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Avaliação de Falhas em Implantes Metálicos Coxo-Femoral e Joelho Retirados de Pacientes

05/02/2007

 

K.B. Fonseca, H.H. Pereira, S.N. Silva

UnilesteMG – Centro Universitário do Leste de Minas Gerais

Av. Presidente Tancredo Neves, 3500 – Bairro Universitário, 35170-056, Coronel Fabriciano, MG, Brasil

e-mail: keilabf@yahoo.com.br, hudsonhenrique@hotmail.com, sidsilva@unilestemg.br

Resumo

Nas últimas décadas, implantes metálicos vêm sendo estudados de maneira sistemática e multidisciplinarmente, visto que sua aplicação tem sido crescente. Estes materiais denominados biomateriais são comumente utilizados como implantes dentários, próteses femurais, próteses cardíacas, etc. numa tentativa de restaurar parte de tecidos danificados por algum tipo de trauma ou doença. Os biomateriais de uma forma geral se caracterizam pela sua biocompatibilidade, ou seja, não causam efeito nocivo ao organismo; devem possuir resistência mecânica adequada além de uma resistência à corrosão, pois o meio fisiológico no qual estarão contidos é sempre extremamente agressivo, podendo induzir algum tipo de degradação. No caso de biomateriais estruturais os mais utilizados na fabricação de próteses atualmente são: titânio (e suas ligas), o Vitalium e o aço Inoxidável 316L, este último, sendo mais empregado aqui no Brasil devido, principalmente, ao seu baixo custo em relação ao titânio. Porém, um número relativamente elevado dessas próteses metálicas utilizadas em pacientes brasileiros, tem apresentado falhas, defeitos superficiais, materiais fora das especificações (normas técnicas e boas práticas de fabricação dentre outros), contribuindo para uma diminuição da vida útil das mesmas, acarretando transtornos anatômicos aos pacientes além de custo adicional ao SUS que em geral paga por estas recolocações. Este trabalho teve como objetivo o levantamento do histórico dos pacientes, as causas clínicas destes defeitos, e por fim, a associação destas informações com os ensaios físicos e químicos das próteses (coxo-femurais) extraídas de pacientes dos serviços de saúde conveniados ao SUS (Ipatinga e Belo Horizonte). Foram realizados ensaios de dureza, microscopia eletrônica de varredura (MEV) e metalografia. Os testes preliminares nos mostram que uma parcela destas próteses analisadas apresentaram inadequação para uso como biomateriais, em função das limitações nas propriedades mecânica e/ou tribológicas do metal e ainda falhas graves no cimento (PMMA) utilizado para sua fixação.

Palavras chaves:    próteses, metais, caracterização.

Evaluation of Failed Metallic Knee and Femoral Implants

abstract

There are over 150,000 such hip replacement operations performed in the Brazil each year. Many people suffer from knee or hip joint pain, caused by the wear and tear produced by walking millions of steps a year. After many years this can lead to osteoarthritis which is the commonest form of degenerative disease of the hip. Accidents may also lead to irreparable damage of the hip. The only solution normally is a total hip replacement operation, the procedure involves sectioning the femoral neck, reaming a cavity into the femoral shaft and cementing or press-fitting a femoral stem component into the bone. The acetabulum is ground away. A plastic cup is then cemented or pressed into the machined hole. The acetabular cup has a metal back enabling the cup to be screwed into the pelvic bone. Although the operation is highly successful in approximately 90% of cases after 8 years, the number of failures is still unacceptably high. In the X-ray on the right we can see a typical form of failure. The implant has moved in the femoral canal. This is as a result of bone loss around the metal stem. There are three main factors which will influence the performance of biomaterials in the human body: biocompatability, mechanical properties and degradation. At present we do not have any materials that can mimic perfectly the mechanical property of bone. Metals (steel, Co-Cr-Mo, Ti-6Al-4V) have sufficient strength and fracture toughness but have relatively high stiffness (Young’s modulus), which can lead to weight shielding problems. Polymers (polymethylmethacylate-PMMA, polyethlene-PE, etc.) have low stiffness values, reasonable fracture toughness but poor strength. Stainless steel is the most commonly used metal for femoral stems in hip replacements. It is an alloy of iron, chromium, nickel and molybdenum. It has extremely high resistance to corrosion, and thus does not degrade in the body. It can be shaped easily which is an important consideration for implant manufacturers who want to minimise production costs. However, problems may arise because of its relatively high stiffness, and the fact that some people may develop an allergic reaction to the nickel content. The metal stem is very stiff compared to bone, and as a consequence carries most of the body weight. This causes bone to think it is not required, resulting in its loss. Bone may also disappear in response to the wear debris produced from the articulation of the femoral head against the acetabular cup. The particles are pumped into the interface between the implant and the bone. Here you can see a worn acetabular cup. Millions of wear particles have been released into the body due to the abrasive action of the femoral head. The head may appear shiny, but if you looked at it under a microscope the surface would appear like a mountain range with lots of small peaks or asperities. These small peaks scratch away the softer plastic cup, releasing debris which causes cell response and eventually bone loss. The metal backing has also come away from the plastic cup. Another, less common, source of failure is the fracture of the metallic femoral stem. This may occur as a result of the implant moving within the bone. You can see scratches on the surface of the implant, these lead to uneven loading and eventually stem fracture. However, this form of failure is less common as a result of using improved metal alloys. An alternative to stainless steel is cobalt chromium alloy (27-30% Cr, 5-7% Mo. rest Co). It has good wear properties and is more resistant to scratching. The fact that it contains no nickel means that it can be used in patients who have nickel sensitivity. The top section of the prosthesis is roughened to increase friction and hence stability. The bottom surface has been polished to prevent the stem from rubbing against the inside of the bone canal, which may lead to wear debris. Developed for the aerospace industry, titanium and its alloys have high strength in relation to their relatively low weight. A titanium implant has a stiffness of less than half that of stainless steel or cobalt chrome, which therefore reduces the effects of weight shielding. Its constituents give it excellent corrosion resistance, but it does suffer from a relatively low fracture toughness and poor wear properties. There are two main uses for them in total hip replacement. The first is as a grouting material in the form of poly(methylmethacylate) (PMMA) bone cement. PMMA bone cement polymerises in situ. Here you can see a surgeon injecting the doughy material into the femoral canal. It is mixed in surgery from a polymer powder and liquid monomer, and forms a hardened material in 10-15 minutes. The main problem with PMMA bone cement is that considerable heat is released to the surrounding bone during the curing process and this causes cell death. The resulting material has poor resistance to fracture. Other problems also include the shrinkage of the cement and the release of toxic monomer into the blood stream. The other major polymer used is polyethylene. Its main advantage is its wear resistance when used as a concave acetabular cup in a total hip replacement. Material scientists are constantly faced with the challenge of producing optimum material properties at minimum cost. The total hip replacement is just one of many examples that illustrates how materials can improve the quality of people's lives. Despite the great advances that have been made, there are still a number of problems that need to be solved if hip replacements are to be 100% successful and last the remaining lifetime of patients. Ultimately we would like to produce a material with identical properties to bone. Brazilian research groups and companies are leaders in the research and development of biomaterials metallic. They not only can contribute to improving the quality life of people but also the economic prosperity of the country. The leading centre for biomedical materials science research in the Brazil is the Interdisciplinary Research Center based at Federal University of Minas Gerais. Why not find much more about our undergraduate courses in http://www.biomaterials.com.br/courses

Keywords:    Implants, metallic, evaluation.

http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10685/


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