Guilherme Bottene Guarda
INFLUÊNCIA DE SUCESSIVAS TERMO-PRENSAGENS NAS
PROPRIEDADES MECÂNICAS, TRANSMISSÃO DA LUZ E
ANÁLISE MICROESTRUTURAL DA CERÂMICA PRENSADA
REFORÇADA POR DI-SILICATO DE LÍTIO
INFLUENCE OF REPEATED HEAT-PRESSING ON MECHANICAL
PROPERTIES, LIGHT TRANSMISSION AND MICROSTRUCTURAL
ANALYSIS OF PRESSABLE CERAMIC REFORCED BY LITHIUM
DISILICATE
PIRACICABA 2015
Universidade Estadual de Campinas
Faculdade de Odontologia de Piracicaba Guilherme Bottene Guarda
INFLUÊNCIA DE SUCESSIVAS TERMO-PRENSAGENS NAS PROPRIEDADES MECÂNICAS, TRANSMISSÃO DA LUZ E ANÁLISE MICROESTRUTURAL DA
CERÂMICA PRENSADA REFORÇADA POR DI-SILICATO DE LÍTIO
INFLUENCE OF REPEATED HEAT-PRESSING ON MECHANICAL
PROPERTIES, LIGHT TRANSMISSION AND MICROSTRUCTURAL ANALYSIS OF PRESSABLE CERAMIC REFORCED BY LITHIUM DISILICATE
Tese apresentada à Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas como parte dos requisitos exigidos para a obtenção do título de Doutor em Materiais Dentários.
Thesis presents to the Piracicaba Dental School of the University of Campinas in partial fulfillment of the requirements for the degree of Doctor in Dental Materials.
Orientador: Prof. Dr. Lourenço Correr Sobrinho
Este exemplar corresponde à versão final da tese defendida por Guilherme Bottene Guarda e orientada pelo Prof. Dr. Lourenço Correr Sobrinho.
_______________________________ Assinatura do Orientador
PIRACICABA 2015
RESUMO
O objetivo neste estudo foi avaliar a dureza Vickers, resistência à flexão biaxial, análise microestrutural e transmissão da luz da cerâmica vítrea prensada por calor após repetidas re-prensagem. Discos de cerâmica de IPS e.max Press (12,0 mm de diâmetro x 0,9 mm de espessura) foram prensadas e usadas como grupo controle (TP1). Sprues e botões cerâmicos restantes da primeira prensagem foram reaproveitados e utilizados para confecionar os grupos testes de re-prensagem (TP2 e TP3). Todos os procedimentos das termoprensagens foram feitas de acordo com as recomendações do fabricante. O teste de resistência à flexão biaxial (BFS) foi realizado para determinar a resistência dos discos cerâmica vítrea prensada (TP1) e re-prensada (TP2). Doze amostras por grupo (n=12) foram testadas, com velocidade de aplicação de carga de 0,5 mm/min., na máquina de ensaio universal (Instron). Para o teste de dureza Vickers foi realizado em um mucrodurômetro HMV 2 (Shimadzu) com carga de 500 gf por 15 segundos. Dez discos de cerâmica vítrea foram foram confeccionadas para primeira prensagem (TP1) e dez para cada reprensagem (TP2 e TP3), realizando 5 endentações por amostras. As características de superfície foram analisadas em MEV. Características de irradiancia e de espectro de luz transmitida através da cerâmica foram medidos utilizando-se um potenciômetro e um espectrômetro de luz. Os dados foram submetidos à analise de variância e teste de Tukey post-hoc (p <0,05). Os valores BFS em MPa (média e desvio-padrão) foram para TP1 (279,7 ±12,5), para TP2 (230,3 ±7,1) e para TP3 (220,8± 8,6). Os valores de TP1 foram significativamente maiores que os grupos TP2 e TP3. Os valores de dureza Vickers foram para TP1 (638,1 ±11,5), para TP2 (592,6± 6,6) e para TP3 (590,4 ±7,6). A dureza de TP1 foi significativamente maior que os grupos TP2 e TP3. As
micrografias do material cerâmico em MEV mostrou aumento no tamanho e concentração dos cristais de disilicato de lítio (LeDiSi) após re-prensagem. A análise das imagens, re-prensagem mostrou homogeneidade na distribuição dos cristais de disilicato de lítio e aumento na densidade dos cristais. Redução na transmissão da luz foi detectada com aumento da espessura da cerâmica, alterações na emissão foi observada após repetidas prensagens. Conclui-se que a reutilização de material em repetidas re-prensagens diminuiu significativamente as propriedades de resistência à flexão biaxial e a dureza da cerâmica reforçada por di-silicato de lítio IPS e.max Press, alterou a microestrutura e influenciou na transmissão da luz através da cerâmica.
Palavras-chave: Cerâmicas, Re-prensagem, Dureza Vickers, Resistência à flexão biaxial.
ABSTRACT
The aim of this study was to evaluate the Vickers hardness, biaxial flexural strength, microstructural analysis and light transmission of heat-pressed glass-ceramic material after repeated pressing. Ceramic discs of IPS e.max Press (12.0 mm in diameter x 0.9 mm thickness) were heat-pressed and used as control (HP1). Sprue and button parts of the heat-pressed material were retrieved and used for repeated heat-pressing to construct discs of two and three re-pressed groups (HP2 and HP3). All the heat-pressed casting procedures were performed according to the manufacturers' instructions. A biaxial flexural strength (BFS) test was performed to determine the strength of pressed (HP1) and re-pressed (HP2 and HP3) glass-ceramic disc specimens (n = 12) at a crosshead speed of 0.5 mm/min., in the universal testing machine (Instron). Vickers hardness test was conducted in a microhardness tester HMV-2 (Shimadzu) with a load of 500 g applied for 15 s. Ten glass-ceramic discs were made for pressed (HP1) and ten for re-pressed (HP2 and HP3) and five indentations were made for each disc. Surface characteristics were examined with SEM. Light irradiance and spectrum characteristics transmitted through ceramic were measured using a power meter and a light spectrometer. Data were submitted to ANOVA and Tukey’s post-hoc test (p<0.05). The BFS values in MPa (mean ± standard deviation) were HP1 (279.7±12.5); HP2 (230.3±7.1) and HP3 (220.8±8.6). HP1 was significantly higher than HP2 and HP3. The VH values were HP1 (638.1±11.5); HP2 (592.6±6.6) and HP3 (590.4±7.6). HP1 was significantly higher than HP2 and HP3. In conclusion, repeated heat-pressing treatment produce a statistically significant decreased in the biaxial flexural strength and Vickers hardness of the IPS e.max Press glass-ceramic, microstructural alterations and alterations in light transmission throughout ceramic discs.
Key words: Ceramic, Repressing, Vickers hardness, Biaxial flexural strength, Light transmission, Surface characterization
SUMÁRIO
DEDICATÓRIA - - - xiii
AGRADECIMENTOS ESPECIAS - - - xv
AGRADECIMENTOS - - - xvii
INTRODUÇÃO - - - - - - - - - 01
CAPÍTULO I –
Influence of Repeated Heat - Pressing on Properties of Pressable Ceramic- - - 05
CAPÍTULO II –
Recycling Heat-Pressed Ceramics: Light Transmission and Microstructural- - - 18
CONCLUSÃO - - - - 30
REFERÊNCIAS - - - 31
APÊNDICE - - - 36
Dedico esse trabalho...
À minha esposa Gabriela
Pela união, compreensão, amor e incentivo...
À minha Família
AGRADECIMENTOS ESPECIAIS
Ao meu orientador Prof. Dr. Lourenço Correr Sobrinho. Pela confiança dada nos principais momentos, sabedoria que soube passar, paciência nos momentos mais difíceis, amizade. Foi muito importante e gratificante estar ao seu lado em todos esses anos de parceria. Foi um grande aprendizado.
AGRADECIMENTOS
Ao Magnífico Reitor da Universidade Estadual de Campinas, Prof. Dr. José Tadeu Jorge.
À Faculdade de Odontologia de Piracicaba – UNICAMP, nas pessoas do Diretor, Prof. Dr. Guilherme Elias Pessanha Henriques e Diretor Associado Prof. Dr. Francisco Haiter Neto.
À Coordenadoria de Pós-Graduação e à Coordenadoria do Programa de Pós-Graduação em Materiais Dentários, pela chance de atualizar meu conhecimento cursando Pós-Graduação em nível de Doutorado.
Ao corpo docente do Programa de Pós-Graduação em Materiais Dentários da Faculdade de Odontologia de Piracicaba – UNICAMP:
Prof. Dr. Simonides Consani, pela serenidade e sabedoria compartilhadas;
Prof. Dr. Mario Fernando de Góes, pela seriedade na transmissão dos conhecimentos;
Prof. Dr. Mário Alexandre Coelho Sinhoreti, pela amizade e por sempre ser solicito com os alunos;
Profª. Drª. Regina Maria Puppin Rontani pela alegria, disposição em ensinar;
Prof. Dr. Marcelo Gianinni pelo exemplo de pesquisador e professor;
Prof. Dr. Rafael Xediek Consani pela contribuição criteriosa durante as aulas do curso de mestrado e doutorado;
Prof. Dr. Luis Roberto Marcondes Martins pela experiência compartilhada.
Aos funcionários, Selma e Marcos, do Programa de Pós-Graduação em Materiais Dentários da Faculdade de Odontologia de Piracicaba – UNICAMP, com quem tive contato durante esse período de formação.
Aos amigos do Programa de Pós-Graduação em Materiais Dentários da Faculdade de Odontologia de Piracicaba – UNICAMP, que tornaram este caminho mais prazeroso e tranqüilo.
Aos professores e alunos da FOP – UNICAMP pela oportunidade de convivência construtiva.
A todos meus amigos que não são dessa faculdade e que sempre me apoiaram para que obtivesse o sucesso.
Ao órgão de fomento à pesquisa que financiou este estudo, CNPq.
À todas as demais pessoas que foram importantes para a execução do trabalho.
INTRODUÇÃO
Dentre os materiais restauradores estéticos, a cerâmica odontológica pode ser considerada o material de escolha para reproduzir os dentes naturais. As cerâmicas são rotineiramente utilizadas em restaurações de cerâmicas puras, próteses parciais e totais fixas e retentores intra-radiculares estéticos. Apesar das cerâmicas serem historicamente muito antigas, o emprego rotineiro como material restaurador estético é mais recente e promoveu nova etapa na Odontologia estética. O uso clínico em diversos procedimentos odontológicos consagrou-a por apresentar várias características desejáveis para restaurar os dentes naturais, destacando-se maior resistência à compressão e abrasão, estabilidade química, biocompatibilidade, menores condutibilidades térmica e elétrica, difusibilidade térmica, translucidez, fluorescência, propriedades estéticas favoráveis e coeficiente de expansão térmica similar ao do dente (Sherril & O’Brien, 1974; Oilo, 1977; Anusavice, 1996; Della Bona, 1996; van Noort, 2002; Borges et al., 2003).
Segundo Conrad et al. (2007), essas características têm promovido crescente aceitação entre os profissionais e pacientes em relação à reabilitação com cerâmica pura. Além disso, tem sido utilizada para confecção de bráquetes estéticos para tratamento ortodôntico (Anusavice, 2005). Recentemente, os avanços tecnológicos possibilitaram o desenvolvimento de novos materiais com maior resistência, como as cerâmicas usinadas, infiltradas por vidro e termo-prensadas.
Dentre as cerâmicas termo-prensadas, na década de 90 foram introduzidas no mercado dois sistemas cerâmicos reforçados por leucita (IPS Empress e
Optimal Pressable Ceramic - OPC) indicados para confecção de coroas unitárias anteriores e posteriores, inlays, onlays e facetas laminadas (Chung et al., 2009). Posteriormente, foi introduzida a cerâmica IPS Empress Esthetic, com grãos de leucita menores, compactados e distribuídos homogeneamente na fase vítrea visando a melhorar as propriedades mecânicas (Ivoclar Vivadent).
Com a finalidade de abranger o uso para confecção de prótese parcial fixa de três elementos envolvendo até o segundo pré-molar foi desenvolvida a cerâmica reforçada com 60% de cristais de disilicato de lítio, conhecida como IPS Empress 2. Recentemente, a cerâmica IPS e.max Press foi desenvolvida com 70% de cristais de dissilicato de lítio objetivando aumentar a resistência mecânica (Guarda et al., 2013).
O sistema IPS e.max Press está disponível em pastilhas com diferentes cores para várias situações clínicas e as restaurações são confeccionadas pelo processo de termo-prensagem. Durante os procedimentos de laboratório, as pastilhas liquefeitas são injetadas no interior do molde de revestimento, com pressão de um êmbolo de alumina, no forno para prensagem. A termo-prensagem aliada à precisão oclusal, adaptação marginal e alta translucidez tornou esse sistema uma das referências como material restaurador cerâmico (Ivoclar Vivadent).
Após o procedimento de prensagem e esfriamento, o material excedente preenchendo os espaços anteriormente ocupados pelo canal de alimentação é retirado e descartado. Entretanto, devido ao alto custo, alguns laboratórios protéticos têm adotado como procedimento de rotina a reutilização do material
excedente para outras prensagens, submetendo-o a maior número de queima do que aquele recomendado pelo fabricante.
Por outro lado, as queimas adicionais além de alterar o comportamento mecânico das cerâmicas reforçadas por leucita podem também influenciar as propriedades estéticas pelo aumento da reflexão da luz (Denry et al., 2001) ou pela variação do coeficiente de transmissão óptico decorrente da maior concentração de leucita. Este fato ocorre porque a fase cristalina apresenta índices luminosos de difração e refração distintos da fase vítrea (Kontonasaki et al., 2008). Repetidas termo-prensagens influenciam a textura do material alterando significativamente a resistência à flexão biaxial e a microesturutra da cerâmica à base de dissilicato de lítio IPS Empress 2 (Chung et al., 2009).
Entretanto, pouco se sabe sobre o comportamento da cerâmica IPS e.max à base de dissilicato de lítio quando submetida às queimas adicionais. Dessa forma seria importante avaliar as propriedades mecânicas, características microestruturais e propriedades ópticas da luz transmitida através de cerâmica vítrea reforçada por dissilicato de lítio IPS e.max Press após repetidas termo-prensagens.
O objetivo neste estudo in vitro foi avaliar:
1 – A dureza Vickers e a resistência à flexão biaxial da cerâmica termo-prensada à base de disilicato de lítio, após repetidas termo-prensagens (Capítulo 1);
2 – As características microestruturais e a transmissão da luz da cerâmica termo-prensada à base de disilicato de lítio, após repetidas termo-prensagens
(Capítulo 2);
A hipótese testada seria que após repetidas termo-prensagens, os materiais reciclados alterariam a dureza Vickers, resistência à flexão biaxial, características microestruturais e a transmissão da luz dos materiais prensado inicialmente.
Capítulo 1
Influence of Repeated Heat-Pressings on Ceramic Properties Artigo submetido à publicação no periódico Brazilian Dental Journal.
Abstract
The purpose of this study was to evaluate the Vickers hardness (VH) and biaxial flexural strength (BFS) of glass-ceramic after repeated heat-pressings. IPS e.max Press Ceramic discs (12 mm in diameter x 0.9 mm thickness) were heat-pressed and used as control (HP1). Retrieved sprue of the single heat-pressed ceramic was used for repeated heat-pressings groups (HP2 and HP3). All the heat-pressed casting procedures were performed according to the manufacturer’s instructions. Biaxial flexural strength (BFS) test was performed for pressed (HP1) and re-pressed (HP2 and HP3) glass-ceramic specimens (n=12) at crosshead speed of 0.5 mm/min. Vickers hardness test was conducted in a microhardness tester HMV-2 (Shimadzu) with a load of 500 gf for 15 s. Ten glass-ceramic specimens were made for each pressed (HP1) and re-pressed (HP2 and HP3). Five indentations were made for each specimen. Data were submitted to two-way ANOVA and Tukey’s post-hoc test (p<0.05). BFS values in MPa (mean ± standard deviation) were HP1 (279.7 ± 12.5); HP2 (230.3 ± 7.1) and HP3 (220.8 ± 8.6). HP1 was significantly higher than HP2 and HP3. VH values were HP1 (638.1 ± 11.5); HP2 (592.6 ± 6.6) and HP3 (590.4 ± 7.6). HP1 was significantly higher than HP2 and HP3. Conclusion: Repeated heat-pressings significantly decreased the BFS and VH for IPS e.max Press glass-ceramic.
Introduction
Dental ceramics are gradually dominating the dental market mainly due their satisfactory mechanical properties, such as higher mechanical resistance, chemical stability, lower thermal conductivity, biocompatibility and esthetic (Höland et al., 2000). Besides, all ceramic restorations are intended to enhance the aesthetic properties of rehabilitation (Anusavice, 1996; van Noort, 2002; Borges et al., 2003, Zhang et al., 2013; Akar et al., 2014). However, ceramics are susceptible to fracture under occlusal forces mainly due the presence of microcracks on the surface (van Noort, 2002, MacLean e Hughes, 1965).
The technique based of heat-pressed ceramic has been used to make of all-ceramic restorations (Chung et al., 2009). The pressable lithium disilicate glass ceramic IPS e.max Press (Ivoclar Vivadent, Schaan, Liechtenstein) was introduced recently with increased strength, durability, translucency with improved esthetics (Heintze et al., 2011). IPS e.max Press is available in ingots with different shades. The ingots are suitable for the fabrication of frameworks or fully restorations. Due to the use of new technologies and optimized processing parameters, the formation of defects in the bulk of the ingot is avoided (Holand et al., 2000).
In laboratory procedures, the ingots of the ceramic are heat-pressed into a mold with plunger under pressure in a furnace. After cooling, the sprue and remaining material (button) are removed. These residual materials should be discarded; however, in some dental laboratories these materials are used for re-pressing. This fact can cause possible microstructural changes after repeated
heat-pressings. Previous studies showed that is important evaluate the influence of repeated heat-pressing on mechanical properties of lithium disilicate glass-ceramic to determine the viability of the ceramic recycling (Heintze et al., 2011; Gehrt et al., 2013; Solá-Ruiz et al., 2013).
Therefore, the aim of this study was to evaluate the Vickers hardness and biaxial flexural strength of pressed glass-ceramic after repeated pressings treatment. The hypotheses tested were that after repeated heat-pressings, the recycled ceramic materials would maintain the biaxial flexural strength and Vickers hardness similar of the original single pressed material.
2. Material and methods 2.1. Ceramic discs
Sixty-six ceramic discs (12 mm in diameter x 0.9 mm thickness) were fabricated with ceramic IPS e.max Press (IvoclarVivadent), in accordance to the manufacturer’s instructions. Disc wax patterns with (12 mm in diameter x 1.2 mm thickness) were prepared using a silicone putty mould (Express STD; 3M ESPE, St Paul, MN), sprued, attached to base former with surrounding paper and invested with phosphate-based material (IPS PressVest Speed, IvoclarVivadent, Schaan, Liechtenstein). The wax was eliminated in an automatic furnace (Vulcan A- 550, Degussa-Ney, Yucaipa, CA, USA) at 850oC for one hour. IPS e.max Press ceramic ingots were heat-pressed at 920°C (into the investment molds in an automatic press furnace (EP 600, Ivoclar Vivadent) and used as control (HP1). The sprues and buttons parts were separated from the discs, adjusted to adequate dimensions
to allow proper insertion into refractory investment molds and used for repeated heat-pressings to make discs for two and three re-pressed groups (HP2 and HP3). The specimens of HP2 and HP3 were pressed following the same procedures adopted for of HP1. In order to obtain flat surface for hardness testing and biaxial flexure tests, the specimens were submitted to wet polishing with 180-, 320-, 400-, 600- and 1200-grit SiC papers (Norton SA, São Paulo, SP, Brazil) in a water-cooled automatic polisher (APL4; Arotec, Cotia, SP, Brazil).
2.2. Vickers Hardness Test
Vickers hardness measurements were performed with a microhardness tester (HMV-2; Shimadzu Corp., Tokyo, Japan) under a load of 500 gf for 15 s. Five indentations were made in two parallel lines and one on the center of the surface of each specimens. Each reference line was located at a distance of 1 mm opposite edges of the disc. With an auxiliary measurement tool, the diagonal of indentation size (d) was measured and the VHN determined according to the equation: DV = 0.1891 F / d² (ISO 6507), where F = applied load. The average values of the five readings was recorded as Vickers hardness (VH). Ten glass-ceramic discs (n=10) were made for pressed (HP1), ten for re-pressed discs HP2, and ten for re-pressed discs HP3. Data were submitted to ANOVA and Tukey’s post-hoc test (p=0.05).
The bi-axial fracture strength for each specimen was determined using a loading on the central polished surface of the specimen using a 10 mm diameter knife-edge support with a spherical ball indenter (4 mm diameter) at a crosshead speed of 0.5 mm/min until failure using a mechanical testing machine (model 4411; Instron; Canton, MA, USA). The bi-axial fracture strength was calculated according to the following equation (ISO 6872):
S = - 0.2387 P(X - Y) / d2 where,
S = bi-axial fracture strength (MPa); P = total load causing the fracture (N); X=(1+v)ln(B/C)2 + [(1-v)/2](B/C)2, Y=(1+v)[1+ln(A/C)2] + (1-v)(A/C)2; where v is the Poisson’s ratio, A is the radius of the support circle (mm), B is the radius of the of the piston tip (mm), C is the radius of the specimen (mm) and D the specimen thickness at fracture origin (mm). Poisson’s ratio= 0.24 for lithium disilicate ceramic (Albakry, 2003). Data were submitted to two-way ANOVA and Tukey’s post-hoc test (p=0.05).
Results
Table 1 presents the results for the biaxial flexural strength (BFS). The statistical analysis showed that the resistance of the specimens subjected to single heat-pressing (HP1) was significantly higher than specimens subjected to two (HP2) or three (HP3) heat-pressings (p<0.00047). No significant difference was found between groups HP2 and HP3.
Table 1 – Mean and standard deviation for biaxial flexural strength (MPa). Group Biaxial Flexural Strength (MPa)
HP1 279.7±12.5 a
HP2 230.3±7.1 b
HP3 220.8±8.6 b
Means followed by different lowercase letters indicate significantly differences by Tukey’s test (p < 0.05).
Table 2 shows the results of the Vickers hardness (VH). Statistical analysis indicated that the Vickers hardness of the specimens subjected to single heat-pressing (HP1) was significantly higher than specimens subjected to two (HP2) and three (HP3) heat-pressings (p<0.00118). No significant difference was found between groups HP2 and HP3.
Table 2 – Mean and standard deviation for Vickers hardness (VH).
Group Vickers hardness (VH)
HP1 638.1±11.5 a
HP2 592.6±6.6 b
HP3 590.4±7.6 b
DISCUSSION
In the present study, the hypothesis that repeated heat-pressings with recycled materials would maintain biaxial flexural strength and Vickers hardness similar to single pressed material was rejected. The results showed that repeated heat-pressings treatment significantly decreased the biaxial flexural strength and Vickers hardness of the IPS e.max Press glass-ceramic. No significant difference was found between HP2 and HP3 groups. The BFS values obtained was 279.7; 230.3 and 220.8 MPa for single (HP1), two (HP2) and three (HP3) heat-pressings, respectively. The values after single heat-pressing is according to the range showed in previous studies, where BFS value ranged from 265.5 to 281.2 MPa (Cattell et al., 2002; Pagniano et al., 2005; Chung et al., 2009). However, these BFS values were lower than those obtained in previous works, as 354.46 MPa (Tang et al., 2014), 400 MPa (Holland et al., 2000) and 407 MPa (Albakry et al., 2003). This difference is probably due to variable, such as specimen prepared with annealing and three point test method. This variable may have contributed to higher flexural strength, as claimed by previous studies (Cattell et al., 2002; Chung et al., 2009).
When the BFS for the three pressings was compared single heat-pressing was significantly higher values than for two or three heat-heat-pressings. Recently, some studies showed that two repeated heat-pressings modified the strength of lithium disilicate glass ceramic material (Albakry et al., 2004a, Chung et al., 2009, Tang et al., 2014). In the current study, no difference was found between
two and three heat-pressings. However, until moment no study in the literature analyzed three repeated heat-pressings.
Decreased BFS occurred when single heat-pressing was compared to two heat-pressings (Albakry et al., 2004a; Tang et al., 2014). According to Tang et al., (2014) after repeated heat-pressings, the microstructure of a crystal size and distribution indicate that intergranular cracks propagate easily through the residual glass matrix, decreasing the strength. Crack microstructure and porosity on the lithium disilicate crystals after two heat-pressings may initiating the fracture and limited the strength of the specimens (Albakry et al., 2004a and 2004b; Tang et al., 2012a). Besides, changes on the lithium disilicate glass-ceramic microstructure occurred after repeated heat-pressings and produced larger grains and different orientation of the crystals (Albakry et al., 2004a and 2004b; Zheng et al., 2008; Chung et al., 2009; Cheng et al., 2010; Yuan et al., 2013).
Conversely, after repeated heat-pressings, the strength of Empress 2 increased significantly (Chung et al., 2009). The XRD showed that no significant change occurred in the nature of the crystalline phases for lithium disilicate after repeated heat-pressings. Higher strength after repeated heat-pressings probably can be attributed to homogeneous crystallization of interlocked needle-like crystals with appropriate ratio (Cattell et al., 2002; Wang et al., 2010; Kang et al., 2013; Yuan et al., 2013). Besides, the glass-ceramic strength can increase significantly after annealing, probably due to residual stress relaxation (Fisher et al., 2005; Denry & Holloway, 2004).
In relation to Vickers hardness, this study showed that after two (HP2) or three (HP3) heat-pressings, the hardness decreased significantly when compared to single (HP1) heat-pressing. This result probably has relation with ceramic porosity and density. It has been claimed that when porosity is lower and density is higher, the hardness is higher for ceramic bulk (Yang et al., 2009; Chavan et al., 2011; Tang et al., 2012a).
The Lithium disilicate based ceramic restorations must have higher hardness and strength to reduce possible restoration problems. However, the present study showed that repeated heat-pressings significantly decreased the biaxial flexural strength and Vickers hardness for IPS e.max Press glass-ceramic. The results of this study using repeated heat-pressings for lithium disilicate glass-ceramic showed decrease values in the biaxial flexural strength and Vickers hardness. Therefore, care should be taken in dental laboratories, regardless the ceramic type submitted to repeated heat-pressings. Future studies should be carried out to clarify other mechanical properties of the lithium disilicate ceramic under effect of two or three heat-pressings.
CONCLUSION
In this study, the effect of repeated heat-pressings treatment significantly decreased the biaxial flexural strength and Vickers hardness of the IPS e.max Press glass-ceramic.
ACKNOWLEDGMENTS
The authors are gratefull to CNPq (National Council for Scientific and Technological Development, Grant # 303928/2009–3) for the study support at Piracicaba Dental School, SP, Brazil.
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6 – Akar GC, Pekkan G, Cal E, Eskitasçıoglu G & Ozcan M (2014) Effects of surface-finishing protocols on the roughness, color change, and translucency of different ceramic systems Journal Prosthetic Dentistry 11(2) 314- 321.
7 - MacLean JW & Hughes TH (1965) The reinforcement of dental porcelain with ceramic oxides Brazilian Dental Journal 119 251-267.
8 - Chung KH, Liao JH, Duh JG, Chan DC (2009) The effects of repeated heat-pressing on properties of pressable glass–ceramics. Journal Oral Rehabilithation 36 132–141.
9 - Heintze, S.D., Albrecht, T., Cavalleri, A &Steiner, M (2011) A new method to test the fracture probability of all-ceramic crowns with a dual-axis chewing simulator. Dental Materials 2 710–19.
10 – Gehrt M, Wolfart S, Rafai N, Reich S &Edelhoff D (2013) Clinical results of lithium-disilicate crowns after up to 9 years of service. Clinic Oral Investigation 17 275–284.
11 – Sola´-Ruiz MF,Lagos-FloresE,Roman-RodriguezJL,Highsmith JdelR,Fons-FontA&Granell-RuizM (2013) Survival rates of a lithium disilicate-based core ceramic for three-unit esthetic fixed partial dentures:a10-year prospective study. International Journal of Prosthodontics 26 175–180.
12 - International Organization for Standardization No 6872, Dental Ceramics. 3rd ed.. Geneva, Switzerland: International Organization for Standardization; 2008. 13 –Albakry M, Guazzato M& Swain MV (2003) Biaxial flexural strength, elastic moduli, and x-ray diffraction characterization of three pressable all-ceramic materials. Journal Prosthetic Dentistry 89 374 - 380.
14 – Cattell MJ, Palumbo RP, Knowles JC, Clarke RL & Samarawickrama DY(2002) The effect of veneering and heat treatment on the flexural strength of Empress R2 ceramics Journal of Dentistry 30 161–169.
15 – Pagniano RP, Seghi RR, Rosenstiel SF, Wang R & Katsube N (2005) The effect of a layer of resin luting agent on the biaxial flexure strength of two all-ceramic systems Journal of Prosthetic Dentistry 93 459 - 466.
16 – Tang X, Tang C, Su H, Luo H, Nakamura T & Yatani H (2014) The effects of repeated heat-pressing on the mechanical properties and microstructure of IPS e.max Press Journal Mech Behav Biomedical Materials 40 390-396
17 –Albakry M, Guazzato M & Swain MV (2004a) Biaxial flexural strength and microstructure changes of two recycled pressable glass ceramics Journal Prosthodontic 13 141 -149.
18 - Albakry M, Guazzato M & Swain MV (2004b) Influence of hot pressing on the microstructure and fracture toughness of two pressable dental glass-ceramics Journal Biomedic Materials Research B Applied Biomaterials 71 99 - 107
19 – Tang X, Nakamura T, Usami H, Wakabayashi K & Yatani H (2012a) Effects of multiple firings on the mechanical properties and microstructure of veneering ceramics for zirconia frameworks Journal of Dentistry 40 372–380.
20 - Zheng X Wen G Song L & Huang X (2008) Effects of P2O5 and heat treatment
on crystallization and microstructure in lithium disilicate glass ceramics. Acta Materials 56 549 – 558
21 - Cheng J, Xiong D, Li H, Wang H (2010) Crystallization behaviors of R2O– Al2O3–SiO2 glass–ceramics for use as anodic bonding materials Journal Alloys Complied 507 531–534.
22 - Yuan K, Wang F, Gao J, Sun X, Deng Z, Wang H, Chen J (2013) Effect of sintering time on the microstructure, flexural strength and translucency of lithium disilicate glass–ceramics Journal Non-Crystallized Solids 362 7–13.
23 – Wang F, Gao J, Wang H & Chen J (2010) Flexural strength and translucent characteristics of lithium disilicate galss-ceramics with different P2O5 content. Mater Des 31 3270 – 3274.
24 –Kang SH, Chang J & Son HH (2013) Flexural strength and microstructure of two lithium disilicate glass ceramics for CAD/CAM restoration in the dental clinic. Restorative Dental Endodontics 38 134 – 140.
25 - Fischer H, Hemelik M, Telle R & Marx R (2005) Influence of annealing temperature on the strength of dental glass ceramic materials. Dental Materials 21 671 - 677.
26 –Denry IL, Holloway JA (2004) Effect of post-processing heat treatment on the fracture strength of a heat-pressed dental ceramic. Journal Biomedical Materials Research B Applied Biomaterials 15 174–179
27 –Yang Y, Wang Y, Tian W, Wang Z, Zhao Y, Wang L & Bian H (2009) Reinforcing and toughening alumina/titania ceramic composites with nano-dopants from nanostructured composite powders. Materials Science Engeeniring A 508 161- 166.
28 – Chavan NM, Ramakrishana M, Phani PS, Rao DS & Sundararajan G (2011) The influence of process parameters and heat treatment on the properties of cold sprayed silver coating. Sur Coat Technol 205 4798-4807.
Capítulo 2
Recycling Heat-Pressed Ceramics: Light Transmission and Microstructural Analysis.
Abstract
The purpose of this study was to evaluate the microstructural morphology and light transmission of heat-pressed glass-ceramic material after repeated pressing. Ceramic discs of IPS e.max Press (12 mm in diameter x 0.9 mm thickness) were pressed and used as control (HP1). Sprue and button parts of the heat-pressed material were retrieved and used for repeated heat-pressing to construct discs of two and three re-pressed groups (HP2 and HP3). All heat-pressed casting procedures were performed according to the manufacturers' instructions. Surface characteristics were examined with secondary electron imaging (SEI) in SEM. Light irradiance and spectrum characteristics transmitted through ceramic were measured using a power meter and a light spectrometer. The SEM micrographs of the glass-ceramic material showed an increase in size and concentration of Lithium Di-Silicate (LiDiSi) crystals after repeated heat-pressings, post-processing imaging analysis showed in homogeneity of LiDiSi crystals distribution and increase in crystal density. Decrease in light transmission was detected with increasing ceramic thickness, and change in the emission profile was observed after repeated heat-pressing. Repeated heat-pressing procedures resulted in microstructural alterations, alterations in light transmission throughout ceramic discs was also detected.
Introduction
Regarding esthetic restorative materials, dental ceramics are gradually dominating the dental market mainly due their satisfactory mechanical properties such as higher compressive strength, chemical stability and lower electrical conductivity. Moreover, all ceramic restorations are intended to enhance the aesthetic properties of rehabilitation (Anusavice, 1996; van Noort, 2002; Borges et al., 2003, Zhang et al., 2013; Akar et al., 2014).
IPS Empress 2 was introduced in 1998 and demonstrated a higher mechanical strength compared to IPS Empress which is leucite reinforced. To combine durability with excellent esthetics, a recent pressable lithium-disilicate glass ceramic named IPS e.max Press with enhanced mechanical properties and improved translucency was launched in 2007 (Heintze et al., 2011).
To improve the characteristics of ceramic restorations made with this new material, different formulations and technical procedures have been developed. IPS e.max Press is available are in pressable ingots are suitable for the fabrication of frameworks or fully anatomical (and partially reduced) restorations. Due to the use of new technologies and optimized processing parameters, the formation of defects in the bulk of the ingot is avoided (Holand et al., 2000). After pressing and cooling, the sprue and remaining ceramic material are removed. The ceramic remnant should be discarded; nonetheless, these residual materials is used for re-pressing in some dental laboratories.
Concerning that repeated thermal treatments may alter some dental ceramics (Anusavice, 2005), it is important to evaluate the influence of repeated heat-pressings on the properties of the lithium disilicate glass-ceramic to determine the recycling reliability (Heintze et al., 2011; Gehrt et al., 2013; Solá-Ruiz et al., 2013). Possible microstructural changes might occur after repeated heat-pressings. In this context, it was hypothesized that after repeated heat-pressings, the recycled material would maintain the same microstructure of the single processed material. The aim of the present study was to evaluate the microstructural morphology and light transmission of repressed lithium disilicate glass-ceramic and to compare with singly processed material.
Material and Methods
The pressable ceramic discs were fabricated from a lithium disilicate based ceramic (IPS E.max press; Ivoclar Vivadent, shade LT A2). Three groups were identified for each evaluation: One heat-pressing cycle (HP1); Two heat-pressing cycles (HP2) and three heat-pressing cycles (HP3). The specimens were submitted to the following evaluations: light irradiance and spectrum characteristics transmitted through ceramic (n=3) for 0.7, 1.4 and 2.0 mm thicknesses. Surface characteristics were evaluated by SEM analysis (n=3) for 0.9 mm thick specimens. Customized discs wax patterns (12mm in diameter and 0.9 mm in thicknesses) were fabricated in a mold made with a silicone putty (Express STD; 3M ESPE, St. Paul, MN). The wax patterns were prepared in dental laboratory according to lost-wax technique. This procedure was firsty described in previous
study (Dong et al., 1992). The main characteristic of this dental lab processing method is viscous flow process. For this purpose, after the wax elimination, the IPS e.max Press glass-ceramic ingots were heated at 920°C into a pressing furnace. The viscous glass-ceramic was then pressed into a mold to form the disc. For successive thermo-pressing, the ceramic remnant from the earlier pressing procedure was adjusted for adequate dimensions that allow proper insertion into the refractory investment molds. The specimens of TP2 and TP3 were pressed following the same procedures of TP1.
Microstructural Analysis (SEM)
To evaluate the effect of the repeated heat-pressings on the morphological aspects of the lithium disilicate ceramic by using SEM, three discs from each heat-pressing (HP1, HP2 and HP3) were analyzed. The glass-ceramic surface was etched with 2.5% HT-aqueous solution for 10 s. This etching procedure was necessary to obtain better surface contrast and best conditions for scanning electron microscopy analysis.
After the surface treatment, the specimens were ultrasonically cleaned in deionized water for 10 min, fixed on metal stubs and gold sputtered (one cycle of 120 s) under a vacuum atmosphere using MED 010 (Balzers, Liechtenstein) sputtering device. The surfaces were examined by SEM (435 VP; LEO Electron Microscopy, Cambridge, UK) for focusing the etching depth, integrity and homogeneity along the treated surfaces. Samples were examined under 50–4000x
magnification. The unit operated at 20KV, WD 5 15 mm, and 25–100pA spotsize range.
Optical Properties – Light Transmittance
The quartz-tungsten-halogen light-curing unit XL2500 (3M ESPE, St. Paul, Minn., USA) was connected to a voltage stabilizer and the irradiance level was checked with a digital power meter (Ophir Optronics, Danvers, Mass., USA). The irradiance value and peak-emission transmitted through each ceramic disk was also obtained. One measurement was carried out for each condition. Furthermore, the spectral distribution of the light transmitted through ceramic was obtained using computer-controlled spectrometer (USB 2000; Ocean Optics, Dunedin, FL, USA). In same conditions, specimens without intervening ceramic were considered control group.
Results
Microstructural Analysis (SEM)
A considered homogeneous crystalline phase was found for HP1 (Fig. 1A and 1D). There was not abnormal crystalline formation for HP2 and HP3. The LiDiSi crystals showed symmetrical tendency with defined needled boundaries (Fig. 1A and 1D).
An increase of the crystalline density or increase of the crystal number per unit of area was detected in HP2 (1B and 1E). Increase in cluster formation was also showed (arrow in Fig. 1E). No homogeneity was noted for HP1 micrographs
(Figs. 1A and 1D), in the distribution of crystals through the glassy phase, and symmetry of the crystals.
All changes showed for HP2 were similar for HP3 (Figs. 1C and 1F); with stronger evident. Presence of crystal twinning (arrow in Fig. 1F) and atypical crystalline formations were also noted (Fig. 1F).
Figure 1 – A: homogeneous crystalline phase was found in HP1 specimens); B: occurrence of abnormal crystalline formation was not found (arrow); C: non-homogeneous crystalline formation D: LiDiSi crystals showed a tendency to be symmetrical, with well defined needled boundaries; E: An increase of the crystalline density, or increasing the number of crystals per unit of area was detected in HP2 specimens. An increase in cluster formation was also detected (arrow in 1E); F: All changes showed in HP2 (B and E) was found in HP3 (F), however in an even more evident way. Presence of crystal twinning (arrow in 1F) and atypical crystalline formations can be noted (1F).
Light Transmission
Table 1 shows the light transmitted spectra through three thicknesses and heat-pressings. Overall, there was a decrease in irradiance for discs interposed between the light source and the digital potentiometer: the thicker, the lower irradiance, regardless of heat-pressing number (control: without the intervening material).
Line 2 shows the spectra transmitted through the HP1, HP2 and HP3 (0.7 mm thick discs). Increasing the number of term-pressing there was a decrease in irradiance, the control group (HP1) showed the higher irradiance (627 mW/cm2) followed by HP2 group (621 mW/cm2) and HP3 group (617 mW/cm2).
Line 3 shows the spectra transmitted through the HP1, HP2 and HP3 (1.4 mm thick discs). Increasing the number of term-pressing there was a decrease in irradiance, the control group (HP1) showed the higher irradiance (523 mW/cm2) followed by HP2 group (519 mW/cm2) and HP3 group (513 mW/cm2).
Line 4 shows the spectra transmitted through the HP1, HP2 and HP3 (2.0 mm thick discs). Increasing the number of term-pressing there was a decrease in irradiance, the control group (HP1) showed the higher irradiance (437 mW/cm2) followed by HP2 group (422 mW/cm2) and HP3 group (419 mW/cm2).
Table 1 - Light transmission for all groups. Irradiance* (mW/cm2) Emission Interval** (nm) Emission Peak** (nm) Heat pressing Group HP1 HP2 HP3 HP1 HP2 HP3 HP1 HP2 HP3 Controle 810 494,5 0.7 mm 627 621 617 482.7 459.7 456.9 1.4 mm 523 519 513 463.7 458.3 455.0 2.0 mm 437 422 419 400-510 463.2 457.1 451.4
*Data obtained using a digital potentiometer. **Data obtained using a digital spectrometer.
Discussion
Agreeing with (Holand et al., 2000), the control group showed interlocking microstructure containing higher content of lithium disilicate layered crystals. The etching protocol for specimen preparation was also significant for illustrating the microstructure of needle-like apatite glass-ceramic and its alteration after successive heat-pressings.
Many needled crystals are shown in the glass matrix for all specimens (Fig. 1). Single heat-pressing showed densely packed and multi-axial interlocking microstructure of numerous needled crystals protruding from the glass matrix. However, after two heat-pressing events, the crystals appeared oriented, wider and larger with a sparser distribution. Moreover, the extremity of the DiSiLi crystals were more rounded compared to the sharp needle shaped ends of the crystals present in the control group. Furthermore, increased porosity and cracks were
seen in the crystals after two heat-pressings. This fact would be related to contaminants included during the ceramic prepare for news recycling procedures, i.e, to reduce the remnants size to allow a new heat-pressing procedure (Naves et al., 2011).
SEM analysis showed also crystal twinning (clustering), which corroborates with the recently published study (Tang et al., 2014). Morphological change on ceramic microstructure after repeated sintering or heat-pressings is commonly claimed in studies with this approach. For the LiDiSi ceramics, alterations in the mechanicals properties seem not follow a pattern in dental literature researches. There are reports showing increased (Tang et al. 2014, Chung et al. 2009) or decreased (Albakry et al. 2004a) in mechanical properties.
Different irradiance levels were transmitted through the different ceramics (HP1, HP2 and HP3). The results of this study suggest that light energy were not great enough to yield significant differences in depth of cure of the composite underneath the ceramics. However, different spectrum profiles were registered and, maybe this fact has some direct relation with the changes shown by microstructural analysis. In addition to the attenuation effect, it is possible to infer that the intervening ceramic could alter the wavelength light spectrum transmitted through the material. Spectrum profiles for some curing units kept the symmetry after light transmission through ceramic, while other units promote higher absorbance at lower wavelengths (Koch et al., 2007).
The current results indicate that LiDiSi ceramics, regardless the thickness, allowed irradiance transmission with minimal and constant interference in the wave
light properties when submitted to a single heat-pressing procedure. Thus more transmittance characteristics of re-pressed materials need to be explored. Moreover, successive heat-pressings changed the wave light properties, suggesting the presence of little translucence and, although not evaluated, shade changes (Cardash et al., 1993).
It is possible to deduce that the results of the study bring clinical implications. Although the decrease of the irradiance due to the ceramic thickness increase be a well-known and widely fact, other changes might interfere with the luting agent activation under ceramics, mainly on the optical/esthetic properties.
Conclusion
After successive heat-pressings changed the orientation, distribution, the symmetry of the microstructure lithium di-silicate crystals in ceramic IPS e.max Press and the porosity increased. Light transmission through ceramic was less effective that in control group.
ACKNOWLEDGMENTS
The authors are gratefull to CNPq (National Council for Scientific and Technological Development, Grant # 303928/2009–3) for the study support at Piracicaba Dental School, SP, Brazil.
References
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10 - Anusavice KJ. Philips – Materiais Dentários. 2005.
11 - Gehrt M, Wolfart S, Rafai N, Reich S, Edelhoff D. Clinical results of lithium-disilicate crowns after up to 9 years of service. Clinic Oral Investigations 2013; 17: 275–284.
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CONCLUSÃO
Dentro das limitações do presente estudo, as seguintes conclusões podem ser definidas:
1 - O efeito de repetidas termo-prensagens promoveu redução na resistência à flexão biaxial e dureza Vickers da cerâmica IPS e.max Press.
2 - Após sucessivas termo-prensagens mudanças na orientação, distribuição, simetria da microestrutura dos cristais de di-silicato de lítio da cerâmica IPS e.max Press e a porosidade aumentaram. A transmissão da luz através da cerâmica foi menos efetiva do que no grupo controle.
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Albakry M, Guazzato M, Swain MV. Influence of hot pressing on the microstructure and fracture toughness of two pressable dental glass-ceramics J Biomedic Mat Res B App Biomat 2004b; 71: 99 - 107
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APÊNDICE METODOLOGIA
1. Confecção das amostras 1.1. Primeira prensagem
A técnica de prensagem do sistema IPS e.max Press utiliza o método da cera perdida. Moldes em forma de disco com 12 mm de diâmetro nas espessuras de 1,2 mm de espessura foram confeccionados com silicone por adição (Express; 3M ESPE, Saint Paul, MN, USA). A cera orgânica (Thowax; Yeti Dentalprodukte, Engen, Alemanha) foi aquecida com chama de lamparina a álcool e vertida no interior dos moldes. Após o esfriamento da cera, o excesso foi removido e as dimensões dos discos foram conferidas com paquímetro digital (Mitutoyo, Tóquio, Japão). Os discos foram unidos aos formadores dos canais de alimentação (sprues) fixados numa base plástica. O anel de silicone foi adaptado à base formadora de cadinho e os padrões em cera incluídos com revestimento à base de fosfato (IPS PressVest Speed; Ivoclar Vivadent), na proporção de 200 g de pó para 27 mL de líquido do material misturado a 27 mL de água destilada e espatulado mecanicamente a vácuo (Multivac 4; Degussa, Hanau, Germany), por 2 minutos.
Figura 1- A: Padrão em cera dos discos a serem injetados - B: Anel de borracha para compensar a expansao do revestimento.
Figura 2 – A: Revestimento sendo vertido no anel de borracha – B- Revestimento em descanso até a presa final.
Após a presa, o cilindro de silicone, o formador do conduto e a base foram removidos e o bloco de revestimento levado ao forno elétrico (7000-5P; EDG
A
B
Equipamentos e Controles Ltda.) na temperatura de 850°C mantida por 90 minutos para eliminação da cera. Decorrido o período de aquecimento, o bloco foi removido do forno e duas pastilhas da cerâmica IPS E.max Press (Ivoclar Vivadent, cor LT) foi posicionada no conduto junto com o êmbolo de alumina preparado no Alox Plunger Separator (Ivoclar Vivadent) e levado ao forno EP600 (Ivoclar Vivadent). O conjunto foi mantido por 20 minutos a 915°C e injetado com pressão de 5 bars por 15 minutos. Depois de esfriado à temperatura ambiente, o bloco de revestimento foi seccionado com disco de carburundum (KG Sorensen, Cotia, SP, Brasil).
Figura 3- A: Anel, base e bloco de revestimento após a presa - B: Bloco de revestimento no forno de queima.
Figura 4 – A: Alox Plunger separator – B – Preparo do bloco de revestimento para injeção no forno EP 600.
Posteriormente, as amostras foram removidas com jatos de partículas de óxido de alumínio de 100 µm (Oxyker Dry; Flli Manfredi), com pressão inicial de 4 bars, na região externa, e posteriormente de 2 bars, na região próxima as amostras, para remoção do revestimento aderido à superfície das amostras.
Após o jateamento, o conduto de alimentação foi removido com disco diamantado (KG Sorensen) e as amostras submetidas ao acabamento com ponta cilíndrica diamantada. Depois foram submetidas ao acabamento com lixas de carbeto de silício granulação 180, 320, 400, 600 e 1200 (Norton S.A., São Paulo, SP, Brasil), com constante refrigeração à água até obter a espessura de 0,9 mm e limpas em ultrassom por 10 minutos.
Figura 5 – A: Remoção das amostras com jateamento com óxido de alumínio B: Polimento das amostras com lixas de carbeto de silício.
As amostras obtidas da cerâmica IPS E.max Press, para cada prensagem (TP1) com 12 mm de diâmetro por 0,9 mm de espessura foram utilizados nos ensaios de resistência à flexão biaxial (n=12) e dureza Vickers (n=10).
Figura 6 – Amostras prontas após a remoção do revestimento e polimento.
1.2. Segunda e terceira prensagens
As sobras de cerâmica do conduto de alimentação foram cortadas em pedaços menores do que 2 mm de diâmetro, imersos em etanol a 50% e limpos com ultrassom, por 10 minutos.
Em seguida foram triturados, reutilizados e submetidos a mais uma (TP2) ou duas (TP3) prensagem levando em consideração o peso original das pastilhas. A segunda e terceira prensagens foram realizadas de forma similar à primeira prensagem, seguindo a recomendação do fabricante.
2. Análise das propriedades mecânicas 2.1. Teste de resistência à flexão biaxial
Discos com 12 mm de diâmetro por 0,9 mm espessura foram mensurados na parte central e periférica utilizando paquímetro digital (Mitutoyo, Tokyo, Japan), com 0,01 mm de precisão. O diâmetro também foi medido em três posições equidistantes e registrado. Os discos foram posicionados num dispositivo composto por um anel de 10 mm de diâmetro. Uma haste com extremidade esférica de 4 mm de diâmetro, acoplada na parte superior da máquina de ensaios mecânicos (Instron), foi posicionada no centro do disco cerâmico e submetida à carga de compressão à velocidade de 0.5 mm/minuto até ocorrer fratura das amostras. A carga de fratura em kgf foi utilizada na equação seguinte para obtenção dos valores de resistência à flexão biaxial:
Onde S = é a resistência à flexão biaxial em (MPa); P = é a carga total em que foi registrada a fratura (N); X=(1+v)ln(B/C)2 + [(1-v)/2](B/C)2, Y=(1+v)[1+ln(A/C)2] + (1-v)(A/C)2; v é coeficiente de Poisson, A é o raio do anel de suporte (mm), B é o raio da área carregada (mm), C é o raio do disco (mm) e D é a espessura do disco (mm). O coeficiente de Poisson considerado foi de 0,25 de acordo com a especificação ISO 6872. Um total de 12 amostras, para cada grupo, foram confeccionadas para cada prensagem (TP1=12, TP2=12 e TP3=12). Os dados foram submetidos à Análise de Variância e ao teste de Tukey post-hoc (p=0,05).
Figura 7: Amostra em forma de disco levada para o teste de flexão biaxial na maquina universal Instron 4411.
2.2. Ensaio de Dureza Vickers
Foram confeccionadas também dez amostras para cada grupo (TP1, TP2 e TP3) e embutidas em tubos de PVC rígido com resina epóxica (Buehler Ltd, Lake Bluff, IL, EUA) (n=10). O teste de dureza Vickers foi realizado num aparelho para ensaio universal de microdureza (HMV-2; Shimadzu, Tóquio, Japão) com aplicação de carga de 500 gf durante 15 segundos. As dimensões das amostras foram similares às das amostras utilizadas no ensaio de resistência à flexão biaxial.
Com auxílio do microscópio mensurador acoplado ao aparelho, a dimensão da diagonal (d) das penetrações foram medidas e a dureza Vickers determinada de acordo com a fórmula:
DV = 0,1891 F/d²
Onde F = carga aplicada. As médias de dureza foram calculadas após realização de 5 penetrações em cada corpo-de-prova. Após comprovada normalidade na distribuição dos dados (teste de Kolmogorov-Smirnov) e igualdade das variâncias, os resultados de dureza foram submetidos à Análise de Variância e teste post-hoc de Tukey, em nível de significância de 5%.
Figura 8 – Amostra embutidas em resina epóxi e submetidas ao teste de dureza Vickers na maquina universal de microdureza HMV 2.
3. Análise Microestrutural (MEV)
Três amostras de cada grupo analisado (TP1, TP3 e TP3) foram submetidas ao condicionamento ácido objetivando expor a microestrutura (ácido hidrofluorídrico 2,5% durante 10 s). Na sequência, as amostras foram limpas em ultrassom com água destilada por 10 minutos, secas com papel absorvente e em estufa a 37°C por 10 minutos e fixados com fita dupla face de carbono (Electron Microscopy Sciences, Hatfield, PA, EUA) em bases metálicas numeradas (stubs). A superfície das amostras foi coberta com liga de ouro-paládio sob alto vácuo (Balzers – SCD 050 sputt coater, Alemanha). A topografia da superfície foi
analisada em aumentos de 5.000,10.000 e 20.000 X. Um total de cinco imagens foram obtidas para cada amostra, totalizando 15 imagens por grupo
Figura 9 – Microscópio Eletrônico de Varredura
4. Propriedades Ópticas - Luz transmitida
O aparelho fotopolimerizador de lâmpada halógena de quartzo de tungstênio XL2500 (3M ESPE, St. Paul, MN, EUA) foi conectada a um estabilizador de voltagem e a irradiância foi aferida com potenciômetro digital (Ophir Optronics, Danvers, MA, EUA). O valor de irradiância transmitido através das três espessuras (0,7; 1,4 e 2 mm) de cerâmica submetida a uma, duas ou três prensagens foi mensurado.
Adicionalmente, a distribuição espectral da luz transmitida através de cada amostra foi obtida usando espectrômetro digital (USB2000; Ocean Optics, Dunedin, FL, EUA) e comparado com o perfil controle (sem o material de interposição).
Figura 10 – A: Aparelho fotopolimerizador B: Potenciômetro e Spectrofotômetro
