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INFLUÊNCIA DA SOLDAGEM (LASER OU TIG) EM
ESTRUTURAS DE TITÂNIO SOBRE O DESAJUSTE MARGINAL
( BI E TRIDIMENSIONAL), FORÇA DE DESTORQUE EM
PARAFUSOS PROTÉTICOS, FADIGA MECÂNICA, CORROSÃO E
TENSÃO INDUZIDA AOS PILARES PROTÉTICOS DE PRÓTESES
IMPLANTOSSUPORTADAS
INFLUENCE OF WELDING (LASER OR TIG) IN TITANIUM
STRUCTURES ON MARGINAL MISFIT (TWO AND
THREE-DIMENSIONAL), DETORQUE STRENGTH IN PROSTHETIC
SCREWS, MECHANICAL FATIGUE, CORROSION AND
STRESS INDUCED TO THE PROSTHETIC ABUTMENT OF
IMPLANT-SUPPORTED PROSTHESES
PIRACICABA 2015
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UNIVERSIDADE ESTADUAL DE CAMPINAS
FACULDADE DE ODONTOLOGIA DE PIRACICABA
SABRINA ALESSANDRA RODRIGUES
INFLUÊNCIA DA SOLDAGEM (LASER OU TIG) EM ESTRUTURAS DE TITÂNIO SOBRE O DESAJUSTE MARGINAL (BI E TRIDIMENSIONAL), FORÇA DE DESTORQUE EM PARAFUSOS PROTÉTICOS, FADIGA MECÂNICA, CORROSÃO E TENSÃO INDUZIDA AOS PILARES PROTÉTICOS
DE PRÓTESES IMPLANTOSSUPORTADOS
INFLUENCE OF WELDING (LASER OR TIG) ON MARGINAL MISFIT (TWO AND THREE-DIMENSIONAL), DETORQUE STRENGTH IN PROSTHETIC SCREWS, MECHANICAL FATIGUE,
CORROSION AND STRESS INDUCED TO THE PROSTHETIC ABUTMENT OF IMPLANT-SUPPORTED PROSTHESES
Orientador: Prof. Dr. Marcelo Ferraz Mesquita
Tese apresentada à Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas como parte dos requisitos exigidos para obtenção do título de Doutora em Clínica Odontológica na Área de Prótese Dental.
Thesis presented to the Piracicaba Dental School of the University of Campinas in partial fulfillment of the requirements for the degree of Doctor in Clinical Denstistry, in Area of Dental Prosthesis.
Este exemplar corresponde à versão final da Tese, defendida pela aluna Sabrina Alessandra Rodrigues, e orientada pelo Prof. Marcelo Ferraz Mesquita.
Assinatura do Orientador
Piracicaba 2015
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Resumo
O objetivo neste estudo foi avaliar o comportamento de estruturas fundidas em titânio, soldadas à LASER (Light Amplification by Stimulated Emission of Radiation) e à TIG (Tungsten Inert Gas), em relação ao desajuste marginal (bi e tridimensional), força de destorque dos parafusos protéticos, fadiga mecânica, corrosão e tensão induzida sobre os análogos de mini pilares protéticos. Cinquenta estruturas foram fundidas em titânio comercialmente puro (Ti-cp); vinte infraestruturas (n=20) simulando próteses fixas múltiplas de três elementos; e trinta halteres (n=30). Para cada infraestrutura foram confeccionados dois tipos de index, simulando um desajuste marginal de 200 µm. Um index em gesso especial, para desenvolvimento da análise de tensão, e outro em resina fotoelástica, para avaliação do desajuste marginal (bi e tridimensional), força de destorque, fadiga mecânica e corrosão. Antes e após os procedimentos de soldagem (LASER ou TIG) foram realizadas as seguintes avaliações: mensuração do desajuste marginal, por meio de microscópio óptico (bidimensional) e microtomógrafo de raio-X (tridimensional); análise da tensão induzida aos mini pilares protéticos, por meio da técnica extensométrica; e mensuração da força de destorque dos parafusos protéticos. Os procedimentos de soldagem foram relizados com os seguintes parâmentros: LASER (370/9ms); TIG (36A/60ms). Após a realização dos procedimentos de soldagem (LASER ou TIG) as infraestruturas foram submetidas à ciclagem mecânica ( 280 N/ 106 ciclos /2Hz), aplicando-se uma carga compressiva no sentido oblíquo, para avaliação da força de destorque dos parafusos protéticos pós-ciclagem. A longevidade das estruturas intactas e soldadas (LASER ou TIG) também foi avaliada, por meio de ensaios mecânicos de fadiga, e o comportamento à corrosão, por meio de testes eletroquímicos de espectroscopia de impedância eletroquímica e polarização potenciodinâmica. Os resultados foram analisados estatisticamente (ANOVA fator único/ Teste de Tukey HSD/Teste t de Student/ (α=0,05)). Ambos os procedimentos de soldagem diminuíram o nível de desajuste marginal (bi e tridimensionalmente), aumentaram a intensidade da força de destorque e reduziram a tensão induzida aos mini pilares protéticos. Contudo, após a ciclagem mecânica houve diminuição da intensidade da força de destorque dos parafusos protéicos. As estruturas soldadas à LASER tiveram maior resistência a fadiga, comparada as estruturas soldadas à TIG. O teste de espectroscopia de impedância foi significativo para as infraestruturas
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soldadas à TIG, comparado com as infraestruturas intactas e soldadas à LASER. Conclui-se que os procedimentos de soldagem tiveram influência sobre a adaptação marginal das infraestruturas, proporcionando maior estabilidade ao sistema implantossuportado. A ciclagem mecânica teve influência sobre a estabilidade dos parafusos protéticos, sendo necessárias proservações periódicas durante o uso da prótese para reaperto e/ou troca dos parafusos. Estruturas fundidas em titânio e soldadas à LASER possuem maior resistência à fadiga do que estruturas soldadas à TIG. Eletroquimicamente, o procedimento de soldagem à LASER possuem melhor comportamento à corrosão comparado ao procedimento de soldagem à TIG.
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Abstract
The aim of this study was to evaluate titanium casted structures behavior, welded by LASER (Light Amplification by Stimulated Emission of Radiation) and TIG (Tungsten Inert Gas), on marginal misfit (two and three-dimensional), prothetic screws detorque strength, mechanical fatigue, corrosion and stress induced to the prosthetic mini abutment analogs. Fifty structures were casted in commercially pure titanium (cp-Ti); twenty frameworks (n=20) simulating multiple prostheses of three elements; and thirty dumbbells (n=30). From the frameworks were fabricated two type of index, simulating a marginal misfit of 200µm. One index was fabricated in special plaster to the development of stress analysis, and other in epoxy resin to the development of marginal misfit analysis (two and three-dimensional), detorque strength, mechanical fatigue and corrosion. Before and after welding procedures (LASER or TIG) were developed the following evalution: marginal misfit analysis, by optical microscope (two-dimensional) and X-ray microtomography (three-dimensional); induced stress analysis on prosthetic mini abutment analogs, by extensometric technique; and measurement of prosthetic screws detorque strength. The welding procedures were made with the following parameters: LASER (370V/9ms); TIG (36A/60ms). After welding procedures (LASER or TIG), the frameworks were submitted to cyclic mechanical (280N; 106 cycles; 2 Hz) by oblique compressive load to evaluation of the detorque strength after the cycling. The longevity of the welded structures was also evaluated by mechanical fatigue testing, and the corrosion behavior by electrochemical tests of electrochemical impedance spectroscopy and potentiodynamic polarization. The results were analyzed statistically (one-way ANOVA / Tukey HSD test / Student-t test (α = 0.05)). Both the welding procedures decreased the marginal misfit level (two and three-dimensionally), increased the detorque strength and decreased of stress induced on prosthetic mini abutment analogs. However, after the mechanical cycling there was decrease of detorque strength of prosthetic screws. The structures welded by LASER had high fatigue resistance than the structures welded by TIG. The impedance spectroscopy test was only significant for frameworks welded by TIG, compared with intact and LASER welded LASER frameworks. It is concluded that welding procedures had influence on marginal fit, providing more stability to the implant-supported system. The mechanical cycling had influence on prothetic screws
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stability, being necessary periodic following up to tighten and/or exchange of prosthetic screws. The titanium casted and LASER welded structures were more fatigue resistance than that TIG welded structures. Electroless, LASER welding procedure have better corrosion behavior compared to TIG welding procedure.
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Sumário
DEDICATÓRIA……….….XIII AGRADECIMENTOS ESPECIAIS………...XV AGRADECIMENTOS………...XVII INTRODUÇÃO………..……...1CAPÍTULO 1: The effect of welding techniques on the biomechanical behavior of implant-supported prosthesis……….……….7
CAPÍTULO 2: Corrosion behavior of titanium frameworks welded by LASER and TIG …….……….……….…23
CAPÍTULO 3: Fatigue behavior of pure titanium structures, welded by LASER or TIG ...……….………...39 CONSIDERAÇÕES FINAIS……….…………....57 CONCLUSÃO ...61 REFERÊNCIAS ...63 APÊNDICE ...71 ANEXO 1...85
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Dedico este trabalho
A Deus por ter me dado força e saúde para concluir esta tese.
Aos meus pais, Mauricio e Laurita, pelo amor, apoio e carinho em todos os momentos da minha vida.
Aos meus irmãos Camila e Leandro, que sempre me apoiaram, fazendo com que eu nunca deixasse de lutar por um futuro melhor.
Muito obrigada!
Amo vocês!
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Agradecimentos Especiais
Ao meu Orientador Prof. Dr. Marcelo Ferraz Mesquita, pela confiança, apoiando-me em todos os momentos. Sua orientação contribuiu e muito para o meu amadurecimento como Mestre, Doutora e ser humano.
Ao meu Orientador de Iniciação Científica Prof. Dr. Luís Geraldo Vaz, pela confiança e apoio durante a minha graduação e pós-graduação. Um super amigo, que me orientou não somente para ser pesquisadora, mas também em relação às dificuldades da vida, me encorajando a nunca desistir dos meus objetivos.
À Profa. Dra. Juliana Maria Costa Nuñez-Pantoja, pela confiança, ensinamento, apoio e carinho.
Aos Professores Luís Rocha, Filipe Samuel Silva, Fatih Toptan e Alexandra
Alves, pela confiança e apoio durante parte do desenvolvimento do meu doutorado na
Universidade do Minho em Guimarães-Azurém, Portugal no Centro de Engenharia Mecânica.
Por fim, agradeço à Minha Família que sempre torceu por mim em todos os momentos da minha vida.
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Agradecimentos
À Faculdade de Odontologia de Piracicaba, nas pessoas do Diretor Prof. Dr.
Guilherme Elias Pessanha Henriques, e Diretor Associado Prof. Dr. Francisco Haiter Neto, pelo acolhimento.
À Profa. Dra. Cínthia Pereira Machado Tabchoury, Coordenadora dos cursos
do Programa de Pós-graduação da Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas, pela atenção e disponibilidade.
À Profa. Dra. Karina Gonzales Silvério Ruiz, Coordenadora do Programa de Pós-graduação em Clínica Odontológica da Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas.
À empresa NEODENT, pela doação de componentes protéticos, em especial à colega Dra. Ivete Sartori, Professora do Instituto Latino-Americano de Pesquisa e Ensino Odontológico de Curitiba (ILAPEO).
À FAPESP, pela concessão da bolsa de estudo no Brasil (Processo no.
2012/14139-8), bolsa de estudo em Portugal (BEPE) (Processo no 2013/21533-7) e auxílio financeiro (Processo no 2012/14141-2) para desenvolvimento da pesquisa.
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Aos amigos do laboratório de Engenharia Mecânica da Universidade do Minho/Azurem- Guimarães, Portugal, Sr. Araújo, Rafaela Santos, Sara Madeira,
Georgina, Mara, Paulo e Paulo Pinto, pelo carinho, apoio, hospitalidade e atenção.
Aos amigos que fiz em Portugal, Ailton Moreira, Brondy Correia, Evelise
Pires Barbosa, Paula Lopes, Sara Tavares, Davy Fonseca, Justino Quadé, Lassana Dafé, Nilton Gomez, Paty Pereira, Vito Seye, Danilo Graça, Sara Tavares, Admilsa Alamô, Daniel Silves, Romina Reis, Suelma Pina, Juseny Borges Damoura, Kleunice Rodrigues, Neryvaldo Galvão, Jair Hungria, Vania Vieira, Héder Sanches e Carla Marques, pelo carinho, paciência, hospitalidade e atenção.
Aos amigos do Laboratório de Prótese Total da Faculdade de Odontologia de Piracicaba, Cláudia Brilhante, Isabella Vieira Marques, Ataís Bacchi, Adaías, Bruno
Zen, Conrado Caetano, Erika Ogawa, Anna Gabriela Camacho Presotto, Júlia Campana, Thamara Beline, Ricardo Caldas e Moisés Nogueira, pela amizade e convívio.
Aos Professores e Doutores Mauro Antônio de Arruda Nóbilo, Rafael
Leonardo Xediek Consani e Valentim Adelino Ricardo Barão, pelos conhecimentos
transmitidos e agradável convivência.
À secretária Eliete A. F. Lima Marim e ao técnico Eduardo Pinez Campos, pela ajuda e boa convivência durante o curso de pós-graduação.
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Aos demais colegas do Departamento de Prótese e Periodontia da Faculdade de Odontologia de Piracicaba, pela amizade e companheirismo.
Aos meus avós maternos Nelita e José Miranda e paternos Anísia e José Amaro
Rodrigues (in memoriam), por sempre acreditarem e incentivarem a minha busca pelo
conhecimento e crescimento profissional. Amo muito vocês!
Aos meus tios e tias, Amauri Amaro, Cleusa Aguiar, Vanderlei Miranda,
Ailton Miranda, Luzia Miranda, Neuza Miranda, Clarice Miranda, Jorge Silva, Lúcia Rodrigues, Ana Lúcia Miranda, Inês Amaro, Henrique Miranda, Adalmo Miranda (in
memoriam), Mário Amaro (in memoriam), Maria Joana Adão (in memoriam), Maurizia
da Silva (in memoriam), pelo carinho, paciência e apoio.
Aos meus primos, Carlos Eduardo, Jacqueline, Karine, Juliana, Adriele,
Miriam, Gabriela, Alisson, Evandro, Enzo Gabriel, Rebeca, Vitor, Gustavo, João Pedro, Rafaela, Isabella, Pedro, Lucas, Andréia, Adriana, Viviani, Fernando, Rafael, Ana Paula, Silvia, Alexandre e Ana Julia, pelo carinho e apoio.
Aos meus amigos, Maria Isabel Previtalle, Leticia Oliveira, Larissa Oliveira,
Mario Sérgio Pedro Silva, Leonardo Nicolau Barros, Fátima N. de Barros, Danielle Nicolau Galdino, Alessandra Borges, Dayane Batista, Priscila Amaro, Alessandra Borges, Isadora Oliveira, pelo carinho, paciência e apoio.
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Por fim, a todos que torceram por mim, e indiretamente me auxiliaram na elaboração desse trabalho.
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Introdução
Atualmente na Odontologia o titânio e suas ligas têm sido muito utilizados para confecção de implantes, componentes protéticos e infraestruturas de próteses implantossuportadas, devido à sua biocompatibilidade com tecido ósseo e periodontal presentes na cavidade bucal. Considerado um biomaterial, possui propriedades mecânicas, físicas e químicas favoráveis como baixa densidade, alta resistência mecânica, alta resistência à corrosão, alta ductibilidade e baixa condutividade térmica (Paiva et al., 2009; Rodrigues et al., 2010; Noguti et al., 2012).
Vários estudos têm avaliado o comportamento biomecânico do titânio e suas ligas, mas poucos têm analisado sua longevidade, quando utilizado para fabricação de infraestruturas de prótese fixas múltiplas implantossuportadas, por meio da técnica de fundição em monobloco (Jemt & Lekholm, 1993; Lindquist et al., 1996; Henriques et. al., 1997; Silveira-Junior et al., 2009; Leles et al., 2010; Roe et al., 2010; Nuñez-Pantoja et al., 2011). Ao fundir o titânio, podem ocorrer distorções, que tem como consequência o comprometimento da adaptação protética, por meio do surgimento de níveis indesejáveis de desajuste marginal entre a interface pilar e/ou prótese (Sahin & Cehreli, 2001; Paiva et al., 2009; Abduo et al., 2010; Rodrigues, 2012).
Nas próteses fixas múltiplas implantossuportadas parafusadas, o desajuste na interface pilar/prótese é um dos fatores significativos de transferência de tensão, desenvolvida durante o torque dos parafusos protéticos (Barbosa et al., 2008; Abduo et al., 2010; Spazzin et al., 2010; Farina et al., 2012). A presença de níveis elevados de desajuste marginal nesta interface pode intensificar o desenvolvimento de tensões e prejudicar a biomecânica do sistema implantossuportado, devido à justaposição existente entre o implante e tecido ósseo, a qual impossibilita a dissipação de esforços mecânicos intensos (Spazzin et al., 2010; Farina et al., 2012; Rodrigues, 2012).
Atualmente, não há definição exata do que seria adaptação protética ideal, apenas sugestões de valores supostamente aceitáveis clinicamente. A literatura sugere que as próteses implantossuportadas devem exibir adaptação passiva aos pilares (Sahin & Cehreli, 2001; Hecker & Eckert, 2003; Karl et al., 2005), com níveis de desajuste marginal variando entre 10 e 150 μm (Branemark, 1983; Jemt, 1991), dependendo do tipo de prótese, unitária
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ou múltipla. Portanto, suspeita-se que níveis de desajuste marginal acima de 150 μm em próteses fixas múltiplas podem gerar complicações mecânicas e biológicas aos tecidos ósseos e moles adjacentes ao sistema implantossuportado (Bauman et al., 1992; Naert et al., 1992; Carlson & Carlsson, 1994; Watanabe et al., 2000; Spazzin et al., 2010; Farina et al., 2012). A adaptação de uma prótese implantossuportada pode ser avaliada por meio de procedimentos clínicos e exames de imagem. Pode-se verificar a presença de adaptação protética quando há ausência de compressão ou dor durante o aparafusamento ou cimentação da prótese sobre os pilares, aperto final dos parafusos protéticos até no máximo 1/3 de volta, controle visual com auxílio de lupa da margem supragengival, e controle radiográfico do ajuste marginal, por meio do teste do parafuso único (Tan et al., 1993; Aparicio, 1994, Rodrigues, 2012).
As complicações mecânicas desenvolvidas durante o uso de próteses com nível de desajuste marginal elevado, podem causar instabilidade ao sistema implantossuportado, por meio do afrouxamento dos parafusos protéticos, pois cargas compressivas de magnitude igual ou superior à pré-carga do parafuso podem ser desenvolvidas, gerando deformações plásticas que reduzem a força de união entre o parafuso e pilar (Alkan, et al., 2004; Youset et al., 2005; Spazzin et al., 2010; Farina et al., 2012). A carga oclusal cíclica associada ao nível de desajuste marginal elevado, também pode induzir tensões prejudiciais, e consequentemente causar micromovimentações entre o implante e a prótese, desgastando as áreas microscópicas de contato entre os sistemas (Jorneus et al., 1992; Duyck et al., 1997; Siamos et al., 2002).
Deste modo, é indispensável durante a confecção de próteses implantossuportadas, a realização de planejamentos cautelosos, para que não haja o desenvolvimento de falhas clínicas, que possam danificar os componentes protéticos, por meio de fraturas de parafusos e próteses, como também causar danos aos tecido duros e moles adjacentes ao sistema implantossuportado (Sahin & Çehreli, 2001; Barbosa et al., 2008, Spazzin et al., 2010; Abduo et al., 2010; Farina et al., 2012). Mesmo seguindo rigorosamente os critérios de confecção, ainda podem ocorrer falhas associadas ao material de moldagem, obtenção do modelo de gesso, confecção do padrão de cera, procedimentos de fundição e aplicação de cerâmica ou resina acrílica sobre a infraestrutura (Wee et al., 1999; Rodrigues, 2010).
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As tensões desenvolvidas no sistema implantossuportado podem ser avaliadas por meio da análise extensométrica, que mensura in vitro e in vivo as deformações ocorridas em áreas específicas dos componentes protéticos, próteses e implantes. Para o desenvolvimento desta técnica, são necessários pequenos resistores (strain gauges) que captam deformações plásticas e/ou elásticas nos locais em que são posicionados (Clelland et al.,1996; Cehreli et al.,2006; Abduo et al., 2010).
Por meio da técnica extensométrica, muitos estudos têm correlacionado o nível de desajuste marginal de próteses múltiplas fixas implantossuportadas com a tensão induzida aos componentes protéticos e/ou implantes. Nestes estudos, o nível de desajuste marginal é sempre mensurado num único eixo da peça, ou seja, a mensuração é sempre bidimensional, realizada em microscópio óptico de precisão (Clelland et al.,1996; Tan et al., 1993; Tramontino, 2008; Abduo et al., 2010; Luthi, 2010, Torres et al., 2011;Rodrigues, 2012). Não há estudos na literatura que correlacionam a tensão com o desajuste marginal mensurado tridimensionalmente em infraestruturas fundidas em titânio comercialmente puro.
Procedimentos de soldagem devem ser utilizados para tentar minimizar o desenvolvimento de tensões, frente a presença de altos níveis de desajuste marginal em próteses fixas múltiplas implantossuportadas. Durante a escolha do procedimento para soldagem de infraestruturas fundidas em titânio, deve-se ter cautela, devido o titânio ser um metal de difícil fundição e rápida oxidação em contato com o ambiente (Rocha et al., 2006; Souza et al., 2008, Nuñez-Pantoja et al., 2011; Rodrigues, 2012). Assim, algumas técnicas de soldagem devem ser descartadas, como a técnica convencional, denominada brasagem por maçarico, pois desenvolve oxidações, porosidades e superaquecimento da peça soldada (Souza, et al., 2000).
Portanto, deve-se optar por técnicas que não comprometam a estrutura do metal titânio, como as técnicas de soldagem LASER (Light Amplification by Stimulated Emission of Radiation) e TIG (Tungsten Inert Gas), que desenvolvem pontos de solda localizados, sem ocorrência de distorção em toda a peça (Wang & Welsch, 1995; Rocha et al., 2006; Souza et al., 2008; Rodrigues et al., 2010). Além disso, ambas técnicas necessitam de proteção da superfície soldada por meio do uso do gás argônio, devido ao fato do titânio ser reativo aos elementos químicos do ar (O2, H2 e N2), quando aquecido em temperaturas acima de 600oC
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A soldagem a LASER usa como fonte de calor um feixe de luz concentrado de alta energia, capaz de gerar aquecimento localizado, o que reduz a possibilidade de ocorrência de deformações na peça soldada (Gordon & Smith, 1972; Jemt & Lindén, 1992; Sousa et al., 2000; Rocha et al., 2006; Cardoso, 2007; Souza et al., 2008; Nuñez-Pantoja et al., 2011). A soldagem a TIG ocorre por meio de um arco elétrico, formado pelo contato da peça a ser soldada com o eletrodo não consumível de tungstênio (Wang & Welsch, 1995; Rocha et al.,2006; Nuñez-pantoja et al.,2011).
Ambas as técnicas de soldagem (LASER ou TIG) demandam menor tempo, quando comparadas à técnica de soldagem convencional, pois são realizadas no próprio modelo protético, o que facilita o trabalho laboratorial, sendo uma vantagem durante a confecção de próteses fixas múltiplas implantossuportadas (Gordon & Smith, 1972; Wang & Welsh 1995; Rocha et al,. 2006; Rodrigues, 2010; Rodrigues, 2012).
As infraestruturas em titânio podem não apresentar a mesma resistência quando soldadas, particularmente quando submetidas a esforços mastigatórios. Estes esforços estão relacionados ao fenômeno de fadiga, que causa alteração na estrutura do material, e que dependendo do número de ciclos pode ser permanente, localizada e progressiva, podendo ou não levar à formação e propagação de trincas, porosidades internas, irregularidades, descontinuidades geométricas e fratura do metal (Chiaverini, 1977; Vallittu & Luotio, 1996; Henriques et. al., 1997, Nuñez-Pantoja et al., 2011).
Alguns estudos mostram que o titânio e suas ligas podem sofrer alterações químicas, como o desenvolvimento de corrosão (Correa et al., 2008; Olmedo et al., 2009; Sartori et al., 2009). No ambiente intrabucal, a corrosão pode estar associada aos elementos químicos presentes na saliva, que em contato com a superfície do titânio ou liga, podem ocasionar o desenvolvimento de reações eletroquímicas (Sartori et al., 2009; Noguti et al., 2012). Essas reações ocorrem por meio da ação de elementos químicos sobre a camada protetora presente na superfície do metal ou liga denominada óxido de titânio (TiO2). Esta
camada é responsável pela resistência do metal ao processo corrosivo, mas que em contato com alguns íons pode ser desestruturada (Correa et al., 2008; Sartori et al., 2009; Noguti et al., 2012; Rosalbino et al., 2012).
Diversos autores estudaram a qualidade dos procedimentos de soldagem (LASER ou TIG) realizadas em titânio e/ou suas ligas (Wang & Welsch, 1995; Chai & Chou,
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1998; Zavanelli et al., 2000; Wiskott et al., 2001; Liu et al., 2002; Hart et al., 2006; Rocha et al., 2006; Watanabe & Topham, 2006); mas poucos analisaram a qualidade dessa união em diferentes situações clínicas e laboratoriais, por meio de ensaios de resistência à fadiga e corrosão (Bezerra et al.,1999; Henriques et al.,1997; Zavanelli et al., 2000; Nuñez-Pantoja et al., 2011).
Estudos que correlacionam a análise bi e tridimensional de desajuste marginal com técnicas de soldagem (LASER ou TIG) também são escassos na literatura, principalmente quando associados à avaliação do comportamento dos parafusos protéticos por meio de simulações de ciclos mastigatórios, e desenvolvimento de tensão sobre os pilares protéticos por meio da técnica extensométria. A análise tridimensional de desajuste marginal é importante, pois permite a obtenção de quantificações representativas do desajuste marginal, por meio de cortes tranversais.
Dessa maneira, ainda há necessidade de pesquisas relacionadas ao comportamento clínico e laboratorial de estruturas soldadas (LASER ou TIG), frente aos ensaios de corrosão e fadiga, como também análises de tensão, força de destorque de parafusos protéticos e mensuração do desajuste marginal (bi e tridimensional).
Portanto, este estudo avaliou e comparou dois tipos de estruturas fundidas em titânio (infraestruturas e halteres), ambas soldadas (LASER ou TIG), em relação ao desenvolvimento de tensão sobre os componentes protéticos, intensidade da força de destorque dos parafusos protéticos, comportamento corrosivo, fadiga mecânica e análise do desajuste marginal (bi e tridimensional).
As infraestruturas simularam uma situação clínica representando próteses fixas múltiplas implantossuportadas de três elementos parafusadas em análogos de mini pilares protéticos. Os halteres simularam uma situação laboratorial, em que foi avaliado somente o comportamento do metal soldado (LASER ou TIG), sem qualquer interferência clínica de ancoragem.
A resistência à corrosão e a propagação de trincas intrínsecas foram avaliadas e comparadas em ambas situações, como também o comportamento mecânico dos halteres, infraestruturas e parafusos, por meio de ensaios mecânicos de fadiga. As análises bi e tridimensional do nível de desajuste marginal foram desenvolvidas somente nas infraestruturas protéticas e comparadas entre si, antes a após o desenvolvimento de ambas
6
soldagens (LASER ou TIG), para avaliar a melhor técnica de soldagem e mensuração do desajuste marginal.
7
Capítulo 1 *
The effect of welding techniques on the biomechanical behavior of implant-supportedprosthesis Authors:
Sabrina Alessandra Rodrigues1, Valentim Adelino Ricardo Barão2, Rafael Leonardo Xediek Consani3, Mauro Antônio Arruda Nóbilo4, Marcelo Ferraz Mesquita4.
1DDS, MSc, PhD Student - Department of Prosthodontics and Periodontology, Piracicaba
Dental School, University of Campinas, Piracicaba, Brazil.
2DDS, MSc, PhD, Assistant Professor - Department of Prosthodontics and Periodontology,
Piracicaba Dental School, University of Campinas, Piracicaba, Brazil.
3DDS, MSc, PhD, Associate Professor - Department of Prosthodontics and Periodontology,
Piracicaba Dental School, University of Campinas, Piracicaba, Brazil.
4DDS, MSc, PhD, Full Professor - Department of Prosthodontics and Periodontology,
Piracicaba Dental School, University of Campinas, Piracicaba, Brazil.
Full address of all authors:
1,2,3,4Piracicaba Dental School – Av. Limeira, 901; Piracicaba – SP, Brazil; 13414-903.
Corresponding author:
Sabrina Alessandra Rodrigues
Av. Limeira, 901; Piracicaba, São Paulo, Brazil; 13414-903. E-mail: +55 (19) 2106 5211 – Fax: +55 (19) 2106 5218 _____________________
*Artigo de acordo com as normas para publicação do periódico The International Journal of Prosthodontics
8
Abstract
Purpose: To evaluate the effect of welding techniques of prosthetic frameworks on the two
and three-dimensional marginal misfits, detorque of prosthetic screws and stress induced on the mini abutment, before and after mechanical cycling Material and Methods: For control group, twenty frameworks were casted in commercially pure titanium (cp-Ti) (n=20). An epoxy resin index was obtained using mini abutment conventional analogs for detorque measurement, cyclic loading and marginal misfit analysis. A plaster index was obtained using mini abutment modified analogs for strain gauge analysis. The indices were made simulating a marginal misfit of 200 µm. The two and three-dimensional marginal misfit analyses were made by precision optical microscope and X-ray microtomography. Afterwards, the frameworks were divided into two experimental groups (n=10), LASER (L) and TIG (T). The welding techniques were performed according to following parameters: LASER (370V/9ms); TIG (36A/60ms). The L and T-groups were reevaluated according to the marginal misfit, detorque strength and stress, and then submitted to oblique compressive load (280N; 106 cycles; 2 Hz).. The results were submitted to one-way ANOVA, followed by Tukey HSD test (α=.05). Results: Welding statistically reduced the two and three-dimensional marginal misfit of prosthetic frameworks (P=.000), the detorque values of prosthetic screws (P=.000), and stress induced on the mini abutment analogs (P=.006). After mechanical cycling, the detorque values decreased (P=.002). Conclusions: The welding techniques improved the biomechanical behavior of implant-supported system.
9
Introduction
Dental implants have been widely used to treat partial and completely edentulous patients, due to the their ability to restore the function and dental aesthetics1,2. Titanium metal are used to fabricate implant-supported prostheses1,2,5. Its use has been favorable due to their biocompatibility and biomechanical characteristics, such as low density, high mechanical strength, high corrosion resistance and high ductility6,7,8. When casted or machined, the marginal fit of titanium frameworks should be investigated as the implant interface and prosthesis/abutment is one of the critical factors of stress transference, which compromise the biomechanical response of implant-supported system5.9.
An implant-supported prosthesis with marginal misfit can induce overloads able to damage the peri-implant soft and bone tissues3. Mechanical complications such as loosening or fracture of retention screws are prone to occur8,9,11,12. This is predictable due to the juxtaposition existing between the bone tissue and implant, which does not allow the dissipation of intensive strength. Therefore, it is extremely important to obtain prostheses with marginal fit for stability of implant-supported system10.
Different techniques have been used to reduce marginal misfit of fixed partial prosthesis, including conventional welding, brazing welding, Light Amplification by Stimulated Emission of Radiation (LASER) welding and Tungsten Inert Gas (TIG) welding. LASER welding and TIG welding are the most common techniques for titanium frameworks2.7,13.
LASER welding uses as heat source a concentrated light beam of high-energy, able to generate localized heating, which decrease the occurrence of deformation of the welded piece1,2,4,14. TIG welding is developed by electric arc, formed by contact welded metal with non consumable electrode of tungsten1,4. In the literature, when comparing the LASER or TIG welding techniques, it appears there is not a better to solve the problem of marginal misfit in implant-supported prosthesis.
Some studies have evaluated stress and strength detorque associated to the marginal misfit of fixed partial prosthesis framework 15,16,17,18,19,20,21. In these studies, the marginal misfit analysis have always been developed on a single-axis (i.e. two-dimensional) in precision optical microscope8,22. Very few studies have investigated the three-dimensional marginal misfit of welded frameworks,. In this study, therefore, X-ray microtomography was
10
used to evaluate the three- dimensional marginal misfit of frameworks welded by LASER or TIG. The marginal misfit evaluated was compared with the two-dimensional marginal misfit to select the best welding technique.
The purpose of this study was to evaluate the effect the welding techniques of prosthetic frameworks on the two and three-dimensional marginal misfits, detorque of prosthetic screws and stress induced on the mini abutment, before and after mechanical cycling. Our hypothesis was that the welding techniques and cyclic loading would affect the marginal fit and biomechanical behavior of frameworks casted in commercially pure titanium (cp-Ti).
Material and Methods
Experimental Design
Twenty frameworks were casted in commercially pure titanium (cp-Ti), simulating a 3-unit mandibular fixed partial prosthesis.. A marginal misfit of 200 µm was simulated using an index (control group). The two and three-dimensional marginal misfits, detorque values of prosthetic screws and stress induced on the mini abutment was investigated. Afterwards, the twenty frameworks of the control group were divided into two experimental groups: LASER (L) and TIG (T) (n=10). The same analyses were performed in the welded groups. Samples were submitted to mechanical cycling to evaluate the prosthetic screws values post-cycling.
Prosthetic frameworks fabrication
A metallic master model in stainless steel was fabricated to simulate a clinical situation of two implants placed on the mandibular region of first premolar (A) and first molar (B) (Fig. 1). Two modified mini abutment analogs of platform 4.1 mm (Neodent, Curitiba, PR, Brazil) were placed for waxing the framework. From the waxing standard, twenty frameworks were casted in commercially pure titanium (cp-Ti). These frameworks represented the control group (n=20).
11
Fig.1. Metallic master model
Vertical marginal misfit simulation
For each framework, two types of indices were fabricated in epoxy resin (Araldite, Araltec, Guarulhos, SP, Brazil) and in plaster (Durone Dentsply; Petropolis, RJ, Brazil). The epoxy resin index was used for two- and three-dimensional marginal misfit analysis, screw detorque analysis before and after mechanical cycling, while the plaster index was used for strain gauge analysis. A rigid index is necessary for the strain gauge measurement due to the fragility of the gauges. The plaster and epoxy resin5 indeces were fabricated containing two 4.1-mm modified mini abutment analogs and two mini abutment analogs, respectively. A marginal misfit of 200 µm was simulated in the indices using 200-µm thickness steel ring placed between the mini abutment analog B (first molar) and the framework. Such misfit level, is considered unacceptable in the literature, due to the development of overstress on the implants and prosthetic components of fixed partial prosthesis 20,21.
Two-dimensional marginal misfit analysis
The two-dimensional marginal misfit analysis was performed using a precision optical microscope (UHL VMM-100-BT; United Kingdom). A digital torque meter (Torque Meter TQ-8800, Lutron, Taipei, Taiwan, Chine) was used to apply the single-screw test protocol22. The vertical marginal misfit between the mini abutment analogs platform (B) and
framework inferior border was measured, considering the vestibular and lingual faces. A total of 10 Ncm torque was applied to the mini abutment analog A (first premolar) for measurement of the vertical marginal misfit on mini abutment analog B (first molar). Afterwards, the screw was loosened and transferred to the mini abutment analog B (first
A A B 18 mm 20 mm 30 mm 15 mm
12
molar) for measurement of the vertical marginal misfit on mini abutment analog A (first premolar). For each framework, a mean value of vertical marginal misfit was obtained. The measurements were performed by a calibrated examiner (intraclass correlation= 0.998).
Three-dimensional marginal misfit analysis
A X-ray microtomography (Micro CT, Sky Scan 1172, Bruker, Kontich, Belgium) was used to evaluate the three-dimensional marginal misfit analysis via the single-screw test protocol22.
The frameworks were scanned with a power source of 100 KV/98 microA, 1-mm filter thickness (Al) and detector pixel size of 9 µm. Optimal images were obtained using rotation step of 0.45º up to 180º. Two images were obtained for each framework ,and the scan time for each image was approximately 1 hour. The images were processed by the CT- analyze software (Skyscan, Bruker, Kontich, Belgium), that is able to develop the exact 3D image of each framework. Therefore, there is no need to cut or alter the framework. to evaluate the internal misfit.
Framework reconstruction in approximately 900 slices was followed by definition and detection of the marginal misfit between the mini abutment analogs platform (A-B) and framework inferior border. Through images analysis of reconstructed axial sections, the total volume (Vt) of marginal misfit between framework and abutment and after screw inlets volume (Vs) were represented. At the end, the misfit real volume (Vr) was obtained by difference between Vt and Vs (Fig.2). The marginal misfit values were measured in volume (mm3).
Fig. 2. Microtomographic of mini abutment analogs A-B (A), Marginal misfit total volume (Vt) (B) and screw inlets volume (Vs).
A B C
A B
B A
13 Detorque analysis
The detorque values were evaluated using titanium prosthetic screws (Neodent) according to Siamos et al. (2002) technique, which allows better stability of the prosthetic screws23. In summary a torque of 10 N cm was applied to the mini abutment analog A (first premolar) and mini abutment analog B (first molar). After 10 minutes, a retorque (10 N cm), following the same sequence (A-B) was performed. After 10 min, the screw detorque was measured. A digital torque meter (Torque Meter TQ-8800; Lutron) was used for development of the analysis and the mean value of detorque was calculated for each framework.
Strain gauge technique
The strain gauge analysis was performed using ADS 2000 equipment (Lynx Tecnologia Eletronica Ltd, SP, Brazil), with data processed by an specific software (AqAnalysis 2000, Lynx Tecnologia Eletronica Ltd). A plaster index associated with modified mini abutment analogs (Neodent) was used. The modified mini abutment analog is a replica of the conventional analog, machined in titanium with an elongated stem (13 mm) and hollow surface. On the elongated stem, strain gauges were positioned near to the fulcrum region of the analogs, which corresponds to the region of strain measurement. The hollow improves the measurement of the elastic deformation (Fig.3).
The strain gauge (PA-06-060-BG-350L – Excel Sensores Ltd, Embu, SP, Brazil) was bonded parallel to the long axis of the each modified analog with cyanoacrylate-based glue (Loctite Super Bonder, Henkel, Düsseldorf, Germany). For stability of analysis, the strain gauge was positioned 10 mm from the upper portion of the analog. The electric circuit was mounted in a 1/4 Wheatstone bridge, and the strain gauges were set at zero before the evaluation of each specimen (Fig. 4). The strain gauge was fixed in the mini abutment analog as it provides more stable results compared to fixing the strain gauge on the epoxy resin. Prior to strain measurement, the sequence of screw torque followed the one described above. A mean value of strain was obtained for each framework.
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Fig.3. Modified mini abutment analog: external (A) and internal (B) part.
Fig.4. Strain gauge set up (A); Electric circuit in ¼ Wheatstone bridge (B).
Welding techniques
After all analyses, the frameworks were divided into two groups (n=10), LASER (L) and TIG (T). On the epoxy resin index, the frameworks were sectioned in pontic area, along to the vertical axis of the analogs. A “I” joint design was obtained and its parts were ultrasonic cleaned and sandblasted with aluminum oxide4.
In the L group, the welding was performed at 370V/9ms, with focus and frequency calibrated at zero (Desktop – F, Dentaurum, Pforzheim, Germany). Four points on the opposite side of the cross-section were developed to stabilize the parts of specimen aligned4, and then other complementary points were done to conclude the welding.
A B
A B
13 mm
3 mm 4.1 mm
15
In the T group, the welding was conducted using the parameter 3:2, where 3 is the potency (current of 36 A), and 2 is the time (60 ms) (NTY 60C machine, Kernit, Industrial Mechathonic Ltd, Indaiatuba, SP, Brazil). Two opposite points were used to stabilize the framework position, and then other complementary points were performed to conclude the welding1.
Afterwards, the 2D and 3D misfits, stress and screw detorque measurements were evaluated.
Mechanical Cycling
The welded groups were submitted to mechanical cycling to investigate the screw detorque. On the epoxy resin index, the frameworks were submitted to 106 mechanical cycles (Mechanical Fatigue Simulator ERIOS, model ER11000 Plus, São Paulo, SP, Brazil)24 with 2 Hz of frequency and 280N oblique compressive load applied to the pontic region in the vestibule-lingual direction. The load intensity represents the mean of maximum strength that a lower second premolar localized in fixed partial prosthesis may support during masticatory cycles25,26. Cyclic loading simulated one year of clinical use as an individual has three episodes of mastication per day, each one with 15 minutes of duration, equivalent to 2.700 cycles per day27. The frameworks remained immersed in artificial saliva (1.5mM Ca, 3.0mM
P, 20.0mM NaHCO3, pH 7.0) at 37ºC during the mechanical cycling. The screw detorque
was measured using digital torque meter (Torque Meter TQ-8800; Lutron). A mean value of detorque value was obtained for each frameworks.
Statistical analysis
One-way ANOVA test was used to verified the effect of welding on the two and three-dimensional marginal misfits, stress and detorque values. Subsequent pairwise comparisons within groups were made using Tukey HSD post hoc analysis. Independent t-test was used to compare the detorque strength values post cycling between L and T groups, and the paired t-test was used to evaluate the effect of loading cycling in the detorque values of screws of welded groups. The level of statistical significance was considered 5% (Statistical Package for the Social Sciences, version 17.0; SPSS, Inc, Chicago, IL, USA).
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Results
Table 1 shows the mean and standard deviation values of 2D and 3D marginal misfit, strain and screw detorque for control and welded groups. Control group exhibited the greatest 2D and 3D marginal misfit values (p<.001). No significant difference was noted between the welded groups (p=.866 for 2D misfit, and p=.185 for 3D misfit). The percentage of reduction of marginal misfit was similar for both 2D (77% and 67% for L and T groups, respectively) and 3D techniques (80% and 75%. for L and T groups, respectively).
Welding reduced the strain values observed in the implant-supported system (p<.01). Similar behavior was noted between L and T groups (p=.213). Welded groups presented greater screw detorque values before and after mechanical cycling when compared to the control group (p<.001). No significant difference was noted between welding techniques (p=.185). Mechanical cycling reduced the screw detorque values for L and T groups (p<.001).
Table 1 – Mean values (standard deviation) of two and three-dimensional marginal misfit, strain and detorque strength according to the groups.
Groups 2D Misfit (μm) 3D Misfit- (mm3) Strain (μstrain) Detorque (N cm) Pre-Cycling Post-Cycling Control (n=20) 214.35 (50.07) a 0.99 (0.40) a 494.00 (154.52) a 4.71 (1.00) ___________ LASER (n=10) 42.60 (23.60) b 0.22 (0.65) b 315.65 (158.33) b 6.60 (0.45) bA 4.00 (2.02) aB TIG (n=10) 52.40 (26.46) b 0.32 (0.16) b 200.54 (109.03) b 6.65 (0.71) bA 3.85 (1.47) aB Lowercase letters indicate differences in a same column. Capital letters indicate differences in a same line.
(p<.05; Tukey's HSD).
Table 2 – Mean values and percentage values of decrease of the two and three-dimensional marginal misfit.
Marginal Misfit Analysis
Groups Two-dimensional Tridimensional
Control (n=20) 214.35 µm (100%) 0.99 mm3 (100%)
LASER (n=10) 42.60 µm (77%) 0.22 mm3 (80%)
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Discussion
In Dentistry, the search of perfectly fitted implant-supported prostheses is constant. It is known that the juxtaposition of two different surfaces, even when smooth and polished, tends to result in a gap in the fit region27. In this study, two welding techniques (LASER and TIG) were used to decrease the marginal misfit and to improve the biomechanical behavior of frameworks casted in cp-Ti. Therefore, our hypothesis was accepted.
Both welding techniques reduced the 2D and 3D marginal misfit between the abutment and framework. LASER or TIG welding techniques reached clinical acceptable marginal misfit values 28,29. This result collaborates with previous studies 1,11,17,20,30,31.
The percentage of marginal misfit reduction were similar between 2D and 3D techniques. Based on that, the X-ray microtomography proved to be an effective tool to evaluate 3D marginal misfit of frameworks. However, such technique present greater cost and processing time of the images, compared to two-dimensional analysis.
As the marginal misfit was reduced after welding, the screw detorque values increased. Therefore, the marginal misfit had influence on the biomechanical behavior of the prosthetic screws. These results are favorable and coherent with previous studies9,11,12 which
showed that the torque and preload may be influenced by the presence of higher marginal misfit values where pre-load strength part used to approximate the surfaces, may cause additional stress to the prosthetic screws10. The overload generated on the screws during the
torque to approximate the misfit parts decrease the detorque values of the screw13.
The induced stress on mini abutment analogs decreased for frameworks welded by LASER and TIG, compared to the intact frameworks. The reduction of the marginal misfit after welding may be the driven force toward such results. When the screwed parts in the implant-supported system are better fitted, the dissipation of stress to the mechanical and biological system is enhanced with consequent implant treatment longevity11,12. Therefore, it is extremely necessary to obtain implant-supported prosthesis with passive fit2,5,6.
After mechanical cycling, a decrease of screw detorque values was noted. This decrease influenced the stability of the prosthetic screws after one year of masticatory simulation. This result is favorable and according to some present studies in the scientific
18
literature11,12,32,33. Therefore, clinically is essentially recommended the tightened of the
prosthetic screws of the implant-supported prosthesis after 12 months of use11,12.
Conclusion
Based on the results obtained in the present study it can be concluded that: • The laser and TIG welding techniques reduced the 2D and 3D marginal misfit of frameworks and the strain in the implant-supported system, and increased the screw detorque values.
• The masticatory simulation, through the development of mechanical cycling reduced the stability of the prosthetic screws.
Acknowledgements
The Sao Paulo Research Foundation (FAPESP grant numbers 2012/14141-2 and 2012/14139-8) supported this study.
References
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13. Wang RR, Welsch GE. Joining titanium materials with tungsten inert gas welding and infrared brazing. J Prosthet Dent. 1995; 74: 521-30.
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29. Jemt T. Failures and complications in 391 consecutively inserted fixed prostheses supported by Branemark implants in edentulous jaws: a study of treatment from the time of prosthesis placement to the first annual checkup. Int J Oral Maxillofac Implants 1991; 3 (6): 270-6.
30. Tossi R, Falcão-Filho H, Aguiar Júnior FA, Rodrigues RC, Mattos M da G, Ribeiro RF. Modified section method for laser-welding of ill-fitting cp Ti and Ni-Cr alloy one-piece cast implant-supported frameworks. J Oral Rehabil. 2010; 37:359-363.
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31. Silva TB, Nobilo MAA, Henriques GEP, Mesquita MF, Guimaraes MB. Influence of laser-welding and electroerosion on passive fit of implant-supported prosthesis. Stomatologija, Baltic Dental and Maxillofac J. 2008; 10:96-100.
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Capítulo 2*
Corrosion behavior of titanium frameworks welded by LASER and TIGAuthors:
Sabrina Alessandra Rodrigues1, Alexandra Alves2, Fatih Toptan3, Luis Augusto Rocha4, Filipe Samuel Silva5, Valentim Adelino Ricardo Barão6, Marcelo Ferraz Mesquita7.
1DDS, MSc, PhD Student - Department of Prosthodontics and Periodontology, Piracicaba
Dental School, University of Campinas, Piracicaba, Brazil.
2BSc, MSc, PhD Student - Center for Mechanics and Materials Technology, Department of
Mechanical Engineering, Minho University, Guimaraes, Portugal.
3BSc, MSc, PhD, Assistant Professor - Center for Mechanics and Materials Technology,
Department of Mechanical Engineering, Minho University, Guimaraes, Portugal.
4BSc, MSc, PhD, Assistant Professor - Department of Physics, School of Sciences, São Paulo
State University, Bauru, Brazil.
5BSc, MSc, Full Professor - Center for Mechanics and Materials Technology, Department of
Mechanical Engineering, Minho University, Guimaraes, Portugal.
6DDS,MSc, PhD, Associated Professor - Department of Prosthodontics and Periodontology,
Piracicaba Dental School, University of Campinas, Piracicaba, Brazil.
7DDS, MSc, PhD, Full Professor - Department of Prosthodontics and Periodontology,
24
Full address of all authors:
1,6,7 Piracicaba Dental School – Limeira Avenue, 901; CEP 13414-903; Piracicaba, São Paulo,
Brazil.
2,3,5 Minho University – Campus Azurém, 4800-058 Guimaraes, Portugal.
4São Paulo State University - Engenheiro Luiz Edmundo C. Coube Avenue, Vargem Limpa,
1401, CEP 17033-360, Bauru, Sao Paulo, Brazil.
Corresponding author:
Sabrina Alessandra Rodrigues
Limeira Avenue, 901; CEP: 13414-903; Piracicaba-SP, Brazil E-mail: +55 (19) 2106 5211 – Fax: +55 (19) 2106 5218
_______________________
*Artigo de acordo com as normas para publicação do periódico Materials Science and Engineering C.
25
Abstract
This study investigated the role of different welding techniques (Light Amplification by Stimulated Emission of Radiation - Laser and tungsten inert gas - TIG) on the corrosive behavior of cp-Ti framework in an oral environment. The specific aim of this study was to understand the corrosion kinetics and surface morphology of welded cp-Ti by using electrochemical impedance spectroscopy (EIS) as well as basic corrosion tests. Standard frameworks of implant-supported prosthesis were fabricated using a metallic matrix. Three frameworks were maintained intact (control group) and six were sectioned to receive the welding (LASER and TIG groups). Standard electrochemical tests, such as open circuit potential, EIS, and potentiodynamic tests were conducted in a controlled environment (artificial saliva at pH 7, 37 oC). The surfaces were examined using scanning electron microscopy (SEM). Data were analyzed by one-way ANOVA followed by Tukey HSD test (α=0.05). For Rox values, no significant difference was noted among groups (p=.078),
although frameworks welded by TIG technique exhibited the lowest values. TIG group had the greatest Qox values (p=.004). Both welding procedures did not affect the corrosion
behavior of cp-Ti as noted by the Ecorr (p=.138) and ipass (p=.659) values. However,
frameworks welded by TIG tended to present later passivation and less noble potential (more negative value). These findings suggest a better corrosion behavior for cp-Ti frameworks welded by Laser.
26
Introduction
Titanium has been widely used to fabricate implant-supported prosthesis (i.e. frameworks, abutments and dental implants) (Cortada et al., 2000; Gil et al., 2000. Mabboux et al., 2004; Correa et al., 2009). Considered a biocompatible metal, titanium has excellent mechanical and electrochemical properties such as good mechanical and corrosion resistance, which contributes to its greater longevity and resistance into the oral environment (Orsi et al., 2011; Barão et al., 2012; Gil et al., 2012). The corrosion resistance of the titanium is due a stable and dense oxide layer (TiO2) formed in its surface, which plays a protection
against the biological, chemical and physical variation of environment (Huang 2003; Orsi et al., 2011; Barão et al., 2015).
Nevertheless, into the oral cavity, metals and alloys suffer constantly pH variations, due the presence of chemical and microbial agents such as saliva, acid, fluoride and bacteria (Rezende et al., 2007; Barao et al., 2012; Souza et al., 2012). These variations in the oral environment often impair the electrochemical stability and the mechanical properties of metals and alloys (Souza et al., 2013; Barao et al., 2015).
Titanium has been machined or casted to fabricate implant-supported prosthesis. Casting procedures can affect the marginal fit of implant-supported restorations (Contreras et al. 2002; Watanabe e t al., 2006; Souza et al., 2008; Pantoja et al., 2011; Nunez-Pantoja et al., 2012). Therefore, welding procedures are necessary to reduce such marginal misfit which improve the restoration longevity (Wang et al., 1995; Wang & Welsh et al., 1998; Rocha et al., 2006; Silveira-Junior et al., 2009; Nunez-Pantoja et al., 2012). Currently, two welding procedures have been commonly used to fabricate implant-supported prosthesis such as the Light Amplification by Stimulated Emission of Radiation (Laser) and the Tungsten Inert Gas (TIG).
Laser welding procedure is characterized by monochrome electromagnetic light, where the energy beam is concentrated in a focal point, resulting in the welding process (Rocha et al., 2006; Nuñez-Pantoja et al., 2011; Nuñez-Pantoja et al., 2012). The procedure relies on the fusion of the parent metal for joint formation (welding area). There is no direct contact with the welded area during the welding process (Nuñez-Pantoja et al., 2011; Orsi et al., 2011; Nuñez-Pantoja et al., 2012). Thus, the welded area is defined with less heating, without the influence of magnetic field on the laser beam (Nuñez-Pantoja et al., 2011; Orsi
27
et al., 2011). TIG welding procedure is characterized by the formation of an electric arc, through contact a non-consumable tungsten electrode and the metal (Rocha et al., 2006; Nuñez-Pantoja et al., 2011). During Laser or TIG welding procedures, a flow of inert gas is sprayed on the titanium joints to minimize the oxidation of the titanium metal that is highly reactive with the atmosphere gases (Nunez-Pantoja et al., 2011; Nunez-Pantoja et al., 2012). The mechanical properties of titanium joints welded by Laser and TIG are different from the intact joints as the welding techniques change the metal microstructure (Nuñez-Pantoja et al., 2011; Orsi et al., 2011; Nuñez-Pantoja et al., 2012). It can affect the electrochemical behavior of the titanium material, compromising its corrosion resistance (Nuñez-Pantoja et al., 2011; Gil et al., 2012; Nuñez-Pantoja et al., 2012).
Very few studies (Orsi et al. 2011; Matos et al. 2015) have investigated the influence of welding procedures on the corrosion behavior of metals used into the oral cavity. Orsi et al. (2011) showed that commercially pure titanium (cp-Ti) welded by TIG is more resistant to local corrosion initiation and propagation than welded by Laser. However, only polarization potentiodynamic test was conducted and authors failed to evaluate the properties of the oxide film formed on the welded area. Others (Matos et al. 2015) observed better corrosion behavior of Ni-Cr framework welded by TIG when compared to conventional brazing.
To authors’ best knowledge, no study has investigated the influence of welding technique on the corrosion kinetics of cp-Ti frameworks. Therefore, in this study, the role of different welding techniques (Laser and TIG) on the corrosive behavior of cp-Ti framework in an oral environment was evaluated. The specific aim of this study was to understand the corrosion kinetics and surface morphology of welded cp-Ti by using electrochemical impedance spectroscopy (EIS) as well as basic corrosion tests. The research hypothesis was that the welding procedures would impair the corrosion resistance of cp-Ti.
28
Materials and Methods
Specimens preparation
A metallic matrix (30×15×20 mm) of stainless steel with two mini abutment analogs (Neodent; Curitiba, PR, Brazil) 18-mm apart was fabricated. A standard framework with compatible anatomy of the lower first premolar to first molar was waxed. Silicone index (Flexitime; Heraeus Kulzer, São Paulo, Brazil) was prepared to standardize the waxing procedure of all the other frameworks in self-polymerized acrylic resin (DuraLay II; Reliance Dental Mfg Co, Chicago, USA). The frameworks were viewed before casting to evaluate any structural flaws.
Casting process of the specimens
The frameworks were casted from cp-Ti (Tritan grade I; Dentaurum, Pforzheim, Germany) by plasma/vacuum-pressure (Rematitan System; Dentaurum, Pforzheim, Germany), which uses plasma as the means for energy transmission to cast metals under a vacuum and argon-inert atmosphere. For each framework, 250 g of powder and 40 mL of liquid of investment (Rematitan Plus Speed Universal Titanium; Dentaurum, Pforzheim, Germany) was used. The specimens were slowly heated (Zavanelli et al., 2000) following identical fabricating procedures for wax removal and casting of cp-Ti (Commercially pure titanium 31g; Dentaurum, Pforzheim, Germany). Afterwards, molds were placed immediately in cold running water and specimens were divested with a pneumatic hammer (M320; Flli Manfredi, Sofia, Italy), and air braded with 100 µm aluminum oxide particles at a pressure of 0.55 MPa (Nuñez-Pantoja et al., 2011; Nuñez-Pantoja et al., 2012). The frameworks were finished and polished using special titanium drills (Rematitan; Dentaurum, Pforzheim, Germany), rubbers (#5001 polisher; Dedeco Dental, New York, USA), and titanium polishing paste (Tiger Brilliant Polier Paste; Dentaurum; Pforzheim, Germany). Radiographies of the specimens were made digitally (Heliodent plus; Sirona, Bensheim, Germany) to verify internal defects (Nuñez-Pantoja et al., 2011). The radiographic examination consisted of exposure of specimens to radiation (70 kV; 7 mA; 0.05s and 12 mm of distance) using an intraoral sensor (XIOS plus, Sirona, Bensheim, Germany). A total of nine frameworks were obtained and divided into three groups (n=3), titanium (Ti - control), Laser and TIG.
29 Welding procedures (LASER and TIG)
From the nine frameworks, three were maintained intact (control group) and six were sectioned to receive the welding (Laser and TIG groups). Section was performed 7 mm away from the mesial area of the first premolar. It was done vertically to the long axis of the mini abutment using a 1-mm thickness disc, forming an “I” design. The aligned framework was fixed with self-polymerized acrylic resin (DuraLay II; Reliance Dental Mfg Co) respecting the welded distances (1 mm). The adjacent part to the gap was abraded with 100 µm airborne aluminum oxide particles at a pressure of 0.55 MPa, and the joints were welded on opposite sides of the specimen in order to stabilize them (Nuñez-Pantoja et al., 2012). All the welding procedure were conducted in a replica of the metallic matrix fabricated with epoxy resin (Araldite; Araltec, Guarulhos, SP, Brazil) (Farina et al., 2012).
The Laser welding was performed at 370V/9ms with focus and frequency calibrated at zero (Desktop-F; Dentaurum, Pforzheim, Germany) (Nuñez-Pantoja et al., 2011; Nuñez-Pantoja et al., 2012). Four points on opposite side of the cross-section were determined to stabilize the framework. Afterwards, the welding was completed using cp-Ti metal filler solder (Dentaurum).
TIG welding (NTY 60 C, Kernit, Industrial Mechatronic Ltda, Indaiatuba, SP, Brazil) was performed using a current of 36 A during 60 ms (Nuñez-Pantoja et al., 2011). Two opposite welding points were done to stabilize the framework; afterwards, the welded area was completed. All the joints were finished and polished using the same procedure done after the casting process.
Electrochemical tests
For the electrochemical tests, the frameworks were sectioned to expose the intact (control group) or the welded area (Laser and TIG groups). Sectioned frameworks were mechanically polished with silicon carbide paper (#600, 800, 1200 e 2400) (Carbimet; Buehler, Lake Bluff, IL, USA) and mirror finished with colloidal silica suspension (0.05 µm) (Master Med, Buehler). Samples were ultrasonically cleaned using 70% propanol for 10 minutes and distilled water for 5 minutes and air dried. An electrical contact was established through a 12-mm long coated copper wire fixed on the samples. The samples were painted
