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UNIVERSIDADE ESTADUAL DE CAMPINAS FACULDADE DE ODONTOLOGIA DE PIRACICABA
Sabrina Alessandra Rodrigues
Efeito do desajuste marginal, tipo de soldagem e material do
parafuso sobre a força de destorque de parafusos protéticos e
tensões induzidas aos pilares
Dissertação de Mestrado apresentada a Faculdade de Odontologia de Piracicaba da UNICAMP para obtenção do título de Mestre em Clínica Odontológica, na Área de concentração em Prótese Dental.
Orientador: Prof. Dr. Marcelo Ferraz Mesquita Este exemplar corresponde à
versão final da Dissertação defendida pela aluna, e orientada pelo Prof. Dr. Marcelo Ferraz Mesquita ______________________________ Assinatura do Orientador
PIRACICABA
2012
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FICHA CATALOGRÁFICA ELABORADA POR MARILENE GIRELLO – CRB8/6159 - BIBLIOTECA DA FACULDADE DE ODONTOLOGIA DE PIRACICABA DA UNICAMP
R618e
Rodrigues, Sabrina Alessandra, 1986-
Efeito do desajuste marginal, tipo de soldagem e material do parafuso sobre a força de destorque de parafusos protéticos e tensões induzidas aos pilares / Sabrina Alessandra Rodrigues. -- Piracicaba, SP : [s.n.], 2012.
Orientador: Marcelo Ferraz Mesquita.
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Odontologia de Piracicaba.
1. Titânio. 2. Ouro. 3. Infraestrutura. 4. Prótese. 5. Torque. I. Mesquita, Marcelo Ferraz, 1967- II. Universidade Estadual de
Campinas. Faculdade de Odontologia de Piracicaba. III. Título.
Informações para a Biblioteca Digital
Título em Inglês: Effect of marginal misfit, welding type and screw material
on the strength of the detorque prosthetic screws and stress induced the abutments Palavras-chave em Inglês: Titanium Gold Infrastructure Prostheses Torque
Área de concentração: Prótese Dental Titulação: Mestre em Clínica Odontológica Banca examinadora:
Marcelo Ferraz Mesquita [Orientador] Luís Geraldo Vaz
Rafael Leonardo Xediek Consani
Data da defesa: 27-04-2012
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DEDICATÓRIA
À Deus por ter me dado força e saúde para concluir esse trabalho.Aos meus pais, Mauricio e Laurita, pelo amor, apoio e carinho em todos os momentos da minha vida. Amo muito vocês.
Aos meus irmãos Camila e Leandro, que sempre acreditaram em mim e me apoiaram, fazendo com que eu nunca deixasse de lutar por um futuro melhor.
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AGRADECIMENTOS ESPECIAIS
Ao meu Orientador Prof. Dr. Marcelo Ferraz Mesquita, pela confiança depositada em mim, apoiando-me em todos os momentos. Sua orientação contribuiu e muito para o meu amadurecimento como mestre 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 uma pesquisadora, mas também em relação às dificuldades da vida, me encorajando a nunca desistir dos meus objetivos.
À minha amiga de laboratório e pesquisadora Dra. Juliana Maria Costa
Nuñez Pantoja, pelo apoio, incentivo e confiança depositada em mim. Agradeço pela
orientação em todos os momentos de desenvolvimento da pesquisa, até nos momentos de dificuldade. Obrigada pelos conselhos e ajuda para tomar algumas decisões e também pelo agradável convívio ao longo desse tempo. Sempre faltarão palavras para expressar a enorme gratidão por tudo o que fez por mim.
Por fim, agradeço a 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.
Jacks Jorge Junior, e do seu diretor associado Prof. Dr. Alexandre Augusto Zaia, pelo
acolhimento.
À Profa. Dra. Renata Cunha Matheus Rodrigues Garcia, coordenadora geral
do programa de pós-graduação da Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas, pela atenção e disponibilidade.
Ao Prof. Dr. Márcio de Moraes, coordenador do programa de pós-graduação em Clínica Odontológica da Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas.
À empresa CONEXÃO, pela doação de componentes protéticos, em especial as colegas Dra. Mônica Nogueira Pigozzo e a consultora Srta. Izis Pires.
À FAPESP, pela concessão da bolsa de estudos (Processo no. 2009/09639-6) e do auxílio financeiro (Processo no 2010/096339-6) para desenvolvimento da pesquisa.
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Ao Prof. Dr. Antônio Francisco Iemma e sua equipe pela paciência no desenvolvimento das análises estatísticas.
Aos amigos do laboratório de Prótese Total da Faculdade de Odontologia de Piracicaba Juliana Maria Costa Nuñez Pantoja, Jessica Mie Ferreira Koyama
Takahashi, Vanessa Tramontino Mesquita, Ana Patrícia Macedo, Caroline Hanada
Odo, Mariana Agustinho Rodrigues, Izabella Pereira, Cláudia Brilhante, Isabella
Vieira Marques, Marco Aurélio de Carvalho, Ataís Bacchi, Manoela Capla, Maíra
Serra e Silva, Brunna Pereira, Leonardo Luthi, Mateus Bertolini, João Paulo da Silva
Neto, Bruno Zen pela amizade e convívio.
Aos Profs. Drs. Guilherme Elias Pessanha Henriques, Mauro Antônio de
Arruda Nóbilo e Rafael Leonardo Xediek Consani, pelos conhecimentos transmitidos e
agradável convivência.
Às secretárias Eliete A. F. Lima Marim, Ketney Ferreira Lopes e ao técnico
Eduardo Pinez Campos pela ajuda e boa convivência durante o curso de pós-graduação.
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 memorian) por sempre acreditarem e incentivarem a minha busca
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Aos meus primos, Jacqueline, Karine, Juliana, Adriele, Miriam, Gabriela,
Alisson, Evandro, Vitor Gustavo, João Pedro, Rafael, Isabella, Pedro, Lucas Andréia,
Adriana, Viviani, Eduardo, Fernando, Gustavo, Rafael, Ana Paula e Ana Julia pelo
carinho, paciência e apoio.
A todos os meus amigos que sempre estiveram torcendo por mim e a todos que indiretamente auxiliaram na elaboração desse trabalho.
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EPÍGRAFE
“Não faças do amanhã o sinônimo de nunca,
nem o ontem te seja o mesmo que nunca mais.
Teus passos ficaram.
Olhes para trás, mas vá em frente
pois há muitos que precisam
que chegues para poderem seguir-te”.
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RESUMO
O objetivo neste estudo foi analisar as tensões induzidas aos análogos de pilares protéticos, utilizando-se strain gauges; e avaliar a força de destorque de parafusos protéticos (ouro e titânio) em infraestruturas fundidas em titânio comercialmente puro (Ti cp), antes e após os procedimentos de soldagem laser e TIG (tungsten inert gas). Foram confeccionadas vinte infraestruturas fundidas em titânio comercialmente puro (Ti cp), simulando uma prótese fixa de três elementos; vinte modelos índex com análogos de pilares protéticos modificados para desenvolvimento da análise de tensão e vinte modelos índex com análogos de pilares protéticos convencionais para realização das soldagens e avaliação da força de destorque de parafusos protéticos. Os dois tipos de modelos índex foram confeccionados simulando desajuste marginal vertical de aproximadamente 200 m. A mensuração do nível de desajuste marginal vertical das infraestruturas foi realizada em microscópio de precisão, por meio do teste do parafuso único (antes e após a soldagem). As soldagens seguiram os seguintes parâmetros: Laser (370V/9ms), TIG (3:36A/2: 60 ms). As tensões induzidas nos análogos modificados de pilares protéticos foram avaliadas por análise extensométrica, utilizando-se strain gauges (antes e após as soldagens). O torque dos parafusos de ouro e titânio foi realizado utilizando-se a técnica de torque (10 N cm) e retorque (10 N cm), após 10 minutos. O destorque dos parafusos foi realizado após 10 minutos do retorque, utilizando-se torquímetro digital. Os resultados foram analisados estatisticamente (ANOVA/Tukey (α=0,05) e teste de correlação de Pearson). Observou-se redução dos níveis de desajuste marginal vertical das infraestruturas, após os procedimentos de soldagem (laser e TIG). Quanto aos níveis de tensão induzidos aos análogos de pilares protéticos pós-soldagem, foram menores para soldagem a laser quando comparados à TIG. Em relação ao destorque dos parafusos, após as soldagens, a força de destorque dos parafusos de ouro foi menor em relação a força de destorque dos parafusos de titânio, que aumentou após os procedimentos de soldagem. Não houve correlação entre desajuste marginal, tensão e força de destorque. Assim, conclui-se que o procedimento de soldagem a laser foi mais eficaz na melhora da adaptação das infraestruturas de três elementos implantossuportadas, quando comparado ao procedimento de soldagem a TIG; e que a osxi
parafusos protéticos de titânio podem ser considerados mais estáveis que os parafusos protéticos de ouro quando torqueados em próteses que possuem satisfatória adaptação. Palavras- chave: Soldagem a laser, Soldagem TIG; Titânio; Ouro, Infraestruturas, Prótese, Torque
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ABSTRACT
The aim of this study was to evaluate the stress induced to analogs of
prosthetic abutment using strain gauges and to analyze the detorque strength of prosthetic screws (gold and titanium) of frameworks casted in commercially pure titanium (cp Ti) before and after laser and TIG (tungsten inert gas) welding procedures. Twenty frameworks were made simulating a three-element fixed prosthesis. Twenty index models with modified analogs of prosthetic abutments were obtained for stress analyses and twenty index models with conventional analogs of prosthetic abutments for welding (laser e TIG) and detorque analyses of the prosthetic screws. Both index models were made with 200 m vertical marginal misfit. An optic microscope was used to measure the vertical marginal misfit following the single-screw test protocol. The welding parameters were the following: laser welding (370V/9ms), TIG welding (3: 36A/2: 60ms). The stress induced on the analogs was evaluated through of the extensometric analysis using strain gauges (before and after welding). The torque of gold and titanium screws was performed with torque (10 Ncm) and retorque (10 Ncm) after 10 minutes. The detorque of the prosthetic screws was performed 10 minutes after the retorque using a digital torquemeter. The results were statistically analyzed (ANOVA/Tukey test (α=0.05)/ Pearson correlation test). A decrease in the levels of vertical marginal misfit was observed after the welding procedures (laser and TIG). Regarding the stress, after the welding procedures, stress levels were lower using laser welding than the TIG. The detorque values were lower for the gold screws than the titanium ones, which presented increased destorque values after welding procedures. There was no correlation between marginal misfit, stress and detorque strength. Thus, it can be concluded that the laser welding procedure was more effective for the improvement of adaptation of three-element implant-supported frameworks, when compared to TIG; and that titanium prosthetic screws can be considered more stable than the gold prosthetic screws when torqued in prostheses that present adaptation satisfactory.Key Words: Laser-welding, TIG-welding, Titanium, Gold, Frameworks, Prostheses, Torque
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SUMÁRIO
1.INTRODUÇÃO... ..1 2. PROPOSIÇÃO ...5 3. CAPÍTULOS ...63.1 CAPÍTULO 1: The effect of marginal misfit and welding type on stress induced on abutment of implant-supported prostheses……… ………, ,…….….……… 7
3.2 CAPÍTULO 2: Effect of marginal misfit, welding type and screw material on the strength of the detorque prosthetic screws… ………..……… ……….…..…...20
4. CONSIDERAÇÕES GERAIS... ...32
5. CONCLUSÃO... ...38
6. REFERÊNCIAS...39
7. APÊNDICE...45
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1. INTRODUÇÃO
Há mais de quarenta anos vem se desenvolvendo pesquisas sobre o tratamento odontológico implantossuportado. Várias técnicas vem sendo estudadas a fim de aperfeiçoar os procedimentos e garantir maior longevidade às próteses (Silveira-Junior et al., 2009). Contudo, alguns problemas ainda persistem como a dificuldade de se obter próteses perfeitamente adaptadas aos implantes (Sahin & Cehreli, 2001; Abduoet al., 2010).
A adaptação da prótese é necessária para a longevidade do tratamento, principalmente em próteses múltiplas. Sua ausência induz o desenvolvimento de estresses contínuos, pois tensões intensas são geradas ao sistema implantossuportado no momento da realização do torque dos parafusos protéticos. Essas tensões comprometem a mecânica do sistema e causam injúrias aos tecidos ósseo e mole circundantes (Burguete et al., 1994; Wee et al., 1999; Sahin & Cehreli, 2001; Kunavisarut et al., 2002; Sousa et al., 2008; Paiva et al., 2009; Silveira-Junior et al., 2009; Abduo et al., 2010).
O afrouxamento dos parafusos protéticos é um dos comprometimentos mecânicos que ocorrem na ausência da adaptação protética no sistema implantossuportado. (Barbosa et al., 2008; Spazzin et al., 2010; Farina, 2011). Esse comprometimento mecânico que ocorre devido ao desenvolvimento de tensões excessivas durante a realização do torque, e tende a reduzir a força compressiva de aperto que mantém os componentes unidos, denominada de pré-carga (Burguete et al., 1994, Mc Glumphy et al., 1994). Essa redução contribui para a diminuição da estabilidade das conexões parafusadas (Patterson & Johns, 1992; Haack et al., 1995; Spazzin et al., 2010).
Quando comparadas aos dentes naturais, as tensões induzidas aos implantes são mais intensas e deletérias devido à ausência de ligamento periodontal, o qual possui a função de amortecer os impactos oclusais e mastigatórios. O implante osseointegrado possui íntimo contato com o tecido ósseo. Assim, apresenta movimentação restrita (10 µm) que limita a capacidade de adaptação de próteses com níveis de desajustes
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marginais elevados (Watanabe et al., 2000; Heckmann et al., 2004; Abduo et al., 2010). Dessa maneira, é extremamente importante obtermos próteses com níveis de desajuste marginal reduzido, já que níveis elevados tendem a aumentar a magnitude das tensões induzidas aos implantes (Millington & Leung, 1995; Uludamar & Leung, 1996). Ainda hoje não há uma definição de desajuste marginal considerada ideal. Alguns autores afirmam que níveis de desajustes marginais variando entre 10 a 150 µm são aceitáveis para que não haja desenvolvimento de tensões que comprometam a prótese e os implantes (Branemark,1983; Jemt,1991). Já outros autores afirmam que níveis de desajustes marginais aceitáveis são os que não permitem a formação de fenda e induções de tensões no momento de instalação da prótese. (Watanabe et al., 2000; Sahin & Cehreli,2001; Karl et al., 2005).
As etapas clínicas e laboratoriais, mesmo quando realizadas criteriosamente, podem acarretar em níveis elevados de desajuste marginal (Wee et al., 1999; Sahin & Cehreli, 2001; Junior et al., 2009). A fundição em monobloco de infraestruturas múltiplas é um dos procedimentos que causa frequentemente distorções, responsáveis pelo aumento nos níveis de desajuste marginal (Longoni et al., 2004, Sousa et al., 2008; Silveira-Junior et al., 2009). Essas distorções são oriundas do processo de fundição ou de falhas no protocolo laboratorial, cujo aumento é proporcional a extensão da infraestrutura fundida (Schieffleger et al., 1985; Ziebert et
al., 1986).
Atualmente um dos metais mais utilizado para fundição de infraestruturas é o titânio comercialmente puro (Ti cp), por suas características de biocompatibilidade, baixa densidade, alta resistência à corrosão e baixo custo, quando comparado às ligas nobres. Contudo, é extremamente reativo aos elementos químicos do ar (O2, H2 e N2)
quando aquecido à temperatura acima de 600oC, além de possuir elevado ponto de fusão (1668°C) (Paiva et al., 2009; Rodrigues et al., 2010). Essas desvantagens podem comprometer a resistência mecânica do Ti cp durante o procedimento de fundição por meio da oxidação do metal e formação de poros na zona termicamente afetada (Paiva et
al., 2009; Rodrigues et al., 2010). Deste modo, durante o procedimento de fundição é
necessário o uso de um equipamento especial que mantenha uma atmosfera inerte de gás argônio, a fim de impedir a fragilização do metal (Watanabe et al., 2006; Fornaini et
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Diferentes técnicas foram desenvolvidas com o objetivo de reduzir as distorções inerentes aos procedimentos de confecção de próteses implantossuportadas, como por exemplo, as técnicas de soldagem a laser e a TIG (gás tungstênio inerte) (Wang & Welsch, 1995; Wiskott et al., 2001 Rocha et al., 2006; Paiva et al., 2009; Pereira et al., 2010).
O procedimento de soldagem a laser começou a ser utilizado na área odontológica na década de 70. Sua vantagem em relação à soldagem convencional é poder ser realizado sobre os próprios modelos, em regiões próximas a resinas e/ou porcelanas, sem danificar a peça. Essa técnica é considerada precisa, pois a energia liberada é dada por meio de um feixe de laser com foco concentrado sobre uma pequena área a ser soldada (Sousa et al., 2000; Cardoso, 2007). Além disso, essa soldagem é muito utilizada para o titânio e suas ligas, visto que é associada à liberação de gás inerte, que protege o metal do processo de oxidação (Bertrand & Poulon-Quintin, 2010; Pereira
et al., 2010; Fornaini et al., 2011). A desvantagem é que necessita de equipamento
específico e de custo elevado, o que dificulta a sua aquisição por alguns profissionais (Silva, 2007).
A aplicabilidade da soldagem TIG é recente na Odontologia (Wang & Welsh, 1995), apesar disso quando comparada ao procedimento de soldagem a laser tem apresentado bons resultados e baixo custo. (Hart & Wilson, 2006; Rocha et al., 2006; Atoui, 2008). À semelhança da soldagem a laser, na soldagem TIG é a liberação de gás inerte ao redor dos eletrodos não-consumíveis e da peça a ser soldada (Wainer et al., 1992; Wang et al., 1999; Rocha et al., 2006). A diferença entre ambas as soldagens (laser e TIG) está no diâmetro do feixe de laser, menor em relação ao diâmetro do eletrodo da soldagem TIG, o que influencia no tamanho da área superficial de metal fundido em cada ponto de solda (Rocha et al.,2006; Nuñez-Pantoja et al., 2011). Também há diferença durante a execução de ambos os procedimentos de soldagem, pois na soldagem TIG há formação de um arco elétrico, por meio do contato entre a peça e o eletrodo. Já no procedimento de soldagem a laser há somente a incidência do feixe sobre a área a ser soldada (Wang & Welsch, 1995; Wiskott et al., 2001. Rocha et al., 2006; Atoui, 2008).
Os dois procedimentos de soldagem apresentam passos laboratoriais que demandam menor tempo, quando comparados à soldagem convencional por brasagem.
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Assim, o tempo de trabalho no laboratório é menor, o que pode ser considerado uma grande vantagem, aliada à melhor adaptação e estabilidade das estruturas protéticas (Rocha et al., 2006; Salles et al., 2008).
As tensões desenvolvidas nos implantes ou análogos protéticos podem ser analisadas por meio de avaliações fotoelásticas, de elementos finitos e extensometria (Waskewickz et al., 1994; Guichet et al., 2000; Sadowsky & Caputo, 2000; Cehreli et
al., 2004).
A extensometria é uma técnica de análise de tensões que pode fornecer mensurações in vivo e in vitro das deformações ocorridas em pontos específicos (Cehreli et al., 2006; Heckmann et al., 2006). Para isso são utilizados pequenos resistores (strain gauges) que captam deformações plásticas e/ou elásticas nos locais em que foram posicionadas (Clelland et al.,1996; Cehreli et al., 2006; Abduo et al., 2010). Estes resistores elétricos podem ser posicionados nos implantes, análogos de pilares protéticos, blocos de poliuretano e de resina fotoelástica (simulam a elasticidade aproximada do osso medular), e próteses dentais (Vasconcellos, 2005; Nishioka, 2006). Para Spiekermann et al., (1995) e Clelland et al., (1996), a aplicação da técnica possibilita a captação de tensões exercidas nas próteses, que são transferidas às estruturas de suporte (implante/osso).
Existe ainda necessidade de se realizar novas pesquisas sobre os procedimentos de soldagem a laser e TIG, para comprovação da efetividade dessas técnicas de soldagem, principalmente em relação à redução do nível de desajuste marginal em infraestruturas metálicas fundidas em monobloco, associado à avaliação da força de destorque dos parafusos protéticos e comportamento dos pilares em relação ao desenvolvimento de tensões. Dessa forma, neste estudo o objetivo foi analisar por meio do uso de strain gauges, o desenvolvimento de tensões induzidas aos análogos de pilares protéticos; e avaliar a força de destorque de parafusos protéticos (ouro e titânio) em infraestruturas fundidas em Ti cp, antes e após os procedimentos de soldagem (laser e TIG).
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2. PROPOSIÇÃO
1. Analisar por meio de strain gauges, as tensões induzidas aos análogos de pilares protéticos em infraestruturas de três elementos fundidas em Ti cp, antes e após os procedimentos de soldagem: a laser e TIG;
2. Avaliar a força de destorque de dois tipos de parafusos protéticos (ouro e titânio), antes e após os procedimentos de soldagem (laser e TIG).
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3. CAPÍTULOS
Esta tese está baseada na resolução CCPG/02/06 UNICAMP que regulamenta o formato alternativo para dissertações de Mestrado e Doutorado.
Dois capítulos contendo artigos científicos compõem este estudo, conforme descrito abaixo:
Capítulo 1:
The effect of marginal misfit and welding type on stress induced on abutment of implant-supported prostheses.
- Artigo de acordo com as normas para publicação no periódico: Journal of Oral
Rehabilitation.
Capítulo 2:
Effect of marginal misfit, welding type and screw material on the strength of detorque prosthetic screws.
- Artigo de acordo com as normas para publicação no periódico: Brazilian Dental
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3.1 CAPÍTULO 1
The effect of marginal misfit and welding type on stress induced
on abutment of implant-supported prostheses
Effect welding type on misfit and stress
Abstract
The aim of this study was to analyze the vertical levels of marginal misfit and the stress induced on prosthetic abutments analogs of frameworks casted in commercially pure titanium (cp Ti) and laser and TIG (tungsten inert gas) welded. Twenty frameworks of three-element prostheses, twenty index models with modified analogs of prosthetic abutment, and twenty index models with convectional analogs of prosthetic abutment, were made. Both index models were made with a standardized vertical marginal misfit of 200 m. An optic microscope was used to measure the vertical marginal misfit, using the single-screw test protocol. The welding parameters used were: laser welding (370V/9ms), TIG welding (3:36A/2:60ms). The stress analysis was developed using strain gauges (before and after welding). The results were statistically analyzed (ANOVA; Tukey test (α=0.05); Pearson correlation test). A decrease in the levels of vertical marginal misfit was observed after the welding procedures (laser and TIG). Regarding stress levels, after the welding procedure, the levels were lower with laser welding when compared to TIG welding. There was no correlation between marginal misfit and stress. Thus, it can be observed that the laser welding procedure was more effective for the improvement of passivity of frameworks, casted in cp Ti, when compared to TIG welding procedure.
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Introduction
Nowadays, fixed implant-supported prostheses have presented high success rates. The rehabilitation of total or partial edentulous patients has proved to be one effective option for dental treatment.
However, in spite of all the advances in implantology, failures still occur, especially related to the adaptation of prostheses to the implant (1). This is because there is not yet a precise clinical and laboratory technique that provides perfect adaptation of the prostheses to the prosthetic components (1,2,3).
Actually, a definition about what is an ideal marginal misfit does not exist. There is only assumptions based on logical deductions and clinical experiences that are considered acceptable (4,5). Some authors define as an acceptable marginal misfit levels those that vary from 10 to 150 μm (6,7).
The misfit of implant-supported prostheses can cause biological complications, like adverse reactions in surrounding tissues, resorption of marginal bone and damage to the integraded bone (8). Moreover, it can also cause numerous mechanical complications, such as loosening of prosthetic screws, loosening of abutment and prosthetic fracture (3,9).
Several procedures related to prostheses manufacturing technique can contribute to the increase of marginal misfit (8). One of these procedures is the one-piece casting that promotes distortions due to contraction of the metallic alloy (10).
In the one-piece casting technique, commercially pure titanium (cp Ti) can be used. This metal is biocompatible, has high corrosion resistance and relative a low cost (11,12). However, it has a high melting point (+1700ºC) and high chemical reactivity with oxygen, hydrogen and nitrogen when heated above 600°C (13).
This high chemical reactivity with air elements can cause brittleness in the metal by modifying the crystal structures that get in touch with the environmental gases during the melting/solidification process. This change in structure has effects on the mechanical properties of the cp Ti. During the casting process, it is necessary to use special equipment that maintains a inert atmosphere of gas (12,13).
In many cases, the one-piece casting process increases the marginal misfit of frameworks, making necessary the use of welding procedures (11,14,15). The welding methods mainly used for cp Ti are laser and TIG (tungsten inert gas) methods.
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These methods use an inert atmosphere of argon gas to protect the metals to be welded (11,14).
Both processes of laser and TIG welding develop minimum dimensional changes, because the affected zone is restricted during the execution of the weld point, concentrated by heating disorders in which the thermal deformation are minimized (13,14).
Different methods have been used to evaluate the relationship between stress and vertical marginal misfit; however, nothing is well established in the literature regarding this relationship. Thus, this study aims to evaluate the vertical marginal misfit levels and the stress induced on analogs of prosthetic abutment, on cp titanium casted frameworks, before and after the welding procedures (laser and TIG). In addition, this study aims to determine if there is a correlation between stress and marginal misfit.
Materials and Methods
A metallic master model was fabricated for fixation of two analogs of prosthetic abutment and the manufacture of three elements frameworks for representation of a multiple fixed prosthesis. Modified analogs of microunit abutment (R. C. P. Prosthetic LTDA, São José dos Campos, São Paulo, Brazil) (A e B), with 4.1 mm prosthetic platform, separated by a distance of 1.8 cm, were fixed to this metallic model, to represent the position of lower first premolar and first molar. The modification of the analogs was necessary to perform the stress analyses. The analog’s body was lengthened to allow the bonding of the strain gauges on the surface of the analogs, allowing the capture of elastic deformations. Using this metallic master model associated to modified analogs, the three element prostheses were waxed, and after that, casted in cp Ti.
After the casting process, two types of index models (n=20) (two types to each framework) were made in type IV stone (Durone, Dentsply; Petrópolis, Rio de Janeiro, Brazil). One of these used modified analogs for strain analysis, and the other type, used conventional analogs of the type microunit abutment (Conexão Prostheses System, São Paulo, São Paulo, Brazil), for the execution of the welding procedures (laser and TIG). The confection of these different index models was necessary because
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of the fragility of the strain gauges. Probably, during the welding procedures, damage would be caused to them.
For both index models, a marginal misfit of 200 µm was simulated. For this simulation, one steel ring of 200 µm was placed between the abutment analog B and the framework. The making of type IV stone index model (Durone, Dentsply; Petrópolis, Rio de Janeiro, Brazil), was done using a silicone mold (Flextime, Heraeus Kulzer, Hanau, Hesse, Germany) of the outer portion of the metallic master model. The positioning of analogs was done using a delineator (Bioart, São Carlos, São Paulo, Brazil).
An optic microscope (UHL VMM-100-BT, Renishaw; Wotton-under-Edge, Gloucestershire, United Kingdom) was used to measure the vertical misfit on both type IV stone index models (Durone, Dentsply; Petrópolis, Rio de Janeiro, Brazil). The technique used to measure the vertical misfit was based on the single-screw test protocol (16), with the use of a digital torquemeter (Torque Meter TQ-8800; Lutron, Taipei, Taiwan, China). The technique involved one titanium screw (Conexão Prostheses System, São Paulo, São Paulo, Brazil) with torque of 10 Ncm in abutment analogs A. Marginal vertical misfits between the platform of the abutment analogs and the inferior border of the framework were measured three times, considering the buccal and lingual faces of the abutment analog B. After, the titanium screw was loosened and transferred to the abutment analog B with 10 Ncm torque, and the marginal vertical misfit of abutment analog A was evaluated, as done previously. A total of 12 marginal vertical misfit values were obtained for each framework, and then the mean of these values was calculated to determine the vertical marginal misfit of the prosthesis.
The stress analyses were measured in the index model that contained the modified analogs of the microunit abutment (R. C. P. Prosthetic LTDA, São José dos Campos, São Paulo, Brazil). Two strain gauges (PA-06-060BG-350L, Sensor Excel Engineering; Embu, São Paulo, Brazil) were glued on the surface of each modified analogs’ body, mesially and distally (Figure 1), to form a ½ Wheatstone bridge. This assembly allowed the sensors to measure compressive and tensile stresses, originated by the static load generated due to the screwing of the frameworks in the analogs. The sequence of torque (10 Ncm) screws was first in the screw of the abutment A, and next, in abutment B. After this, the torque in the reverse sequence was done. The torque
11
development in abutment was done using a digital torquemeter (Torque Meter TQ-8800; Lutron, Taipei, Taiwan, China). Stress analysis was obtained after ten minutes of torque on the frameworks. A total of six sequences of analyses of stress were obtained for each framework, and then the mean of these values was calculated to determine the comportment of the prosthesis. The stresses were registered for the equipment control to computer (ASD0500; Lynx Tecnologia Eletrônica Ltda, São Paulo, São Paulo, Brazil) and process for the specific software (AqDados 7; Aq Analyse- Lynx, Orion Township, Michigan, United States) (17,18).
Figure 1. Strain gauges placed in modified analogs.
After the initial stress analyses, the frameworks were sectioned on the type IV stone index model (Durone Dentsply; Petrópolis, Rio de Janeiro, Brazil) with conventional analogs (Conexão Prostheses System, São Paulo, São Paulo, Brazil). The section was localized between the abutment of analogue A and the pontic, 7 mm from the mesial of the lower premolar. The section was made vertically along the axis of the analog, to form an “I” design of the joint. The sectioned parts were cleaned using an ultrasonic tub and blasted with abrasive particles of aluminum oxide(19).
In ten frameworks, the laser welding procedure was made, using the following parameters: 370V/9ms with focus and frequency calibrated at zero. Four points on opposite sides of the cross-section were determined to stabilize the parts of specimen aligned (11), and then the welding was completed. The machine used in this process was Desktop – F (Dentaurum, Pforzhein, Baden-Wurttemberg, Germany). In another group (n=10), the TIG welding procedure was made, using the parameter 3/2; that means, number 3 of potency (current of 36 A), and number 2 of time (60 ms), in a
12
machine NTY 60C (Kernit, Industrial Mechathonic Ltda, Indaiatuba, São Paulo, Brazil). Two opposite welding points were made to stabilize the position of framework, and then another two points for completion of the joint (20). The parameters of the two welding procedures were determined and executed by a trained and competent professional.
The measurement of the vertical marginal misfit was again developed following the same protocol used to analyze the vertical misfit before welding. The strain gauge analysis was also developed in the same way. The results obtained in all the tests were subjected to analysis of variance (ANOVA) followed by Tukey test (α = 0.05) and Pearson’ correlation test for correlation between vertical marginal misfit and stress. All tests were performed with the assistance of SAS program (Statistical Analysis System Version 9.2 TS Level 2M2).
Result
Table 1 shows the mean values and standard deviation of vertical marginal misfit of cp Ti in frameworks before and after welding procedures (laser and TIG).
Table 1. Means values (standard deviation) of vertical marginal misfit (µm) according to welding type and welding moment.
Welding Type Pre-welding Post-welding
TIG 227.69 (81.78)Aa 125.71 (77.96)Ab
Laser 252.69 (74.83) Aa 85.59 (130.18) Bb
Capital letters indicate differences in a same column. Lowercase letters indicate differences in a same line (Tukey test / p≤0.05).
The vertical marginal misfit decreased in both groups of frameworks after the welding procedures (laser and TIG). For laser welding, when it is compared to pre-welding (252.69 m) and post-welding values (85.59 m) there is a 66.13% of decrease (p < 0.0001). For TIG welding, there was also a decrease (44.81% / p=0.0045). Comparing the two welding procedures, in post-welding, it can be noted that laser welding showed higher reduction of vertical marginal misfit (31.91%/ p < 0.0001).
Table 2 shows the mean values and standard deviation of stress induced to analog of prosthetic abutment before and after welding procedures (laser and TIG).
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Table 2. Mean values (standard deviation) of stress (N) values according to welding type and welding moment
Welding Types Pre-welding Post-welding
TIG 10624 (6428.8) Ab 15733(8498.5) Aa
Laser 9072 (3390.5) Aa 6202.8 (4573.5) Ba
Capital letters indicate differences in a same column. Lowercase letters indicate differences in a same line (Tukey test / p≤0.05).
The laser welding frameworks behaved similarly with the induction of stress on modified analogs the prosthetic abutments (p=0.3434), when compared this stress pre and post-welding moments. For TIG welding procedures, there was an increase in the values of stress induction (32.47% / p=0.0331). Comparing the two welding procedures, in the post-welding moment, it can be noticed that laser welding showed less induction of stress (60.57% / p=0.0142).
Table 3 shows the correlation between vertical marginal misfit and induced stress on the prosthetic abutment. In both welding procedures (laser and TIG), there was no correlation between vertical marginal misfit and stress.
Table 3. Pearson´s Correlation Coefficient (r) between marginal misfit and stress for both welding procedures.
Stress
Welding Types R p-value
Misfit Laser - 0.35 0.3252
TIG 0.33 0.3555
Pearson’s correlation test level of significance - 5%
Discussion
The search for an implant-supported prosthesis perfectly fitted to implants is still a challenge for researchers in Implantology. It is known that the juxtaposition of two different surfaces, even when smooth and polished, tends to result in a gap in the region of the fit (21). In this study, two welding procedures (laser and TIG) were
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tested, aiming to reduce the vertical marginal misfit and stress induction on analogs of prosthetic abutment of cp Ti prostheses.
Analyzing the laser welded frameworks, a decrease of the vertical marginal misfit (from 252.69 to 85.59 µm) could be observed. This result corroborates with other scientific studies in the dental literature (15,22,23,24). Thus, it can be considered that the laser welding procedure is effective in achieving higher levels of framework adaptation.
For TIG welding procedure there was also a reduction on the vertical marginal misfit levels (from 227.79 to 125.71µm). This data shows that this welding procedure is also effective in achieving a well-fitted framework, which has been very little studied up until now in the literature (14,25). Both welding procedures achieved acceptable levels of vertical marginal misfit, between 10 and 150 µm (6,7). However, the laser welding procedure was more effective in reducing the level of vertical marginal misfit of frameworks, compared to TIG welding procedures (p < 0.0001). This accuracy of laser welding procedure can be associated to the smaller diameter of the laser beam (0.2 mm) (11), compared to the electrode diameter of the TIG welding procedure (1.6 mm) (14). In the laser welding procedure, four points on opposite sides of the cross-section were developed to stabilize the parts of specimen aligned, and then the welding was complete. In the TIG welding procedure, two opposite welding points were made to stabilize the position of the framework and then another two for completion of the joint. Despite the lower number of points made during the TIG welding procedure, it was observed that there was a higher area of molten metal for each point, due to the larger diameter of the electrode (14). This may have led to major distortions in the metal during and after the solidification (14).
Regarding induced stress on the analogs of prosthetic abutment, after both welding procedures (laser and TIG), it can be observed that with laser welding there was no alteration and with TIG welding procedure there was an increased stress. Through this result, it is suspected that during both welding procedures (laser and TIG) there was dimensional alteration of the cp Ti frameworks. This alteration was higher during the TIG welding procedure, in which there is increase of stress in approximately 32%, compared to the pre-welding moment. This may be explained by the size of the solder point, which is proportional to the diameter of the beam (0.2 mm - laser welding)
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(11) or diameter of electrode (1.6 mm - TIG welding) (14). In the laser welding procedure, four points on opposite sides of the cross-section were developed to stabilize the parts of specimen aligned, and then the welding was completed. In the TIG welding procedure, two opposite welding points were made to stabilize the position of framework and then another two, for completion of the joint. Despite the lower number of points made during the TIG welding procedure, it was observed that there was a higher superficial area of molten metal for each point, due to the larger diameter of the electrode (14), which may have led to major distortions in the metal during and after its solidification (14).
There was no correlation between marginal misfit and induced stress on the analogs of prosthetic abutment in both welding procedures (laser and TIG). Despite of some studies have found correlation positive between marginal misfit and stress (26,27). However, in these studies there was not a necessary proportionality between the variables. This leads us to affirm that vertical misfit measured of isolated manner is not a good method of analysis of marginal adaptation, because lack of linearity shows that the misfit can be considered to be, in some cases, one of the factors to induce stress to the implants, but cannot be attributed to a direct relationship with the magnitude of the increase in stress.
In this study, horizontal marginal misfit values were not verified. Only the vertical changes were analyzed because of the fact that the term prosthetic misfit is constantly utilized as vertical marginal misfit (15). In addition, the methodology used in this study that uses an optic microscopic and the single-screw test protocol (16) does not allow the measurement of horizontal deformation. This methodology is also currently, in the literature as the most used for the analysis of marginal misfit (3,8,15,16).
In this study it is essential not to associate lower marginal misfit with passive settlement, and consequently lower induced stress to the implants. These are not necessarily of the same magnitude, despite numerous scientific studies associating both (4,13,28). Because many times there can be one minimal marginal misfit at the expense of little passivity, through of tightening of the prosthetic screws (29), forcing the fit of the frameworks on the abutment.
This study confirmed that both welding procedures are effective in the improvement of vertical marginal misfit, despite some methodological limitations.
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However, when passivity of frameworks is the question to be discussed, the TIG welding procedure cannot be considered an effective method. The laser welding procedure was considered the better method. It is suggested that future studies involving three-dimensional analysis of marginal misfit, through the use of photogrammetric technique and coordinate measurement machines (3) similar to the CAD-CAM be used to better evaluate the effectiveness of both welding procedures.
Conclusion
The laser welding procedure was more effective for the improvement of passivity of implant-supported three element frameworks, casted in cp Ti, when compared to TIG welding.
Acknowledgements
This study was supported by FAPESP grants number 2010/096339-6 and 2009/09639-6.
References
1. Sahin S, Cehreli M. The significance of passive fit in implant prosthodontics: Current status. Implant Dent. 2001; 10:85–92.
2. Kunavisarut C, Lang LA, Stoner BR, Felton DA. Finite Element Analysis on Dental Implant-supported Prostheses without Passive Fit. J Prosthodont. 2002; 11:30-40.
3. Abduo J, Bennani V, Waddell N, Lyons K, Swain M. Assessing the Fit of Fixed Prostheses: A Critical Review. Int J Oral Maxillofac Implants. 2010; 25: 506-515. 4. Karl M, Rosch S, Graef F, Taylor T, Heckman S. Strain situation after fixation of 3
unit ceramic veneered implant superstructures. Implant Dent. 2005; 14:157–165. 5. 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. 6. Branemark PI. Osseointegration and its experimental background. J Prosthet Dent.
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7. Jemt T. Failures and complications in 391 consecutively inserted fixed prostheses supported by Brånemark 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; 6(3):270-276.
8. Barbosa GA, das Neves FD, de Mattos M da G, Rodrigues RC, Ribeiro RF. Implant/abutment vertical misfit of one-piece cast frameworks made with different materials. Braz Dent J. 2010; 21(6):515-9.
9. Michalakis K, Hirayama H, Garefis P. Cement-retained versus screw-retained implant restorations: A critical review. Int J Oral Maxillofac Implants. 2003;18: 719–728.
10. Guichet DL, Yoshinobu D, Caputo AA. Effect of splinting and interproximal contact tightness on load transfer by implant restorations. J Prosthet Dent. 2002; 87 (5) 528-35.
11. Nuñez-Pantoja JMC, Vaz LG, Nóbilo MAA, Henriques GEP, Mesquita MF.f Effects of laser-weld joint opening size on fatigue strength ofTi-6Al-4V structures with several diameters. J Oral Rehabil. 2011; 38: 196–201.
12. Orsi IA, Raimundo LB, Bezzon OL, Nóbilo MAA, Kuri SE, Rovere CAD, Pagnano VO. Evaluation of Anodic Behavior of Commercially Pure Titanium in Tungsten Inert Gas and Laser Welds. J Prosthodont. 2011; 00:1–4.
13. Watanabe I, Topham DS: Laser welding of cast titanium and dental alloys using argon shielding. J Prosthodont. 2006; 15: 102-107.
14. Rocha R, Pinheiro AL, Villa Verde AB. Flexural Strength of Pure Ti, Ni-Cr and Co-Cr Alloys Submitted to Nd:YAG Laser or TIG Welding. Braz Dent J. 2006; 17(1): 20-23.
15. Sousa SA, Nóbilo MAA, Henriques GEP, Mesquita MF. Passive fit of frameworks in titanium palladium-silver alloys submitted the laser welding. J Oral Rehabil. 2008; 35:123-127.
16. Tan KB, Rubenstein JE, Nicholls JI, Yuodelis RA. Three-dimensional analysis of the casting accuracy of one-piece, osseointegrated implant-retained prostheses. Int J Prosthodont. 1993; 6:346-363.
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17. Koke U, Wolf A, Lenz P, Gilde H. In vitro investigation of marginal accuracy of implant-supported screw-retained partial dentures. J Oral Rehabil. 2004; 31: 477-82.
18. Naconecy MM, Teixeira ER, Shinkai RS, Frasca LC, Cervierri A. Evaluation of the accuracy of 3 transfer techniques for implant techniques for implant-supported prostheses with multiple abutments. Int J Oral Maxillofac Implants. 2004; 19 (2):192-8.
19. Liu J, Watanabe I, Yoshida K, Atsuta M. Joint strength of laser-welded titanium. Dent Mater. 2002; 18:143-148.
20. Nuñez-Pantoja JM, Farina AP, Vaz LG, Consani RL, de Arruda Nóbilo MA, Mesquita MF. Fatigue strength effect of welding type and joint design executed in Ti-6Al-4V structures. Gerodontology. 2011; Nov 24. dói:10.1111/j.1741-2358.2011.00598.x.[Epud ahead of print].
21. Contreras EFR, Henriques GEP, Giolo SR, Nóbilo MAA. Fit of cast commercially pure titanium and Ti-6Al-4V alloy crowns before and after marginal refinement by electrical discharge machining. J Prosthet Dent. 2002; 88 (5) 467-472.
22. 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.
23. Silveira-Junior CD, Neves FD, Fernandes-Neto AJ, Prado CJ, Sumamoto-Junior PC. Influence of different tightening forces before laser welding to the implant/framework fit. J Prosthodont. 2009; 18:337-341.
24. Jemt T, Lindén B. Fixed implant-supported prostheses with welded titanium frameworks. In J Periodontics Restorative Dent.1992; 12(3)177-184.
25. Wang RR, Welsch GE. Joining titanium materials with tungsten inert gas welding and infrared brazing. J Prosthet Dent. 1995; 74: 521-30.
26. Millington ND, Leung T. Inaccurate fit of implant superstructures. Part 1: Stresses generated on the superstructure relative to the size of fit discrepancy. Int J Prosthodont. 1995; 8:511-516.
27. Clelland N, Carr A, Gilat A. Comparison of strains transferred to a bone simulant between as-cast and postsoldered implant frameworks for a five-implant-supported fixed prosthesis. J Prosthodont. 1996; 15(3):193–200.
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28. Watanabe F, Uno I, Hata Y, Neuendorff G, Kirsch A. Analysis of stress distribution in a screw-retained implant trosthesis. Int J Oral Maxillofac Implants. 2000; 15(2): 209-18.
29. Jem T, Lekholm U. Measurements of bone and frawe-work deformations induced by misfit of implant superstructures. Clin Oral Implants Res. 1998; 9:272-80.
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____________________________________________________________
3.2 CAPÍTULO 2
Effect of marginal misfit, welding type and screw material on the
detorque of prosthetic screws
Effect welding type on detorqued screws
Abstract
The aim of this study was to evaluate the vertical marginal misfit and the detorque strength of prosthetic screws on cp Ti frameworks, before and after two welding procedures (laser and TIG). Twenty frameworks were made simulating a three-element fixed prosthesis, and twenty index models, simulating 200 m of vertical marginal misfit. An optic microscope, associated with the single-screw test protocol was used to measure the vertical marginal misfit. The welding parameters used were the following: laser welding (370V/9ms) and TIG welding (3: 36A / 2: 60 ms). The torque of titanium and gold screws was performed by the technique of torque (10 Ncm) and retorque (10 Ncm / after 10 minutes). The detorque was performed ten minutes after the retorque, using a digital torquemeter. The results were statistically analyzed (ANOVA/Tukey test/ Pearson’s correlation coefficient; α=0.05). A decrease in the levels of misfit, after both welding procedures was observed. The detorque values were lower for the gold screws when compared to titanium ones, which increased after welding procedures. There was no correlation between marginal misfit and destorque strength. Thus, it can be observed that both welding procedures were effective for the improvement of adaptation of frameworks, casted in cp Ti; and that screws prosthetic of titanium can be considered more stable that prosthetic screws of gold when torque in prostheses that presents adaptation satisfactory.
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Introduction
The interface between abutment/prosthesis has been an important factor in the transfer of stress, adverse biological response and mechanical complications in prosthetic restorations on implants (1,2). This is especially so when there is marginal misfit between the implant and prosthesis (1,3).
The absence of adaptation in the prostheses associated to the development of occlusal strength can be the result of loosening of the prosthesis screw (4,5). The loosening of the screw can develop mechanical complications, such as fractures of other prostheses components and marginal bone loss around of the implant (1,4,5).
The joint between the screws and prosthesis abutments is often reduced by compressive loads action of magnitude equal or superior to preload of the screw, that result in development of stress and plastic deformation of the abutments and screws (5).
The material of the screws can also influence the loosening of prosthetic screws (5,6). Some studies suggest that titanium screws have more stability when compared with gold ones, and other studies affirm that the application of retorque after ten minutes from the initial torque can reduce the loosening (5,7).
Often, to achieve a prosthesis fit, it is necessary to execute welding procedures (8,9). Nowadays, the welding procedures that are used to weld titanium are laser and TIG welding (tungsten inert gas). This is because of the inert argon atmosphere, which protects the metal from oxidation by air elements, avoiding titanium’s fragility (8,10).
The two welding types can improve the fit of partial or complete prostheses supported by implants, and can prevent problems related to fractures and loosening of prosthesis screws (11). In addition, this is considered the simple method, because there is no necessity of inclusion of the piece on coat, and the assembly (weld) is processed in the model that improves the lower rate of distortion (8,9,11).
Regarding problems posed related to the development of prosthetic misfit on partial or complete implant-supported prostheses, the present study aims to evaluate the vertical marginal misfit and detorque strength of prosthetic screws in cp Ti frameworks, before and after the procedures of laser and TIG welding.
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Material and Methods
A metallic master model was fabricated for fixation of two analogs of prosthetic abutment and for making of three-element frameworks. Analogs of microunit abutments (Conexão Prostheses System, São Paulo, São Paulo, Brazil) (A e B), with 4.1 mm of prosthetic platform, separated at a distance of 1.8 cm, were fixed to this metallic model, to represent the position of a lower first premolar and first molar. Using this metallic master model, the three-element prostheses were waxed, and after that, casted in cp Ti.
After the casting procedure, twenty index models (n=20) (one for each framework) were made in type IV stone (Durone, Dentsply; Petrópolis, Rio de Janeiro, Brazil) using analogs of microunit abutment (Conexão Prostheses System, São Paulo, São Paulo, Brazil). This index model simulated a marginal misfit of 200 µm. For this simulation, one steel ring of 200 µm was placed between the abutment analogue B and framework. After the making of a type IV stone index model (Durone, Dentsply; Petrópolis, Rio de Janeiro, Brazil), was done using a silicone mold (Flextime, Heraeus Kulzer, Hanau, Hesse, Germany) of the outer portion of the metallic master. The positioning of analogs was done using a delineator (Bioart, São Carlos, São Paulo, Brazil).
An optic microscope (UHL VMM-100-BT, Renishaw; Wotton-under-Edge, Gloucestershire, United Kingdom) was used to measure the vertical misfit. The technique used to measure the vertical misfit was based on the single-screw test protocol (12), with the use of a digital torquemeter (Torque Meter TQ-8800; Lutron, Taipei, Taiwan, China). The technique involved one titanium screw (Conexão Prostheses System, São Paulo, São Paulo, Brazil) with torque of 10 Ncm in abutment of the analog A. Marginal vertical misfit between the platform of the abutment analog and the inferior border of the framework were measured three times, considering the buccal and lingual faces of the abutment analog B. After, the titanium screw was loosened and transferred to the abutment of the analog B, and the evaluation was done as previously, in another analog. A total of 12 values were obtained for each framework, and then the mean of these values was calculated to determine the vertical marginal misfit of the prostheses.
23
For measurement of detorque strength of the prostheses screws, a precision digital torquemeter (Torque Meter TQ-8800; Lutron, Taipei, Taiwan, China) was used connected to a positioning dispositive. This dispositive permitted the vertical positioning of the torquemeter to the long axis of the analog, without inducing lateral stress that could confound the values of strength destorque (Figure 1).
Figure 1. Digital torquemeter connected on the dispositive
The detorque strength was evaluated using titanium and gold screws (Conexão Prostheses System, São Paulo, São Paulo, Brazil) after the application of the retorque technique (7). The technique was made as follows: 1st torque of the prosthetic screw of analog A (10 Ncm) followed by the analog B; 2nd retorque (10 Ncm) of the prosthetic screws, after ten minutes, following the same sequence; 3rd detorque of the screws, ten minutes after the last torque. This protocol was repeated using the reverse order, and the prosthetic screws were used only once.
After the initial detorque evaluation, the frameworks were sectioned localized between the abutment of analog A and the pontic, 7 mm from the mesially of the premolar. The section was made vertically along the axis of the analog, to form an “I” design of the joint. The sectioned parts were cleaned using an ultrasonic tub and blasted with abrasive particles of aluminum oxide (13).
In a group of ten frameworks the laser welding procedure was made, using the following parameters: 370V/9ms with focus and frequency calibrated at zero. Four
24
points on opposite sides of the cross-section were determined to stabilize the parts of specimen aligned (8), and then the welding was complete. The machine used in this process was Desktop – F (Dentaurum, Pforzhein, Baden-Wurttemberg, Germany). In another group (n=10), the TIG welding procedure was made, using the parameter 3: 36 A/2: 69 ms; that means, number 3 of potency (current of 36A), and number 2 of time (60 ms), in a machine NTY 60C (Kernit, Industrial Mechathonic Ltda, Indaiatuba, São Paulo, Brazil). Two opposite welding points were made to stabilize the position of the framework and then another two points, for the completion of the joint (14). The parameters of the two welding procedures were determined and executed by a trained and competent professional.
The measurement of the vertical marginal misfit was again developed following the same protocol used in the earlier welding situation. The detorque was also evaluated. The results obtained in all the tests were subjected to analysis of variance (ANOVA) followed by a Tukey test (α = 0.05) and Pearson’ correlation test for correlation between vertical marginal misfit and strength destorque. All tests were performed with the assistance of SAS program (Statistical Analysis System Version 9.2 TS Level 2M2).
Results
Table 1 shows the mean values and standard deviation of the frameworks’ vertical marginal misfit before and after welding procedures (laser and TIG).
Table 1. Means values (standard deviation) of vertical marginal misfit (µm) according to welding type and welding moment.
Welding Type Pre-welding Post-welding
TIG 227.69 (81.78)Aa 125.71 (77.96)Ab
Laser 252.69 (74.83) Aa 85.59 (130.18) Bb
Capital letters indicate differences in a same column. Lowercase letters indicate differences in a same line (Tukey test / p≤0.05).
The vertical marginal misfit of the frameworks decreased in both the welding procedures (laser and TIG). For laser welding there was 66.13% of decrease (p
25
< 0.0001). The values of vertical marginal misfit of 256.69 m (pre-welding) decreased to 85.59 m (post-welding). For TIG welding, there was also a decrease (44.81%/ p=0.0045) (227.69 m to 125.71 m). Comparing the two welding procedures, in the post-welding, it can be noted that laser welding showed a higher reduction of vertical marginal misfit (p < 0.0001).
Table 2 and 3 show the mean values and standard deviation of detorque strength of the prosthetic screws (titanium and gold), before and after both welding procedures (laser and TIG, respectively).
Table 2. Mean values (standard deviation) of detorque strength of the prosthetic screws (Ncm) for laser welding procedure
Screws Pre-welding Post- welding
Au 3.90 (0.72) Aa 4.58 (1.08) Ba
Ti 4.68 (0.91) Ab 6.33 (0.70) Aa
Capital letters indicate differences in a same column. Lowercase letters indicate differences in a same line (Tukey test / p≤0.05).
Table 3. Mean values (standard deviation) of detorque strength of the prosthetic screws (N cm) for TIG welding procedure.
Screws Pre-welding Post-welding
Au 3.90 (0.72) Ba 4.45 (1.24) Ba
Ti 4.95 (0.73) Ab 6.13 (0.91) Aa
Capital letters indicate differences in a same column. Lowercase letters indicate differences in a same line (Tukey test / p≤0.05).
After both welding procedures (laser and TIG), the detorque strength level of titanium screws increased. For laser welding, when comparing pre-welding (4.68 Ncm) and post-welding (6.33 Ncm) detorque level, there is a 26.07% increase (p=0.0003). For TIG welding, there was also an increase (19.25% / p=0.0053). The detorque strength level of gold screws after both welding procedures (laser and TIG) was not altered.
26
Comparing both prosthetic screws (titanium and gold), after welding procedures, it can be noticed that the detorque strength level of titanium screws was higher. For laser welding, when comparing the detorque strength level of titanium screws (6.33 Ncm) with the gold screws (4.58 Ncm), there was a 27.65% of increase (p=0.0004). For TIG welding the same thing happened (27.41% / p=0.0028).
Table 4 shows the correlation between vertical marginal misfit and strength detorque (gold and titanium screws). In both welding procedures, there is no correlation between vertical marginal misfit and strength detorque level of gold and titanium screw.
Table 4. Pearson’s Correlation Coefficient (r) between marginal misfit and destorque strength for both welding procedures.
Destorque of gold screw Destorque of titanium screw Misfit r p-value r p-value Welding laser - 0.62 0.5709 - 0.41 0.2454
Welding TIG - 0.17 0.6325 - 0.32 0.3676 Pearson’s correlation test level of significance - 5%
Discussion
Analyzing the welded frameworks by the laser welding procedure, a decrease of the vertical marginal misfit level (Table 1) could be observed. This result is consistent compared to scientific studies of literature (11,15,16). Thus, it can be considered that the laser welding procedure is effective in achieving lower levels of vertical marginal misfit of cp Ti framework.
For the TIG welding procedure, there was also a decrease on the vertical marginal misfit level (Table 1). This results shows that this welding process is also effective in achieving of vertical marginal misfit decrease of the frameworks, which is yet very little studied in the worldwide literature (11,17).
Both welding procedures achieved acceptable levels of vertical marginal misfit, between 10 and 150 µm (18,19). However, the laser welding procedure was more effective in reducing the level of vertical marginal misfit of frameworks, compared to TIG welding procedures (p < 0.0001). This accuracy of laser welding procedure can be associated to the smaller diameter of the laser beam (0.2 mm) (8),
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compared to the electrode diameter of the TIG welding procedure (1.6 mm) (11). In the laser welding procedure, four points on opposite sides of the cross-section were developed to stabilize the parts of specimen aligned, and then the welding was complete. In the TIG welding procedure, two opposite welding points were made to stabilize the position of the framework and then another two for completion of the joint. Despite the lower number of points made during the TIG welding procedure, it was observed that there was a higher area of molten metal for each point, due to the larger diameter of the electrode (11). This may have led to major distortions in the metal during and after the solidification (11).
The welding procedures (laser and TIG) only influenced the detorque strength of titanium screws. There was no alteration in detorque strength of the gold screws. The presence of a higher level of vertical marginal misfit, in the pre-welding moment (laser and TIG), decreased the level of detorque strength of titanium screws.
When vertical marginal misfit level decreased, in the post-welding moment (laser and TIG), the detorque strength of titanium screws increased. These results are consistent with scientific studies of literature (1,4,5). The torque and pre-load can be influenced by misfit of prostheses, where part of the preload is used to approximate the surfaces, causing additional stress to the screw (3). The overload generated to the screw may lead to additional stress on other system components causing the reduction of detorque strength of the prosthetic screws (20). Thus can be noticed in this study that decrease of vertical marginal misfit level of frameworks, casted in cp Ti , after both welding procedures (laser and TIG), influenced in increase of the detorque strength, through reducing the residual stress on the prosthetic screws of titanium.
The difference of behavior between the screws may have occurred because of the outflow lower resistance limit of gold alloy, which has a higher malleability and ductility when compared to a titanium alloy (5,22,23,24). This lower outflow resistance limit commits the comportment of gold screws, because decrease in the destorque strength level developed by elastic deformation of screw submitted to the torque (5,22,23). Thus, regardless of the vertical marginal misfit amplitude, there will always be a reduction of gold screws detorque (5,22).
Thus was observed that increased of detorque strength of titanium prosthetic screws associated decrease of the vertical marginal misfit level, after both welding
