High-strength CAD/CAM-fabricated veneering material sintered to zirconia copings--a new fabrication mode for all-ceramic restorations.

OBJECTIVES: With this in vitro study the fracture strength of zirconia-based crown copings being veneered with a CAD/CAM generated high-strength ceramic cap by sintering is compared with anatomically identical zirconia-based crowns, which were either overpressed or veneered by the layering technique for completion. METHODS: A 1.2mm, 360 degrees chamfer preparation was performed on a second maxillary molar and was dublicated 15 times in a cobalt-chromium-alloy. A sample of 45 zirconia copings was produced and divided into three groups. In the first group (VT) zirconia copings received conventional veneering in layering technique, in the second group the veneering porcelain was pressed over the zirconia coping (PT), and for the third group (ST) a CAD/CAM-fabricated high-strength anatomically shaped veneering cap was sintered onto the zirconia coping. All crowns were cemented conventionally onto their dies and tested in the universal testing machine until clinical failure. The fracture load data were compared by a one-way analysis of variance and a multiple comparison posthoc test (alpha<0.05). RESULTS: Specimens from group VT showed a mean (S.D.) fracture load of 3700.39 (1238.72) N, group OT 3523.73 (1181.11) N and group ST 6262.67 (2257.42) N. The difference between groups VT/OT and ST were statistically significant (P<0.001). SIGNIFICANCE: The new CAD/CAM-fabricated bilayered restorations (ST) were superior to the present techniques (VT and OT) in terms of fracture load and offer the possibility to produce cost-effective crowns and fixed partial dentures with a potential lower risk of chippings.

Corrosion resistance measurements of dental alloys, are they correlated?

OBJECTIVES: The aim was to assess in vitro the resistance to corrosion of eight commercial dental alloys by two quantitative methods, electrochemical and immersion tests, then to statistically test the hypothesis of possible correlation between the polarization resistance (R(p)) and the elemental release. METHODS: Two quantitative methods; electrochemical and immersion test, were used. From the first, after recording the OCP during 24h immersion in acidified artificial saliva (pH 2.3), R(p) was obtained using the linear polarization in anodic path and applying the Mansfeld's method. From the static immersion test, using the same test solution, the elemental release from was analysed and determined using the ICP-AES. Thereafter, the two measurements were used to plot the regression line and to determine the correlation coefficient. The significance of the correlation was tested using F-test at a confidence interval of 0.99. RESULTS: : The resistance to corrosion results obtained from the two methods were ranked and compared; an inverse relation between them was evident. Then, the obtained coefficient of correlation (R(2)) was 0.886. With the F-test at 0.99 confidence interval, the hypothesis was accepted as the calculated F was about 44 against critical F=13.7. CONCLUSION: The correlation between the two measurements, R(p) and mass loss, was proved statistically significant. This result may provide a new approach to predict the corrosion behaviour of dental alloys by firstly using the easier methods.

[In vitro fracture resistance tests of 3-unit posterior bridges produced by the Procera process]. / In-vitro-Bruchlasttests von 3-gliedrigen Seitenzahnbrücken hergestellt im Procera-Verfahren.

The aim of this investigation was to evaluate the fracture resistance of three-unit fixed partial dentures manufactured with the Procera-System in vitro. Metal abutments representing the teeth 25 and 27 were prepared with a 90 degrees circumferential internally rounded shoulder. The abutments were duplicated with polyvinysiloxane, and master dies were fabricated. Using the Procera-System, 21 aluminiumoxid frameworks were manufactured. Twelve of these were veneered with porcelain in a standardized manner. All non-veneered and veneered frameworks were cemented on the original metal abutments and loaded to fracture. The mean fracture loads of the non-veneered and veneered frameworks were 477 N and 645 N, respectively. The Weibull modules of the test series were 4.7 for the non-veneered and 4.0 for the veneered frameworks. The in vitro results of this investigation seem to indicate that the use of fixed partial dentures manufactured with the Procera-System are clinically questionable. Furthermore, the low Weibull modules point to a limited fracture predictability. Assuming that chewing forces up to 600 N may occur in the molar region and loading of dental ceramics causes fatigue and weakening over time, the clinical application may not be recommended.

Corrosion susceptibility of lingual wire extensions in removable appliances. An in vitro study.

MATERIAL AND METHODS: The corrosion resistance of ten different round orthodontic wires as plastic-encased extensions was evaluated in vitro. Following the production of defined test samples with free as well as with acrylic-embedded wire segments, ten samples of each product were subjected to gap provocation by deflecting the free-running wire ends with an electric drive, while 10 samples remained provocation-free. The tests were run under standardized conditions (7 days, 37 degrees C, pH 2.3, 0.1 mol NaCl/CH3CHOHCOOH), and the findings were analyzed by stereo light microscopy and scanning electron microscopy. RESULTS: The following alterations were observed on the wire surfaces: a) crevice corrosion with substantial linear surface erosion and opaque discolorations, b) localized pitting corrosion, and c) mild surface erosion with translucent discolorations. The reactions of the test samples were similar in both those that were subjected to addition mechanical loading in the electrolyte and those that were not. Cr-Ni steels were susceptible to corrosion, whereas Co-Cr alloys as well as low-nickel manganese steels had only slight visible alterations, if any, in the embedded wire sections. CONCLUSIONS: Clinically occurring discolorations in the acrylic can be attributed to corrosion processes on the embedded wire extensions. These processes can be simulated and observed in vitro. During the processing of acrylic materials now customary in the orthodontic laboratory, crevices are inevitably formed between the wire and the acrylic, furthering the corrosion process. Our results suggest that, being more resistant to crevice corrosion, Co-Cr alloys or low-nickel steels can be considered as alternatives.