True crystalline ceramic materials presently used in restorative dentistry are Al2O3 (alumina) and yttrium-oxide stabilised tetragonal polycrystalline zirconium-dioxide (Y-TZP). To ensure optimal clinical performance, the dimensions of the Fixed Partial Denture (FPD) framework in general and of the connectors in particular, must be adequate. Considered recommendations for connector dimensions for Y-TZP FPDs vary from 2 to 4 mm in occluso-gingival height and 2 to 4 mm in bucco-lingual width. In order to reduce the fracture probability when designing all-ceramic FPDs, the shape of the connector is an important factor to consider. The radius of curvature at the gingival embrasure plays a significant role in the load-bearing capacity. FPDs with small gingival embrasure radii are subjected to high stress concentrations in the connector area during loading, compared to FPDs with large embrasure radii. The aim of this in-vitro study was to investigate how different radii of curvature in the embrasure of the connector area and different connector dimensions could affect the fracture resistance of 3-unit all-ceramic FPDs made of Y-TZP. Forty-eight FPDs in 6 groups of 8 FPDs with different connector design were produced in Procera Zirconia Bridge material. The FPD cores were subjected to heat treatment to simulate veneering. Following cementation, the FPDs were firstly thermocycled for 5,000 cycles, then preloaded for 10,000 cycles and finally loaded to fracture. All the FPDs fractured in the connector area. All the crack propagation which led to fracture started at the gingival embrasure of the connector. Within the limitations of this in-vitro study,the recommended minimum dimension of an anterior 3-unit all-ceramic FPD of Y-TZP is 3 mm in incisal-cervical direction and 2 mm in buccal-lingual direction. By increasing the radius of the gingival embrasure from 0.6 to 0.9 mm, the fracture strength for a Y-TZP FPD with connector dimension 3 x 3 mm increases by 20%.

Objective: When a new material is released, clinical studies are indicated. For the clinical studies to be defensible, in-vitro studies, as clinically relevant as possible, must be performed. The aim of the present study was to investigate how the choice of material used for supporting tooth analogues and support complexity influence test results concerning the fracture strength of FDPs made of 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 5,000 cycles to simulate ageing. The FDPs were divided into 3 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 and 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 standardised, 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 e-modulus of aluminium or higher) tooth analogues give high and unrealistic load-at-fracture values together with adverse fracture modes compared to FDPs failing in clinical situations.

Brotthållfasthet hos helkeramiska (Y-TZP) tre och fyra-leds broskelett: betydelsen av connectordesign och produktionshistorik. Yttriumoxidstabiliserad zirkoniumdioxid (Y-TZP) används frekvent inom tandvården. Helkeramiska broar måste dimensioneras och designas med hänsyn till bromaterialets specifika materialegenskaper. Särskilt kritisk är utformningen av connectorområdet. Rekommendationerna avseende connectordimensionerna för anteriora 3-4 ledsbroar av Y-TZP varierar mellan 2 till 4 mm i vertikal riktning och mellan 2 till 4 mm i horisontell riktning. Utöver dimensioneringen måste hänsyn tas till connectorns utformning då connectorns gingivala radie har en stor betydelse för konstruktionens totala brotthållfasthet. Vid belastning utsätts broar med liten gingival radie för högre spänningskoncentrationer i connectorområdet jämfört med broar med större gingival radie. Den gingivala utformningen av connectorer kan ha en ännu större betydelse för brotthållfastheten om antalet hängande led utökas från ett till två. Syftet med föreliggande studie var att undersöka hur förinställningarna avseende design av connectorer i två olika CAD/CAM system påverkar brotthållfastheten, hur den gingivala radiens storlek påverkar brotthållfasthet och frakturmönster samt undersöka hur brospannets längd, dvs antalet hängande led påverkar brotthållfastheten hos 3- och 4-ledsbroar av Y-TZP. Två olika CAD/CAM system användes för att framställa totalt 32 broskelett av Y-TZP, varav 16 st 3-ledsbroar med ett hängande led och 16 st 4-ledsbroar med två hängande led. Broskeletten delades in i 4 grupper baserat på antal led och utformning av connectorernas gingivala radie. För framställa grupperna 3Z:1, 4Z:1 användes en mekanisk scanner, Procera®Forte och för grupperna 3Z:2, 4Z:2 användes en optisk scanner, NobelProcera® Scanner. Samtliga broar utsattes för värmebehandling (simulerad porslinspåbränning), termocykling (5°C – 55°C / 5000 cykler), cyklisk förbelastning (30 – 300N / 10 000 cykler) och belastades slutligen till brott. Samtliga broar fakturerade i connectorområdet och efter okulär besiktning kunde man konstatera att samtliga frakturer startat i connectorns gingivala del. Medelvärdena för last vid brott var i grupp 3Z:1 734 N, grupp 4Z:1 405 N, grupp 3Z:2 910 N samt i grupp 4Z:2 520 N. Med reservation för de begränsningar som en In-vitro studie innebär dras följande Slutsatser: Förinställningarna i de olika CAD/CAM systemen hade avgörande betydelse för brotthållfastheten. Eftersom connectorområdet utgör den svagaste punkten i en helkeramisk brokonstruktion är det avgörande att CAD/CAM systemens standardinställningar tillåter en konstruktionsdesign som uppfyller de mekaniska och kliniska kraven. Storleken på den gingivala connectorradien har avgörande betydelse både för brotthållfastheten och för frakturmönster. Genom att öka antalet hängande led från ett till två hängande led minskar brokonstruktionens brotthållfasthet med ca 50 %.

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 is the design of the radius of the gingival embrasures. Increasing the number of pontics from three to four decreases the load-bearing capacity nearly twice.

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.