Production tolerance of conventional and digital workflow in the manufacturing of glass ceramic crowns.

OBJECTIVES: To measure and compare the size of the cement gap of wax and polymer copings and final glass-ceramic crowns, produced from conventional and digital workflows, one additive and one subtractive. METHODS: Thirty wax copings were made by conventional manual layering technique and modeling wax on stone models with spacer varnish simulating a cement spacer. The wax copings were embedded and press-cast in lithium disilicate glass ceramic. Thirty wax copings were produced by milling from a wax blank, i.e. subtractive manufacturing, and thirty polymer burn-out copings were produced by stereolithography, i.e. additive manufacturing. These copings were embedded and press-cast in lithium disilicate glass ceramic in the same manner as the conventional group. The fit of the wax/polymer copings and subsequent crowns was checked using an impression replica method. Mean values for cement gap for marginal, axial, and occlusal areas were calculated and differences were analyzed using Student's t-test. RESULTS: There were significant differences in mean values for accuracy/production tolerance among different manufacturing techniques for both production stages: wax and polymer copings and final pressed glass-ceramic crowns. In general, crowns produced from a digital additive workflow showed smaller mean cement gaps than crowns produced from a conventional workflow or a digital subtractive workflow. Additive polymer copings showed significantly smaller cement gaps than milled wax copings (p≤.001) and conventional wax copings (p≤.001) in the axial area. In the occlusal area, both additive polymer copings and conventional wax copings showed significantly smaller cement gaps than milled wax copings (p=.002 and p≤.001 respectively). Crowns produced from conventional manual build-up wax copings showed significantly larger mean cement gaps than crowns produced from milled wax and additively manufactured polymer copings in the marginal and axial areas (p≤.001). Among the crowns with smaller cement gaps, crowns produced from additively manufactured polymer copings showed significantly smaller mean cement gaps than crowns produced from milled wax in the marginal and axial areas (p≤.001). In the occlusal areas, the differences in mean cement gaps were only statistically significant between crowns produced from conventional manual build-up wax copings and crowns produced from milled wax where the latter ones showed smaller mean cement gaps (p=.025). SIGNIFICANCE: The present study suggests that an additive manufacturing technique produces smaller mean cement gaps in glass-ceramic crowns than a conventional or subtractive manufacturing technique.

Accuracy of surgical guides from 2 different desktop 3D printers for computed tomography-guided surgery.

STATEMENT OF PROBLEM: Different factors influence the degree of deviation in dental implant position after computed tomography-guided surgery. The surgical guide-manufacturing process with desktop 3D printers is such a factor, but its accuracy has not been fully evaluated. PURPOSE: The purpose of this in vitro study was to evaluate the deviation in final dental implant position after the use of surgical guides fabricated from 2 different desktop 3D printers using a digital workflow. MATERIAL AND METHODS: Twenty 3D-printed resin models were prepared with missing maxillary premolar. After preoperative planning, 10 surgical guides were produced with a stereolithography printer and 10 with a digital light-processing (DLP) printer. A guided surgery was performed; 20 dental implants (3.8×12 mm) were installed, and a digital scan of the dental implants was made. Deviations between the planned and final position of the dental implants were evaluated for both the groups. RESULTS: A statistically significant difference between stereolithography and DLP were found for deviation at entry point (P=.023) and the vertical implant position (P=.009). Overall lower deviations were found for the guides from the DLP printer, with the exception of deviation in horizontal implant position. CONCLUSIONS: The tested desktop 3D printers were able to produce surgical guides with similar deviations with regard to the final dental implant position, but the DLP printer proved more accurate concerning deviations at entry point and vertical implant position.

Fracture load of colored and non-colored high translucent zirconia three-unit fixed dental prosthesis frameworks.

Aim: The use of colored translucent zirconia may enable restorations of a more natural tooth-like appearance than previous opaque white zirconia. The shift from non-colored to colored zirconia may however entail a risk of reduced strength. The aim of the present study was to compare fracture load and fracture mode of fixed dental prostheses frameworks made of colored translucent zirconia to that of non-colored controls. Methods: A total of forty three-unit FDP frameworks were manufactured from two different high translucent zirconia materials (Zenostar, Wieland Dental, and DD cubeX2, Dental Direkt). Each group contained two subgroups, one colored and one non-colored. Coloring was performed before final sintering using two different infiltration techniques. All FDPs underwent an artificial aging process in the form of heat treatment, thermocycling and preloading whereafter the specimens were subjected to load until fracture. Fracture load and mode was registered. Results: For one of the zirconia materials, Zenostar, the non-colored frameworks showed significantly higher fracture loads (p < .0001) compared to its colored counterpart. No significant difference (p > .05) was found between colored and non-colored frameworks in the other zirconia material, DD cubeX2. All FDPs fractured through the connector. Some fractures ran through the mesial and some through the distal side of the connector but there were no significant differences in fracture mode between groups. Conclusion: Coloring before sintering of high-translucent zirconia may decrease the fracture load of FDP frameworks for certain materials and techniques. Fracture mode however, does not appear to be affected.

Influence of core design, production technique, and material selection on fracture behavior of yttria-stabilized tetragonal zirconia polycrystal fixed dental prostheses produced using different multilayer techniques: split-file, over-pressing, and manually built-up veneers.

AIM: To investigate and compare the fracture strength and fracture mode in eleven groups of currently, the most commonly used multilayer three-unit all-ceramic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) fixed dental prostheses (FDPs) with respect to the choice of core material, veneering material area, manufacturing technique, design of connectors, and radii of curvature of FDP cores. MATERIALS AND METHODS: A total of 110 three-unit Y-TZP FDP cores with one intermediate pontic were made. The FDP cores in groups 1-7 were made with a split-file design, veneered with manually built-up porcelain, computer-aided design-on veneers, and over-pressed veneers. Groups 8-11 consisted of FDPs with a state-of-the-art design, veneered with manually built-up porcelain. All the FDP cores were subjected to simulated aging and finally loaded to fracture. RESULTS: There was a significant difference (P<0.05) between the core designs, but not between the different types of Y-TZP materials. The split-file designs with VITABLOCS(®) (1,806±165 N) and e.max(®) ZirPress (1,854±115 N) and the state-of-the-art design with VITA VM(®) 9 (1,849±150 N) demonstrated the highest mean fracture values. CONCLUSION: The shape of a split-file designed all-ceramic reconstruction calls for a different dimension protocol, compared to traditionally shaped ones, as the split-file design leads to sharp approximal indentations acting as fractural impressions, thus decreasing the overall strength. The design of a framework is a crucial factor for the load bearing capacity of an all-ceramic FDP. The state-of-the-art design is preferable since the split-file designed cores call for a cross-sectional connector area at least 42% larger, to have the same load bearing capacity as the state-of-the-art designed cores. All veneering materials and techniques tested in the study, split-file, over-press, built-up porcelains, and glass-ceramics are, with a great safety margin, sufficient for clinical use both anteriorly and posteriorly. Analysis of the fracture pattern shows differences between the milled veneers and over-pressed or built-up veneers, where the milled ones show numerically more veneer cracks and the other groups only show complete connector fractures.

Fracture strength of all-ceramic (Y-TZP) three- and four-unit fixed dental prostheses with different connector design and production history.

AIMS: To investigate how different default settings for connector design of two different CAD/CAM systems, i.e. to compare how different radii of curvature in the embrasure area of the connector affect the fracture strength and the fracture mode of 3-unit and 4-unit all-ceramic FDPs made from Y-TZP and to investigate how the number of pontics affect the fracture strength of Y-TZP. MATERIAL AND METHODS: 32 all-ceramic three (3Z) and four (4Z)-unit, Y-TZP, FDP cores were made and divided in 4 groups, with 8 FDP in each group. The groups 3Z:1 and 4Z:1 were generated with a mechanical scanner, Procera Forte and the FDPs in group 3Z:2 and 4Z:2 were generated with an optical scanner, NobelProcera Scanner. The connector dimensions were set to 3 mm x 3 mm and core was set to 0.7 mm. The design of radius of the gingival and occlusal embrasures in the connector areas was set according to default settings and the manufacturer's recommendations. All the FDP cores were subjected to heat treatment, thermocycled for 5,000 cycles, preloaded for 10,000 cycles to simulate ageing and finally loaded to fracture. RESULTS: Regarding connector design a significant difference was found between group 3Z:1 and 3Z:2 (P<0.05), and group 4Z:1 (50% of the FDPs fractured during preloading 30-300N) and 4Z:2 (P<0.05). An extra pontic decreased the fracture strength up to 45%. CONCLUSIONS: The default settings of the two different CAD/CAM systems had a great impact on the fracture strength. It is important that a CAD/CAM system is equipped with possibilities to design a connector that fulfils the clinical demands of mechanical function and longevity. The most crucial factor for the load-bearing capacity isthe design of the radius of the gingival embrasures. Increasing the number of pontics from three to four decreases the load-bearing capacity nearly twice.

The influence of support properties and complexity on fracture strength and fracture mode of all-ceramic fixed dental prostheses.

Abstract Objective. When a new material is released, clinical studies are indicated. For the clinical studies to be defensible, in-vitro studies, which are as clinically relevant as possible, must be performed. The aim of this study was to investigate how the choice of material used for supporting tooth analogues and support complexity influences test results concerning the fracture strength of fixed dental prostheses (FDPs) made from a brittle material: Y-TZP. Material and methods. Twenty-four FDPs were produced in Y-TZP. The FDP cores were subjected to heat treatment to simulate veneering and then thermocycled for 5000 cycles to simulate ageing. The FDPs were divided into three groups and were cemented on tooth-supporting analogues made from aluminium, polymer and DuraLay( ). The FDPs were preloaded for 10,000 cycles and finally loaded to fracture. Results. There were no significant differences in load to fracture or fracture mode between the groups cemented on polymer and DuraLay tooth analogues. The FDPs cemented on aluminium tooth analogues showed a significantly higher load at fracture and a different fracture mode. Conclusions. Within the limitations of this in-vitro study, the following could be concluded. To achieve mutually comparable results there is a need for a standardized, simple test set-up for in-vitro testing of all-ceramic FDPs intended for cementation upon natural teeth. Resilient, non-complex and resilient, complex tooth analogues give comparable test results when the test set-up is unchanged in all other aspects. Non-resilient (with an elastic modulus equivalent to or higher than that of aluminium) tooth analogues give high and unrealistic load-at-fracture values together with adverse fracture modes compared to FDPs failing in clinical situations.