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.

Evaluation of the influence of different build angles on the surface characteristics, accuracy, and dimensional stability of the complete denture base printed by digital light processing.

Purpose: This study aims to investigate the influence of the build angle on the surface characteristics, accuracy, and dimensional stability of digital light processing (DLP) printed resin bases. Material and methods: Rectangular and complete denture base samples were fabricated at 0, 45, and 90-degree angles (n = 5 for rectangular samples; n = 10 for maxillary and mandibular denture base samples) using a DLP printer. Surface morphology and roughness were assessed using a profilometer, followed by measuring hydrophilicity with a contact angle meter. Accuracy (trueness and precision) and dimensional stability were evaluated at intervals of 1, 3, 7, 14, 28, and 42 days after base printing using best-fit-alignment and deviation analysis in 3D software. Statistical analysis was performed using one-way ANOVA for surface characteristics (α = 0.05), multi-way ANOVA for accuracy and dimensional stability data, and Tukey's test for post-hoc comparisons. Results: The 0-degree group exhibited significantly lower mean roughness (1.27 ± 0.19 µm) and contact angle (80.50 ± 3.71°) (P < 0.001) compared to the 90-degree and 45-degree groups. The 0-degree build angle led to superior trueness (maxilla: 77.80 ± 9.35 µm, mandible: 61.67 ± 10.32 µm) and precision (maxilla: 27.51 ± 7.43 µm, mandible: 53.50 ± 15.16 µm) compared to other groups (P < 0.001). Maxillary base precision was superior to mandibular base precision (P < 0.001). The maxillary base exhibited less dimensional deviation than the mandibular base. The 90-degree group showed the highest deviation compared to the other two groups, and all groups' deviations increased over time (P < 0.001). Conclusions: The build angle significantly influences the surface characteristics, accuracy, and dimensional stability of DLP-printed denture bases. A 0-degree build angle provides the most favorable performance. The maxillary base displayed superior precision and dimensional stability than the mandibular base.

Evaluation on Surface Characteristics, Accuracy, and Dimensional Stability of Tooth Preparation Dies Fabricated by Conventional Gypsum and 3D-Printed Workflows.

PURPOSE: To evaluate the surface characteristics, accuracy (trueness and precision), and dimensional stability of tooth preparation dies fabricated using conventional gypsum and direct light processing (DLP), stereolithography (SLA), and polymer jetting printing (PJP) techniques. MATERIALS AND METHODS: Gypsum preparation dies were replicated according to the reference data and imported into DLP, SLA, and PJP printers, and the test data were obtained by scanning after 0, 1, 3, 7, 14, 28, and 42 days. After analyzing the surface characteristics, a best-fit algorithm between the test and the reference data was used to evaluate the accuracy and dimensional stability of the preparation dies. The data were analyzed by one-way analysis of variance and Tukey test or Kruskal-Wallis H test (α = .05). RESULTS: Compared with the gypsum group (3.61 ± 0.59 µm), the root mean square error (RMSE) values of the SLA group (5.33 ± 0.48 µm) was rougher (P < .05), the PJP group (2.43 ± 0.37 µm) was smoother (P < .05), and the DLP group (2.92 ± 0.91 µm) had no significant difference (P > .05). For trueness, the RMSE was greater in the PJP (34.90 ± 4.91 µm) and SLA (19.01 ± 0.95 µm) groups than in the gypsum (16.47 ± 0.47 µm) group (P < .05), and no significant difference was found between the DLP (17.10 Å} 1.77 µm) and gypsum groups. Regarding precision, the RMSE ranking was gypsum = DLP = SLA < PJP group. The RMSE ranges in the gypsum, DLP, PJP, and SLA groups at different times were 6.79 to 8.86 µm, 5.44 to 10.17 µm, 10.16 to 11.28 µm, and 10.94 to 32.74 µm, respectively. CONCLUSION: Although gypsum and printed preparation dies showed statistically significant differences in surface characteristics, accuracy, and dimensional stability, all tooth preparation dies were clinically tolerated and used to produce fixed restorations.

Effect of different tooth preparation designs on the marginal and internal fit discrepancies of cobalt-chromium crowns produced by computer-aided designing and selective laser melting processes.

PURPOSE: To evaluate the impact of five different tooth preparation designs on the marginal and internal fit discrepancies of cobalt-chromium (CoCr) crowns produced by computer-aided designing (CAD) and selective laser melting (SLM) processes. MATERIALS AND METHODS: Five preparation data were constructed, after which design crowns were obtained. Actual crowns were fabricated using an SLM process. After the data of actual crowns were obtained with structural light scanning, intaglio surfaces of the design crown and actual crown were virtually superimposed on the preparation. The fit-discrepancies were displayed with colors, while the root means square was calculated and analyzed with one-way analysis of variance (ANOVA), Tukey's test or Kruskal-Wallis test (α =.05). RESULTS: The marginal or internal color-coded images in the five design groups were not identical. The shoulder-lip and sharp line angle groups in the CAD or SLM process had larger marginal or internal fit discrepancies compared to other groups (P < .05). In the CAD process, the mean marginal and internal fit discrepancies were 10.0 to 24.2 µm and 29.6 to 31.4 µm, respectively. After the CAD and SLM processes, the mean marginal and internal fit discrepancies were 18.4 to 40.9 µm and 39.1 to 47.1 µm, respectively. The SLM process itself resulted in a positive increase of the marginal (6.0 - 16.7 µm) and internal (9.0 - 15.7 µm) fit discrepancies. CONCLUSION: The CAD and SLM processes affected the fit of CoCr crowns and varied based on the preparation designs. Typically, the shoulder-lip and sharp line angle designs had a more significant effect on crown fit. However, the differences between the design groups were relatively small, especially when compared to fit discrepancies observed clinically.

Titanium clasp fabricated by selective laser melting, CNC milling, and conventional casting: a comparative in vitro study.

PURPOSE: This study aimed to investigate the effects of selective laser melting (SLM), milling methods, and casting on the behavior of titanium clasp. METHODS: The clasp and its die simulating the molar were designed using 3D software. Clasp specimens were fabricated using SLM approaches (SLM Ti) and computerized numerical control (CNC) milling technology (Milling CPTi). Cast clasps of the same forms were also prepared as controls using titanium alloy powder (Cast Ti) and commercial pure titanium (Cast CPTi), following the conventional casting methods. The surface roughness and accuracy of clasps were analyzed. The changes in retentive force and permanent deformation were measured up to 10,000 insertion/removal cycles. One-way analysis of variance and Tukey's test or Kruskal-Wallis H test were performed for data analysis and comparisons. RESULTS: The Milling CPTi clasps had a smoother inner surface than the other groups (p<0.05). The accuracy of the inner surface showed no significant difference among the groups, whereas that of the outer surface showed significant differences (p<0.05). The SLM Ti clasp had significantly higher retentive forces than the other groups (p<0.05), but it rapidly reduced after 2000 insertion/removal cycles until the fracture of all specimens was at 4000 cycles. The Milling CPTi clasps had more permanent deformation, but the rate of reduction of retentive force was only 9.5% (at 10,000 cycles). CONCLUSIONS: Milling has the potential to replace casting for fabricating removable partial denture (RPD) titanium clasps. However, SLM should be further improved for fabricating RPD titanium clasps before clinical application.

Accuracy and reproducibility of 3D digital tooth preparations made by gypsum materials of various colors.

PURPOSE: The study aimed to identify the accuracy and reproducibility of preparations made by gypsum materials of various colors using quantitative and semi-quantitative three-dimensional (3D) approach. MATERIALS AND METHODS: A titanium maxillary first molar preparation was created as reference dataset (REF). Silicone impressions were duplicated from REF and randomized into 6 groups (n=8). Gypsum preparations were formed and grouped according to the color of gypsum materials, and light-scanned to obtain prepared datasets (PRE). Then, in terms of accuracy, PRE were superimposed on REF using the best-fit-algorithm and PRE underwent intragroup pairwise best-fit alignment for assessing reproducibility. Root mean square deviation (RMSD) and degrees of similarity (DS) were computed and analyzed with SPSS 20.0 statistical software (α=.05). RESULTS: In terms of accuracy, PREs in 3D directions were increased in the 6 color groups (from 19.38 to 20.88 µm), of which the marginal and internal variations ranged 51.36 - 58.26 µm and 18.33 - 20.04 µm, respectively. On the other hand, RMSD value and DS-scores did not show significant differences among groups. Regarding reproducibility, both RMSD and DS-scores showed statistically significant differences among groups, while RMSD values of the 6 color groups were less than 5 µm, of which blue color group was the smallest (3.27 ± 0.24 µm) and white color group was the largest (4.24 ± 0.36 µm). These results were consistent with the DS data. CONCLUSION: The 3D volume of the PREs was predisposed towards an increase during digitalization, which was unaffected by gypsum color. Furthermore, the reproducibility of digitalizing scanning differed negligibly among different gypsum colors, especially in comparison to clinically observed discrepancies.

[An experimental study on the effect of different optical impression methods on marginal and internal fit of all-ceramic crowns].

OBJECTIVE: To study the effect of different optical impression methods in Cerec 3D/Inlab MC XL system on marginal and internal fit of all-ceramic crowns. METHODS: A right mandibular first molar in the standard model was used to prepare full crown and replicated into thirty-two plaster casts. Sixteen of them were selected randomly for bonding crown and the others were used for taking optical impression, in half of which the direct optical impression taking method were used and the others were used for the indirect method, and then eight Cerec Blocs all-ceramic crowns were manufactured respectively. The fit of all-ceramic crowns were evaluated by modified United States Public Health Service (USPHS) criteria and scanning electron microscope (SEM) imaging, and the data were statistically analyzed with SAS 9.1 software. RESULTS: The clinically acceptable rate for all marginal measurement sites was 87.5% according to USPHS criteria. There was no statistically significant difference in marginal fit between direct and indirect method group (P > 0.05). With SEM imaging, all marginal measurement sites were less than 120 microm and no statistically significant difference was found between direct and indirect method group in terms of marginal or internal fit (P > 0.05). But the direct method group showed better fit than indirect method group in terms of mesial surface, lingual surface, buccal surface and occlusal surface (P < 0.05). The distal surface's fit was worse and the obvious difference was observed between mesial surface and distal surface in direct method group (P < 0.01). CONCLUSION: Under the conditions of this study, the optical impression method had no significant effect on marginal fit of Cerec Blocs crowns, but it had certain effect on internal fit. Overall all-ceramic crowns appeared to have clinically acceptable marginal fit.