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Case study and analysis of a fatigue failure in a THA stem
Engineering Failure Analysis 28 (2013) 166–175
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Case study and analysis of a fatigue failure in a THA stem Sandro Griza a,⇑, Silvando Vieira dos Santos a, Marcelo Massayoshi Ueki a, Fabiano Bertoni b, Telmo Roberto Strohaecker b a b
Programa de Pós-Graduação em Ciência e Engenharia dos Materiais, Universidade Federal de Sergipe, Brazil Programa de Pós-Graduação em Engenharia de Minas, Metalúrgica e de Materiais, Universidade Federal do Rio Grande do Sul, Brazil
a r t i c l e
i n f o
Article history: Received 3 April 2012 Accepted 18 October 2012 Available online 2 November 2012 Keywords: Hip stem Fatigue ASTM F 745 ASTM F 138
a b s t r a c t This paper presents a case of a collared, polished and cemented hip stem which presented premature fracture due to ‘‘cantilever beam’’ effect. The study consisted of radiographic analysis, fracture analysis, material characterization and S–N fatigue tests. Numerical simulation of the stem was also carried out in two ways: under the ISO 7206-4 standard test and under a well stabilized arthroplasty hypothesis. The failure was predicted through both numerical simulations. The mechanical aspect of the premature failure is associated to the use of the cast stainless steel. This article aims at reinforcing the idea that the use of the ‘‘as cast’’ austenitic stainless steel ASTM F 745 is harmful for implants that have potential for fatigue failure. The article also discusses the application of collars in polished and cemented stems whose stability is based on the distal migration. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The modern technique of total hip arthroplasty proposed by Charnley is one of the most successful practices in orthopedics [1]. It is a widely used strategy to restore the normal function of the hip joint disrupted by fracture or disease. This approach has been extremely successful and its usage has grown to several hundred thousand primary joint replacements per year worldwide. In early years, the rate of revision was higher due to mechanical and biological concerns. Many of these 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 continuous evolution of the technique for many years produced sufficient knowledge to allow for a decrease in stem fatigue rate nowadays. In particular, the introduction of the endurance fatigue standard ISO 7206-4 in the 1980s, resulted in a significant reduction of the failure rates. From this test, the designer can evaluate the material behavior besides other factors that can enhance the failure risk of the stem subjected to the severe condition of the proximal aseptic loosening. In the design process to find the ideal hip prosthesis geometry, pre-clinical tests must be carried out to verify the mechanical endurance to physiological loads. An elegant alternative approach would be the use of the virtual finite element method (FEM) to have a first glimpse of the expected mechanical behavior of the candidate design. It does not mean that the physical pre-clinical tests can be excluded. It is just a rapid way to discard potentially low performance designs. Unfortunately, some premature stem fractures are still occurring nowadays. The need of hip revision is dangerous for the patient since it is a more complicated surgical intervention with an increased difficulty of rehabilitation. In recent papers, different stem failures were presented [2–5]. Now, this paper shows a case of a collared, polished and cemented hip stem which presented premature fracture in the half height level due to the use of as cast stainless steel and favored by the proximal aseptic loosening.
⇑ Corresponding author. Tel.: +55 79 21056888; fax: +55 79 21056845. E-mail address: [email protected] (S. Griza). 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2012.10.011
S. Griza et al. / Engineering Failure Analysis 28 (2013) 166–175
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2. Materials and methods 2.1. Preliminary analysis A fractured total hip arthroplasty stem was revised from a female patient (67 years old, 64 kgf), after only 2.5 years in use. The radiographic analysis demonstrated stem fracture. Radiographies obtained previously, just after primary arthoplasty (post operative radiography) and that obtained after fracture were compared (Fig. 1). It is possible to clearly see a fracture of the femoral component at the half height level between the prosthesis upper body and the stem tip. After revision, the two sections of the fractured stem were cleaned in auto-clave to be subjected to analysis. The stem is polished, collared, and has a rectangular section with rounded corners from distal up to proximal aspect. The stem head has 28 mm diameter. The two parts of the stem were measured in a three-dimensional coordinate measurement machine (Zeiss Vista, Carl Zeiss Industrielle Messtechnik GmbH 73446 Oberkochen). These drawing parts were then joined to produce a solid for the numerical simulation model (Fig. 2). 2.2. Fracture analysis The fracture surfaces presented scratches and dents commonly found in scenarios of contact under compression. The best preserved fracture surface was chosen for analysis and the failure mechanisms were identified in low magnification stereo microscope (Carl Zeiss Stemi 2000). The fracture surface was also analyzed by scanning electron microscopy (SEM Philips XL20) to detect the failure micromechanisms. 2.3. Material characterization A metallographic sample was taken from the fracture initiation point along the longitudinal plane of symmetry of the stem and it was analyzed by optical microscopy (Carl Zeiss Axioscope A1). Electrolytic etching of 10% oxalic acid diluted
Fig. 1. Frontal radiographies taken immediately after arthroplasty (left) and before revision (right). It can be seen the implantation of the stem slightly in valgus (dashed lines) and radioluscent line at the proximal aspect. White arrow at right points to the initiation site of the fracture, at the lateral aspect of the stem. Black lines denote proximal support thickness reduction from 16 to 13 mm.
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Fig. 2. General view of the collared and polished stem.
Fig. 3. Fatigue test sample used to achieves wrought stainless steel performance.
in water was used to reveal the microstructural features of the sample. Chemical analysis was performed with an optical spectrometer (Spectro, Spectrolab).
2.4. Fatigue tests Test samples were machined from ASTM F 138 wrought stainless steel commercial rods (Fig. 3). All samples were polished to an average roughness of 1 lm. Tension fatigue tests were carried out through a servo hydraulic material test system (MTS 810, MTS Corporation, Eden Prairie, MN) following the ASTM E 466 standard. An applied load rate R = 0.1 at 30 Hz was used during test. The results were compared with the fatigue performance of the ASTM F 745 published early [3].
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Fig. 4. General view of the container and the stem mesh used to model assembly and boundary conditions for ISO 7206-4. Frontal view (left), lateral view (right). Stem is enveloped for a cement mantle inside a container. Load is applied at the shell and the base of container is constrained.
Table 1 Constitutive properties and mesh characteristics of the simulated systems individual components. Part
Elastic modulus (GPa)
Poisson
Shell
–
–
Stem
205
0.3
Container
205
0.3
Cement (first model)
2.8
0.33
Cement (second model) Cancellous bone Cortical bone
2.8 0.33 15.5
Number of elements
Number of nodes
Element type
2092
2153
35,210
42,241
1824
2484
42,781
44,027
5741 linear tetrahedral 37,040 linear hexahedral
41,745
14,906
38,357 linear tetrahedral 3388 linear hexahedral
43,694
10,103
43,133 linear tetrahedral 561 linear hexahedral
13,791
5248
12,399 linear tetrahedral 1392 linear hexahedral
0.33 0.32 0.3
Rigid quadrilateral shell 14,498 linear tetrahedral 27,743 linear hexahedral linear hexahedral
2.5. Numerical simulation 2.5.1. ISO 7206 simulation The aim of the first computational simulation was to evaluate, through the finite element method (FEM), the stem stress levels when a vertical load is applied over the stem head. The protocol simulating the ISO 7206-4 endurance fatigue test was the same as in an earlier study [3]. The resulting stress level can be compared to the fatigue performance of the material. The standard determines that the main axis of the stem should be sloped respectively 9° and 10°, in the antero-posterior and in the medio-lateral direction. Then, the stem should be inserted in a container and cemented using some polymer such as PMMA up to 60% of the distance between head center and stem tip (Fig. 4). The container was designed to provide at least 5 mm cement thickness. The material properties needed for simulation agree with linear elastic regime and are shown in Table 1. A shell of 28 mm curvature radius was used for the stress distribution at the surface of the head 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. Constraining the lower surface of the container and the 2300 N vertical load applied in the head through the shell were used as boundary conditions. Net characteristics used also are shown in Table 1. Tetrahedral and hexahedral, linear interpolation elements were applied to the stem, the cement and the container. Rigid quadratic elements were applied to the shell. As it can be seen in Fig. 4, net refinement was done in the half height stem region, reaching convergence in a reasonable computational time.
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Fig. 5. General view of the well stabilized arthroplasty model. Stem was implanted with 2 mm cement mantle.
Fig. 6. General view of the femur mesh used into the arthroplasty model.
2.5.2. Stable arthroplasty simulation A second simulation was performed to verify the stem stress level in the case of a well stabilized primary arthroplasty. The stem was cemented in a composite femur model 3306 (Pacific Research Labs., Vashon Island, WA, USA). The cement mantle and cortical bone were both modeled with 5 and 2 mm constant thickness respectively (Fig. 5). The femur was sectioned and restricted in the plane of its half height. The stem slope, interface conditions and external load were evaluated in the same manner as in the previous model. Tetrahedral, hexahedral and linear interpolation elements were applied to the new cement mantle and the femur (Fig. 6). Both cortical and cancellous bones were assumed as isotropic and homogeneous and their constitutive properties are shown in Table 1.
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Fig. 7. Stem fracture surface observed under low magnification. Gross fracture aspect and beach marks can be observed. Nucleation (at left) and final fracture (right).
Fig. 8. Fatigue striations observed at the stem fracture surface. SEM fractography.
3. Results 3.1. Preliminary analysis The postoperative radiography shows the implantation of the stem slightly in valgus. Radiolucency at the proximal medial aspect denotes the stem was supported proximally by a thick cement mantle and cancellous bone wall. The cement mantle thickness decreases as it follows the distal portion where the stem tip reaches direct contact with the cortical bone. However, as the radiography obtained after fracture shows, the proximal medial wall experienced thickness reduction from 16 mm to 13 mm, due to the compressive stresses imposed for flexural displacement of the stem. 3.2. Fracture analysis The macroscopic analysis shows beach marks indicating crack propagation and a rough aspect commonly associated with coarse microstructure (Fig. 7). Fatigue nucleated at the lateral part of the stem. The fatigue process commonly takes place at this site in total hip stems due to highest positive stresses under in-service loads. A fatigue crack propagated in the most of the fracture surface leaving beach marks until the last minor part when final rupture can be correlated to shear lips formation. High magnification analysis showed fatigue striations at the propagation surface (Fig. 8). 3.3. Material characterization The microstructure of the material is in the ‘‘as cast’’ condition, with delta ferrite at boundaries of austenitic primary grains (Fig. 9). Chemical composition of the material was compared with the ASTM F 745 – ‘‘Standard Specification for 18
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Fig. 9. Microstructure of the material with delta ferrite at boundaries of austenitic primary grains.
Table 2 Fractured stem chemical analysis (weight%).
Sample ASTM F 745-00
C
Mn
P
S
Si
Cr
Ni
Mo
0.022 0.06 max
1.07 2.0 max
0.03 0.045 max
0.005 0.03 max
0.45 1.0 max
19.46 17.0–19.0
12.08 11.0–14.0
2.25 2.0–3.0
450
y = 1525.6x-0.106 R² = 0.853
Max. Stress (MPa)
400 350 300
Cast
250 200 150
wrought
y = 1282.6x-0.125 R² = 0.8591
100 50 0 10000
100000
1000000
10000000
Cicles (log N) Fig. 10. Austenitic stainless steel ASTM F 745 (cast) and ASTM F 138 (wrought) fatigue curves.
Chromium–12.5 Nickel–2.5 Molybdenum Stainless Steel for Cast and Solution-Annealed Surgical Implant Applications’’. Only the Cr content was slightly higher than specified (Table 2). 3.4. Fatigue tests The fatigue performance of the cast stainless steel ASTM F 745 as well as the wrought stainless steel ASTM F 138 is presented in Fig. 10. The stress vs. number of cycles to failure curves can be used to predict number of cycles to failure at any specific stress level. As expected, wrought stainless steel has about 50% higher fatigue resistance than the ‘‘as cast’’ stainless steel. 3.5. Numerical simulation 3.5.1. ISO 7206 simulation The endurance test ISO 7206 numerical simulation results indicated that the highest stress which is detrimental for fatigue crack nucleation is coincident with the fracture initiation region observed during the fracture analysis. For the simulation, the lateral part of the stem surface experienced 250 MPa of maximum principal stress component and 237 MPa of Von Mises stress. The medial part also experienced 237 MPa of Von Mises stress (Fig. 11).
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Fig. 11. Results of the ISO 7206 simulation. Maximum principal stress (left) and Von Mises stress (middle) at the lateral part of the stem. Von Mises stress at the medial part (right).
Fig. 12. Results of the well stabilized arthroplasty. Stem experienced 180 MPa at the dorsal neck.
3.5.2. Stable arthroplasty simulation The stable arthroplasty simulation resulted in a stress decrease at the fracture site, approaching 100 MPa of von Mises stress. Moreover, the stress increased in the dorsal neck site where it was observed 180 MPa of Von Mises stress (Fig. 12). 4. Discussion This study evaluated the analysis of premature fatigue failure that takes place at the half height level of the collared, polished and cemented total hip stem. It was verified fatigue fracture initiated from the lateral of the stem. This region is expected to experience the highest tension in service, especially when preceded for the occurrence of proximal loosening. The use of collar in a polished stem is subject of discussion. The radiographic analysis demonstrated the reduction of the proximal support during the stem use. This means the occurrence of proximal loosening, either by cement creep or by cancellous bone compaction. The polished and tapered stem remains along the years the international standard for comparative analysis with any innovations in hip articulate substitutions [6–12]. Studies showed that the uncollared, polished and tapered cemented stem presents a mechanical behavior in that the stem distal migration, provided from the gradual cement creep strain experienced for reconstruction under in vivo loads, generates radial compressive loads that stabilize the implant
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[13–16]. Therefore, the success of design conception depends on the migration for a stable position. Furthermore, this stem concept has shown better adaptation to cement mantle deficiencies [12]. However, the distal migration was, in a certain moment at the past, considered the prelude for aseptic loosening. Then, design changes were proposed in an attempt to avoid it. Some of the most outstanding modifications occurred in middle 1970s were the introduction of collars and dorsal flanges for cemented stem artroplasty. Beyond the migration arrestment, collars would also present the beneficial effect of calcar compression, reducing the effects of bone resorption [17]. The beneficial of this alteration is still discussed. However, the long term clinical follow-ups of the uncollared, polished and tapered stems denote the success of this conception and deserve consideration [18–24]. If the stem of the present study was uncollared, it could be subsided to compensate for the proximal support loosening, perhaps reducing the risk of cantilever beam effect and the short term fracture. The implantation of the stem in valgus increases the flexural moment, especially after proximal loosening. Furthermore, the maintenance of a thick layer of cancellous bone in the calcar provided by the implantation in valgus reduces stiffness in this region and again, increases the stem flexion. Studies indicate that achieving a more rigid proximal medial support by removal of weak proximal medial cancellous bone from the calcar region reduces the risk of the stem fracture and improves the long-term fixation of cemented femoral components [25–28]. Despite the slightly higher chromium content obtained from chemical analysis, it is possible to assume the results of the present study as applied to stainless steel ASTM F 745, since the chromium excess is unable to provide a significant solution hardening and it was not observed chromium carbide formation. Metallographic analysis shows ‘‘as cast’’ microstructure, with delta ferrite surrounding austenite primary grains. The ISO 5832-1 standard which describes the requirements of mechanical resistance, chemical composition and microstructure of wrought austenitic stainless steels for biomedical applications, does not allow second phase particles in the austenitic matrix, perhaps due to the negative effect of delta ferrite and microstructural heterogeneities, such as pores, voids, segregation and cracks on the fatigue and corrosion properties of the austenitic stainless steels. According to the endurance fatigue standard ISO 7206 part 4, a hip stem should hold up more than 5 million cycles even after proximal loosening. The use of numerical simulation to predict the ISO 7206 – 4 standard results is well established [29]. In the ISO 7206 numerical simulation performed in this study, it was observed 250 MPa of maximum principal stress and 237 Von Mises stress in the lateral and medial region of the stem. This value is near the fatigue limit of the cast austenitic stainless steel found in the literature [30,31]. Furthermore, according to the fatigue tests of ASTM F 745 samples carried out in the early study [3], the stress level would produce the stem failure around 0.46 million cycles. According to some researches, 1 million cycles correspond to the minimum number of cycles of an individual walking for 1 year [32,33]. Then, more time should have been spent previously to the fracture, since cement mantle initially did some proximal support, allowing the 2.5 years prior to failure. If the manufacturer’s choice had been ISO 5832-1 stainless steel, fatigue endurance limit probably should be reached from this first simulation. The simulation of the well stabilized arthroplasty also presented an important result on the stem fracture prediction. In this case, although the stress level in the fracture region was reduced to values lower than 100 MPa, which is below the fatigue limit of the as cast steel, the higher stress level moved to the stem dorsal neck where it was reached 130 MPa of maximum principal stress and 180 MPa of Von Mises stress. From the S–N fatigue curve, 180 MPa corresponds to 6.6 million cycles to failure. Moreover, the endurance limit would be reached if the stem had manufactured from wrought stainless steel. Although it may be more feasible and possibly cheaper to produce orthopedic devices by casting, this paper states that manufacturers should rather prefer using ISO 5832-1 instead of ASTM F 745, especially for those components subjected to cyclic loads whose expected fatigue life is high. 5. Conclusions The premature fatigue failure of the THA stem was enhanced by the proximal loosening and the use of ‘‘as cast’’ stainless steel which shows lower fatigue resistance than wrought stainless steel. Even if the arthroplasty had not experienced proximal loosening and had not implanted in valgus, the numerical simulation allowed predicting the potential of stem neck fracture. The stem performance could have been improved still in the design conception, through numerical simulation. The use of collar in polished stems should be matter of more discussion, since the subsidence arrestment can enhance cantilever beam effect. Acknowledgments The authors would like to acknowledge the FINEP, CAPES, CNPq and FAPITEC financial support. References [1] Ek ET, Choong M. Comparison between triple-tapered and double-tapered cemented femoral stems in total hip arthroplasty – a prospective study comparing the C-stem versus the Exeter universal early results after 5 years of clinical experience. J Arthroplasty 2005;20(1):94–100. [2] 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 2008;30:1325–32.
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[3] Griza S, Zanon G, Silva EP, Bertoni F, Reguly A, Strohaecker TR. Design aspects involved in a cemented THA stem failure case. Eng Fail Anal 2009;16:512–20. [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;15:981–8. [5] Griza S, Reguly A, Strohaecker TR. Total hip stem failure due to weld procedure and material selection misuse. Eng Fail Anal 2010;17:555–61. [6] Charnley J. Arthroplasty of the hip, a new operation. Lancet 1961;1:1129. [7] Charnley J. The long-term results of low-friction arthroplasty of the hip, performed as a primary intervention. J Bone Joint Surg 1972;54-B:61. [8] Charnley J. Low friction arthroplasty of the hip, theory and practice. Heidelberg: Springer; 1979. [9] Older J. Low-friction arthroplasty of the hip. A 10–12-year follow-up study. Clin Orthop 1986;211:36. [10] Wroblewski BM, Siney PD, Fleming PA. Charnley low-frictional torque arthroplasty in patients under the age of 51 years. Follow-up to 33 years. J Bone Joint Surg Br 2002;84(4):540–3. [11] Older J. Charnley low-friction arthroplasty: a worldwide retrospective review at 1–20 years. J Arthroplasty 2002;17(6):675–80. [12] Fowler JL, Gie GA, Lee AJC, Ling MA. Experience with the Exeter total hip replacement since 1970. Orthop Clin North Am 1988;19(3):477–89. [13] Shen G. Femoral stem fixation. An engineering interpretation of the long-term outcome of Charnley and Exeter stems. J Bone Joint Surg Br 1999;81(2):372. [14] Huiskes R, Verdonschot N, Nivbrant B. Migration, stem shape, and surface finish in cemented total hip arthroplasty. Clin Orthop Relat Res 1998;355:103–15. [15] Huiskes R, Boeklagen R. Mathematical shape optimization of hip prosthesis design. J Biomech 1989;22(8/9):793–804. [16] Verdonschot N, Huiskes R. Subsidence of THA stems due to acrylic cement creep is extremely sensitive to interface friction. J Biomech 1996;29(12):1569–75. [17] 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. [18] Neumann L, Freund KG, Sorenson KH. Long-term results of Charnley total hip replacement: review of 92 patients at 15–20 years. J Bone Joint Surg Br 1994;76-B:245–51. [19] Berry DJ, Harmsen WS, Cabanela ME, Morrey BF. Twenty-five-year survivorship of two thousand consecutive primary Charnley total hip replacements: factors affecting survivorship of acetabular and femoral components. J Bone Joint Surg Am 2002;84-A(2):171–7. [20] Collis DK, Mohler CG. Comparison of clinical outcomes in total hip arthroplasty using rough and polished cemented stems with essentially the same geometry. J Bone Joint Surg 2002;84-a(4):586–92. [21] Halley DK, Glassman AH. Twenty- to twenty-six-year radiographic review in patients 50 years of age or younger with cemented Charnley low-friction arthroplasty. J Arthroplasty 2003;18(7, Suppl. 1):79–85. [22] Chiu KH, Shen WY, Cheung KW, Tsui HF. Primary Exeter total hip arthroplasty in patients with small femurs, a minimal of 10 years follow-up. J Arthroplasty 2005;20(03):275–81. [23] Glyn-Jones S, Gill HS, Beard DJ, McLardy-Smith P, Murray DW. Influence of stem geometry on the stability of polished tapered cemented femoral stems. J Bone Joint Surg 2005;87-b(7):921–7. [24] Williams HDW, Browne G, Gie GA, Ling RSM, Timperley AJ, Wendover NA. The Exeter universal cemented femoral component at 8–12 years – a study of the first 325 hips. J Bone Joint Surg Br 2002;84-B:324–34. [25] Andriacchi TP, Galante JO, Belytschko TB, Hampton S. A stress analysis of the femoral stem in total hip prostheses. J Bone Joint Surg Am 1976;58:618–24. [26] Ayers D, Mann K. The importance of proximal cement filling of the calcar region: a biomechanical justification. J Arthroplasty 2003;18(7 Suppl.):1. [27] Wroblewski BM, Siney PD, Fleming PA, Bobak P. The calcar femorale in cemented stem fixation in total hip arthroplasty. J Bone Joint Surg Br 2000;82B:842–5. [28] Ebramzadeh E, Sarmiento A, McKellop HA, Llinas A, Gogan W. The cement mantle in total hip arthroplasty analysis of long-term radiographic results. J Bone Joint Surg Am 1994;76:77–87. [29] 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. [30] Teoh SH. Fatigue of biomaterials: a review. Int J Fatigue 2000;22:825–37. [31] Niinomi M. Fatigue characteristics of metallic biomaterials. Int J Fatigue 2007;29:992–1000. [32] 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. [33] Clarke IC. Wear of artificial joint materials. I. Friction and wear studies: validity of wear-screening protocols. Eng Med 1981;10:115–22.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Case study and analysis of a fatigue failure in a THA stem Sandro Griza a,⇑, Silvando Vieira dos Santos a, Marcelo Massayoshi Ueki a, Fabiano Bertoni b, Telmo Roberto Strohaecker b a b
Programa de Pós-Graduação em Ciência e Engenharia dos Materiais, Universidade Federal de Sergipe, Brazil Programa de Pós-Graduação em Engenharia de Minas, Metalúrgica e de Materiais, Universidade Federal do Rio Grande do Sul, Brazil
a r t i c l e
i n f o
Article history: Received 3 April 2012 Accepted 18 October 2012 Available online 2 November 2012 Keywords: Hip stem Fatigue ASTM F 745 ASTM F 138
a b s t r a c t This paper presents a case of a collared, polished and cemented hip stem which presented premature fracture due to ‘‘cantilever beam’’ effect. The study consisted of radiographic analysis, fracture analysis, material characterization and S–N fatigue tests. Numerical simulation of the stem was also carried out in two ways: under the ISO 7206-4 standard test and under a well stabilized arthroplasty hypothesis. The failure was predicted through both numerical simulations. The mechanical aspect of the premature failure is associated to the use of the cast stainless steel. This article aims at reinforcing the idea that the use of the ‘‘as cast’’ austenitic stainless steel ASTM F 745 is harmful for implants that have potential for fatigue failure. The article also discusses the application of collars in polished and cemented stems whose stability is based on the distal migration. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The modern technique of total hip arthroplasty proposed by Charnley is one of the most successful practices in orthopedics [1]. It is a widely used strategy to restore the normal function of the hip joint disrupted by fracture or disease. This approach has been extremely successful and its usage has grown to several hundred thousand primary joint replacements per year worldwide. In early years, the rate of revision was higher due to mechanical and biological concerns. Many of these 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 continuous evolution of the technique for many years produced sufficient knowledge to allow for a decrease in stem fatigue rate nowadays. In particular, the introduction of the endurance fatigue standard ISO 7206-4 in the 1980s, resulted in a significant reduction of the failure rates. From this test, the designer can evaluate the material behavior besides other factors that can enhance the failure risk of the stem subjected to the severe condition of the proximal aseptic loosening. In the design process to find the ideal hip prosthesis geometry, pre-clinical tests must be carried out to verify the mechanical endurance to physiological loads. An elegant alternative approach would be the use of the virtual finite element method (FEM) to have a first glimpse of the expected mechanical behavior of the candidate design. It does not mean that the physical pre-clinical tests can be excluded. It is just a rapid way to discard potentially low performance designs. Unfortunately, some premature stem fractures are still occurring nowadays. The need of hip revision is dangerous for the patient since it is a more complicated surgical intervention with an increased difficulty of rehabilitation. In recent papers, different stem failures were presented [2–5]. Now, this paper shows a case of a collared, polished and cemented hip stem which presented premature fracture in the half height level due to the use of as cast stainless steel and favored by the proximal aseptic loosening.
⇑ Corresponding author. Tel.: +55 79 21056888; fax: +55 79 21056845. E-mail address: [email protected] (S. Griza). 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2012.10.011
S. Griza et al. / Engineering Failure Analysis 28 (2013) 166–175
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2. Materials and methods 2.1. Preliminary analysis A fractured total hip arthroplasty stem was revised from a female patient (67 years old, 64 kgf), after only 2.5 years in use. The radiographic analysis demonstrated stem fracture. Radiographies obtained previously, just after primary arthoplasty (post operative radiography) and that obtained after fracture were compared (Fig. 1). It is possible to clearly see a fracture of the femoral component at the half height level between the prosthesis upper body and the stem tip. After revision, the two sections of the fractured stem were cleaned in auto-clave to be subjected to analysis. The stem is polished, collared, and has a rectangular section with rounded corners from distal up to proximal aspect. The stem head has 28 mm diameter. The two parts of the stem were measured in a three-dimensional coordinate measurement machine (Zeiss Vista, Carl Zeiss Industrielle Messtechnik GmbH 73446 Oberkochen). These drawing parts were then joined to produce a solid for the numerical simulation model (Fig. 2). 2.2. Fracture analysis The fracture surfaces presented scratches and dents commonly found in scenarios of contact under compression. The best preserved fracture surface was chosen for analysis and the failure mechanisms were identified in low magnification stereo microscope (Carl Zeiss Stemi 2000). The fracture surface was also analyzed by scanning electron microscopy (SEM Philips XL20) to detect the failure micromechanisms. 2.3. Material characterization A metallographic sample was taken from the fracture initiation point along the longitudinal plane of symmetry of the stem and it was analyzed by optical microscopy (Carl Zeiss Axioscope A1). Electrolytic etching of 10% oxalic acid diluted
Fig. 1. Frontal radiographies taken immediately after arthroplasty (left) and before revision (right). It can be seen the implantation of the stem slightly in valgus (dashed lines) and radioluscent line at the proximal aspect. White arrow at right points to the initiation site of the fracture, at the lateral aspect of the stem. Black lines denote proximal support thickness reduction from 16 to 13 mm.
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Fig. 2. General view of the collared and polished stem.
Fig. 3. Fatigue test sample used to achieves wrought stainless steel performance.
in water was used to reveal the microstructural features of the sample. Chemical analysis was performed with an optical spectrometer (Spectro, Spectrolab).
2.4. Fatigue tests Test samples were machined from ASTM F 138 wrought stainless steel commercial rods (Fig. 3). All samples were polished to an average roughness of 1 lm. Tension fatigue tests were carried out through a servo hydraulic material test system (MTS 810, MTS Corporation, Eden Prairie, MN) following the ASTM E 466 standard. An applied load rate R = 0.1 at 30 Hz was used during test. The results were compared with the fatigue performance of the ASTM F 745 published early [3].
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Fig. 4. General view of the container and the stem mesh used to model assembly and boundary conditions for ISO 7206-4. Frontal view (left), lateral view (right). Stem is enveloped for a cement mantle inside a container. Load is applied at the shell and the base of container is constrained.
Table 1 Constitutive properties and mesh characteristics of the simulated systems individual components. Part
Elastic modulus (GPa)
Poisson
Shell
–
–
Stem
205
0.3
Container
205
0.3
Cement (first model)
2.8
0.33
Cement (second model) Cancellous bone Cortical bone
2.8 0.33 15.5
Number of elements
Number of nodes
Element type
2092
2153
35,210
42,241
1824
2484
42,781
44,027
5741 linear tetrahedral 37,040 linear hexahedral
41,745
14,906
38,357 linear tetrahedral 3388 linear hexahedral
43,694
10,103
43,133 linear tetrahedral 561 linear hexahedral
13,791
5248
12,399 linear tetrahedral 1392 linear hexahedral
0.33 0.32 0.3
Rigid quadrilateral shell 14,498 linear tetrahedral 27,743 linear hexahedral linear hexahedral
2.5. Numerical simulation 2.5.1. ISO 7206 simulation The aim of the first computational simulation was to evaluate, through the finite element method (FEM), the stem stress levels when a vertical load is applied over the stem head. The protocol simulating the ISO 7206-4 endurance fatigue test was the same as in an earlier study [3]. The resulting stress level can be compared to the fatigue performance of the material. The standard determines that the main axis of the stem should be sloped respectively 9° and 10°, in the antero-posterior and in the medio-lateral direction. Then, the stem should be inserted in a container and cemented using some polymer such as PMMA up to 60% of the distance between head center and stem tip (Fig. 4). The container was designed to provide at least 5 mm cement thickness. The material properties needed for simulation agree with linear elastic regime and are shown in Table 1. A shell of 28 mm curvature radius was used for the stress distribution at the surface of the head 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. Constraining the lower surface of the container and the 2300 N vertical load applied in the head through the shell were used as boundary conditions. Net characteristics used also are shown in Table 1. Tetrahedral and hexahedral, linear interpolation elements were applied to the stem, the cement and the container. Rigid quadratic elements were applied to the shell. As it can be seen in Fig. 4, net refinement was done in the half height stem region, reaching convergence in a reasonable computational time.
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Fig. 5. General view of the well stabilized arthroplasty model. Stem was implanted with 2 mm cement mantle.
Fig. 6. General view of the femur mesh used into the arthroplasty model.
2.5.2. Stable arthroplasty simulation A second simulation was performed to verify the stem stress level in the case of a well stabilized primary arthroplasty. The stem was cemented in a composite femur model 3306 (Pacific Research Labs., Vashon Island, WA, USA). The cement mantle and cortical bone were both modeled with 5 and 2 mm constant thickness respectively (Fig. 5). The femur was sectioned and restricted in the plane of its half height. The stem slope, interface conditions and external load were evaluated in the same manner as in the previous model. Tetrahedral, hexahedral and linear interpolation elements were applied to the new cement mantle and the femur (Fig. 6). Both cortical and cancellous bones were assumed as isotropic and homogeneous and their constitutive properties are shown in Table 1.
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Fig. 7. Stem fracture surface observed under low magnification. Gross fracture aspect and beach marks can be observed. Nucleation (at left) and final fracture (right).
Fig. 8. Fatigue striations observed at the stem fracture surface. SEM fractography.
3. Results 3.1. Preliminary analysis The postoperative radiography shows the implantation of the stem slightly in valgus. Radiolucency at the proximal medial aspect denotes the stem was supported proximally by a thick cement mantle and cancellous bone wall. The cement mantle thickness decreases as it follows the distal portion where the stem tip reaches direct contact with the cortical bone. However, as the radiography obtained after fracture shows, the proximal medial wall experienced thickness reduction from 16 mm to 13 mm, due to the compressive stresses imposed for flexural displacement of the stem. 3.2. Fracture analysis The macroscopic analysis shows beach marks indicating crack propagation and a rough aspect commonly associated with coarse microstructure (Fig. 7). Fatigue nucleated at the lateral part of the stem. The fatigue process commonly takes place at this site in total hip stems due to highest positive stresses under in-service loads. A fatigue crack propagated in the most of the fracture surface leaving beach marks until the last minor part when final rupture can be correlated to shear lips formation. High magnification analysis showed fatigue striations at the propagation surface (Fig. 8). 3.3. Material characterization The microstructure of the material is in the ‘‘as cast’’ condition, with delta ferrite at boundaries of austenitic primary grains (Fig. 9). Chemical composition of the material was compared with the ASTM F 745 – ‘‘Standard Specification for 18
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Fig. 9. Microstructure of the material with delta ferrite at boundaries of austenitic primary grains.
Table 2 Fractured stem chemical analysis (weight%).
Sample ASTM F 745-00
C
Mn
P
S
Si
Cr
Ni
Mo
0.022 0.06 max
1.07 2.0 max
0.03 0.045 max
0.005 0.03 max
0.45 1.0 max
19.46 17.0–19.0
12.08 11.0–14.0
2.25 2.0–3.0
450
y = 1525.6x-0.106 R² = 0.853
Max. Stress (MPa)
400 350 300
Cast
250 200 150
wrought
y = 1282.6x-0.125 R² = 0.8591
100 50 0 10000
100000
1000000
10000000
Cicles (log N) Fig. 10. Austenitic stainless steel ASTM F 745 (cast) and ASTM F 138 (wrought) fatigue curves.
Chromium–12.5 Nickel–2.5 Molybdenum Stainless Steel for Cast and Solution-Annealed Surgical Implant Applications’’. Only the Cr content was slightly higher than specified (Table 2). 3.4. Fatigue tests The fatigue performance of the cast stainless steel ASTM F 745 as well as the wrought stainless steel ASTM F 138 is presented in Fig. 10. The stress vs. number of cycles to failure curves can be used to predict number of cycles to failure at any specific stress level. As expected, wrought stainless steel has about 50% higher fatigue resistance than the ‘‘as cast’’ stainless steel. 3.5. Numerical simulation 3.5.1. ISO 7206 simulation The endurance test ISO 7206 numerical simulation results indicated that the highest stress which is detrimental for fatigue crack nucleation is coincident with the fracture initiation region observed during the fracture analysis. For the simulation, the lateral part of the stem surface experienced 250 MPa of maximum principal stress component and 237 MPa of Von Mises stress. The medial part also experienced 237 MPa of Von Mises stress (Fig. 11).
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Fig. 11. Results of the ISO 7206 simulation. Maximum principal stress (left) and Von Mises stress (middle) at the lateral part of the stem. Von Mises stress at the medial part (right).
Fig. 12. Results of the well stabilized arthroplasty. Stem experienced 180 MPa at the dorsal neck.
3.5.2. Stable arthroplasty simulation The stable arthroplasty simulation resulted in a stress decrease at the fracture site, approaching 100 MPa of von Mises stress. Moreover, the stress increased in the dorsal neck site where it was observed 180 MPa of Von Mises stress (Fig. 12). 4. Discussion This study evaluated the analysis of premature fatigue failure that takes place at the half height level of the collared, polished and cemented total hip stem. It was verified fatigue fracture initiated from the lateral of the stem. This region is expected to experience the highest tension in service, especially when preceded for the occurrence of proximal loosening. The use of collar in a polished stem is subject of discussion. The radiographic analysis demonstrated the reduction of the proximal support during the stem use. This means the occurrence of proximal loosening, either by cement creep or by cancellous bone compaction. The polished and tapered stem remains along the years the international standard for comparative analysis with any innovations in hip articulate substitutions [6–12]. Studies showed that the uncollared, polished and tapered cemented stem presents a mechanical behavior in that the stem distal migration, provided from the gradual cement creep strain experienced for reconstruction under in vivo loads, generates radial compressive loads that stabilize the implant
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[13–16]. Therefore, the success of design conception depends on the migration for a stable position. Furthermore, this stem concept has shown better adaptation to cement mantle deficiencies [12]. However, the distal migration was, in a certain moment at the past, considered the prelude for aseptic loosening. Then, design changes were proposed in an attempt to avoid it. Some of the most outstanding modifications occurred in middle 1970s were the introduction of collars and dorsal flanges for cemented stem artroplasty. Beyond the migration arrestment, collars would also present the beneficial effect of calcar compression, reducing the effects of bone resorption [17]. The beneficial of this alteration is still discussed. However, the long term clinical follow-ups of the uncollared, polished and tapered stems denote the success of this conception and deserve consideration [18–24]. If the stem of the present study was uncollared, it could be subsided to compensate for the proximal support loosening, perhaps reducing the risk of cantilever beam effect and the short term fracture. The implantation of the stem in valgus increases the flexural moment, especially after proximal loosening. Furthermore, the maintenance of a thick layer of cancellous bone in the calcar provided by the implantation in valgus reduces stiffness in this region and again, increases the stem flexion. Studies indicate that achieving a more rigid proximal medial support by removal of weak proximal medial cancellous bone from the calcar region reduces the risk of the stem fracture and improves the long-term fixation of cemented femoral components [25–28]. Despite the slightly higher chromium content obtained from chemical analysis, it is possible to assume the results of the present study as applied to stainless steel ASTM F 745, since the chromium excess is unable to provide a significant solution hardening and it was not observed chromium carbide formation. Metallographic analysis shows ‘‘as cast’’ microstructure, with delta ferrite surrounding austenite primary grains. The ISO 5832-1 standard which describes the requirements of mechanical resistance, chemical composition and microstructure of wrought austenitic stainless steels for biomedical applications, does not allow second phase particles in the austenitic matrix, perhaps due to the negative effect of delta ferrite and microstructural heterogeneities, such as pores, voids, segregation and cracks on the fatigue and corrosion properties of the austenitic stainless steels. According to the endurance fatigue standard ISO 7206 part 4, a hip stem should hold up more than 5 million cycles even after proximal loosening. The use of numerical simulation to predict the ISO 7206 – 4 standard results is well established [29]. In the ISO 7206 numerical simulation performed in this study, it was observed 250 MPa of maximum principal stress and 237 Von Mises stress in the lateral and medial region of the stem. This value is near the fatigue limit of the cast austenitic stainless steel found in the literature [30,31]. Furthermore, according to the fatigue tests of ASTM F 745 samples carried out in the early study [3], the stress level would produce the stem failure around 0.46 million cycles. According to some researches, 1 million cycles correspond to the minimum number of cycles of an individual walking for 1 year [32,33]. Then, more time should have been spent previously to the fracture, since cement mantle initially did some proximal support, allowing the 2.5 years prior to failure. If the manufacturer’s choice had been ISO 5832-1 stainless steel, fatigue endurance limit probably should be reached from this first simulation. The simulation of the well stabilized arthroplasty also presented an important result on the stem fracture prediction. In this case, although the stress level in the fracture region was reduced to values lower than 100 MPa, which is below the fatigue limit of the as cast steel, the higher stress level moved to the stem dorsal neck where it was reached 130 MPa of maximum principal stress and 180 MPa of Von Mises stress. From the S–N fatigue curve, 180 MPa corresponds to 6.6 million cycles to failure. Moreover, the endurance limit would be reached if the stem had manufactured from wrought stainless steel. Although it may be more feasible and possibly cheaper to produce orthopedic devices by casting, this paper states that manufacturers should rather prefer using ISO 5832-1 instead of ASTM F 745, especially for those components subjected to cyclic loads whose expected fatigue life is high. 5. Conclusions The premature fatigue failure of the THA stem was enhanced by the proximal loosening and the use of ‘‘as cast’’ stainless steel which shows lower fatigue resistance than wrought stainless steel. Even if the arthroplasty had not experienced proximal loosening and had not implanted in valgus, the numerical simulation allowed predicting the potential of stem neck fracture. The stem performance could have been improved still in the design conception, through numerical simulation. The use of collar in polished stems should be matter of more discussion, since the subsidence arrestment can enhance cantilever beam effect. Acknowledgments The authors would like to acknowledge the FINEP, CAPES, CNPq and FAPITEC financial support. References [1] Ek ET, Choong M. Comparison between triple-tapered and double-tapered cemented femoral stems in total hip arthroplasty – a prospective study comparing the C-stem versus the Exeter universal early results after 5 years of clinical experience. J Arthroplasty 2005;20(1):94–100. [2] 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 2008;30:1325–32.
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