Marc Philipp Dittmer

Influence of cooling rate on zirconia/veneer interfacial adhesion.

Slow cooling firing schedules have recently been introduced by some manufacturers to reduce chipping complications in zirconia-based core/veneer composites. The aim of this study was to test the hypothesis that these firing schedules may influence the bond strength between the zirconia core and veneering ceramic. Four different veneering ceramics recommended for zirconia (Lava Ceram, Triceram, VM9 and Zirox) were fired onto rectangular shaped Y-TZP specimens (Lava Frame) and cooled using a rapid or a slow cooling rate. The resulting bilayer specimens were notched, loaded in a four-point bending test and load-displacement curves were recorded. The critical load to induce stable crack extension at the core/veneer interface was determined, in order to calculate the strain energy release rate (G, Jm⁻²). Additionally, dilatometric measurements of the veneering ceramics were performed to determine the coefficient of thermal expansion (α, ppm.K⁻¹) between 50 and 450°C (α₁) and in the temperature region above the glass transition temperature (α₂). Discrepancies between α₂ and α₁ (Δα) were calculated. For all core/veneer compositions G values were lower for the slowly cooled specimens than for the rapidly cooled specimens. Significant differences with respect to the firing schedule were found in the Triceram and VM9 groups (P<0.05). The reductions in G values correlated with Δα. The bond strength between the zirconia core and the veneer decreased with the slow cooling rate. These results indicate that slow cooling of zirconia restorations may increase the risk of adhesive delamination failures between the core and veneer.

Influence of the veneering process on the marginal fit of zirconia fixed dental prostheses

Distortions in the marginal region during manufacture are detrimental to the long-term success of fixed dental prostheses (FDPs). The aim of the present in vitro study was to evaluate changes in marginal fit because of the veneering process of four-unit FDPs made from different zirconia materials. Two groups of FDPs with 10 specimens each were machined from white body zirconia blanks (VITA In-Ceram YZ Cubes, KaVo Everest ZS-Blanks) by means of computer-aided design/computer-aided manufacturing systems and subsequently sintered to their final density. The marginal and internal gaps of the frameworks were determined using a replica technique. Afterwards, frameworks were veneered with the recommended ceramics (VITA VM9, GC Initial Zr), and fitting accuracy was evaluated again. Statistical analyses were performed by Students t-test with the level of significance chosen at 0.05. With one framework/veneering system (Everest ZS-Blanks/Initial Zr), both marginal gap (P = 0.019) and internal gap (P = 0.001) at the premolar retainer were significantly affected by the veneering process. The resulting distortions were directed towards the lumina of the retainers and the mean values of the measured gaps decreased by about 23.8 mum (marginal gap) and 27.3 mum (internal gap). For the other system (In-Ceram YZ Cubes/VM9), no distortions because of the veneering were found. The veneering of zirconia restorations may lead to significant changes in the marginal fit, while the combination of core and veneering material used influences the resulting distortions.

Stresses and distortions within zirconia-fixed dental prostheses due to the veneering process

Changes in the marginal and internal fit during manufacture are detrimental to the long-term success of fixed dental prostheses (FDPs). Hence, the aim of this study was to investigate the distortion of four-unit zirconia bridges induced by the veneering process with in vitro and finite element analyses (FEA). Ten all-ceramic FDPs with zirconia frameworks were prepared. The marginal and internal fit of the restorations were determined prior to and after veneering by means of a replica technique. Additionally, a three-dimensional finite element model of the restoration was constructed and cooling after the veneering process was virtually simulated. Statistical analysis revealed significant changes in the marginal and internal fit due to the veneering process. FEA verified these observations and displayed tensile stresses (up to 65MPa) within the framework and compressive stresses (up to 10MPa) within the veneering layer. The present study showed that stresses and distortions, occurring due to the veneering process, may influence the marginal and internal fit and therefore the clinical success of dental restorations.

Effect of Implant–Abutment Connection Design on Load Bearing Capacity and Failure Mode of Implants

PURPOSE In this in vitro study, six implant-abutment connection designs were compared and evaluated regarding load bearing capacities and failure modes. MATERIALS AND METHODS Five implants of Astra Tech, Bego, Camlog, Friadent, Nobel Biocare, and Straumann were separately embedded in stainless steel tubes using polyurethane, for a total of 30 specimens. Specimens were statically loaded under an angle of 30° with respect to the implant axis in a universal testing machine using a test setup according to ISO 14801. Failure was indicated by a load drop of 100 N in force. Load-displacement curves were analyzed, and maximum force and force at which permanent deformation occurred were recorded. Statistical analysis was performed using one-way ANOVA with the level of significance set at 0.05. RESULTS Statistical analysis revealed that the type of implant-abutment connection design has a significant influence on load bearing capacity (p < 0.001). The mean maximum forces ranged between 606 N (Straumann) and 1129 N (Bego); the forces where plastic deformation set in ranged between 368 N (Friadent) and 955 N (Bego). Failure modes differed between the various implant-abutment connection types tested. CONCLUSIONS Implant-abutment connection design has a significant influence on load bearing capacity and failure mode of implants; however, all implant-abutment connection designs tested would be expected to withstand clinically relevant forces.

Finite element analysis of a four-unit all-ceramic fixed partial denture.

All-ceramic restorations are known to be prone to brittle fracture. However, a previously performed in vitro study indicates that four-unit fixed partial dentures (FPDs) with a zirconia framework are sufficiently strong to withstand occlusal forces in the posterior region. The aim of this study was to determine the stress distribution in such a four-unit FPD made of yttria-stabilized polycrystalline tetragonal zirconia (Y-TZP), under an occlusal load. A three-dimensional finite element model was constructed and a stress analysis performed with a force of 1630 N applied at the centre of the middle connector area. The location of maximum tensile stress according to finite element analysis coincided with the fracture origin of all 10 specimens fractured within the previous in vitro study. The maximum tensile stress in the area of the middle connector amounted to 633 MPa. It increased with the load being applied from the oral towards the buccal side (648 MPa) and decreased with the load being applied from the buccal towards the oral side (570 MPa). These stresses are of the same order as the flexural strength of Y-TZP, determined under standardized test conditions to be 600-1000 MPa. The model presented is intended to be used for further investigations, including thermally induced stresses during veneering.

Influence of the interface design on the yield force of the implant–abutment complex before and after cyclic mechanical loading

PURPOSE The aim of this in vitro study was to evaluate the implant-abutment assembly of Astra Tech (AST), Bego (BEG), Camlog (CAM), Friadent (FRI), Nobel Biocare (NOB) and Straumann (STR) with respect to yield force before and after cyclic fatigue, using a static overload test with a test set-up according to ISO 14801. METHODS Ten specimens of each type were split into two homogenous groups: one half was tested for static yield force without any further treatment (control), whereas the other one underwent one million cycles of mechanical loading with 100N as the upper load limit. For load-to-failure testing, specimens were then placed in a stainless steel jig and loaded in a universal testing machine under an angle of 30° with respect to the implant axis until failure. Load-displacement curves were analyzed and the yield forces at which non-linear behaviour set in (Fp) were recorded. Statistical analysis was performed using one-way ANOVA and t-test, respectively, with the level of significance set at 0.05. RESULTS Statistical analysis revealed that the type of implant-abutment connection has a significant influence on Fp (p<0.001). Furthermore, dynamic loading proved to significantly influence Fp of BEG and CAM (p<0.001). CONCLUSION None of the implant-abutment types tested would be expected to fail under clinically relevant forces, but the type of implant-abutment connection significantly influences the yield force Fp.

Comparative analysis of slot dimension in lingual bracket systems

BackgroundOrthodontic treatment with fixed appliances requires - among others - the correct clinical expression of torque, which depends on the precise fitting of archwire and slot. Especially in the lingual technique torque problems become clinically more evident than in labial appliances also with respect to the vertical alignment of teeth due to different distances from the center of resistance. The purpose of the present study was to compare the preciseness of slot dimensions of different lingual bracket systems.MethodsThree lingual bracket systems were included in the study (7th Generation and STb, Ormco, Glendora, CA, USA; Incognito, TOP-Service/3 M Unitek, Monrovia, CA, USA). Non destructive analysis of vertical slot dimensions was performed using precision pin gauges (Azurea, Belprahon, Switzerland) that were tapered in increments of 0.002 mm (0.00008 inch). The sizes of 240 incisor and canine brackets were measured per system (total: 720). Data were compared using one-way ANOVA. A p-value < 0.05 was considered statistically significant.ResultsAverage slot dimensions were 0.467 mm ± 0.007 mm (0.0184 inch ± 0.0003 inch) for the 7th Generation bracket system, 0.466 mm ± 0.004 mm (0.0183 inch ± 0.0001) inch for the STb bracket system and 0.459 mm ± 0.004 mm (0.0181 inch ± 0.0001) inch for the Incognito bracket system. Differences between systems were statistically significant (p < 0.05).ConclusionsThe analyzed bracket systems for lingual treatment exhibited significant differences in slot dimension that will clinically result in torque play. These aspects must be considered in lingual orthodontic treatment.

Stress analysis of an all-ceramic FDP loaded according to different occlusal concepts

The aim of this study was to investigate the influence of four different occlusal concepts on stress distribution in a four-unit fixed dental prosthesis (FDP) made of zirconia. A three-dimensional finite element (FE) model was created and four model versions with between 15 and 42 contact points were developed. A force of 300 N was distributed over the chewing surface according to the different occlusal concepts and virtually applied perpendicularly to the occlusal plane in each version. Then, FE analysis was performed and stress distribution evaluated. Generally, highest tensile stresses were located at the point of load application within the veneering layer and ranged from 104 to 204 MPa. The highest tensile stresses within the framework were located at the basal side of the connectors and ranged between 12 and 39 MPa. This study shows that the distribution of occlusal contact points significantly influences the stresses induced by occlusal forces in four-unit all-ceramic fixed dental prostheses and strongly suggests that the number and distribution of occlusal contacts may be important to minimise fatigue effects caused by cycling.

Tensile Properties of Orthodontic Elastomeric Chains

AbstractAim:Orthodontic elastomeric chains are a main component in orthodontic therapy employing labial or lingual appliances. The aim of this study was to investigate the tensile properties of orthodontic elastomeric chains with a test setup according to ISO 21606:2007.Material and Methods:Orthodontic elastomeric chains of eight manufacturers with and without an intermodular link were obtained from commercially-available stock, yielding 23 groups with ten specimens each. Samples were mounted in a universal testing machine and extended at a rate of 100 mm/min to four times the initial length and held for 5 seconds. After 5 seconds, the chain lengths were reduced to an extension of three times the initial length and held for 30 seconds before extension until failure. Forces at four times the test length (Fmax1), three times the initial length (Fmin) as well as force (Fmax2) and length (Lmax) at failure were recorded. Data were statistically analyzed by one-way analysis of variance using SPSS® 17. The level of significance was set at p = 0.05.Results:Statistical analysis revealed significant differences in Fmax1, Fmin, Fmax2 and Lmax among the various manufacturers. Fmax1 ranged between 9.1 N and 23.2 N, Fmin ranged between 1.5 N and 3.0 N, Fmax2 ranged between 15.7 N and 34.0 N, and Lmax ranged between 425% and 629% of the initial length.Conclusions:The tensile properties of different manufacturers of elastomeric orthodontic chains differ statistically significantly. Hence the amount of orthodontic force that is applied depends significantly on the type of elastomeric chains used in combination with labial or lingual appliances.ZusammenfassungZiel:Orthodontische elastomere Ketten sind fester Bestandteil der Therapie mittels festsitzender vestibulärer oder lingualer Apparaturen. Ziel dieser Studie war es, die werkstoffkundlichen Eigenschaften aktuell auf dem Markt befindlicher Ketten in einem an ISO 21606:2007 angelehnten Zugversuch zu untersuchen.Material und Methodik:Orthodontische elastomere Ketten acht verschiedener Hersteller sowohl mit als auch ohne einen intermodularen Steg wurden in 23 Gruppen mit jeweils zehn Probekörpern aufgeteilt. Die Probekörper wurden auf individuellen Halterungen in einer Universalprüfmaschine platziert und mit einer Vorschubgeschwindigkeit von 100 mm/min auf die vierfache Länge gedehnt und 5 Sekunden lang gehalten. In diesem Zustand wurde die Kraft Fmax1 ermittelt. Nach 5 Sekunden wurde der Dehnungszustand der Ketten auf die dreifache Länge reduziert. Nach 30-sekündigem Halten dieses Zustands wurde die Kraft Fmin ermittelt und anschließend die Probe bis zum Versagen gedehnt, wobei zum Zeitpunkt des Versagens die Kraft Fmax2 und die Länge Lmax ermittelt wurden. Die Daten wurden statistisch mittels einer einfaktoriellen Varianzanalyse mit Hilfe von SPSS® 17 ausgewertet, wobei das Signifikanzniveau auf p = 0,05 festgelegt wurde.Ergebnisse:Die ermittelten Werte Fmax1, Fmin, Fmax2 und Lmax unterschieden sich je nach Hersteller signifikant voneinander. Fmax1 betrug zwischen 9,1 N und 23,2 N, Fmin betrug zwischen 1,5 N und 3,0 N, Fmax2 betrug zwischen 15,7 N und 34,0 N und Lmax betrug zwischen 425% und 629% der initialen Länge.Schlussfolgerungen:Die elastomeren Eigenschaften orthodontischer Ketten unterscheiden sich in Abhängigkeit vom Hersteller signifikant voneinander. Daher hängt die Höhe der applizierten orthodontischen Kraft signifikant von der Auswahl der Kette ab.

Load-bearing capacity of implant-supported, tooth-supported, and combined zirconia-fixed dental prostheses.

Aim:The aim of this in vitro study was to investigate the influence of heterogeneous abutment supports on the load-bearing capacity of zirconia posterior four-unit fixed dental prostheses. Methods:A total of 18 specimens were fabricated and divided into 3 groups. Specimens in the first group were supported by teeth with simulated periodontal resilience, in the second group by the combination of a tooth with simulated periodontal resilience and an implant, whereas in the last group by implants only. The bridgeworks were loaded in a universal testing machine until fracture, and the load-bearing capacity was recorded. Statistical analysis was performed by one-way analysis of variance, with the level of significance set at 0.05. Results:Implant-supported zirconia restorations exhibited the highest loads at fracture (2689.3 N), followed by combined restorations (2144.2 N) and tooth-retained prostheses (2009.4 N). Statistical analysis revealed a significant influence of abutment support on the load-bearing capacity (P = 0.014). Visual inspection of the tested specimens showed different fracture patterns in the 3 groups. Conclusion:For all support scenarios, the recorded loads were much higher than maximum natural bite forces, so that all the tested prosthesis designs could be recommended for application in posterior areas. The long-term prognosis of such restorations should however be further evaluated within in vivo studies to recommend their use in the clinic.