Anna Gabriella Camacho Presotto
“Avaliação do desajuste marginal e tensão em próteses
fixas implantossuportadas de 3 elementos: efeito da
infraestrutura protética e do design do implante”
“Evaluation of marginal misfit and stress in 3-unit
implant-supported prostheses: the role of prosthetic
framework and implant design”
Piracicaba/SP 2016
“Avaliação do desajuste marginal e tensão em próteses fixas
implantossuportadas de 3 elementos: efeito da infraestrutura
protética e do design do implante”
“Evaluation of marginal misfit and stress in 3-unit implant-supported
prostheses: the role of prosthetic framework and implant design”
Dissertação apresentada à Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas como parte dos requisitos exigidos para a obtenção do título de Mestra em Clínica Odontológica, na Área de Prótese Dental.
Dissertation presented to the Piracicaba Dental School of the University of Campinas in partial fulfillment of the requirements for the degree of Master in Clinical Dentistry in Dental Prosthesis area.
Orientador: Prof. Dr. Valentim Adelino Ricardo Barão Coorientador: Prof. Dr. Marcelo Ferraz Mesquita
Este exemplar corresponde à versão final da dissertação defendida pela aluna Anna Gabriella Camacho Presotto, e orientada pelo Prof. Dr. Valentim Adelino Ricardo Barão.
Piracicaba/SP 2016
Aos meus pais, Odair Antonio Presotto e Ana Maria Borilho Camacho
Presotto, que são meus exemplos de força, dedicação e amor. Muito obrigada por todo o
amor, carinho, apoio e compreensão em todos os momentos. Obrigada por todo esforço que fizeram para que eu pudesse alcançar meus sonhos e objetivos. Agradeço ainda por todo apoio e incentivo nos momentos difíceis. Minha eterna gratidão por tudo que vocês fizeram e ainda tem feito por mim. Sem vocês nada disso se concretizaria. Vocês são os grandes responsáveis pelo sucesso da minha trajetória.
Ao meu irmão, João Gabriel Camacho Presotto, pelo amor, carinho e grande amizade. Obrigada por me apoiar sempre e por ser um irmão tão especial para mim.
Ao meu namorado, Diego Tetzner Fernandes, por me incentivar, me apoiar, me fazer crescer com ensinamentos que tanto contribuem para meu desenvolvimento pessoal e profissional. Muito obrigada por todo amor, carinho e companheirismo.
A Deus, pela proteção, por guiar meu caminho e minhas decisões.
Ao meu Orientador, Prof. Dr. Valentim Adelino Ricardo Barão, Professor Assistente da Área de Prótese Total da Faculdade de Odontologia de Piracicaba – UNICAMP, pelo grande exemplo de competência e dedicação. Agradeço imensamente pelos conhecimentos transmitidos, pelo imprescindível apoio e contribuição para a execução deste trabalho.
Ao meu Coorientador, Prof. Dr. Marcelo Ferraz Mesquita, Professor Titular da Área de Prótese Total da Faculdade de Odontologia de Piracicaba – UNICAMP, pela seriedade, competência e ensinamentos transmitidos. Obrigada pela confiança depositada em mim desde a época da Graduação. Agradeço por todo o apoio, incentivo e, sobretudo, pela imensa contribuição para o meu crescimento pessoal e profissional.
À minha colega de Pós-Graduação, Cláudia Lopes Brilhante Bhering, por toda contribuição para a execução deste trabalho. Agradeço por toda disposição em me ajudar, pelo apoio e conselhos. Torço muito por seu sucesso.
À Universidade Estadual de Campinas (UNICAMP), na pessoa do Magnífico Reitor, Prof. Dr. José Thadeu Jorge e à Faculdade de Odontologia de Piracicaba –
UNICAMP, na pessoa do seu Diretor, Prof. Dr. Guilherme Elias Pessanha Henriques e
Diretor Associado, Prof. Dr. Francisco Haiter Neto.
À Profa. Dra. Cínthia Pereira Machado Tabchoury, coordenadora geral dos programas de Pós-graduação da Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas.
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 Fundação de Amparo a Pesquisa do Estado de São Paulo, pela concessão de bolsa de estudo no período de janeiro de 2015 a fevereiro de 2016, Processo nº 2014/19264-0, fundamental para o desenvolvimento desta pesquisa.
À Coordenação de Aperfeiçoamento de Pessoal de Nível Superior –
CAPES, pela concessão de bolsa de estudo no período de agosto de 2015 a dezembro de
2015, Processo PROEX AUX-PE-PROEX nº 1778/2014.
À empresa Amann Girrbach, pelo fornecimento dos blocos de CoCr e todo material necessário para confecção das infraestruturas em CAD/CAM utilizadas neste trabalho.
Ao Técnico em Prótese Dental, Ricardo Takeshi Nagahisa do Artwork
Pesqueira, Profa. Dra. Marina Xavier Pisani de Souza, Prof. Dr. Valentim Adelino Ricardo Barão, pelas considerações, sugestões e correções fundamentais para o
aprimoramento deste trabalho.
Aos docentes do Departamento de Prótese e Periodontia, da área de concentração de Prótese Dentária da Faculdade de Odontologia de Piracicaba – UNICAMP, Profa. Dra. Altair Antoninha Del Bel Cury, Prof. Dr. Frederico Andrade e
Silva, Prof. Dr. Mauro Antônio de Arruda Nóbilo, Prof. Dr. Rafael Leonardo Xediek Consani, Profa. Dra. Renata Cunha Matheus Rodrigues Garcia, Prof. Dr. Wander José da Silva, e Prof. Dr. Wilkens Aurelio Buarque e Silva, pelo convívio e
conhecimentos transmitidos que contribuíram à minha formação acadêmica.
À Profa. Dra. Célia Marisa Rizzatti Barbosa, pela amizade, confiança e ensinamentos que contribuíram à minha formação acadêmica.
À Dra. Vanessa Tramontino Mesquita, por compartilhar ensinamentos, experiências, pelo apoio e incentivo.
À Cristina Tetzner Fernandes e Carlos Fernandes, pela receptividade e agradável convívio durante esses anos.
Às minhas amigas, Priscila Alves Giovani, Priscila Jardim, Raisa Zago
Falkine, Thaís Harder de Palma, e Verônica Polo Perez, pela amizade verdadeira, por
compreenderem os momentos de ausência, por compartilharem tantas alegrias comigo e por sempre me incentivarem.
Gustavo Corradini, Heloísa Pantaroto, Isabella Marques, Jairo Cordeiro, Júlia Campana, Marina Pisani, Moisés Nogueira, Paolo Di Nizo, Raisa Falkine, Ricardo Caldas, Sabrina Rodrigues, Sales Barbosa, Simone Valenga, Thamara Beline, e Vagner Reginato, pelo convívio e experiências trocadas.
Aos alunos de Graduação pela convivência, especialmente à Gabriela
Silveira, pelo carinho e incentivo constante.
Às colegas e amigas de Graduação e Pós-Graduação que conheci através do engajamento com a causa animal, Amanda Fabião, Erika Harth, Fabiana Facco
Casarotti, Fabiana Furtado Freitas, e Lívia Rodrigues. Vocês são muito especiais.
À secretária Eliete Lima Marim, e ao técnico de laboratório Eduardo Pinez
Campos, pela ajuda e agradável convivência durante o curso de Pós-Graduação.
Agradeço também aos técnicos de laboratório, Márcia Broglio, Neide de Paula, e
Reinaldo Casagrande, pela agradável convivência e auxílio, desde a época da minha
Graduação.
À todos os meus familiares, amigos e pessoas que indiretamente torceram por mim, contribuindo para a concretização deste trabalho.
A presença de desajustes protéticos e tensão no sistema implantossuportado podem levar ao comprometimento da longevidade do tratamento. Sendo assim, os objetivos deste estudo in vitro foram avaliar a influência do método de obtenção de infraestrutura protética (CAD/CAM e sobrefundidas) sobre o desajuste marginal e tensão transmitida aos implantes, e o efeito de distintos designs de implantes (comprimento e diâmetro) e desajuste marginal sobre os níveis de tensão de próteses parciais fixas (PPFs). Infraestruturas de PPFs de 3 elementos foram confeccionadas utilizando os métodos CAD/CAM (n=10) e sobrefundição (n=10). As infraestruturas foram enceradas a fim de simular uma PPF de primeiro pré-molar (pilar P) a primeiro molar superior (pilar M) utilizando cilindros para sobrefundição. Os enceramentos foram sobrefundidos (grupo sobrefundido) ou escaneados (grupo CAD/CAM) para obtenção das infraestruturas. Todas as infraestruturas foram confeccionadas em liga de CoCr. Dois modelos fotoelásticos foram obtidos: modelo C com dois implantes convencionais padrão Branemark (4.1 × 11 mm); e modelo S com um implante curto (5 × 6 mm) e um convencional padrão Branemark (4.1 × 11 mm). O desajuste marginal foi analisado de acordo com o protocolo do teste do parafuso único, obtendo-se um valor médio de desajuste por implante e por infraestrutura. Para avaliação do efeito do design dos implantes, diferentes níveis de desajuste marginais foram selecionados de acordo com as médias de desajuste obtidas por infraestrutura: baixo (< 20 µm), médio (> 20 e < 40 µm) e alto (> 40 µm). Para comparação dos tipos de infraestrutura, a tensão foi mensurada quantitativamente pela análise fotoelástica após a fixação das infraestruturas sobre o modelo fotoelástico C com um torque padronizado de 10 Ncm. Para comparação do design dos implantes, foi realizada análise qualitativa nos modelos C e S sobre condições sem carregamento e após a aplicação de carga de 280 N. Os resultados foram submetidos ao Teste T, ANOVA 2 fatores e correlação de Pearson (α=0,05) para avaliação do efeito do tipo de infraestrutura sobre as variáveis estudadas (desajuste e tensão). O tipo de infraestrutura e o local avaliado (implante M e P) não influenciaram os valores de desajuste marginal (p=0,466 e p=0,153, respectivamente) e de tensão (p=0,602 e p=0,746, respectivamente). Houve forte correlação positiva entre o desajuste e a tensão (CAD/CAM: r=0,922 p<0,0001; Sobrefundição: r=0,908 p<0,0001).
marginal e tensão para confecção de PPFs implantossuportadas de 3 elementos. O aumento do desajuste marginal induz maior tensão no sistema implantossuportado. Pequenos aumentos nos valores de desajuste marginal aumentam os níveis de tensão independentemente do tipo do design de implante utilizado e o uso de um implante posterior curto e de maior diâmetro pode ser uma abordagem de tratamento para a reabilitação de pacientes com altura óssea reduzida.
Palavras–chave: Próteses e implantes. Prótese dentária parcial fixa. Projeto auxiliado
The presence of prosthetic misfit and stress in implant-supported system can compromise the longevity of treatment. Therefore, this in vitro study evaluated the influence of prosthetic framework fabrication method (CAD/CAM and overcasted) on marginal misfit and stress transmitted to implants and the effect of implant design (length and diameter) and marginal misfit on the stress levels of fixed partial denture (FPD). Three-element prosthetic frameworks were made using the CAD/CAM (n=10) and overcasted (n=10) methods. The frameworks were waxed to simulate a superior first pre-molar (pillar P) to first molar (pillar M) FPD using overcasted mini abutment cylinders. The wax patterns were overcasted (overcasted group) or scanned to obtain the frameworks (CAD/CAM group). All frameworks were fabricated in CoCr alloy. Two photoelastic models were obtained: model C with two standard Branemark implants (4.1 × 11 mm); and model S with a short implant (5 × 6 mm) and a standard Branemark implant (4.1 × 11 mm). The marginal misfit was analyzed according to the single-screw test protocol,obtaining an average value for each implant site and each framework. For implant design evaluation, different levels of marginal misfit were selected based on the misfit average of frameworks: low (< 20 µm), medium (> 20 and < 40 µm) and high (> 40 µm). The stress was measured by quantitative photoelastic analysis after tightening of frameworks to the photoelastic model C with a standardized 10-Ncm torque to evaluate the influence of framework type. The qualitative analysis in models C and S was used to compare the implant design under non-loaded condition and after 280-N load application. The results were submitted to T-test, 2-way ANOVA, and Pearson correlation test (α=.05) to evaluated the effect of framework type on the variables studied (marginal misfit and stress). The framework type and evaluation site (implant M and P) did not affect the marginal misfit values (p=.466 and p=.153, respectively) and stress (p=.602 and p=.746, respectively). Positive correlations between marginal misfit and stress was observed (CAD/CAM: r=.922 p<.0001; overcasted: r=.908 p<.0001). Under loaded condition, the short and wide implant reduced the transmitted stress to the system.It can be concluded that overcasted and CAD/CAM methods present similar marginal misfit and stress values for 3-unit implant-supported FPDs. Increasing the marginal misfit of frameworks induces greater stress in the implant-supported system.
Key-words: Prostheses and implants. Partial dentures, fixed. Computer-aided design.
1 Introdução ... 15
2 Capítulos
2.1 Capítulo 1:
Marginal misfit and photoelastic stress analysis of CAD/CAM and overcasted 3-unit implant-supported frameworks... 20
2.2 Capítulo 2:
Photoelastic stress analysis of 3-unit frameworks supported by different designs of implants under different level of marginal misfit... 37
3 Considerações Gerais ... 53
4 Conclusões ... 57
Referências ... 58
Apêndice 1 ... 64
Apêndice 2 ... 76
Anexo 1 ... 77
Anexo 2 ... 78
Introdução
O reconhecimento da existência de união biocompatível entre o tecido ósseo e a superfície do titânio criou novas aplicações na Odontologia (Hecker e Eckert, 2003). O sucesso das reabilitações implantossuportadas, confirmado por meio de estudos que comprovam a longevidade do tratamento em pacientes totalmente ou parcialmente desdentados (Adell et al., 1990; Jemt e Lekholm, 1993; Aparicio, 1994; Lindquist et al., 1996) possibilitou alternativa às próteses convencionais dento ou mucossuportadas.Apesar das altas taxas de sucesso das reabilitações implantossuportadas, a longevidade do tratamento pode ser comprometida se os passos necessários para a confecção das próteses não forem devidamente controlados a fim de se obter níveis aceitáveis de adaptação entre a plataforma dos implantes e pilares protéticos (Romero et al., 2000; Spazzin et al., 2011). Caso contrário, uma série de complicações biológicas e mecânicas podem ser geradas, comprometendo o processo de osseointegração, incluindo reação adversa aos tecidos circundantes, dor, reabsorção óssea, fratura de pilares intermediários, soltura ou fratura do parafuso protético ou de fixação do pilar, ou até mesmo sobrecarga e fratura da infraestrutura metálica (Adell et al., 1981; Skalak, 1983; Zarb e Schmitt, 1991; Naert et al., 1992). Isso ocorre porque os implantes têm mobilidade limitada à resiliência óssea. Sendo assim, a presença de desajustes protéticos pode gerar altos níveis de tensão na interface osso-implante (Skalak, 1983; Roberts et al., 1984) mesmo sem a aplicação de cargas funcionais sobre o sistema (Jemt, 1991), como por exemplo, após o torqueamento dos parafusos de retenção. A fixação das infraestruturas sobre os implantes gera tensões que são transmitidas a todas as partes do sistema implantossuportado, que permanece unido pela ação destes parafusos (Burguete et al., 1994; Nishioka et al., 2010).
Jemt, em 1991, definiu adaptação passiva como nível de desajuste que não causasse qualquer complicação biológica ou mecânica ao longo do tempo, sugerindo que valores de desajuste menores que 150 μm seriam clinicamente aceitáveis. Apesar dele e outros autores (Branemark, 1983; Jemt, 1991) tentarem definir um nível de desajuste marginal aceitável para as próteses implantossuportadas, ainda não há um consenso na literatura a respeito do valor de desajuste que pode ser considerado clinicamente aceitável (Abduo e Lyons, 2012). Deste modo, ao tratar-se da confecção de prótese fixas
implantossuportadas, valores mínimos de desajuste devem ser almejados (Spazzin et al., 2009).
Apesar da importância do nível de adaptação, as variáveis clínicas e laboratoriais envolvidas no processo de confecção da prótese podem ser consideradas como obstáculo para obtenção do ajuste passivo, uma vez que a maioria das distorções é inerente aos materiais e técnicas atualmente disponíveis (Romero et al., 2000; Hollweg et al., 2012). Desse modo, com a prerrogativa de melhorar os níveis de adaptação das próteses implantossuportadas e consequentemente reduzir os níveis de tensão exercidos sobre o sistema implantossuportado, diversas técnicas têm sido sugeridas, como a eletroerosão (Sartori et al., 2004; Nakaoka et al., 2011), técnicas de impressão alternativas (Del’Acqua et al., 2008), soldagem (Tiossi et al., 2010), sobrefundição (Bhering et al., 2013) e uso de sistemas CAD/CAM (Karl e Holst, 2012; Abduo, 2014).
Na tentativa de minimizar alterações na base dos componentes protéticos decorrentes do processo de fundição, foram desenvolvidos componentes metaloplásticos, cuja superfície de encaixe à plataforma do implante é previamente usinada em metal e somente a porção plástica do cilindro é fundida em laboratório (Bhering et al., 2013; Bhering et al., 2015). Estes componentes têm como finalidade o favorecimento da obtenção de infraestruturas protéticas com melhores níveis de adaptação (Bhering et al., 2013; Bhering et al., 2015).Apesar de terem sido inicialmente idealizados para fundição com metais preciosos, são atualmente confeccionados em liga de CoCr (Bhering et al., 2013). Bhering et al., em 2015, avaliaram o efeito da utilização de cilindros para sobrefundição, em relação a totalmente calcináveis, e observaram que o uso de cilindros para sobrefundição em próteses extensas (protocolo Branemark) não apresentou vantagens em relação ao desajuste marginal, enquanto para infraestruturas de PPFs de 3 elementos mostrou níveis de desajuste marginal aceitáveis, além de maior estabilidade dos parafusos protéticos.
Outro método desenvolvido a fim de promover reabilitações implantossuportadas com melhores níveis de adaptação é a tecnologia CAD/CAM (Computer-Aided Design/Computer-Aided Manufacturing). Inicialmente instituído para confecção de restaurações dentossuportadas por volta de 1980 (Patzelt et al., 2015), o método CAD/CAM tem sido amplamente utilizado para confecção de próteses dentossuportadas e implantossuportadas, tendo como vantagens a previsibilidade e precisão da técnica (Kapos et al., 2009). Infraestruturas fabricadas a partir da tecnologia
CAD/CAM demonstraram melhor adaptação comparadas à peças confeccionadas por fundição em monobloco e estruturas soldadas a laser (Abduo, 2014). O processo CAD/CAM permite a redução de vários passos utilizados na técnica convencional, como enceramento, revestimento, fundição, polimento; tais procedimentos introduzem imprecisões na peça que podem tornar-se ainda mais evidentes em infraestruturas mais extensas (Abduo et al., 2011).
Embora a tecnologia CAD/CAM elimine várias etapas, introduz outros passos que também dependem do domínio do operador e equipamentos utilizados, como o escaneamento, modelagem de software e usinagem (Abduo, 2014), além de envolver maior custo em relação às técnicas convencionais. Além disso, apesar de diversos estudos demonstrarem a superioridade da técnica CAD/CAM quanto à obtenção de melhores valores de adaptação protética (Abduo et al., 2011; Abduo e Lyons, 2012; Karl e Holst, 2012; Abduo, 2014; De França et al., 2015), tais estudos comparam o método com técnicas convencionais de fundição enquanto há uma tecnologia superior para fundição, a sobrefundição, que mostrou níveis melhores de adaptação em PPFs de 3 elementos quando comparada às técnicas convencionais (Bhering et al., 2015). Deste modo, o custo-benefício da técnica deve ser ponderado na hora da seleção de acordo com as necessidades de cada caso clínico, uma vez que a magnitude e a distribuição das tensões geradas sobre a infraestrutura, implantes e no tecido peri-implantar irá variar também por outros fatores, como por exemplo, o comprimento do implante utilizado (Hasan et al., 2013).
O uso de implantes curtos (< 10 mm) parece ser uma alternativa para o tratamento em áreas com altura óssea insuficiente como a região posterior de maxila e mandíbula (Fugazzotto et al., 2004; Renouard e Nisand, 2005; Atieh et al., 2012; Hasan et al., 2013; Şeker et al., 2014; Pellizzer et al., 2015). Quando utilizados sob protocolos clínicos rigorosos, como a otimização da oclusão da restauração final e a ausência de carregamento lateral dos implantes (Hasan et al., 2013), os implantes curtos oferecem a possibilidade de simplificação da técnica cirúrgica tornando o tratamento mais seguro, devido à redução dos riscos de interferência com estruturas anatômicas, tais como o seio maxilar (Atieh et al., 2012). Esta opção evitaria procedimentos cirúrgicos de enxertia óssea que podem implicar em tratamentos mais extensos, maior desconforto e dor pós-operatória, e outras complicações pós-cirúrgicas (Hasan et al., 2013; Chang et al., 2012).
Em relação ao aspecto biomecânico, estudos demonstram elevada taxa de sobrevivência dos implantes curtos por um período de 2 a 3 anos, associada à reabsorção do osso marginal comparável àquela em implantes convencionais (Hasan et al., 2013). Adicionalmente, o aumento do diâmetro dos implantes curtos tem demonstrado melhor distribuição de tensão gerada sobre o sistema implantossuportado (Santiago et al., 2013). Estudos utilizando análise de elementos finitos têm mostrado que para próteses unitárias (Chang et al., 2012) ou reabilitações com PPFs (Şeker et al., 2014), implantes curtos e com maior diâmetro reduzem a transmissão de tensão no tecido ósseo ao redor dos implantes na região posterior de maxila em comparação a instalação de implantes mais longos associados a levantamento de seio maxilar.
Com relação às metodologias de análise de tensão três são mais comumente aplicadas: a extensometria, o método dos elementos finitos e a análise fotoelástica. A análise fotoelástica tem sido amplamente utilizada na Odontologia a fim de estudar a interação de resposta dos tecidos peri-implantares e as características físicas da prótese e dos implantes (Assunção et al., 2009; Pereira et al., 2015). O princípio da análise fotoelástica é baseado em uma propriedade única de alguns materiais transparentes que é o efeito de anisotropia ótica, assim quando submetidos a algum estado de tensão, demonstram propriedades ópticas. A luz polarizada que atravessa o modelo, emitida por um aparelho denominado polariscópio, permite determinar as direções e os gradientes das tensões principais por meio da interpretação dos parâmetros ópticos observados (Bernardes, 2004). Atualmente, o método tem sido aplicado para analisar tensões induzidas por próteses implantossuportadas (Dwivedi et al., 2013; Celik e Uludag, 2014; Pellizzer et al. 2015), permitindo a avaliação do efeito do desajuste nos tecidos peri-implantares (Pereira et al., 2015).
Baseado nas considerações acima, o primeiro objetivo deste estudo in vitro foi verificar a influência de duas técnicas (CAD/CAM e sobrefundição) para confecção de infraestruturas de PPFs de 3 elementos com relação ao desajuste marginal e tensão gerada através de análise fotoelástica quantitativa. Caso ambas as técnicas de confecção de infraestruturas avaliadas demostrem características biomecanicamente semelhantes, poderiam ser recomendadas técnicas satisfatórias para a confecção de infraestruturas de PPFs de 3 elementos, cabendo ao profissional optar pela mais conveniente de acordo com as limitações que o mesmo apresentar. Ainda, uma vez que a presença de desajustes protéticos é uma realidade clínica (Spazzin et al., 2011) e não há estudos que avaliem a
influência de diferentes níveis de desajuste marginal sobre diferentes designs de implantes, foi realizada a comparação por análise fotoelástica qualitativa da influência de diferentes designs de implantes e diferentes níveis de desajuste marginal de PPFs sobre os níveis e distribuição de tensão no sistema implantossuportado.
Capítulo 1*
Marginal misfit and photoelastic stress analysis of CAD/CAM and overcasted 3-unit implant-supported frameworks
Anna Gabriella Camacho Presotto, DDSa, Cláudia Lopes Brilhante Bhering, DDS, MScb, Marcelo
Ferraz Mesquita, DDS, MSc, PhDc, Valentim Adelino Ricardo Barão, DDS, MSc, PhDd.
Piracicaba Dental School, University of Campinas, Piracicaba, Sao Paulo, Brazil.
Supported by grant #2014/19264-0 from the Sao Paulo Research Foundation (FAPESP) and grant #1778/2014 from the Coordination for the Improvement of Higher Education Personnel (CAPES), Brazil.
a MSc Student, Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo, Brazil.
b PhD Student, Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo, Brazil.
c Full Professor, Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo, Brazil.
d Assistant Professor, Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas (UNICAMP), Piracicaba, Sao Paulo, Brazil.
Corresponding author:
Anna Gabriella Camacho Presotto
Piracicaba Dental School, University of Campinas
Limeira Avenue, 901; CEP: 13414-903; Piracicaba, SP, Brazil E-mail: annapresotto@gmail.com
Phone: 55 (19) 2106 5211 - Fax: 55 (19) 2106 5218
Acknowledgements
The authors are grateful to Ricardo Takeshi Nagahisa of Artwork Dental Lab for his assistance in CAD/CAM frameworks fabrication, to Professor Mauro A. A. Nobilo for the polariscope facility and to Amann Girrbach for supplying the CoCr blocks.
________________
* Artigo de acordo com as normas para publicação no periódico The Journal of Prosthetic
Abstract
Statement of the Problem: The superiority of the CAD/CAM method in relation to conventional casting has been shown by several studies. However, there is an advanced technology for casting procedures (i.e., the overcasting technique) that may present similar characteristics to CAD/CAM for 3-unit implant-supported fixed partial dentures (FPDs). Purpose: This in vitro study evaluated the influence of the prosthetic framework fabrication method (CAD/CAM and overcasting) on marginal misfit and stress transmitted to implants using quantitative photoelastic analysis. The correlation between marginal misfit and stress was also investigated. Materials and Methods: Three-element denture frameworks were made using the CAD/CAM (n=10) and overcasting (n=10) methods. The frameworks were waxed to simulate an superior first premolar (pillar P) to first molar (pillar M) FPD using overcasted mini abutment cylinders. The wax patterns were overcasted (overcasted group) or scanned to obtain the frameworks (CAD/CAM group). All frameworks were fabricated from CoCr alloy. The marginal misfit was analyzed according to the single-screw test protocol, obtaining an average value for each implant site and each framework. The stress was measured by quantitative photoelastic analysis after the tightening of frameworks for the photoelastic model with a standardized 10-Ncm torque. The results were submitted to T-test, 2-way ANOVA, and Pearson correlation test (α=.05). Results: The framework type and evaluation site (implant M and P) did not affect the marginal misfit values (p=.466 and p=.153, respectively) and stress (p=.602 and p=.746, respectively) in the implant-supported system. Positive correlations between marginal misfit and stress were observed (CAD/CAM: r=.922 p<.0001; overcasted: r=.908 p<.0001). Conclusions: CAD/CAM and overcasting methods present similar marginal misfit and stress values for 3-unit FPD frameworks. Increasing the marginal misfit of frameworks induces greater stress in the implant-supported system.
Keywords: CAD/CAM, overcasting, multiple prosthesis, marginal misfit, stress analysis,
Clinical Implications
Overcasting is an advanced technology for casting procedures with effectiveness similar to CAD/CAM technology according to marginal misfit and stress values. A 3-unit overcasted FPD framework is an acceptable and affordable option for clinicians.
Introduction
Despite the high success rates of implant-supported rehabilitations,1–6 the longevity of treatment may be compromised if the steps to fabricate an implant-supported prosthesis are not properly controlled to obtain acceptable levels of misfit between the platform of the implants and the abutments.7–11
The presence of misfit in a prosthesis rigidly connected to multiple implants can generate high levels of stress at the bone-implant interface,12,13 risking the osseointegration process and leading to biological or mechanical complications.14 These levels of stress can be generated even before the application of functional loads on the system.14 During the torque application to the prosthetic screws, forces are generated on the osseointegrated system and the stresses are transferred to all parts of this system, which remain united by the action of these screws.15,16
To improve the fit level of implant-supported prosthesis and consequently to reduce the stresses transmitted to the implant-supported system, several techniques such as welding,17 electroerosion,18,19 overcasting20,21 and Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) systems22–24 have been used. The overcasted components consisted of a premachined bottom metal strap, so that only the plastic cylinder is subject to casting.20 Frameworks fabricated via CAD/CAM system demonstrate better fit and passivity than those made by conventional castings23 as it decreases laboratory variables generated by the inconsistency of volumetric and linear expansion of the materials used, including impression material, gypsum products, waxes, investments, and casting metal.7 It leads to frameworks with improved fit than one-piece casting, even those welded by laser.25 Other proposed advantages of CAD/CAM are its predictability and the consistency of the technique.26,27
While CAD/CAM technology eliminates several steps, it introduces others such as scanning, software modeling and machining,22 which also depend on the domain of the operator and equipment used. Differences in the accuracy of fit between the CAD/CAM and cast frameworks have been observed, showing the superiority of the CAD/CAM method.22-25,28 However, these studies compared CAD/CAM with conventional casting, and there is an advanced technology (i.e., overcasting technique) for casting procedures that has revealed better fit levels for implant-supported FPD frameworks than the conventional casting technique.21
Based on these considerations, the objective of this in vitro study was to evaluate the marginal misfit and stress generated around the implants by the quantitative photoelastic analysis of CAD/CAM and overcasted 3-unit implant-supported frameworks. The correlation between marginal misfit and stress was also investigated. The research hypotheses were as follows: [1] no difference in the marginal misfit and stress values of frameworks manufactured by different techniques (CAD/CAM and overcasting methods) would be found, and [2] a positive correlation between marginal misfit and stress transmitted to implants would be found.
Materials and Methods
Frameworks Fabrication
The frameworks simulating a fixed partial denture (FPD) of an superior first premolar to a first molar were waxed with a low-shrinkage acrylic resin (Duralay II Reliance Dental Mfg. Co.) using overcasted mini abutment cylinders (SIN – Sistema de Implante). The cylinders were screwed onto a steel master model with dimensions of 30 × 20 × 15 mm, with two drillings 18 mm from each other (center to center) and two mini abutment analogs screwed onto the master itself (Figure 1A).
The master model/waxed framework set was impressed (Flexitime Easy Putty Correct Flow Heraeus-Kulzer) and duplicated to obtain the overcasted frameworks (n=10). All waxed patterns were sectioned and connected with a low-shrinkage acrylic resin to verify a full fit on the master model.
The frameworks were included in the investment (Gilvest HS – BK Giulini), and were overcasted in CoCr alloy (Starloy C – Degudent, Dentsply) for the overcasted
group. Subsequently, the frameworks were blasted with 100-μm particles of aluminum oxide at a pressure of 0.55 MPa, followed by finishing and polishing with tungsten carbide drills at a low speed with the exception of the metallic strap region.
For the fabrication of CAD/CAM frameworks, the master model was scanned (Ceramill Map 300 Scanner, Amann Girrbach). Thereafter, one waxed framework of acrylic resin was scanned on the master model. The images obtained were transferred to a software program (Ceramill map v2.7.05., Amann Girrbach), and the virtual model was obtained. The frameworks were milled with a milling machine (Ceramill Motion 2; Amann Girrbach) using CoCr blocks. After milling, the specimens were sintered at 1280 °C for a total of 5 hours in the sintering box (Ceramill Sinter box; Amann Girrbach) of the sintering furnace (Ceramill Argotherm; Amann Girrbach).
Photoelastic Model Fabrication
Mini pillar transfers were tightened on the master model and attached with a drill and a low shrinkage acrylic resin (Duralay II Reliance Dental Mfg. Co.). The photoelastic model was obtained by using a silicone mold (Silibor Industria e Comercio Ltd.) of the master model/transfer set.
The photoelastic resin (Araldite GY 279 BR, catalyst Aradur HY 2963, Araltec Chemicals Ltd.) was manipulated with a proportion of 2 parts of resin to 1 part of catalyst for 1 minute, leaving a homogeneous mixture. To prevent bubbles in the material, the mixture was placed in a pressure chamber coupled to an air injection tube and a pressure of 60 kgf/cm2 was applied for 20 minutes. Thereafter, the Branemark standard implants (external hexagon (EH) 4.1 × 11 mm; SIN – Sistema de Implante) with mini pillars (Mini Abutment - EH 4.1 × 2 mm; SIN – Sistema de Implante) were tightened upon transfer of silicone mold. The photoelastic resin was slowly poured over the mold. After the resin cure time (72 hours), the transfers were removed from the silicone mold and the photoelastic model was obtained. The ideal characteristics of the photoelastic model that were suitable for photoelastic analysis were verified, such as for translucency and appropriate surface finish.
Marginal Misfit Evaluation
The existing marginal misfit between abutments and cylinders was quantified using a 1.0-μm precision microscope and 120 times magnification (VMM-100-BT – Walter UHL). The microscope was equipped with a digital camera (KC-512NT - Kodo BR Eletronics Ltd.) and analyzer unit (QC 220-HH Quadra-Check 200, Metronics Inc.).
The procedures were performed by a blinded calibrated examiner - intraclass correlation coefficient of .995 (p<.0001) - according to the single-screw test protocol,19,21 which determines the marginal misfit reading of the loop presented while the screw of the opposite pillar is tightened.
For the evaluation, the pillars of the master model and the photoelastic model were designated as pillar M (molar) and P (premolar) (Figure 1). The reading location was standardized in the central portion of the mini abutment platform between the top edge and the prosthetic framework. Prosthetic frameworks were positioned on the model and pillar M readings on the buccal and lingual sides in diametrically opposite positions were obtained after the pillar P screw was tightened and vice versa. The prosthetic screw was tightened using 10 Ncm using a 0.1-Ncm precision digital torque meter (Torque Meter TQ-8800, Lutron).
The measurements were performed on both framework extremities and an average value of misfit was obtained for each pillar and for each framework.
Figure 1. Master model (A) and photoelastic model (B) dimensions and pillars
designation.
Photoelastic Analysis
The photoelastic analysis was performed 72 hours after the photoelastic model fabrication. A horizontal transmission polariscope was used. It was developed in the Mechanical Design Laboratory Henner Alberto Gomide, School of Mechanical Engineering of the Federal University of Uberlandia and consisted of two ¼-retardant wave filters and two polarizing filters, called the polarizer and the analyzer. The position of the photoelastic models was standardized by markings on the polariscope platform. The frameworks were tightened to the photoelastic model with 10-Ncm standardized torque, always following the tightening sequence P-M. To improve the view of the fringes, a layer of mineral oil was applied on the photoelastic model.
After obtaining the images using a digital camera (Canon SX50HS - Canon Inc.), a software program (Fringes, Mechanical Design Laboratory, FMEC, Federal University of Uberlandia) was used to quantify the stress in the model. Ten points of interest were determined around each implant (Figure 2) to obtain the stress average for each implant (implant M and P) and framework (𝑖𝑚𝑝𝑙𝑎𝑛𝑡 𝑀+𝑖𝑚𝑝𝑙𝑎𝑛𝑡 𝑃2 ). For each point, the maximum shear stress (τ) was calculated by using the formula: τ=KN/2b, where K=11.271 N/mm is the optical constant of the photoelastic resin (such value was
determined through previous calibration), N is the fringe order, and b=15 mm is the thickness of the model.
Figure 2. Ten points (in red) of interest determined in the software around each
implant for a CAD/CAM (A) and overcasted framework (B).
Statistical Analyses
All data included in this study were tested regarding normality using the Kolmogorov–Smirnov method. T-test was used to compare the marginal misfit values of the master model and the photoelastic model. It was conducted in an effort to evaluate the reproducibility of the transfer technique. Two-way ANOVA was used to evaluate the influence of framework type, evaluation site and their interaction regarding marginal misfit and stress values. Pearson correlation test was used to verify the correlation between marginal misfit and stress data. All analyses were conducted at a 5% level of significance (SPSS v. 20.0; SPSS Inc.).
The sample size of each test was determined statistically to achieve a large effect according to Cohen’s effect size (> 0.8) and Partial Eta Squared (Ƞp2>.692) analyses.
Results
There was no difference in the marginal misfit between the master model and the photoelastic model (CAD/CAM: t=-.687, df=18, p=.501; overcasted: t=-.745,
df=18, p=.466; T-test).
The framework type, evaluation site (implant M and P) and their interaction did not affect the marginal misfit values (p<.05, 2-way ANOVA) (Table 1). Figure 3 shows the median and interquartile range of the marginal misfit for the CAD/CAM and the overcasted groups according to evaluation site. Considering the implant 𝑀 + 𝑃/2 data, a median misfit of 39 and 31 μm was observed for the CAD/CAM and the overcasted groups, respectively.
Table 1. Two-way ANOVA results of marginal misfit and stress values as a function of
framework type, evaluation site and their interaction.
Sum of Squares df Mean Squares F value P* Marginal misfit Framework .033 1 .033 .538 .466 Site .237 2 .118 1.945 .153 Framework × Site .005 2 .003 .041 .959 Total 130.428 60 Stress Framework 9928.049 1 9928.049 .275 .602 Site 21269.622 2 10634.811 .294 .746 Framework × Site 38130.625 2 19065.313 .548 .593 Total 6529782.607 60
Figure 3. Box plot of marginal misfit values (µm) for CAD/CAM and overcasted
frameworks according to evaluation site.
Two-way ANOVA shows no influence of the framework type, evaluation site (implant M and P) and their interaction on the stress values in the implant-supported system (p<.05) (Table 1). The median and interquartile range of the stress values for the CAD/CAM and the overcasted groups according to the evaluation site is presented in Figure 4. The median for the CAD/CAM group was 287.53 MPa and the median was 231.06 MPa for the overcasted group when the stress average for each framework type (Implant M+P/2) was observed.
Figure 4. Box plot of stress values (MPa) for CAD/CAM and overcasted frameworks
according to evaluation site.
Positive correlations between marginal misfit and stress were observed for all interactions (Table 2).
Table 2. Pearson correlation analysis between misfit and stress as a function of
framework type in different site.
Framework type Misfit/Stress Correlation
r p value* CAD/CAM Implant M r=.678 p=.031 Implant P r=.926 p<.0001 Implant M+P/2 r=.922 p<.0001 Overcasted Implant M r=.667 p=.035 Implant P r=.974 p<.0001 Implant M+P/2 r=.908 p<.0001
Discussion
According to the results, the first research hypothesis was accepted because no difference in the marginal misfit (CAD/CAM: 41.55 ± 18.72 µm; overcasted: 41.65 ± 28.21 µm) and stress values (CAD/CAM: 261.30 ± 200.34 MPa; overcasted: 287.02 ± 135.23 MPa) between CAD/CAM and overcasted frameworks was found. Regarding the marginal misfit, the findings of this study corroborate that the pre-machined base of the overcasted cylinders is effective in promoting a higher level of fit because the overcasted group obtained similar levels of CAD/CAM. This can be explained by the presence of the pre-machined metal strap so that only the plastic cylinder is subject to casting, minimizing the distortions in the cylinder base during the casting process, resulting in a more intimate contact between the abutment platform and the cylinder base.20 Bhering et al.21 evaluated the effect of the use of overcasted cylinders compared with totally calcinable cylinders and observed that the use of overcasted cylinders for 3-unit FPD frameworks showed more acceptable levels of misfit (average misfit of 57.93±22.79 µm) and greater stability of prosthetic screws.
The superiority of the CAD/CAM technique regarding the marginal misfit was reported in other in vitro studies.22–24 Nonetheless, in vivo data showed an insignificant difference between CAD/CAM and conventional cast frameworks.6 Other studies stated that, despite the unknown clinical significance of the difference in fit between the two systems, the consistency and favorable technique sensitivity of CAD/CAM frameworks were reported.27 In this present study, although no difference between the techniques and the evaluation site (implant M and P) was found, the consistency of the CAD/CAM technique can be observed in Figure 3. Observing the average misfit of each framework (𝑖𝑚𝑝𝑙𝑎𝑛𝑡 𝑀+𝑖𝑚𝑝𝑙𝑎𝑛𝑡 𝑃2 ), the data in the 2nd and 3rd quartiles showed that 50% of the CAD/CAM framework group presented a lower distribution of misfit values compared with the overcasted group. The 2nd and 3rd quartiles of the overcasted group showed a higher data distribution, and the highest standard deviation error was also observed for this group.
The absence of a totally passive fit framework was expected, as shown by previous data.21,23,27 This can be explained by the framework having been made on the master model, and the evaluation was performed on the working model (photoelastic model). Although no significant difference between the misfit values of the master model
and the photoelastic model was found, the transference modified the misfit values. Clinically, the same situation occurs due to the impression procedure and framework fabrication on the working model. These findings confirm that misfits are a clinical reality.9 Some authors have attempted to define an acceptable level of fit, suggesting values between 10 and 150 μm as being clinically acceptable.9,14 Although the preceding values have been reported and used as reference, they are empirical in nature.10 Thus, for the production of implant fixed prosthesis, those with minimum misfit values should be targeted.9 Therefore, regardless of technique used, all steps involved in the fabrication of the prosthesis should be conducted with the necessary care.
The second hypothesis of this study was accepted because a positive correlation between marginal misfit and stress values was found. This can be justified because, after fixing the framework on the photoelastic model with a standardized tightening of 10 Ncm, the distortion level of each framework generated the transference of internal stress in the photoelastic model, which could be observed as visible light patterns. Thus, some correlation between these values could be expected. Considering that qualitative photoelastic analysis is subjective and depends on the interpretation of the examiner,11 the application of the quantitative method was indispensable to apply the correlation test between marginal misfits and stress data as performed herein.
Regarding the stress transmitted to the implants, there is some scientific evidence that the cyclic load application can change the fit of implant frameworks.8,11 The objective of this study was to compare the two framework types, so the photoelastic analysis was intentionally applied without loading. Thus, the frameworks could present the stresses related to existing misfits in frameworks, allowing the comparison as well as the verification of correlation between misfit and stress. Furthermore, the load application would result in higher levels of stress, which could compromise the quantitative analysis application, given that the software has a reading limitation to the 4th fringe order and its corresponding maximum shear stress (1,500 MPa).
According to Figure 4, in the 2nd and 3rd quartiles, one can observe the average stress of each framework. There were marginal misfit values of 50% for the CAD/CAM group that presented a lower distribution of values compared with the overcasted group. The similar distribution between misfit and stress values was expected because a positive correlation between the variables was found.
There is an increasing amount of interest in the development of new technologies and materials in digital dentistry. However, advances in conventional
techniques such as overcasting show similar effectiveness of new technologies for 3-unit FPD frameworks, being an acceptable and affordable option for clinicians with limited access to CAD/CAM technologies. Herein, the values of stress obtained are overestimated to extrapolate them to a clinical scenario. However, this quantitative analysis provides the correlation between stress and misfit and permits the comparison between overcasted and CAD/CAM techniques. However, the long-term behavior of frameworks cannot be determined. Further research simulating longer periods of clinical use are required to clarify the influence of dynamic loading on the analyzed variables. Additionally, longitudinal follow-ups of implant-supported prosthetic rehabilitations with 3-unit FPDs fabricated by the evaluated techniques could be a valid procedure to clarify the relevance of the findings of this study.
Conclusions
Based on the results obtained in the present study, the following can be concluded for 3-unit FPD frameworks:
- CAD/CAM and overcasting techniques allow for FPD frameworks with acceptable fit levels;
- Similar stress values were noted for CAD/CAM and overcasted frameworks; - As the misfit value for the frameworks increase, the stress levels on the implant-supported system increase.
References
1. Adell R, Eriksson B, Lekholm U, Brånemark PI, Jemt T. Long-term follow-up study of osseointegrated implants in the treatment of totally edentulous jaws. Int J Oral Maxillofac Implants 1990;5:347–59.
2. Jemt T, Lekholm U. Oral implant treatment in posterior partially edentulous jaws: a 5-year follow-up report. Int J Oral Maxillofac Implants 1993;8:635–40.
3. Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseointegrated dental implants in posterior partially edentulous patients. Int J Prosthodont 1993;6:189–96.
4. Aparicio C. A new method to routinely achieve passive fit of ceramometal prostheses over Brånemark osseointegrated implants: a two-year report. Int J Periodontics Restorative Dent 1994;14:404–19.
5. Lindquist LW, Carlsson GE, Jemt T. A prospective 15-year follow-up study of mandibular fixed prostheses supported by osseointegrated implants. Clinical results and marginal bone loss. Clin Oral Implants Res 1996;7:329–36.
6. Jemt T, Bäck T, Petersson A. Precision of CNC-milled titanium frameworks for implant treatment in the edentulous jaw. Int J Prosthodont 1999;12:209–15.
7. Romero GG, Engelmeier R, Powers JM, Canterbury AA. Accuracy of three corrective techniques for implant bar fabrication. J Prosthet Dent 2000;84:602–7.
8. Hecker DM, Eckert SE. Cyclic loading of implant-supported prostheses: Changes in component fit over time. J Prosthet Dent 2003;89:346–51.
9. Spazzin AO, Henriques GEP, de Arruda Nóbilo MA, Consani RLX, Correr-Sobrinho L, Mesquita MF. Influence of prosthetic screw material on joint stability in passive and non-passive implant-supported dentures. Open Dent J 2009;3:245–9.
10. Spazzin AO, Abreu RT, Noritomi PY, Consani RLX, Mesquita MF. Evaluation of Stress Distribution in Overdenture-Retaining Bar with Different Levels of Vertical Misfit. J Prosthodont 2011;20:280–5.
11. De Torres ÉM, Barbosa GAS, Bernardes SR, de Mattos MDGC, Ribeiro RF. Correlation between vertical misfits and stresses transmitted to implants from metal frameworks. J Biomech 2011;44:1735–9.
12. Skalak R. Biomechanical considerations in osseointegrated prostheses. J Prosthet Dent 1983;49:843–8.
13. Roberts WE, Smith RK, Zilberman Y, Mozsary PG, Smith RS. Osseous adaptation to continuous loading of rigid endosseous implants. Am J Orthod 1984;86:95–111.
14. 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;6:270–6.
15. Burguete RL, Johns RB, King T, Patterson EA. Tightening characteristics for screwed joints in osseointegrated dental implants. J Prosthet Dent 1994;71:592–9.
16. Nishioka RS, Nishioka LNB de M, Abreu CW, de Vasconcellos LGO, Balducci I. Machined and plastic copings in three-element prostheses with different types of implant-abutment joints: a strain gauge comparative analysis. J Appl Oral Sci 2010;18:225–30.
17. Tiossi R, FalcãO-Filho H, Aguiar JÚNior FA, Rodrigues RC, Mattos MDG, 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;37:359–63.
18. Nakaoka MM, Nuñez-Pantoja JMC, Takahashi JMFK, Consani RLX, Mesquita MF. Misfit of pure titanium frameworks: effect of veneer coverage and spark erosion process. Acta Odontol Scand 2011;69:238–42.
19. Sartori IADM, Ribeiro RF, Francischone CE, De Mattos MDGC. In vitro comparative analysis of the fit of gold alloy or commercially pure titanium implant-supported prostheses before and after electroerosion. J Prosthet Dent 2004;92:132–8.
20. Bhering CLB, Takahashi JMFK, Luthi LF, Henriques GEP, Consani RLX, Mesquita MF. Influence of the casting technique and dynamic loading on screw detorque and misfit of single unit implant-supported prostheses. Acta Odontol Scand 2013;71:404–9.
21. Bhering CLB, Marques IDSV, Takahashi JMFK, Barão VAR, Consani RLX, Mesquita MF. Fit and Stability of Screw-Retained Implant-Supported Frameworks Under Masticatory Simulation: Influence of Cylinder Type. J Prosthodont 2015; doi: 10.1111/jopr.12349.
22. Abduo J. Fit of CAD/CAM implant frameworks: a comprehensive review. J Oral Implantol 2014;40:758–66.
23. Karl M, Holst S. Strain development of screw-retained implant-supported fixed restorations: procera implant bridge versus conventionally cast restorations. Int J Prosthodont 2012;25:166–9.
24. De França DGB, Morais MHST, das Neves FD, Barbosa GAS. Influence of CAD/CAM on the fit accuracy of implant-supported zirconia and cobalt-chromium fixed dental prostheses. J Prosthet Dent. Editorial Council for the Journal of Prosthetic Dentistry 2015;113:22–8.
25. Abduo J, Lyons K. Effect of vertical misfit on strain within screw-retained implant titanium and zirconia frameworks. J Prosthodont Res 2012;56:102–9.
26. Kapos T, Ashy LM, Gallucci GO, Weber H-P, Wismeijer D. Computer-aided design and computer-assisted manufacturing in prosthetic implant dentistry. Int J Oral Maxillofac Implants 2009;24:110–7.
27. Hjalmarsson L, Örtorp A, Smedberg JI, Jemt T. Precision of Fit to Implants: A Comparison of CrescoTM and Procera® Implant Bridge Frameworks. Clin Implant Dent Relat Res 2010;12:271–80.
28. Abduo J, Lyons K, Bennani V. Fit of Screw-Retained Fixed Implant Frameworks Fabricated. Int J Prosthodont 2011;24:207–20.
Capítulo 2*
Photoelastic stress analysis of 3-unit frameworks supported by different designs of implants under different levels of marginal misfit
Authors:
Anna Gabriella Camacho Presotto1, Cláudia Lopes Brilhante Bhering2, Rafael Leonardo Xediek Consani3, Valentim Adelino Ricardo Barão4, Marcelo Ferraz Mesquita5.
1DDS, MS Student - Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas, Piracicaba, Sao Paulo, Brazil.
2DDS, PhD Student - Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas, Piracicaba, Sao Paulo, Brazil.
3DDS, MS, PhD, Associate Professor - Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas, Piracicaba, Sao Paulo, Brazil.
4DDS, MS, PhD, Assistant Professor - Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas, Piracicaba, Sao Paulo, Brazil.
5DDS, MS, PhD, Full Professor - Department of Prosthodontics and Periodontology, Piracicaba Dental School, University of Campinas, Piracicaba, Sao Paulo, Brazil.
Full address of all authors:
1,2,3,4,5 Piracicaba Dental School - Limeira Avenue, 901; CEP 13414-903; Piracicaba - SP, Brazil
Corresponding author:
Anna Gabriella Camacho Presotto
Limeira Avenue, 901; CEP: 13414-903; Piracicaba-SP, Brazil E-mail: annapresotto@gmail.com
Phone: 55 (19) 2106 5211 - Fax: 55 (19) 2106 5218 ______________________
Abstract
We investigated the influence of marginal misfit levels and implant design (length and diameter) on the stress levels and distribution of fixed partial dentures (FPDs) using photoelastic analysis. Two photoelastic models were obtained: model C with two standard Branemark implants (4.1×11 mm); and model S with a short implant and a standard Branemark implant (5×6 mm, 4.1×11 mm). Three-unit CoCr frameworks were fabricated, simulating a superior first pre-molar to first molar FPD. Different levels of marginal misfit (in µm) were selected based on the misfit average of frameworks obtained by the single-screw test protocol: low (<20), medium (>20 and <40) and high (>40). The stress level and distribution were measured by qualitative photoelastic analysis after tightening the frameworks to the models and after application of a 280-N load. Increasing the marginal misfit generates higher levels of stress. Under the loaded condition, the short and wide implant reduced the transmitted stress to the implant-supported system.It can be concluded that small increments in marginal misfit increase the stress levels independently of the supporting condition, and the use of a posterior short and wide implant may be an approach to rehabilitate patients with reduced bone height.
Keywords: Short implants, multiple prosthesis, implant-supported prostheses, marginal
Introduction
Obtaining acceptable levels of misfit between the platform of the implants and abutments is very important to the longevity of implant-supported treatment.1–3 The presence of prosthetic misfits can lead to biological and mechanical complications4 due to the high levels of stress generated for the osseointegrated system.5 The distribution of the stress is influenced not only by the level of misfit but also by the type of implant.6
The use of short implants (< 10 mm) is an alternative for treatment regions with insufficient bone height such as the posterior maxilla and mandible.6–12 Although widely used, techniques such as ridge augmentation and sinus floor elevation involve greater morbidity for the patient, more extensive treatments, and higher costs.6–13 The option of short implants would avoid these surgical procedures. Short implants are a predictable alternative when used under rigorous clinical protocols,9 such as the optimization of occlusion of the definitive prostheses,6 avoiding occlusal overloading, which is a common cause of failure of implant-supported rehabilitation.14 In addition, the increase in diameter of short implants shows the greatest benefit for stress distribution.15 Finite element analysis (FEA) studies have shown that for the placement of a single implant13 and in FPD rehabilitation,10 the short and wide implant reduced the transmitted stress and resultant strain in the surrounding bone in the posterior maxilla in comparison to long implants placed in the grafted sinus.
Some studies, in evaluating the biomechanical behavior of rehabilitations with short implants, showed a high survival rate and comparable marginal bone resorption to the conventional implants.6 There was a decrease of the transmitted stress around the implants,10,13 but the influence of marginal misfit in this design of implants was not evaluated by these studies. Because misfit is a clinical reality16 and based on these considerations, the aim of this in vitro study was to evaluate the stress distribution to implants with different designs under different marginal misfit conditions of three-element FPD using photoelastic analysis. The research hypotheses were as follows: i. Increasing the marginal misfit generates higher levels of stress in the implant-supported system; and ii. The increase of marginal misfit is less critical for short and wide implants under loaded and non-loaded conditions.
Materials and Methods
Study design
Two simulated clinical conditions were evaluated: 1) model C with two standard Branemark implants (external hexagon (EH), 4.1 × 11 mm); and 2) model S with one conventional implant and one short and wide implant (EH, 5 × 6 mm), simulating the placement of a short implant in posterior maxilla. Frameworks simulating 3-unit fixed partial dentures (FPDs) were obtained by overcasting method (n=10). Three frameworks with different levels of marginal misfit were selected based on their misfit average obtained by the single-screw test protocol. The three levels were low (< 20 µm), medium (> 20 µm and < 40 µm) and high (> 40 µm).Qualitative photoelastic analysis was used to evaluate the stress level and distribution under two situations: non-loaded, after tightening of frameworks to the models; and loaded, after applying a 280-N load to the molar.
Frameworks Fabrication
A steel master model with dimensions of 30 × 20 × 15 mm was fabricated with two drill holes 18 mm from each other (center to center) and two mini abutment analogs screwed on the model. Overcasted mini abutment cylinders (SIN – Sistema de Implante, Sao Paulo, SP, Brazil) were tightened on the master model. The frameworks simulating FPDs for superior first pre-molar (pillar P) to first molar (pillar M) were waxed with a low-shrinkage acrylic resin (Duralay – Duralay II Reliance Dental Mfg. Co., Chicago, USA). All waxed patterns were sectioned and reunited with a low-shrinkage acrylic resin. The frameworks (n=10) were overcasted in CoCr alloy (Starloy C – Degudent, Dentsply, Hanau-Wolfgang, Hesse, Germany) after including in investment material (Gilvest HS–BK Giulini, Ludwigshafen, Rheinland-Pfalz, Germany).
Photoelastic Model Fabrication
A silicone mold (Silibor – Silibor Industria e Comercio Ltd., Sao Paulo, SP, Brazil) was obtained from the master model/transfer set. The photoelastic resin
(Araldite GY 279 BR, catalyst Aradur HY 2963 - Araltec Chemicals Ltd., Guarulhos, SP, Brazil) was manipulated with the proportion of 2 parts of resin to 1 part of catalyst for 1 minute, leaving a homogeneous mixture. The Branemark standard implants (EH 4.1 × 11 mm; SIN – Sistema de Implante, Sao Paulo, SP, Brazil) with mini pillars (Mini Abutment - EH 4.1 × 2 mm; SIN – Sistema de Implante, Sao Paulo, SP, Brazil) were tightened on transfers of silicone mold. The resin was placed for 20 minutes in a pressure chamber coupled to an air injection tube and at a pressure of 60 kgf/cm2toprevent bubbles in the material. The photoelastic resin was slowly poured over the mold. The same procedure was performed with short and wide implant (EH 5 x 6 mm; SIN – Sistema de Implante, Sao Paulo, SP, Brazil) and conventional implants to obtain the second photoelastic model. After 72 hours, the transfers were removed from the silicone mold, and photoelastic models were obtained for evaluation. Thus, a photoelastic model for each clinical situation was obtained and identified as model C (conventional) that was two standard Branemark implants, and model S (short) that was a short and wide implant and a standard Branemark implant.
Marginal Misfit Evaluation
The procedures were performed at 120X magnification using a 1.0-μm precision microscope (VMM-100-BT – Walter UHL, Asslar, Germany) equipped with a digital camera (KC-512NT - Kodo BR Eletronics Ltd, Sao Paulo, Sao Paulo, Brazil) and analyzer unit (QC 220-HH Quadra-Check 200 - Metronics Inc., Bedford, Massachusetts, USA). The procedures also involved a calibrated examiner with an intraclass correlation coefficient of 0.995 (p<.0001), according to the single-screw test protocol,17,18 which shows the marginal misfit reading of the loop while the screw of the opposite pillar is tightened.
The pillars were designated as pillar M (molar) and pillar P (pre-molar). The frameworks were positioned on the model and tightened with 10 Ncm using a 0.1-Ncm precision digital torque meter (Torque Meter TQ-8800 – Lutron, Taipei, Taiwan). The readings for pillar M were performed on the buccal and lingual sides in diametrically opposite positions after the pillar P screw was tightened and vice versa. The measurements were performed on both pillars, and an average value of misfit was obtained for each framework.
Three of the ten frameworks were selected according to their average values of marginal misfit obtaining three groups: low (misfit average < 20 µm, model C = 18 µm, model S = 21 µm); medium (misfit average > 20 µm and < 40 µm, model C = 32 µm, model S = 33 µm); and high (misfit average > 40 µm, model C = 52 µm, model S = 55 µm). No difference in the marginal misfit between model C and S was found (t=-.351, df=18, p=.730; T-test).
Photoelastic Analysis
A horizontal transmission polariscope developed in the Mechanical Design Laboratory Henner Alberto Gomide, School of Mechanical Engineering of Federal University of Uberlandia consisted of two ¼-retardant wave filters and two polarizing filters, called polarizer and analyzer.
A standard position for the photoelastic models was obtained by markings on the polariscope platform. Each framework was tightened to the photoelastic model with 10-Ncm standardized torque, always following the tightening sequence P-M. A layer of mineral oil was applied on the photoelastic models to improve the view of the fringes. The photoelastic models were positioned on the polariscope, and the images were obtained using a digital camera (Canon SX50HS - Miyazaki Daishin Canon Inc., Miyazaki, Japan) after tightening of frameworks to the photoelastic models, and after applying a load of 280-N19 to pillar M.
The analysis of each image was performed with a graphic software (Adobe Photoshop CS5®; Adobe Systems, San Jose, CA) according to the visualization of isochromatic fringe order where fringe order of 0 = black; 1 = violet/blue transition; and 2, 3, 4 = red/green transition.20 All images were evaluated by the same operator. The analysis was separated according to the intensity (fringe order) and distribution of stress for different misfit levels and implant designs.
Between the analyses, the models were kept under 37 °C for 10 minutes until no stress was observed using the polariscope, avoiding the masking of readings during the experiments generated by residual stress.
Results
Table I and Figure 1 show the stress levels and distribution, respectively, in the models C and S for different misfits under loaded and non-loaded conditions. The stress intensity is presented according to the higher fringe order observed for each implant. After the tightening of frameworks, the stress levels and distribution of stress was similar between the models for all situations of misfit. There was an increase of stress from the apical region of implants of the M model that increased according to higher misfit values. However, for model S, as the implant M was short, the stresses were located close to its coronal region. Under loading conditions, compared to model C, model S showed similar or lower values of stress around the implants for all misfit levels. Model C showed a higher concentration of stress around implant M, which was distributed to implant P, whereas for model S, the stress was lower and located almost entirely around implant M.
Figure 1. Stress distribution in model C and S without load and with load for different
