- Email: [email protected]
Design aspects involved in a cemented THA stem failure case
Engineering Failure Analysis 16 (2009) 512–520
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Design aspects involved in a cemented THA stem failure case S. Griza *, G. Zanon, E.P. Silva, F. Bertoni, A. Reguly, T.R. Strohaecker Departamento de Metalurgia, Universidade Federal do Rio Grande do Sul, Porto Alegre 90035-190, Brazil
a r t i c l e
i n f o
Article history: Received 11 March 2008 Accepted 6 June 2008 Available online 21 June 2008 Keywords: Total hip arthroplasty Fatigue ASTM F 745 Design
a b s t r a c t Fatigue failure of total hip arthroplasty stem is today a rare case due to improvements of the technique along many decades. However, any case of stem fracture should be treated with attention due to the problems that can result in the revision for the patient. The most common cause of stem fracture is the proximal aseptic loosening. This scenario is reproduced in endurance test protocol ISO 7206-4 and should be applied for all design changes to prevent fracture of the stem even in the event of mechanical or biological problem that result in the aseptic loosening. This paper presents a cemented hip stem failure analysis. The study consisted of fracture analysis, material characterization, numerical simulation of the stem under the conditions of the endurance test, as well as fatigue tests of the ASTM F 745 stainless steel. The failure, in the mechanical aspect, is associated to the use of the cast stainless steel and could have been predicted through testing prior commercialization. Some design aspects are discussed. The use of cylindrical distal portion, matt surface and collar can reduce the stability in a loosened cemented hip stem. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The modern technique of total hip arthroplasty elaborated by Charnley [1] is one of the more successful practices in orthopedics. In the early years, the rate of revision was elevated due to mechanical and biological problems. Many of these problems induced the proximal aseptic loosening, a scenario in that the stem act as a ‘‘cantilever beam”, increasing its stress level thus promoting the stem fatigue fracture. The continuum evolution of the technique for many years produced sufficient knowledge to allow for a decrease in stem fatigue rate nowadays [2]. In particular, the introduction of the endurance fatigue standard ISO 7206-4 in the 80s, resulted in a significant reduction of the failure rates. From this test, the designer has conditions of evaluate the material behavior, the presence of stress raisers and surface transformations that can promote the failure of the stem submitted to the severe condition of the proximal aseptic loosening. Unfortunately, the negligence regarding the validation tests of the product has caused premature stem fracture. The need of hip revision it is dangerous for the patient because is a more complicated surgical intervention with an increased difficulty of rehabilitation. In articles recently published different stem failures manufactured in cast stainless steel were associated to some design deficiencies that increase stress risers [3,4]. This paper present a case of a cemented hip stem supplied with collar, cylindrical, not tapered, distal portion and matt surface, which presented premature fracture in the medial level. The study consisted of fracture analysis, material characterization, numerical simulation of the stem under the ISO 7206-4 test conditions, as well as fatigue tests of the stainless steel. The mechanical aspect of the premature failure is associated to poor stem design and the use of the cast stainless steel. This failure could have been predicted through test prior commercialization.
* Corresponding author. Tel.: +55 51 3308 4251, fax: +55 51 3308 3565. E-mail address: [email protected] (S. Griza). 1350-6307/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2008.06.016
S. Griza et al. / Engineering Failure Analysis 16 (2009) 512–520
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2. Materials and methods The female old patient (T.N. 75 kgf) had signs of pain when the fracture of the stem was detected after 5.5 years in use. According to radiographies taken prior revision (Fig. 1), cephalic component diameter is uneven to the acetabular socket thickness. It can be noted areas of osteolisys at proximal and medial regions, characterized by loses of trabecular bone. The cement thickness is very thin in several regions, including those coincident with osteolisys. Radiolucent line in the shoulder is an evidence of the proximal loosening of the stem. The two sections of the broken stem were measured in a three-dimensional measurement machine (Zeiss Vista, Carl Zeiss Industrielle Messtechnik GmbH 73446 Oberkochen). These drawing parts were then joined to produce the numerical simulation model (Fig. 2). Most of the stem, from collar until the distal tip, was shot penned to enhance the surface roughness. The roughness was measured by surface measuring instrument (Surfcom 130 Carl Zeiss Industrielle Messtechnik GmbH 73446 Oberkochen), found 5.22 Ra (1.11 SD). The main stem measurements are shown in Fig. 3. The most prominent geometrical characteristics are the medial stem section, the cylindrical distal shape having groves at anterior and posterior regions as well as a collar in the proximal body over the calcar. The failure mechanisms were identified in low magnification as well as by scanning electron microscopy (SEM Philips XL-20). A metallographic sample was taken from longitudinal plane of symmetry of the stem, at fracture level, and it was analyzed by optical microscopy. Electrolytic etching in a 10% oxalic acid solution was used to reveal the microstructural features of the sample. Chemical analysis was performed with an optical spectrometer (Spectro, Spectrolab). The aim of the computational simulation was to provide, through finite element method (FEM), the stress levels proposed for the ISO 7206-4 fatigue test of the stem. Basically, the standard determines that the main axis of the stem should be slopped 90 in the antero-posterior direction and 100 in the medio-lateral direction. Then, the stem should be inserted in a container and cemented using some material as PMMA up to 60% of the distance between cephalic center and stem tip. Finally, a cyclic sinusoidal vertical load varying from 230 to 2300 N is applied over the cephalic component. The container, the stem and the cement mantle were modeled by commercial software SolidWorks (Solidworks Corp., Boston, MA, USA) as shown in Fig. 4. The container was designed to provide at least 5 mm cement thickness. The net generation, boundary conditions as well as numerical analysis were done by commercial software Abaqus CAE 6.5 (Abaqus 6.5, Hibbit, Karlsson e Sorensen, Inc., Pawtucker, RI). Input data about material properties needed for simulation (Table 1) agree with linear elastic regime and were taken from the literature [5–7]. A shell of 28-mm curvature radius was used for the stress distribution at the surface of cephalic component and to simulate contact condition with the test machine. Tangential and normal contact conditions were selected, using a 0.25 friction coefficient. Other interactions, i.e. interfaces between stem and cement as well as cement and container were kept as in adhesion [8]. Constraining the lower surface of the container and the 2300 N vertical load applied in the cephalic component trough the shell were used as boundary conditions. Net characteristics used are shown in Table 1. Tetrahedral, linear interpolation elements were applied to the stem, the cement and the container. Rigid quadratic elements were applied
Fig. 1. Radiographies obtained prior revision. Acetabular socket is uneven to cephalic component. Cement mantle is very thin in several regions. Black arrows point to fracture at lateral (left) and frontal (right) view. It is possible to see radiolucent lines in several osteolisys sites as well as at stem shoulder (white narrows).
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Fig. 2. Solid produced from measured parts of the broken stem.
to the shell. Net refinement was done in the medial stem region (Fig. 4), reaching convergence in a limited computational time. Fatigue test samples were provided from ASTM F 745 cast stainless steel. Fatigue tests were carried out through a servo hydraulic material test system (MTS 810, MTS Corporation, Eden Prairie, MN) according to standard ASTM E 466 ‘‘Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials”. The applied load rate was 0.1 at 30 Hz. 3. Results 3.1. Fracture analysis Fracture presents a rough macroscopic aspect, commonly associated with metals with large grain size. Furthermore, it can be noted several stretched regions due to the contact between crests during the compressive loads. The failure nucleation may have occurred in several points of the surface as in medial or in lateral side (Fig. 5). At higher magnification it was possible to observe striation characteristics of fatigue propagation (Fig. 6). 3.2. Material characterization The microstructure of the material is in the ‘‘as cast’’ condition, with austenitic and delta ferrite gross columnar dendrites growing from the surface to the core (Fig. 7). The shot penning process do not provided significant amount of slip bands near the surface which indicates the treatment was not effective in inducing significant residual stresses (Fig. 8). Chemical com-
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Fig. 3. Main stem measurements.
Fig. 4. Model assembly and net refinement in the medial part of the stem.
Table 1 Constitutive properties and mesh characteristics of the simulated system individual components
Stem Cement Container
Elastic modulus (GPa)
Poisson
Nodes
Elements
Type of element
205 2.8 205
0.3 0.33 0.3
15,371 7236 29,668
13,511 13,796 26,072
11829 hexahedral and 1682 tetrahedral 3378 hexahedral and 10418 tetrahedral All hexahedral
516
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Fig. 5. Stem fracture surface observed under stereomicroscope. Stretched areas and gross fracture aspect can be observed.
Fig. 6. Fatigue striations observed at the stem fracture surface. SEM fractograph.
Fig. 7. Austenitic and delta ferrite gross columnar dendrites (left) growing from the surface to the core (right). The arrow points to fracture surface which grain boundary is seem as pathway for fracture propagation.
position of the material was confronted with the ASTM F 745 – ‘‘Standard Specification for 18 Chromium –12.5 Nickel–2.5 Molybdenum Stainless Steel for Cast and Solution-Annealed Surgical Implant Applications’’. Only Ni content is slightly above to specification (Table 2). 3.3. Numerical simulation The numerical simulation result indicated that the higher stress, which is detrimental for fatigue crack nucleation, is coincident with the fracture initiation region observed during the fracture analysis. Fig. 9 shows the lateral part of the stem sur-
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Fig. 8. Cross-section micrograph indicating no significant amount of slip bands near the shot penned surface of the stem.
Table 2 Fractured stem chemical analysis (weight%)
Sample ASTM F745-00
C
Mn
P
S
Si
Cr
Ni
Mo
0.053 0.06 máx.
0.71 2.0 máx.
0.018 0.045 máx.
0.005 0.03 máx.
0.51 1.0 máx.
17.31 17.0–19.0
14.94 11.0–14.0
2.57 2.0–3.0
Fig. 9. Maximum main stress and Von mises stress at the lateral part of the stem.
face, which experienced 197 MPa of maximum main stress component and 210 MPa of Von Mises stress. Fig. 10 shows the medial part, which experienced 235 MPa of Von Mises stress. 3.4. Fatigue tests The fatigue performance of the stainless steel in the cast condition (ASTM F 745) is presented in Fig. 11. The stress vs number of cycles to failure curves can be used to predict time to failure at any specific stress level. 4. Discussions The failure analysis methodology applied indicated fatigue of the stem, nucleated in several points at the lateral and medial regions of the stem. This aspect is common in stems subjected to flexural and torsional loads which undergo proximal aseptic loosening. Indeed, the frontal radiography has shown a radiolucent line at stem shoulder region, indicating the loos-
518
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Fig. 10. Von Mises stress 235 MPa in the medial part of the stem.
400
Stress (MPa)
y = 1282,6x-0,1255 R2 = 0,8591 300
200
100 10000
100000
1000000
10000000
Cycles(N) Fig. 11. Cast austenitic stainless steel ASTM F 745 fatigue curve.
ening. It can be noted sites of osteolisys at the proximal and medial region, characterized by loss of trabecular bone. Frequently osteolisys is attributed to organic reaction to polythene, cement or metal wear particles [9–13]. Although wear of the polythene could not be noted, the cephalic component diameter is uneven along the thickness of the acetabular socket, which can generate high amount of debris. Thin cement mantle layer in several regions and the matt surface of the stem, associated to the possibility of relative movement to the cement mantle are indicative of wear particles formation. The material of the stem, despite Ni content, is in the range of ASTM 745 standard. In the metallographic analysis it was found an ‘‘as cast” microstructure, with austenitic and delta ferrite aligned dendritic formation. This gross structure facilitates the nucleation and the aligned boundaries favor the fatigue propagation. The matt surface design concept was developed early in 70s and is applied to improve stem/cement stability. The numerical simulation model used did not consider the effect of the matt surface obtained by shot penning. This procedure could enhance the endurance limit of the stem due to induced compressive residual stresses. Moreover, in a recent paper, it was noted that the fatigue strength of type 304 stainless steel is increased only around 15% comparing emery paper polishing process and shot penning applied on surface of the test samples [14]. In another paper, it was noted that the surface roughness effect is decreased with increasing grain size because of the change in the crack initiation mode [15]. Then, considering the dendritic structure and low amount of slip bands near surface of the stem, it is possible, for this case, to neglect the bene-
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fic effect of shot penning to fatigue resistance improvement. According to the endurance fatigue standard ISO 7206 part 4, a hip stem should hold up 5 million cycles even after proximal loosening. In the numerical simulation it was observed a maximum main stress of 197 MPa and 210 Von Mises stress in the lateral region of the stem. In the medial region, 235 MPa Von Mises stress was noted. This value is near to the fatigue limit of the cast austenitic stainless steel found in the literature [16,17] and indicates that the fatigue of a stem submitted to the standard test conditions can have diverse nucleation points along the perimeter, including the medial region, in agreement with results from previous work [3]. Furthermore, according to the fatigue tests of ASTM F745 samples carried out in this study, the stress level would produce the stem failure around 0.75 millions cycles. Obviously, some time was consumed previously to the proximal loosening become significant allowing the 5.5 years prior to failure, since 1 million cycles correspond to the minimum number of cycles of an individual walking for one year [18–20]. Finally, some design aspects can be discussed in the light of the stem stability under ‘‘in vivo” conditions: the better surface condition for a cemented stem, the cylindrical, not tapered distal shape and the use of collar. Some authors indicate that the polished surface is more adequate because of the low amount of cement debris it produces. Others argue that the rough surface is more adequate to enhance adhesion with the cement. In our opinion, a matt surfaced stem is more dependent of the cementation quality, since roughness promotes more cement damage due to the relative movement between the stem and the cement [21]. Interfacial damage can increase cracking and reduce the time for crack propagation throughout cement mantle, becoming pathways for debris to be transported to the cement/bone interface [22]. Another aspect is the use of collar and the cylindrical distal shape of cemented stems. The collar was developed to reduce stem subsidence as well as increase compressive stress in the calcar, reducing the effect of stress shielding [23]. Moreover, considering that cement/bone interfacial gaps will arise in the proximal region by cement creep under load or by loss of stiffness of the bone tissue, the subsidence restraint make difficult for the stem to adjust to a more stable position. Thus promoting elevated stress level in the medial region of the stem as is proposed by test standard. 5. Conclusion The failure analysis indicated fatigue of the stem. The event takes place after proximal loosening. The collar, matt surface as well as cylindrical distal shape were restraints to subsidence after proximal loosening, enhancing the stress level in the medial section. The use of as cast stainless steel was detrimental to the mechanical facilitating the failure of the stem. Moreover, if this stem had been submitted to endurance tests before commercialization, this failure could be predicted. Acknowledgements The authors would like to acknowledge the financial support of CNPq and Dr. Gomes L.S.M., for the support in radiographic analysis. References [1] Charnley J. Arthroplasty of the hip, a new operation. Lancet 1961;1:1129. [2] Heck DA, Partridge CM, Reuben JD, Lanzer WL, Lewis CG, Keating EM. Prosthetic component failures in hip arthroplasty surgery. J Arthroplast 1995;10(5). [3] Griza S, Kwietniewski C, Tarnowski GA, Bertoni F, Reboh Y, Strohaecker TR, et al. Fatigue failure analysis of a specific total hip prosthesis stem design. Int J Fatigue 2007. doi:10.1016/j.ijfatigue.2007.11.005. [4] Griza S, Reis M, Reboh Y, Reguly A, Strohaecker TR. Failure analysis of uncemented total hip stem due to microstructure and neck stress riser. Eng Fail Anal 2008;15(7):981–8. [5] Estok DM, Harris WH. A stem design change to reduce peak cement strains at the tip of cemented total hip arthroplasty. J Arthroplast 2000;15(5):584–9. [6] Hedia HS, Barton DC, Fisher J, Elmidany TT. A method for shape optimization of a hip prosthesis to maximize the fatigue life of the cement. Med Eng Phys 1996;18:647–54. [7] Huiskes R, Boeklagen R. Mathematical shape optimization of hip prosthesis design. J Biomechan 1989;22(8/9):793–804. [8] Raimondi MT, Pietrabissa R. Modelling evaluation of the testing condition influence on the maximum stress induced in a hip prosthesis during ISO 7206 fatigue testing. Med Eng Phys 1999;21:353–9. [9] Jasty MJ, Floyd WE, Schiller AL, Goldring SR, Harris WH. Localized osteolysis in stable, non-septic total hip replacement. J Bone Joint Surg Am 1986;68:912–9. [10] Harris WH, Schiller AL, Scholler JM, Freiberg RA, Scott R. Extensive localized bone resorption in the femur following total hip replacement. J Bone Joint Surg Am 1976;58:612–8. [11] Anthony PP, Gie GA, Howie CR, Ling RSM. Localized endosteal bone lysis in relation to the femoral components of cemented total hip arthroplasties. J Bone Joint Surg [Br] 1990;72-B:971–9. [12] Tanzer M, Maloney WJ, Jasty M, Harris WH. The progression of femoral cortical osteolysis in association with total hip arthroplasty without cement. J Bone Joint Surg Am 1992;74:404–10. [13] Maloney WJ, Jasty M, Rosenberg A, Harris WH. Bone lysis in well-fixed cemented femoral components. J Bone Joint Surg [Br] 1990;72-B:966–70. [14] Hayashi M, Enomoto K. Effect of preliminary surface working on fatigue strength of type 304 stainless steel at ambient temperature and 288 °C in air and pure water environment. Int J Fatigue 2006;28:1626–32. [15] Lee JM, Nam SW. Effect of crack initiation mode on low cycle fatigue life of type 304 stainless steel with surface roughness. Mater Lett 1990;10(6):223–30. [16] Teoh SH. Fatigue of biomaterials: a review. Int J Fatigue 2000;22:825–37. [17] Niinomi M. Fatigue characteristics of metallic biomaterials. Int Journal of Fatigue 2007;29:992–1000. [18] Seedhom BB, Dowson D, Wright V. Wear of solid phase formed HDPE in relation to the life of artificial hips and knees. Wear 1973;24:35–51. [19] Clarke IC. Wear of artificial joint materials. I. Friction and wear studies: validity of wear-screening protocols. Eng Med 1981;10:115–22.
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[20] Stolk J, Verdonschot N, Murphy BP, Prendergast PJ, Huiskes R. Finite element simulation of anisotropic damage accumulation and creep in acrylic bone cement. Eng Fract Mech 2004;71:513–28. [21] Verdonschot N, Huiskes R. Surface roughness of debonded straight-tapered stems in cemented THA reduces subsidence but not cement damage. Biomaterials 1998;19:1773–9. [22] Mann KA, Gupta S, Race A, Miller MA, Cleary RJ, Ayers DC. Cement microcracks in thin-mantle regions after in vitro fatigue loading. J Arthroplasty 2004;19(5):605–12. [23] Oh I, Harris WH. Proximal strain distribution in the loaded femur. An in vitro comparison of the distributions in the intact femur and after insertion of different hip-replacement femoral components. J Bone Joint Surg Am 1978;60:75–85.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Design aspects involved in a cemented THA stem failure case S. Griza *, G. Zanon, E.P. Silva, F. Bertoni, A. Reguly, T.R. Strohaecker Departamento de Metalurgia, Universidade Federal do Rio Grande do Sul, Porto Alegre 90035-190, Brazil
a r t i c l e
i n f o
Article history: Received 11 March 2008 Accepted 6 June 2008 Available online 21 June 2008 Keywords: Total hip arthroplasty Fatigue ASTM F 745 Design
a b s t r a c t Fatigue failure of total hip arthroplasty stem is today a rare case due to improvements of the technique along many decades. However, any case of stem fracture should be treated with attention due to the problems that can result in the revision for the patient. The most common cause of stem fracture is the proximal aseptic loosening. This scenario is reproduced in endurance test protocol ISO 7206-4 and should be applied for all design changes to prevent fracture of the stem even in the event of mechanical or biological problem that result in the aseptic loosening. This paper presents a cemented hip stem failure analysis. The study consisted of fracture analysis, material characterization, numerical simulation of the stem under the conditions of the endurance test, as well as fatigue tests of the ASTM F 745 stainless steel. The failure, in the mechanical aspect, is associated to the use of the cast stainless steel and could have been predicted through testing prior commercialization. Some design aspects are discussed. The use of cylindrical distal portion, matt surface and collar can reduce the stability in a loosened cemented hip stem. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The modern technique of total hip arthroplasty elaborated by Charnley [1] is one of the more successful practices in orthopedics. In the early years, the rate of revision was elevated due to mechanical and biological problems. Many of these problems induced the proximal aseptic loosening, a scenario in that the stem act as a ‘‘cantilever beam”, increasing its stress level thus promoting the stem fatigue fracture. The continuum evolution of the technique for many years produced sufficient knowledge to allow for a decrease in stem fatigue rate nowadays [2]. In particular, the introduction of the endurance fatigue standard ISO 7206-4 in the 80s, resulted in a significant reduction of the failure rates. From this test, the designer has conditions of evaluate the material behavior, the presence of stress raisers and surface transformations that can promote the failure of the stem submitted to the severe condition of the proximal aseptic loosening. Unfortunately, the negligence regarding the validation tests of the product has caused premature stem fracture. The need of hip revision it is dangerous for the patient because is a more complicated surgical intervention with an increased difficulty of rehabilitation. In articles recently published different stem failures manufactured in cast stainless steel were associated to some design deficiencies that increase stress risers [3,4]. This paper present a case of a cemented hip stem supplied with collar, cylindrical, not tapered, distal portion and matt surface, which presented premature fracture in the medial level. The study consisted of fracture analysis, material characterization, numerical simulation of the stem under the ISO 7206-4 test conditions, as well as fatigue tests of the stainless steel. The mechanical aspect of the premature failure is associated to poor stem design and the use of the cast stainless steel. This failure could have been predicted through test prior commercialization.
* Corresponding author. Tel.: +55 51 3308 4251, fax: +55 51 3308 3565. E-mail address: [email protected] (S. Griza). 1350-6307/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2008.06.016
S. Griza et al. / Engineering Failure Analysis 16 (2009) 512–520
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2. Materials and methods The female old patient (T.N. 75 kgf) had signs of pain when the fracture of the stem was detected after 5.5 years in use. According to radiographies taken prior revision (Fig. 1), cephalic component diameter is uneven to the acetabular socket thickness. It can be noted areas of osteolisys at proximal and medial regions, characterized by loses of trabecular bone. The cement thickness is very thin in several regions, including those coincident with osteolisys. Radiolucent line in the shoulder is an evidence of the proximal loosening of the stem. The two sections of the broken stem were measured in a three-dimensional measurement machine (Zeiss Vista, Carl Zeiss Industrielle Messtechnik GmbH 73446 Oberkochen). These drawing parts were then joined to produce the numerical simulation model (Fig. 2). Most of the stem, from collar until the distal tip, was shot penned to enhance the surface roughness. The roughness was measured by surface measuring instrument (Surfcom 130 Carl Zeiss Industrielle Messtechnik GmbH 73446 Oberkochen), found 5.22 Ra (1.11 SD). The main stem measurements are shown in Fig. 3. The most prominent geometrical characteristics are the medial stem section, the cylindrical distal shape having groves at anterior and posterior regions as well as a collar in the proximal body over the calcar. The failure mechanisms were identified in low magnification as well as by scanning electron microscopy (SEM Philips XL-20). A metallographic sample was taken from longitudinal plane of symmetry of the stem, at fracture level, and it was analyzed by optical microscopy. Electrolytic etching in a 10% oxalic acid solution was used to reveal the microstructural features of the sample. Chemical analysis was performed with an optical spectrometer (Spectro, Spectrolab). The aim of the computational simulation was to provide, through finite element method (FEM), the stress levels proposed for the ISO 7206-4 fatigue test of the stem. Basically, the standard determines that the main axis of the stem should be slopped 90 in the antero-posterior direction and 100 in the medio-lateral direction. Then, the stem should be inserted in a container and cemented using some material as PMMA up to 60% of the distance between cephalic center and stem tip. Finally, a cyclic sinusoidal vertical load varying from 230 to 2300 N is applied over the cephalic component. The container, the stem and the cement mantle were modeled by commercial software SolidWorks (Solidworks Corp., Boston, MA, USA) as shown in Fig. 4. The container was designed to provide at least 5 mm cement thickness. The net generation, boundary conditions as well as numerical analysis were done by commercial software Abaqus CAE 6.5 (Abaqus 6.5, Hibbit, Karlsson e Sorensen, Inc., Pawtucker, RI). Input data about material properties needed for simulation (Table 1) agree with linear elastic regime and were taken from the literature [5–7]. A shell of 28-mm curvature radius was used for the stress distribution at the surface of cephalic component and to simulate contact condition with the test machine. Tangential and normal contact conditions were selected, using a 0.25 friction coefficient. Other interactions, i.e. interfaces between stem and cement as well as cement and container were kept as in adhesion [8]. Constraining the lower surface of the container and the 2300 N vertical load applied in the cephalic component trough the shell were used as boundary conditions. Net characteristics used are shown in Table 1. Tetrahedral, linear interpolation elements were applied to the stem, the cement and the container. Rigid quadratic elements were applied
Fig. 1. Radiographies obtained prior revision. Acetabular socket is uneven to cephalic component. Cement mantle is very thin in several regions. Black arrows point to fracture at lateral (left) and frontal (right) view. It is possible to see radiolucent lines in several osteolisys sites as well as at stem shoulder (white narrows).
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Fig. 2. Solid produced from measured parts of the broken stem.
to the shell. Net refinement was done in the medial stem region (Fig. 4), reaching convergence in a limited computational time. Fatigue test samples were provided from ASTM F 745 cast stainless steel. Fatigue tests were carried out through a servo hydraulic material test system (MTS 810, MTS Corporation, Eden Prairie, MN) according to standard ASTM E 466 ‘‘Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials”. The applied load rate was 0.1 at 30 Hz. 3. Results 3.1. Fracture analysis Fracture presents a rough macroscopic aspect, commonly associated with metals with large grain size. Furthermore, it can be noted several stretched regions due to the contact between crests during the compressive loads. The failure nucleation may have occurred in several points of the surface as in medial or in lateral side (Fig. 5). At higher magnification it was possible to observe striation characteristics of fatigue propagation (Fig. 6). 3.2. Material characterization The microstructure of the material is in the ‘‘as cast’’ condition, with austenitic and delta ferrite gross columnar dendrites growing from the surface to the core (Fig. 7). The shot penning process do not provided significant amount of slip bands near the surface which indicates the treatment was not effective in inducing significant residual stresses (Fig. 8). Chemical com-
515
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Fig. 3. Main stem measurements.
Fig. 4. Model assembly and net refinement in the medial part of the stem.
Table 1 Constitutive properties and mesh characteristics of the simulated system individual components
Stem Cement Container
Elastic modulus (GPa)
Poisson
Nodes
Elements
Type of element
205 2.8 205
0.3 0.33 0.3
15,371 7236 29,668
13,511 13,796 26,072
11829 hexahedral and 1682 tetrahedral 3378 hexahedral and 10418 tetrahedral All hexahedral
516
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Fig. 5. Stem fracture surface observed under stereomicroscope. Stretched areas and gross fracture aspect can be observed.
Fig. 6. Fatigue striations observed at the stem fracture surface. SEM fractograph.
Fig. 7. Austenitic and delta ferrite gross columnar dendrites (left) growing from the surface to the core (right). The arrow points to fracture surface which grain boundary is seem as pathway for fracture propagation.
position of the material was confronted with the ASTM F 745 – ‘‘Standard Specification for 18 Chromium –12.5 Nickel–2.5 Molybdenum Stainless Steel for Cast and Solution-Annealed Surgical Implant Applications’’. Only Ni content is slightly above to specification (Table 2). 3.3. Numerical simulation The numerical simulation result indicated that the higher stress, which is detrimental for fatigue crack nucleation, is coincident with the fracture initiation region observed during the fracture analysis. Fig. 9 shows the lateral part of the stem sur-
517
S. Griza et al. / Engineering Failure Analysis 16 (2009) 512–520
Fig. 8. Cross-section micrograph indicating no significant amount of slip bands near the shot penned surface of the stem.
Table 2 Fractured stem chemical analysis (weight%)
Sample ASTM F745-00
C
Mn
P
S
Si
Cr
Ni
Mo
0.053 0.06 máx.
0.71 2.0 máx.
0.018 0.045 máx.
0.005 0.03 máx.
0.51 1.0 máx.
17.31 17.0–19.0
14.94 11.0–14.0
2.57 2.0–3.0
Fig. 9. Maximum main stress and Von mises stress at the lateral part of the stem.
face, which experienced 197 MPa of maximum main stress component and 210 MPa of Von Mises stress. Fig. 10 shows the medial part, which experienced 235 MPa of Von Mises stress. 3.4. Fatigue tests The fatigue performance of the stainless steel in the cast condition (ASTM F 745) is presented in Fig. 11. The stress vs number of cycles to failure curves can be used to predict time to failure at any specific stress level. 4. Discussions The failure analysis methodology applied indicated fatigue of the stem, nucleated in several points at the lateral and medial regions of the stem. This aspect is common in stems subjected to flexural and torsional loads which undergo proximal aseptic loosening. Indeed, the frontal radiography has shown a radiolucent line at stem shoulder region, indicating the loos-
518
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Fig. 10. Von Mises stress 235 MPa in the medial part of the stem.
400
Stress (MPa)
y = 1282,6x-0,1255 R2 = 0,8591 300
200
100 10000
100000
1000000
10000000
Cycles(N) Fig. 11. Cast austenitic stainless steel ASTM F 745 fatigue curve.
ening. It can be noted sites of osteolisys at the proximal and medial region, characterized by loss of trabecular bone. Frequently osteolisys is attributed to organic reaction to polythene, cement or metal wear particles [9–13]. Although wear of the polythene could not be noted, the cephalic component diameter is uneven along the thickness of the acetabular socket, which can generate high amount of debris. Thin cement mantle layer in several regions and the matt surface of the stem, associated to the possibility of relative movement to the cement mantle are indicative of wear particles formation. The material of the stem, despite Ni content, is in the range of ASTM 745 standard. In the metallographic analysis it was found an ‘‘as cast” microstructure, with austenitic and delta ferrite aligned dendritic formation. This gross structure facilitates the nucleation and the aligned boundaries favor the fatigue propagation. The matt surface design concept was developed early in 70s and is applied to improve stem/cement stability. The numerical simulation model used did not consider the effect of the matt surface obtained by shot penning. This procedure could enhance the endurance limit of the stem due to induced compressive residual stresses. Moreover, in a recent paper, it was noted that the fatigue strength of type 304 stainless steel is increased only around 15% comparing emery paper polishing process and shot penning applied on surface of the test samples [14]. In another paper, it was noted that the surface roughness effect is decreased with increasing grain size because of the change in the crack initiation mode [15]. Then, considering the dendritic structure and low amount of slip bands near surface of the stem, it is possible, for this case, to neglect the bene-
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fic effect of shot penning to fatigue resistance improvement. According to the endurance fatigue standard ISO 7206 part 4, a hip stem should hold up 5 million cycles even after proximal loosening. In the numerical simulation it was observed a maximum main stress of 197 MPa and 210 Von Mises stress in the lateral region of the stem. In the medial region, 235 MPa Von Mises stress was noted. This value is near to the fatigue limit of the cast austenitic stainless steel found in the literature [16,17] and indicates that the fatigue of a stem submitted to the standard test conditions can have diverse nucleation points along the perimeter, including the medial region, in agreement with results from previous work [3]. Furthermore, according to the fatigue tests of ASTM F745 samples carried out in this study, the stress level would produce the stem failure around 0.75 millions cycles. Obviously, some time was consumed previously to the proximal loosening become significant allowing the 5.5 years prior to failure, since 1 million cycles correspond to the minimum number of cycles of an individual walking for one year [18–20]. Finally, some design aspects can be discussed in the light of the stem stability under ‘‘in vivo” conditions: the better surface condition for a cemented stem, the cylindrical, not tapered distal shape and the use of collar. Some authors indicate that the polished surface is more adequate because of the low amount of cement debris it produces. Others argue that the rough surface is more adequate to enhance adhesion with the cement. In our opinion, a matt surfaced stem is more dependent of the cementation quality, since roughness promotes more cement damage due to the relative movement between the stem and the cement [21]. Interfacial damage can increase cracking and reduce the time for crack propagation throughout cement mantle, becoming pathways for debris to be transported to the cement/bone interface [22]. Another aspect is the use of collar and the cylindrical distal shape of cemented stems. The collar was developed to reduce stem subsidence as well as increase compressive stress in the calcar, reducing the effect of stress shielding [23]. Moreover, considering that cement/bone interfacial gaps will arise in the proximal region by cement creep under load or by loss of stiffness of the bone tissue, the subsidence restraint make difficult for the stem to adjust to a more stable position. Thus promoting elevated stress level in the medial region of the stem as is proposed by test standard. 5. Conclusion The failure analysis indicated fatigue of the stem. The event takes place after proximal loosening. The collar, matt surface as well as cylindrical distal shape were restraints to subsidence after proximal loosening, enhancing the stress level in the medial section. The use of as cast stainless steel was detrimental to the mechanical facilitating the failure of the stem. Moreover, if this stem had been submitted to endurance tests before commercialization, this failure could be predicted. Acknowledgements The authors would like to acknowledge the financial support of CNPq and Dr. Gomes L.S.M., for the support in radiographic analysis. References [1] Charnley J. Arthroplasty of the hip, a new operation. Lancet 1961;1:1129. [2] Heck DA, Partridge CM, Reuben JD, Lanzer WL, Lewis CG, Keating EM. Prosthetic component failures in hip arthroplasty surgery. J Arthroplast 1995;10(5). [3] Griza S, Kwietniewski C, Tarnowski GA, Bertoni F, Reboh Y, Strohaecker TR, et al. Fatigue failure analysis of a specific total hip prosthesis stem design. Int J Fatigue 2007. doi:10.1016/j.ijfatigue.2007.11.005. [4] Griza S, Reis M, Reboh Y, Reguly A, Strohaecker TR. Failure analysis of uncemented total hip stem due to microstructure and neck stress riser. Eng Fail Anal 2008;15(7):981–8. [5] Estok DM, Harris WH. A stem design change to reduce peak cement strains at the tip of cemented total hip arthroplasty. J Arthroplast 2000;15(5):584–9. [6] Hedia HS, Barton DC, Fisher J, Elmidany TT. A method for shape optimization of a hip prosthesis to maximize the fatigue life of the cement. Med Eng Phys 1996;18:647–54. [7] Huiskes R, Boeklagen R. Mathematical shape optimization of hip prosthesis design. J Biomechan 1989;22(8/9):793–804. [8] Raimondi MT, Pietrabissa R. Modelling evaluation of the testing condition influence on the maximum stress induced in a hip prosthesis during ISO 7206 fatigue testing. Med Eng Phys 1999;21:353–9. [9] Jasty MJ, Floyd WE, Schiller AL, Goldring SR, Harris WH. Localized osteolysis in stable, non-septic total hip replacement. J Bone Joint Surg Am 1986;68:912–9. [10] Harris WH, Schiller AL, Scholler JM, Freiberg RA, Scott R. Extensive localized bone resorption in the femur following total hip replacement. J Bone Joint Surg Am 1976;58:612–8. [11] Anthony PP, Gie GA, Howie CR, Ling RSM. Localized endosteal bone lysis in relation to the femoral components of cemented total hip arthroplasties. J Bone Joint Surg [Br] 1990;72-B:971–9. [12] Tanzer M, Maloney WJ, Jasty M, Harris WH. The progression of femoral cortical osteolysis in association with total hip arthroplasty without cement. J Bone Joint Surg Am 1992;74:404–10. [13] Maloney WJ, Jasty M, Rosenberg A, Harris WH. Bone lysis in well-fixed cemented femoral components. J Bone Joint Surg [Br] 1990;72-B:966–70. [14] Hayashi M, Enomoto K. Effect of preliminary surface working on fatigue strength of type 304 stainless steel at ambient temperature and 288 °C in air and pure water environment. 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