mill CAD/CAM veneers. Marginal gap thickness and cement space thickness were measured using light microscopy at four locations: marginal, cervical internal, middle internal, and incisal internal. RESULTS: No significant difference existed between groups for marginal adaptation (p = 0.058) or incisal internal adaptation (p = 0.076). The control group had significantly lower values for middle internal adaptation (p = 0.023) and cervical internal adaptation (p = 0.019). CONCLUSIONS: Guided preparation evaluation showed no significant differences in marginal or incisal internal adaptation. The CAD/CAM group had significantly lower middle and cervical internal adaptation values.

Computer-aided design software-enabled preclinical prosthodontic training: A digital education technique.

The prosthodontics preclinical training modules involve textbook-based two-dimensional (2D) ideal images and practicing on manikin models to emulate ideal tooth preparations and teeth arrangements. Relying solely on 2D images as objectives for preclinical exercises limits the trainee's creative skills to instructions of textbooks and clinical instructions received. With advancements in digital dentistry, dental trainees should have early exposure to the three-dimensional (3D) rendering of ideal preclinical objectives. A dental education technique using computer-aided design software and smartphones is described that will allow 3D rendering of ideal prosthodontic training assignments allowing early exposure to digital dentistry for dental training students.

Effect of cement gap and drill offset on the marginal and internal fit discrepancies of crowns designed with different tooth preparations.

AIM: The aim of the present study was to evaluate the effect of cement gap and drill offset on the marginal and internal fit discrepancies of crowns designed with different tooth preparations. MATERIALS AND METHODS: Five tooth preparations were constructed, and crowns with different cement gaps and drill offsets were obtained. Then, best-fit alignment was performed on the crowns with the corresponding tooth preparations, and the fit discrepancies were expressed by color-coded difference images and root mean square (RMS) values. The RMS values of each group were analyzed by the rank-based Scheirer-Ray-Hare test (α = 0.05). RESULTS: The color segments in the sharp line angles area of the Sharp line angles group changed significantly before and after the drill offset. The cement gap had a significant effect on the marginal, internal, or overall fit discrepancies of the five design groups (P < 0.001), while the drill offset had a significant effect on the marginal fit discrepancies of the Shoulder-lip group and the internal or overall fit discrepancies of the Sharp line angles group (P < 0.001). Additionally, the interaction effect between cement gap and drill offset was significant for the marginal fit discrepancies of the Shoulder-lip group and the internal or overall fit discrepancies of the Sharp line angles group (P < 0.01). CONCLUSIONS: The cement gap and drill offset had a significant adverse effect on the marginal or internal fit discrepancies of the crowns designed with the shoulder-lip and sharp line angles designs. Tooth preparation designs with intense curvature changes such as shoulder-lip and sharp line angles should be avoided clinically.

Impact of marginal defects on the accuracy of automated finish line detection in tooth preparation.

AIM: The present study aimed to evaluate the accuracy of automated detection of preparation finish lines in teeth with defective margins. MATERIALS AND METHODS: An extracted first molar was prepared for a full veneer crown, and marginal defects were created and scanned (discontinuity of finish line: 0.5, 1.0, and 1.5 mm; additional line angle: connected, partially connected, and disconnected). Six virtual defect models were entered into CAD software and the preparation finish line was designated by 20 clinicians (CAD-experienced group: n = 10; CAD-inexperienced group: n = 10) using the automated finish line detection method. The accuracy of automatic detection was evaluated by calculating the 3D deviation of the registered finish line. The Kruskal-Wallis and Mann-Whitney U tests were used for between-group comparisons (α = 0.05). RESULTS: The deviation values of the registered finish lines were significantly different according to conditions with different amounts of finish line discontinuity (P < 0.001). There was no statistical difference in the deviation of the registered finish line between models with additional line angles around the margin. Moreover, no statistical difference was found in the results between CAD-experienced and CAD-inexperienced operators. CONCLUSIONS: The accuracy of automated finish line detection for tooth preparation can differ when the finish line is discontinuous. The presence of an additional line angle around the preparation margin and prior experience in dental CAD software do not affect the accuracy of automated finish line detection.

[Prediction, analysis and application of learning curve of tooth preparation for all ceramic crowns of maxillary central incisors].

OBJECTIVE: To predict the learning curve of tooth preparation for all ceramic crowns of maxillary central incisors on phantom head simulators for graduate students participating in standardized dental resident training based on the modified Wright learning curve model, then to analyze and applicate the learning curve. METHODS: Twelve graduate students participating in standardized dental resident training were selected to prepare the resin maxillary central incisors on phantom head simulators for all ceramic crowns 4 times. The results of preparation were evaluated by 3 prosthetic experts with at least 10 years' experience focusing on the reduction, contour, taper, shoulder, finish line, margin placement, adjacent tooth injury, and preparation time for tooth preparation. The learning rate of tooth preparation was calculated by scores of tooth preparation of 4 times. The learning curve of tooth preparation was predicted based on the modified Wright learning curve model. According to the criteria of standardized training skill examinations for dental residents in Beijing, 80 was taken as the qualified standard score. The minimum training times for tooth preparation to satisfy the qualified standard score (80) was calculated, to analyze the characteristics of learning curve and evaluate the effectiveness of tooth preparation. RESULTS: The scores of 4 tooth preparation were 64.03±7.80, 71.40±6.13, 74.33±5.96, and 75.98±4.52, respectively. The learning rate was (106±4)%, which showed the learning curve an upward trend. There were no significant differences between the qualified standard score and the predicted scores of tooth preparation from the 5th preparation to the 13th preparation (P > 0.05). The predicted score of the 14th preparation was higher than the qualified standard score (P < 0.05). CONCLUSION: The trend of the learning curve of tooth preparation for all ceramic crowns of maxillary central incisors on phantom head simulators for graduate students participating in standardized dental resident training is upward, which predicts the minimum training times higher than the qualified standard score is 14 times.

Reliability of automatic finish line detection for tooth preparation in dental computer-aided software.

PURPOSE: This study aimed to investigate the accuracy of automatic tooth finish line registration compared to manual registration with regard to various finish line configurations and dental computer-aided design (CAD) software. METHODS: Finish line registrations were performed on 15 digital tooth models with different finish line configurations (edge roundness radius = 0 mm, 0.2 mm, and 0.4 mm; edge angle = 30°, 60°, 90°, 120°, and 150°) using automatic and manual methods for designing virtual copings (N = 150). The discrepancies between the registered finish line extracted from the copings and the actual finish line segmented from the digitized tooth model were measured. Three-way analysis of variance (ANOVA) and post-hoc analyses with Bonferroni correction (α = 0.05) were used to analyze the results. RESULTS: The finish line configurations, registration methods, and CAD software interacted with the accuracy of the registered finish line (p = 0.001). The automatic finish line registration method exhibited larger error values than the manual method, especially at high finish line edge roundness and obtuse edge angles for both EXOCAD and R2CAD software (p < 0.001). The difference in dental CAD software affected the registration accuracy in the automatic method (p < 0.001), but not in the manual method (p = 0.676). CONCLUSIONS: Finish line registration errors may occur when the automatic registration method is applied to the indistinct edge of tooth preparation. The accuracy of the automatic finish line registration could differ according to the CAD software program.

Prediction, analysis and application of learning curve of tooth preparation for all ceramic crowns of maxillary central incisors / 北京大学学报(医学版)

OBJECTIVE@#To predict the learning curve of tooth preparation for all ceramic crowns of maxillary central incisors on phantom head simulators for graduate students participating in standardized dental resident training based on the modified Wright learning curve model, then to analyze and applicate the learning curve.@*METHODS@#Twelve graduate students participating in standardized dental resident training were selected to prepare the resin maxillary central incisors on phantom head simulators for all ceramic crowns 4 times. The results of preparation were evaluated by 3 prosthetic experts with at least 10 years' experience focusing on the reduction, contour, taper, shoulder, finish line, margin placement, adjacent tooth injury, and preparation time for tooth preparation. The learning rate of tooth preparation was calculated by scores of tooth preparation of 4 times. The learning curve of tooth preparation was predicted based on the modified Wright learning curve model. According to the criteria of standardized training skill examinations for dental residents in Beijing, 80 was taken as the qualified standard score. The minimum training times for tooth preparation to satisfy the qualified standard score (80) was calculated, to analyze the characteristics of learning curve and evaluate the effectiveness of tooth preparation.@*RESULTS@#The scores of 4 tooth preparation were 64.03±7.80, 71.40±6.13, 74.33±5.96, and 75.98±4.52, respectively. The learning rate was (106±4)%, which showed the learning curve an upward trend. There were no significant differences between the qualified standard score and the predicted scores of tooth preparation from the 5th preparation to the 13th preparation (P > 0.05). The predicted score of the 14th preparation was higher than the qualified standard score (P < 0.05).@*CONCLUSION@#The trend of the learning curve of tooth preparation for all ceramic crowns of maxillary central incisors on phantom head simulators for graduate students participating in standardized dental resident training is upward, which predicts the minimum training times higher than the qualified standard score is 14 times.

Marginal Integrity of CAD/CAM Ceramic Crowns Using Two Different Finish Line Designs.

OBJECTIVE: Research on evaluation of crowns made by the latest contemporary dental computer-aided design/computer-aided manufacturing (CAD/CAM) systems for their marginal adaptation is scarce. The purpose of this in vitro study was to evaluate the marginal integrity of crowns fabricated by the latest Chairside Economical Restorations of Esthetic Ceramic (CEREC) system using 2 different finish line preparation designs: chamfer and shoulder. MATERIALS AND METHODS: Typodont teeth were divided equally into 2 groups, A and B. The teeth were prepared for full coverage crowns with a shoulder (group A) and chamfer (group B) finish line design. An experienced prosthodontist prepared all crown preparations. Evaluation of 6 sites per sample was completed by 2 calibrated, experienced prosthodontists using the modified US Public Health Services (USPHS) criteria. The descriptive statistics and Z-test were used to evaluate the results. RESULTS: A total of 180 teeth were included in the study (90 teeth in each group). Only 2 crowns in group A and 1 crown in group B were clinically unacceptable. There was no statistical significance (p = 0.282) between the 2 groups regarding finish line design. CONCLUSIONS: The CEREC system provides clinically acceptable crowns and can safely be utilized in dental treatment. Therefore, CAD/CAM restorations could be considered as a safe treatment modality by dental professionals.

Fit of tooth-supported zirconia single crowns-A systematic review of the literature.

PURPOSE: The purpose of this study is to systematically map all the factors that influence the fit and adaptation of zirconia crowns and/or copings. MATERIALS AND METHODS: The investigational strategy involved carrying out an electronic search between December 1, 2009 and September 1, 2019 through the Embase and Medline databases using Boolean operators to locate appropriate articles. RESULTS: A total of 637 articles were discovered after the removal of duplicates, and 46 of these were selected for evaluation. Further, a quality assessment was performed using GRADE evaluation criteria. CONCLUSIONS: Shoulder finish line preparations had slightly better marginal fit compared to chamfer finish lines. Crowns obtained from digital impressions had comparable to superior marginal adaptation compared to conventional impressions. Increasing cement space showed to improve zirconia crown adaptation. Cementation and veneering zirconia frameworks found to increase the marginal and internal gaps. Limited information is available on the effect of the alteration of sintering time/Temperature and/or sintering techniques on the adaptation of zirconia crowns. Most of the selected studies had a moderate quality assessment evaluation. Future studies could investigate the chair-side, ultra-fast sintering effect on the marginal gap of zirconia crowns.

3D Printed Teeth for the Preclinical Education of Dental Students.

A need for more realistic tooth models for education has often been expressed by dental students. The aim of this study was to design and create 3D printed teeth with anatomical details for use in preclinical dental education. A tooth with realistic carious lesions and pulp cavity was designed, and this tooth was used in 2018 with 47 dental students for preclinical education in caries excavation, direct capping of the pulp, core build-up, and crown preparation. The students had the ability to identify the carious lesions by a simulated radiograph and by tactile sense of the consistency. The benefits of the 3D printed tooth were evaluated by a questionnaire. The printed tooth was evaluated by grades (1=excellent, 2=good, 3=satisfactory, 4=adequate, 5=poor). The students gave the tooth an overall mean grade of 1.9, with a grade of 2.0 for the haptic impression, 1.5 for the exercise, 1.9 for the examination, and 1.5 for high practical relevance in contrast to a standard model tooth. The new features of the printed tooth were given a mean grade of 2.0 for the radiograph, 2.3 for consistency of the caries, 2.0 for the tooth filling, and 1.7 for the pulp capping as realistic. The students had the possibility to generate a complete concept for prosthodontic tooth treatment on an artificial 3D printed tooth.

Lean Methods Applied to CAD/CAM Pedagogy in the Dental Simulation Laboratory.

Dental education is incorporating computer-assisted design/computer-assisted manufacturing (CAD/CAM) into patient care delivery. The aim of this study was to determine if lean production methods applied to the preclinical phase of dental education would reduce the students' time (efficiency) to complete CAD/CAM indirect restorations (CAD/CAM IR) without compromising the desired quality of the CAD/CAM tooth preparations (effectiveness). In 2016, all third-year students at one U.S. dental school were randomly assigned to control and research groups (approximately 33 per group). The control group was taught using existing methods, while the course was redesigned to incorporate lean methods for the research group. Cycle times of the CAD/CAM IR process for both groups were collected and statistically analyzed to compare the effectiveness of the redesign. Standardized grading forms and tools were used to assess the quality of the CAD/CAM tooth preparations. The research group demonstrated significantly faster preparation times with both crowns and onlays (p<0.05) than the control group. The research group also produced CAD/CAM crown and onlay designs at a significantly faster pace than the control group (p<0.05). There was no significant difference between the control and research groups associated with the scanning process times (p>0.05) nor in the quality of the CAD/CAM tooth preparations (p>0.05). This study demonstrated that lean methods applied to pedagogy significantly decreased preparation and design times in a CAD/CAM restorative dentistry course without compromising the quality of the CAD/CAM tooth preparations.

In vitro 3D and gravimetric analysis of removed tooth structure for complete and partial preparations.

PURPOSE: To quantify and to compare a gravimetric and three-dimensional (3D) analysis of the removed tooth structure for different complete crown preparations. METHODS: A total of 80 molar resin teeth and 8 preparation finishing lines were chosen: 1 for metal ceramic crowns (MCC); 3 for zirconia all-ceramic crowns: knife edge (ZirKnE), chamfer (ZirCha), and shoulder (ZirSho); 4 for lithium disilicate: light chamfer (LDLCha), chamfer (LDCha), shoulder (LDSho) and table top. Teeth were individually weighed to high precision and then prepared following the preparation guidelines. The teeth were reweighed after preparation, and the amount of structural reduction was calculated. In addition, all teeth were scanned before and after preparation, and the 3D volume of removed dental tissue was calculated, superimposing the two .stl files, as a difference of the volumes before and after the preparation. Kruskal-Wallis statistical analysis was carried out to determine significant differences among the groups with a significance level of p<0.05. RESULTS: Both analyses showed that LDLCha, ZirKnE and table-top preparations produced the smallest amount of removed structure, whereas the preparations for MCC, ZirSho and LDSho were more destructive. For MCC, 2.6 times more tooth structure must be removed than for table top. ZirKnE was 17.82% and LDLCha was 21.51% more conservative than MCC. The data obtained through the volumetric method were similar with those obtained by gravimetric analysis. CONCLUSIONS: ZirKnE, LDLCha, and table-top preparations produced the least amount of tooth tissue removal. Three-dimensional volumetric analysis can be a possible alternative to gravimetric analysis.