Femoral stem fractures in total hip arthroplasty (THA) are a problem in clinical practice that results in great morbidity and high cost of revision hip surgery. Stem fractures are multifactorial events that are usually related to a combination of factors that increase the mechanical stress on the stem or decrease the mechanical strength of the implant. Failure analyses of hip prosthesis have identified that the presence of inadequate grain size may lead to implant failure. The aim of this article is to develop a rational to set specific sites to perform grain size measurements along stems used in THA as well as appropriate procedures to evaluate the heterogeneity of the microstructure related to the grain size distribution. In the present study, nonmodular femoral stems from three manufacturers with different wrought materials were chosen: stainless steel ISO 5832-1 (Manufacturer I), high nitrogen stainless steel ISO 5832-9 (Manufacturer II), and cobalt-chromium-molybdenum alloy ISO 5832-12 (Manufacturer III). The results of this study showed a great variability of grain size number depending on the cross section and fields evaluated. Therefore, the current technical standards for evaluating THA stems need to be modified. Analyses of grain sizes at different cross sections and inside each cross section of the stem is necessary to ensure the safety of hip stems.
Prior to clinical use, the corrosion resistance of new prosthesis system must be verified. The fretting‐corrosion mechanisms of total hip arthroplasty (THA) implants generate metal debris and ions that can increase the incidence of adverse tissue reactions. For cemented stems, there are at least two interfaces that can be damaged by fretting‐corrosion: stem‐head and stem‐cement. This investigation aimed to evaluate, through in vitro and in silico analyses, fretting‐corrosion at the stem‐head and stem‐cement interfaces, to determine which surface is most affected in pre‐clinical testing and identify the causes associated with the observed behavior. Unimodular stems and femoral heads of three different groups were evaluated, defined according to the head/stem material as group I (SS/SS), group II (CoCr/SS), and group III (CoCr/CoCr). Seven pairs of stems and heads per group were tested: three pairs were subjected to material characterization, three pairs to in vitro fretting‐corrosion testing, and one pair to geometric modeling in the in silico analysis. The absolute area of the stem body degraded was more than three times higher compared with the trunnion, for all groups. These results were corroborated by the in silico analysis results, which revealed that the average micromotion at the stem‐cement interface (9.65–15.66 μm) was higher than that at the stem‐head interface (0.55–1.08 μm). In conclusion, the degradation of the stem‐cement interface is predominant in the pre‐clinical set, indicating the need to consider the fretting‐corrosion at the stem‐cement interface during pre‐clinical implant evaluations.
ResumoO objetivo do presente trabalho foi avaliar e comparar a composição química e microestrutura de dois fabricantes (nacional e importado) de haste femoral fabricados em aço inoxidável austenítico ABNT NBR ISO 5832-9. As análises químicas dos principais elementos foram realizadas utilizando espectrometria de emissão ótica por descarga luminescente. A microestrutura das hastes foi caracterizada por microscopia óptica, microscopia eletrônica de varredura e difração de raios X. Medidas de dureza Vickers também foram realizadas neste estudo comparativo. As hastes nacionais e importadas exibiram pequena quantidade de partículas de nitreto (fase Z) dispersas na matriz austenítica e tamanhos de grão similares da austenita. Por outro lado, o aumento de 32% observado nos resultados de dureza da haste importada em relação à haste nacional foi atribuído às diferenças de composição química entre estas hastes. Foram identificadas microinclusões ricas em Al nas hastes importadas e microinclusões ricas em Mg nas hastes nacionais. Embora tenham sido observadas algumas diferenças entre as hastes nacionais e importadas, ambas atenderam os critérios de composição e microestrutura da norma ABNT NBR ISO 5832-9. Palavras-chave: Implante de quadril; Haste femoral; Aço inoxidável; Microestrutura.
CHEMICAL AND MICROSTRUCTURAL CHARACTERIZATION OF MODULAR FEMORAL STEMS MADE OF STAINLESS STEEL ISO 5832-9. AbstractThe aim of this study was to evaluate and compare the chemical composition and microstructure of two models of femoral stem (national and imported) manufactured from ISO 5832-9 austenitic stainless steel. Chemical analyses of the key elements were performed by glow discharge optical emission spectrometry. The microstructure of the stems was characterized by optical microscopy, scanning electron microscopy and X-ray diffraction. Vickers hardness measurements were also performed in this comparative study. The national and imported stems exhibited small amount of nitride particles (Z-phase) dispersed in the austenitic matrix and similar grain sizes of austenite. On the other hand, the hardness was 32% higher in the imported stem, which was attributed to the differences in composition between the stems. Al-rich and Mg-rich inclusions were identified in the imported and national stems, respectively. Although some differences have been observed between national and imported stems, both met the criteria of composition and microstructure of the ISO 5832-9 standard.
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