The effect of thermal and mechanical cycling on bond strength of a ceramic to nickel-chromium (Ni-Cr) and cobalt-chromium (Co-Cr) alloys.

Purpose: This study evaluates the effect of thermo-mechanical cycling (TMC) on the bond strength of a ceramic to three cobalt-chromium (Co-Cr) and two nickel-chromium (Ni-Cr) alloys. Materials and Methods: One hundred metal-ceramic specimens were prepared. While half of the specimens from each metal-ceramic combination (n = 10) were tested after storage in water at 37°C for 24 hours, the other half were subjected to TMC before testing. The bond strength was evaluated by the flexural strength test according to ISO 9693:1999 (E) recommendations. Results: TMC decreased the bond strength of the tested metal-ceramic systems as compared to the water storage (control groups) (P=0.04). Although metal alloys were significantly different from each other in their bond strength with porcelain (P<0.001), the effect of TMC on the various metal-ceramic systems was not significantly different (P=0.99). Conclusion: It may be concluded that base metal-ceramic bond strength is affected by aging and the effect is relatively the same for all the tested porcelain-metal systems.

The effect of remelting various combinations of new and used cobalt-chromium alloy on the mechanical properties and microstructure of the alloy.

Context: Remelting previously cast base metal alloy can adversely affect the mechanical properties of the alloy and necessitates addition of new alloy. Aims: To study the effect of remelting different combinations of new and used cobalt-chromium (Co-Cr) alloy on its mechanical properties and microstructure. Materials and Methods: Using induction casting, 24 tensile test specimens were prepared for eight different combinations of new and used Co-Cr alloy. The test specimens were assessed for yield strength and percentage elongation. Microhardness was evaluated using Vickers's hardness tester. The tensile testing was carried out on a 50 kN servo-hydraulic universal testing machine. Microstructure analysis was done using an optical photomicroscope on the fractured samples after acid etching. Statistical Analysis: The mean values (±standard deviation) and coefficient of variation were calculated. Student's 't' test was used for statistical analysis. Statistical significance was assumed at P=.05. Results: The mean yield strength of eight different combination groups were as follows: group A: 849 MPa, group B 1 : 834 MPa, group B 2 : 915 MPa, group B 3 : 897 MPa, group C 1 : 874 MPa, group C 2 : 859 MPa, group D 1 : 845 MPa, and group D 2 : 834 MPa. The mean percentage elongation for the different groups were as follows: group A: 7%, group B 1 : 7%, group B 2 : 8%, group B 3 : 7%, group C 1 : 8%, group C 2 : 7%, group D 1 : 7%, and group D 2 : 8%. The mean hardness values were as follows: group A: 373 VHN, group B 1 : 373 VHN, group B 2 : 346 VHN, group B 3 : 346 VHN, group C 1 : 364 VHN, group C 2 : 343 VHN, group D 1 : 376 VHN, and group D 2 : 373 VHN. Conclusion: Repeated remelting of base metal alloy for dental casting without addition of new alloy can affect the mechanical properties of the alloy. Microstructure analysis shows deterioration upon remelting. However, the addition of 25% and 50% (by weight) of new alloy to the remelted alloy can bring about improvement both in mechanical properties and in microstructure.

Averiguaçäo radiográfica de possíveis defeitos de fundiçäo em armaçöes de próteses parciais removíveis / Quality evaluation of removable partial denture framework by x-rays

Cientes das grandes variáveis que podem determinar a ocorrência de fraturas em armaçöes de cobalto-cromo no momento de sua adaptaçäo pelo cirurgiäo-dentista e/ou durante o uso pelos pacientes, procurou-se, por meio de uma técnica radiográfica simples, identificar a presença de defeitos estruturais (áreas de maior ou menor radioluscência), a fim de evitar perda de tempo e despesas envolvidas com o uso de laboratório, protético, ou a responsabilizaçäo do paciente pela fratura. Foram realizadas armaçöes de P. P. R. para os pacientes atendidos na Clínica de Prótese da Faculade de Odontologia de Bauru-USP, utilizando-se as ligas Still-Dent e Alloy, respectivamente nos anos de 1990 e 1991, tendo sido radiografadas 82 armaçöes previamente identificadas e numeradas. O processamento das películas radiográficas expostas foi realizado, obedecendo-se rigorosamente às recomendaçöes do fabricante, e as películas foram avaliadas por 2 examinadores previamente calibrados. As armaçöes fundidas com liga Alloy apresentaram uma melhor qualidade que as em Still-Dent. Somente por intermédio de radiografias das armaçöes, feitas rotineiramente, poder-se-ía identificar possíveis defeitos estruturais que culminariam em fracasso do aparelho

Biocompatibilidade de algumas ligas de Cr-Ni: estudo experimental em ratos / Biocompatibility of some Cr-Ni alloys: experimental study in rats

Os autores implantaram, subcutaneamente, corpos de prova metálicos em ratos. Exames histológicos foram feitos após 10, 20, 40 e 80 dias. Das três ligas utilizadas, duas mostraram melhor biocompatibilidade, formando cápsulas fibrosas. Uma das ligas apresentou aspecto de corrosäo superficial