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Metallurgical failure analysis of acetabular metal-backed screws
Engineering Failure Analysis 32 (2013) 178–187
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Metallurgical failure analysis of acetabular metal-backed screws Sandro Griza a,⇑, Thiago Figueiredo Azevedo a, Silvando V. dos Santos a, Eduardo K. Tentardini a, Telmo R. Strohaecker b a b
Programa de Pós-Graduação em Ciência e Engenharia dos Materiais, Universidade Federal de Sergipe, São Cristóvão, Brazil Programa de Pós-Graduação em Engenharia Metalúrgica, de Minas e Materiais, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil
a r t i c l e
i n f o
Article history: Received 9 January 2013 Received in revised form 6 March 2013 Accepted 31 March 2013 Available online 8 April 2013 Keywords: THA Metal-backed screws Fatigue Ti6Al4V Cr–Co
a b s t r a c t Screws are often used in total hip arthroplasty to increase the cementless acetabular metalbacked fixation stability. This study aimed at identifying metallurgical factors related to metal-backed screw fractures in two cases of premature total hip arthroplasty revision. A preliminary analysis of radiographs and retrieved parts was performed. Fractured screws were subjected to fracture analysis, energy dispersive X-ray spectroscopy (EDS) and metallographic analysis. A finite element analysis was performed to predict the stresses experienced by the screws. Two factors were identified: titanium alloy microstructure, consisting of alpha phase plates with oxygen-rich alpha case and as cast Cr–Co alloy with dendritic microstructure and unfavorably aligned shrinkage formation. Finite element analysis has showed enough level of stress to predict screw fracture in the case of unbounded metalbacked and bone interface. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The actual model of total hip arthroplasty consists basically of a femoral stem which has a spherical head under load bearing contact with an acetabular component, called liner. The stem is usually made of austenitic stainless steel, Cr–Co alloy or Ti6Al4V, whereas the liner is made of a hollow hemispherical ultra high molecular weight polyethylene (UHMWPE), or metal (Cr–Co or steel), or ceramic. This device must possess low friction among moving parts, maintaining the degree of freedom of a healthy hip. The acetabular component should be well fixed into the hip to ensure long term stability. Nowadays there are two concepts of fixation well accepted by the academic and medical community: the mechanical and the biological system. The first one is the current standard and consists on using bone cement to fix the polymeric liner into the spongy bone cavity previously rammed by the surgeon [1]. However, some difficulties have been reported to this concept, such as the fact that procedure success is dependent of two factors: the surgeon technique and the reduced time to complete the operation, since the bone cement becomes solid during the polymerization and loses ability to be quickly molded [2]. The biological fixation technique consists on connecting the liner to a cementless and metallic hemispherical shell, called metal-backed. The outer surface of the metal-backed is functionalized from techniques such as porous coating, sintered bead or plasma spray in order to improve bone growth and achieve short term stability. This prosthetic option was developed with the advantage to present higher stiffness, better stress distribution along the bearing surface and superior modularity, which allows significant flexibility in revision surgery, permitting for surgeons the ability to change femoral head size, liner offset and liner build up with preservation of a well fixed metal-backed. Besides, it reduces the obligation to stock several head and stem sizes and allows the final choice of neck length and head diameter to be done after stem implantation. A press fit ⇑ Corresponding author. Tel.: +55 79 21056888; fax: +55 79 21056845. E-mail address: [email protected] (S. Griza). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.03.021
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assembly is achieved when the acetabular bony cavity is rammed with a curvature radius smaller than the metal-backed. On the other hand, when the two sides have the same curvature, they provide an exact fit condition [3]. Screws are sometimes used to enhance the biological fixation stability. In some cases, when the surgeon feels that the metal-backed are well placed in the bone, he may choose to discard the screw use [4]. The long-term stability of the acetabular component is limited by wear of the liner which must be replaced before metal/ metal contact [5]. However, some cases of premature failures have been previously reported [5–13], many of them associated to displacements between the metal-backed and bone which becomes an interfacial loosening manifestation. As a result, structural integrity of the screws plays an important role in delaying the premature failure event. The common practice of metal-backed screw fixation is to use only two proximal screws in the posterior column of ileum. This is applied to avoid the problems with potential neurovascular injuries, frequently associated with bicortical screws in the ischium and pubis regions [13]. This study was dedicated to analyze two cases of acetabular metal-backed screw fractures in two different prosthesis materials (Ti6Al4V and Cr–Co alloy) occurred less than 5 years before revision. The microstructural analyses of the broken screws were evaluated by OM, SEM, EDS and finite element analysis. 2. Materials and methods 2.1. Preliminary analysis This paper analyzes two cases of acetabular metal-backed screw failures, each one from different patients: Case 1: male patient M.S.M., 82 kgf weight, 71 years old, experienced revision 2.5 years after the primary arthroplasty. Case 2, male patient A.H.R., 76 kgf weight, 62 years old, experienced revision 4.2 years after the primary arthroplasty. In general cases, 5 years before revision is a short term period for total hip arthroplasty [5–14]. To the specific case of cementeless acetabular using porous metal-backed in younger patients, Sharp et al. [4] confirm a useful life before revision of 5.1 years. The material used in the metal-backed manufacture was Ti6Al4V and Cr–Co alloy for cases 1 and 2, respectively. There were six screws in total (three in each shell), one fractured screw in case 1 (pubis region) and two fractured in case 2 (both in ileum). Failures were observed from radiographies before total hip replacement revision. The post retrieval protocol consisted in identify the previous fractured screw positions regarding to the hip reconstruction and cleaning parts with tap water. All the retrieval elements were then subjected to autoclave cleaning followed by preliminary visual analysis. The radiographies taken before revision were also analyzed. 2.2. Fracture and EDS analyses Screws fracture analysis was performed by using a low magnification stereomicroscope (Zeiss Stemi 2000 C) and a scanning electron microscope (SEM – Philips XL-20) for high magnification. The alloys were qualitatively analyzed by energy dispersive X-ray spectroscopy (EDS microprobe). Screws were embedded in thermocouple resin (Bakelite) and grinded until reached their longitudinal cross section. Metallographic samples were performed in order to show the respective fracture initiation point of the screws. The outer screw diameters were measured by a caliper (0.05 mm resolution) before sample mounting. During grinding, the sample thicknesses were controlled to reach the mean section and after grinding, the samples were polished with diamond suspension (1 lm) in an appropriate polishing cloth. The EDS analyses were performed over the polished surface of the samples. 2.3. Microstructural analysis Metallographic samples were obtained from the fracture initiation point along the longitudinal plane of symmetry of the screws (same samples as described before) and microstructural analyses were performed from optical microscopy (Carl Zeiss Axioscope A1). 2.4. FE analysis The components were measured using a caliper with resolution of 0.05 mm to obtain its geometrical features and to perform 2D finite element analyses of the acetabular reconstruction, as showed in Fig. 1. The thread curvature radius of the screws was reproduced from the measurement of the metallographic samples in optical microscopy. Head, polymeric liner, metal-backed and acetabulum were made as connecting hemispheres of 28, 38, 42 and 72 mm, respectively. Two screws disposed diametrically opposed at 45° from the axis of symmetry of the reconstruction were considered to fix the metal-backed to the bone tissue. For simplification, the screws were modeled using Abaqus software (Abaqus 6.5, Hibbit, Karlsson e Sorensen, Inc., Pawtucker) with only the thread where the fracture occurred. The parameters software applied were: element type used was the quadratic 2D and mesh refinement was evaluated in the thread region. The refinement was done by section
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Fig. 1. Hemispherical acetabular model. The external bone tissue was restricted and the load (L) was applied from the head to the liner in each step angle. The figure shows only three steps: 0°, 45° and 75°.
Table 1 Mesh attributes and material’s constitutive properties. Material
E (GPa)
Poisson
N° elements
N° nodes
Bone tissue Ti6Al4V Cr–Co UHMWPE
0.33 110 240 1.07
0.32 0.34 0.29 0.41
1382 1573 1573 100
4325 5048 5048 359
partition in the limits of the curvature radius. By the way, this specific screw part was refined apart of the other regions of the screw. It was assumed convergence within 5% error. The two failure cases were simulated, constitutive properties obtained from the literature [15] and mesh characteristics are summarized in Table 1. After visual inspection, the external side of the acetabulum appears constrained. An external load of 2300 N was applied from the head to the liner [16] and it was applied in steps of 15°, beginning in the axis of symmetry (0°) and following up to 75°. Two interfacial conditions between the metalbacked and the bone tissue were simulated: (i) bonded and (ii) with friction coefficient of 0.15. These simulations were prepared to verify the effect of the stable fixation in short term and the interfacial loosening manifestation on the screw stress level, respectively. All the other interfaces were considered as bonded. The maximum Von Mises stress experienced by the screw thread was correlated to the load angle, material (Ti6Al4V or Cr–Co) and the interface condition between the metalbacked and the bone tissue (bonded or 0.15 friction coefficient).
3. Results 3.1. Preliminary analysis The radiographic analysis showed similar features for both cases. The metal-backed loosening and screw fractures were observed. The radiography achieved prior to revision of the case 1 shows wear of the liner, since the stem head was displaced upward in the proximal direction, as showed in Fig. 2. Rotation of the metal-backed was encouraged by a distal screw rupture. The image also shows the screw tip lodged into the trabecular bone tissue. Visual analysis indicated both metal-backed are porous coated. Fig. 3a shows the metal-backed of case 1. Despite some wear degree of the liners, they had not reached the metal/metal contact yet, as showed in Fig. 3b. The rupture occurred in the cross section corresponding to the first thread from the screw hub in one screw for case 1 (Fig. 4a) and in two screws for case 2 (Fig. 4b). Thread crest fractures were also observed in a third screw for case 2, as illustrate in Fig. 4c. Another evident aspect in these screws of case 2 is scoring marks at the longitudinal axis and secondary crack at the threads. The heads and femoral stems showed no signs of significant damage related to the revision.
3.2. Fracture and EDS analyses The screw in case 1 presented a unidirectional bending fracture which started in a specific region at the thread root and propagated by a large amount until the final rupture was reached. It was related to shear of the thread section (Fig. 5a). Fig. 5b shows an SEM image of the nucleation site. Fatigue features (striations) were identified by high magnification analysis (SEM) in the fracture surface near the nucleation site (Fig. 5c), whereas dimples formation was observed next to the
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Fig. 2. Radiography obtained just before the revision of the case 1. It is possible to observe the metal-backed rotation. The tip of the fractured distal screw is lodged in the acetabulum (arrow).
Fig. 3. Images showing (a) case 2 porous coated metal-backed, and (b) the case 1 liner concavity. Both metal-backed have 50 mm outside diameter.
propagation end (Fig. 5d). Dimples are often seen at the end of the fatigue propagation and it may occur due to increased stress intensity factor at the crack tip. Both screws fractured in the cross section of the case 2 showed a rough and brittle fracture. This feature is commonly seen in coarse grain material fractures. Linear steps also were seen on the fracture surface in both screws of case 2. These steps have similarity with beach marks; however they are straight and not semi-elliptical as expected. Fig. 6 shows a SEM analysis
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Fig. 4. Image (a) shows the fractured screw of the case 1. Fracture takes place in the region of the first thread. Image (b) shows the screws of the case 2: the cross section of two fractured screws and the remaining 24 mm length screw. Image (c) shows fractures in the thread crests of the remaining screw. Longitudinal scoring marks also can be seen.
Fig. 5. Images of case 1 screw: (a) the smoothness aspect of the fracture surface. The arrow points to the fracture initiation site. (b) Radial lines emanating from the fracture initiation. (c) The appearance of fatigue propagation on the surface near the fracture initiation, with striations (white arrows) and some shear between the deformed grains (black arrows). (d) Formation of dimples on the fracture surface near the end of the fatigue propagation.
and can be observed the separation without plastic deformation, the fatigue appearance and the dimples formation. The remaining screw of the case 2 had fractures in the thread crests. Fig. 7 shows the formation of interdendritic fracture (striations which indicate fatigue propagation), the scoring marks and longitudinal secondary cracks on the thread crests. Fig. 8 shows the EDS analyses for the screw in case 1, indicated titanium, aluminum and vanadium and for case 2 screws, the analysis shows peaks in chromium, cobalt, molybdenum and silicon. 3.3. Microstructural analysis The case 1 screw presented a typical Ti6Al4V alloy microstructure transformed at temperatures above the beta transition, with the formation of platelike alpha phase and beta intergranular. It was observed the formation of a lighter, thin surface
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Fig. 6. Images at left showing brittle aspect of cross section fracture of both case 2 screws. The roughness appearance and flat, parallel steps over the surfaces can be seen. The images at right show the respective fracture micromechanisms, with particle separation, fatigue appearance and dimples outlining carbide particles (white arrows).
Fig. 7. Thread crest fractures, secondary cracks and the transversal scoring marks of the remained case 2 screw (a). High magnification of interdendritic separation and fatigue striations of the fractured crests (b).
Fig. 8. Microprobe EDS spectrum of the case 1 screw (left) and the case 2 screw (right).
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Fig. 9. Fracture initiation of case 1 screw associated to a lighter surface layer (a). The microstructure consists of alpha phase plates and intergranular beta phase (b). Etchant: 0.2%, 0.4% HF and HNO3 diluted in water.
Fig. 10. As polished metallographic transversal section of the case 2 screw head showing shrinkages alignment.
Fig. 11. Representative microstructures of the case 2 screws. Austenite dendritic matrix contains eutectic carbide chains and scattered carbide particles. The right image shows in detail the patch of lamellar constituent at a grain boundary. The globular inclusions are chiefly sulfide and silicate. 10 mL HNO3, 20 mL HCl, 30 mL glycerol.
layer, indicating a greater amount of alpha phase. This feature is common for oxygen-rich atmosphere heat treatment in titanium alloys. The fracture nucleation is related to this layer, as could be observed in Fig. 9. The analysis of the screws of the case 2 indicated the formation of shrinkages oriented across the section, as showed in Fig. 10. The microstructure is typical of the as cast, Co–Cr high temperature resistant alloys, consisting in cobalt-rich solidsolution matrix containing carbide within grains and at grain boundaries (Fig. 11).
3.4. FE analysis The results of the FE simulation are shown in Fig. 12. The friction produces increased stress to values around 150 MPa and 200 MPa, respectively, for the Ti6Al4V and Cr–Co alloy. Von Mises stress remains almost the same with angle up to
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Fig. 12. The graphic shows the Von Mises stress experienced by de screw thread regarding to the load angle, metallic material tested (Ti6Al4V or Cr–Co alloy) and interface condition between the metal-backed and bone tissue (bonded or with friction coefficient l = 0.15).
30°. Moreover, the stress dramatically increases with increasing the angle above 45°, especially for the case of the Ti6Al4V alloy in friction.
4. Discussion Total hip arthroplasty is considered one of the safest and most successful orthopedic procedures. Polished and tapered prosthetic stem models such as original Charnley and Exeter have been showing excellent results in hip function restore and patient satisfaction, with survivor rate higher than 90% for more than 20 years [17–25]. However, the greatest current challenge in total hip arthroplasty is to increase the survival rate for younger and more active people. Innovative projects of cementeless components have been tried in the last years, as the acetabular component using metal-backed. As was exposed before, previous studies confirm a useful life of only 5.1 years in this case. This rate still not satisfactory and point to the necessity of improve the knowledge in prosthesis project. This paper analyzed two cases of failed metal-backed screws with five patients in less than 5 years after the surgery. The radiographic analysis showed metal-backed displacement after screw fractures. The removal of the screw tip inside the bone may lead to further bone loss and the necessity for bone grafting, impairing the interface quality of a new revision. The biological fixation using metal-backed is very dependent on short-term interaction between bone and metal. Screws are used to increase the stability and provide the initial short-term fixation. In cases where biological fixation deficiency manifests, screws function is maintain the implant stability. Therefore, the materials and processes used to manufacturing the screws should be chosen from those that produce the higher fatigue performance. There are some contributions in the literature dealing with the study of the fatigue performance of biomaterials. The Cr–Co alloys may have increased twice their fatigue performance if manufactured from forging or Hot Isostatic Pressure (HIP) instead of conventional casting [26–28]. The Ti6Al4V alloy shows something as 40% fatigue strength reduction of the lamellar or platelike microstructure when compared to the finely dispersed. In all cases studied here the fracture occurred near the first thread from the screw hub. In this specific region is expected to have higher stress concentration due to tightening and to loads applied when the metal tries to sliding against to the bone [29]. In case 1, the screw had aspect of bending stress fatigue, with striations and some shear between the deformed platelike grains. The failure was encouraged by the oxygen-rich Ti6Al4V alloy high temperature transformation microstructure. According to the literature, it has a low performance on fatigue nucleation [30]. Furthermore, transformed lamellar Ti6Al4V alloy presents corrosion fatigue strength reduction in tests simulating body environment [31], denoting a tendency to high sensitivity to environmentally assisted cracks [32,33]. Perhaps due to these features, the ASTM F136 standard determines the microstructure of the Ti6Al4V alloy to use in surgical implants should be a fine dispersion of alpha and beta phase, resulting from processing in the alfa + beta field. The microstructure should not contain neither continuous alpha network in beta primary grain boundary nor coarse elongated alpha plates. Indeed, the material with a fine alpha + beta dispersion has better fatigue properties. The cross section rupture of two screws was seen in case 2. The fractures had rough aspect with straight and parallel steps along the surface. The fracture surface morphology as well as the micromechanics indicates brittle fracture. It can be seen smooth regions of fracture propagation, with striations in the matrix phase and some dimples outlining carbides. The metallographic showed typical as cast cobalt–chromium alloys gross dendritic structure and the formation of shrinkages outlining the dendrites in some regions. Shrinkages are typical casting defects and substantially reduce the fatigue performance of the material by acting as stress risers. This crosswise aligned dendritic structure may be related to the steps formation on the fracture surface and was also responsible for the scoring marks and the brittle fractures. Fatigue propagation mechanism
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was also found between the dendrites of some of the thread crests of the third screw, indicating it was supporting a high load and was also bound to break or to be pulled out of the bone. The manufacture using this alloy was for compatibility with the acetabular metal-backed which is also chrome-cobalt. Materials in load bearing contact must be compatible (preferably be the same material) to prevent the formation of galvanic cell corrosive processes. However, the screws would have resisted more if they had been made from a finer cobalt–chromium alloy structure as those obtained by forging or Hot Isostatic Pressure (HIP) to prevent fractures and scoring marks in machine process. The 2D model used in the simulation was sufficient to evaluate the stresses in the screw thread. We have noted that the effect of the load applied in one side of the hemisphere do not affect significantly the stresses of the screw in the other side. The stress experienced by the screws is lower when the interface between the metal-backed and the bone is bonded and in the angle range between 0° and 30°. The friction coefficient produces increased stress to values around 150 MPa and 200 MPa, respectively, for the Ti6Al4V and Cr–Co alloy. The fatigue strength of the as cast Cr–Co alloy is in the range of 250–300 MPa [26]. If one considers the fatigue strength reduction due to the shrinkages, the stress of 200 MPa could be enough to provide the fatigue failure. The same analysis can be evaluated for the case of the Ti6Al4V alloy if it is considered the published results of the fatigue strength reduction due to microstructural transformation and body environment effect [26– 28]. The simulation showed also that the Von Mises stress dramatically increases with increasing the angle above 45°, especially for the case of Ti6Al4V alloy in friction. The condition when the load is imposed by the head in the periphery of the liner simulates the prelude for dislodgement of the head and it is considered a critical situation in the clinical practice. The results of the present study indicate that this situation enhances the stress and can promote the screws fracture in a very short term. 5. Conclusions Screws are often used in cementless acetabular metal-backed to increase the fixation stability. Although they are sometimes seen as secondary elements, the rupture of the screws can provide a premature failure. Therefore, the most resistant materials and manufacturing process should be selected in the design of these elements. In the two cases it could be observed microstructural features that lead to poor performance in fatigue and corrosion fatigue: (a) titanium alloy microstructure consisting of gross plates of alpha phase with oxygen-rich alpha case, (b) cobalt chrome as cast alloy with dendritic microstructure and formation of unfavorably aligned microvoids.
Acknowledgements The authors would like to thank the financial support of CNPq, CAPES, FINEP and FAPITEC. References [1] Schulte KR, Callaghan JJ, Kelley SS, Johnston RC. The outcome of Charnley total hip arthroplasty with cement after a minimum twenty-year follow-up. The results of one surgeon. J Bone Joint Surg Am 1993;75:961–75. [2] Rodriguez Jose´ A. Acetabular fixation options: notes from the other side. J Arthroplasty 2006;21(4) [Suppl. 1]. [3] Spears Iain R, Peiderer Martin, Schneider Erich, Hille Ekkehard, Morlock Michael M. The effect of interfacial parameters on cup bone relative micromotions, a finite element investigation. J Biomech 2001;34:113–20. [4] Sharp RJ, O’Leary ST, Falworth M, Cole A, Jones J, Marshall RW. Analysis of the results of the C-Fit uncemented total hip arthroplasty in young patients with hydroxyapatite or porous coating of components. J Arthroplasty 2000;15(2). [5] Berry DJ, Barnes CL, Scou RD, Cabanela ME, Poss R. Catastrophic failure of the polyethylene liner of uncemented acetabular components. Bone Joint Surg [Br] 1994;76-B:575–8. [6] Heaton-Adegbile P, Russery B, Taylor L, Tong J. Failure of an uncemented acetabular prosthesis – a case study. Eng Fail Anal 2006;13:163–9. [7] Barrack RL, Burke DW, Cook SD, Skinner HB, Harris WH. Complications related to modularity of total hip components. J Bone Joint Surg [Br] 1993;75B:688–92. [8] Stiehl JB, Mahfouz MR. Catastrophic failure of a modular revision total hip polyethylene insert. J Arthroplasty 2007;22(1). [9] Ito H, Minami A, Kondo E, Fujita M, Ubayama Y, Matsuno T. Destruction of acetabular bone caused by early failure of a constrained acetabular component. J Arthroplasty 2001;16(7). [10] Brien WW, Salvati EA, Wright TM, Nelson CL, Hungerford DS, Gilliam DL. Dissociation of acetabular components after total hip arthroplasty – report of four cases. J Bone Joint Surg Am 1990;72:1548–50. [11] Yun AG, Padgett D, Pellicci P, Dorr LD. Constrained acetabular liners mechanisms of failure. J Arthroplasty 2005;20(4). [12] Cheung K, Yung S, Wong K, Chiu K. Early failure of smooth hydroxyapatite-coated press-fit acetabular cup – 7 years of follow-up. J Arthroplasty 2005;20(5). [13] Ronald R. Hugate, Ian D. Dickey, Qingshan Chen, Christina M. Wood, Franklin H. Sim, Michael G. Rock. Fixed-angle screws vs standard screws in acetabular prosthesis fixation – a cadaveric biomechanical study. J Arthroplasty 2009;24(5). [14] 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 Failure Anal 2008;15:981–8. [15] Griza S, Cê AN, Silva EP, Bertoni F, Reguly A, Strohaecker TR. Acetabular metal-backed fatigue due to severe wear before revision. Eng Failure Anal 2009;16:2036–42. [16] Griza Sandro, dos Santos Silvando Vieira, Ueki Marcelo Massayoshi, Bertoni Fabiano, Strohaecker Telmo Roberto. Case study and analysis of a fatigue failure in a THA stem. Eng Failure Anal 2013;28:166–75. [17] Older J. Low-friction arthroplasty of the hip. A 10–12-year follow-up study. Clinic Orthopaed 1986;211:36. [18] Older J. Charnley low-friction arthroplasty – a worldwide retrospective review at 15–20 years. J Arthroplasty 2002;17(6):675–80. [19] 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 [Jul].
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Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Metallurgical failure analysis of acetabular metal-backed screws Sandro Griza a,⇑, Thiago Figueiredo Azevedo a, Silvando V. dos Santos a, Eduardo K. Tentardini a, Telmo R. Strohaecker b a b
Programa de Pós-Graduação em Ciência e Engenharia dos Materiais, Universidade Federal de Sergipe, São Cristóvão, Brazil Programa de Pós-Graduação em Engenharia Metalúrgica, de Minas e Materiais, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil
a r t i c l e
i n f o
Article history: Received 9 January 2013 Received in revised form 6 March 2013 Accepted 31 March 2013 Available online 8 April 2013 Keywords: THA Metal-backed screws Fatigue Ti6Al4V Cr–Co
a b s t r a c t Screws are often used in total hip arthroplasty to increase the cementless acetabular metalbacked fixation stability. This study aimed at identifying metallurgical factors related to metal-backed screw fractures in two cases of premature total hip arthroplasty revision. A preliminary analysis of radiographs and retrieved parts was performed. Fractured screws were subjected to fracture analysis, energy dispersive X-ray spectroscopy (EDS) and metallographic analysis. A finite element analysis was performed to predict the stresses experienced by the screws. Two factors were identified: titanium alloy microstructure, consisting of alpha phase plates with oxygen-rich alpha case and as cast Cr–Co alloy with dendritic microstructure and unfavorably aligned shrinkage formation. Finite element analysis has showed enough level of stress to predict screw fracture in the case of unbounded metalbacked and bone interface. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The actual model of total hip arthroplasty consists basically of a femoral stem which has a spherical head under load bearing contact with an acetabular component, called liner. The stem is usually made of austenitic stainless steel, Cr–Co alloy or Ti6Al4V, whereas the liner is made of a hollow hemispherical ultra high molecular weight polyethylene (UHMWPE), or metal (Cr–Co or steel), or ceramic. This device must possess low friction among moving parts, maintaining the degree of freedom of a healthy hip. The acetabular component should be well fixed into the hip to ensure long term stability. Nowadays there are two concepts of fixation well accepted by the academic and medical community: the mechanical and the biological system. The first one is the current standard and consists on using bone cement to fix the polymeric liner into the spongy bone cavity previously rammed by the surgeon [1]. However, some difficulties have been reported to this concept, such as the fact that procedure success is dependent of two factors: the surgeon technique and the reduced time to complete the operation, since the bone cement becomes solid during the polymerization and loses ability to be quickly molded [2]. The biological fixation technique consists on connecting the liner to a cementless and metallic hemispherical shell, called metal-backed. The outer surface of the metal-backed is functionalized from techniques such as porous coating, sintered bead or plasma spray in order to improve bone growth and achieve short term stability. This prosthetic option was developed with the advantage to present higher stiffness, better stress distribution along the bearing surface and superior modularity, which allows significant flexibility in revision surgery, permitting for surgeons the ability to change femoral head size, liner offset and liner build up with preservation of a well fixed metal-backed. Besides, it reduces the obligation to stock several head and stem sizes and allows the final choice of neck length and head diameter to be done after stem implantation. A press fit ⇑ Corresponding author. Tel.: +55 79 21056888; fax: +55 79 21056845. E-mail address: [email protected] (S. Griza). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.03.021
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assembly is achieved when the acetabular bony cavity is rammed with a curvature radius smaller than the metal-backed. On the other hand, when the two sides have the same curvature, they provide an exact fit condition [3]. Screws are sometimes used to enhance the biological fixation stability. In some cases, when the surgeon feels that the metal-backed are well placed in the bone, he may choose to discard the screw use [4]. The long-term stability of the acetabular component is limited by wear of the liner which must be replaced before metal/ metal contact [5]. However, some cases of premature failures have been previously reported [5–13], many of them associated to displacements between the metal-backed and bone which becomes an interfacial loosening manifestation. As a result, structural integrity of the screws plays an important role in delaying the premature failure event. The common practice of metal-backed screw fixation is to use only two proximal screws in the posterior column of ileum. This is applied to avoid the problems with potential neurovascular injuries, frequently associated with bicortical screws in the ischium and pubis regions [13]. This study was dedicated to analyze two cases of acetabular metal-backed screw fractures in two different prosthesis materials (Ti6Al4V and Cr–Co alloy) occurred less than 5 years before revision. The microstructural analyses of the broken screws were evaluated by OM, SEM, EDS and finite element analysis. 2. Materials and methods 2.1. Preliminary analysis This paper analyzes two cases of acetabular metal-backed screw failures, each one from different patients: Case 1: male patient M.S.M., 82 kgf weight, 71 years old, experienced revision 2.5 years after the primary arthroplasty. Case 2, male patient A.H.R., 76 kgf weight, 62 years old, experienced revision 4.2 years after the primary arthroplasty. In general cases, 5 years before revision is a short term period for total hip arthroplasty [5–14]. To the specific case of cementeless acetabular using porous metal-backed in younger patients, Sharp et al. [4] confirm a useful life before revision of 5.1 years. The material used in the metal-backed manufacture was Ti6Al4V and Cr–Co alloy for cases 1 and 2, respectively. There were six screws in total (three in each shell), one fractured screw in case 1 (pubis region) and two fractured in case 2 (both in ileum). Failures were observed from radiographies before total hip replacement revision. The post retrieval protocol consisted in identify the previous fractured screw positions regarding to the hip reconstruction and cleaning parts with tap water. All the retrieval elements were then subjected to autoclave cleaning followed by preliminary visual analysis. The radiographies taken before revision were also analyzed. 2.2. Fracture and EDS analyses Screws fracture analysis was performed by using a low magnification stereomicroscope (Zeiss Stemi 2000 C) and a scanning electron microscope (SEM – Philips XL-20) for high magnification. The alloys were qualitatively analyzed by energy dispersive X-ray spectroscopy (EDS microprobe). Screws were embedded in thermocouple resin (Bakelite) and grinded until reached their longitudinal cross section. Metallographic samples were performed in order to show the respective fracture initiation point of the screws. The outer screw diameters were measured by a caliper (0.05 mm resolution) before sample mounting. During grinding, the sample thicknesses were controlled to reach the mean section and after grinding, the samples were polished with diamond suspension (1 lm) in an appropriate polishing cloth. The EDS analyses were performed over the polished surface of the samples. 2.3. Microstructural analysis Metallographic samples were obtained from the fracture initiation point along the longitudinal plane of symmetry of the screws (same samples as described before) and microstructural analyses were performed from optical microscopy (Carl Zeiss Axioscope A1). 2.4. FE analysis The components were measured using a caliper with resolution of 0.05 mm to obtain its geometrical features and to perform 2D finite element analyses of the acetabular reconstruction, as showed in Fig. 1. The thread curvature radius of the screws was reproduced from the measurement of the metallographic samples in optical microscopy. Head, polymeric liner, metal-backed and acetabulum were made as connecting hemispheres of 28, 38, 42 and 72 mm, respectively. Two screws disposed diametrically opposed at 45° from the axis of symmetry of the reconstruction were considered to fix the metal-backed to the bone tissue. For simplification, the screws were modeled using Abaqus software (Abaqus 6.5, Hibbit, Karlsson e Sorensen, Inc., Pawtucker) with only the thread where the fracture occurred. The parameters software applied were: element type used was the quadratic 2D and mesh refinement was evaluated in the thread region. The refinement was done by section
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Fig. 1. Hemispherical acetabular model. The external bone tissue was restricted and the load (L) was applied from the head to the liner in each step angle. The figure shows only three steps: 0°, 45° and 75°.
Table 1 Mesh attributes and material’s constitutive properties. Material
E (GPa)
Poisson
N° elements
N° nodes
Bone tissue Ti6Al4V Cr–Co UHMWPE
0.33 110 240 1.07
0.32 0.34 0.29 0.41
1382 1573 1573 100
4325 5048 5048 359
partition in the limits of the curvature radius. By the way, this specific screw part was refined apart of the other regions of the screw. It was assumed convergence within 5% error. The two failure cases were simulated, constitutive properties obtained from the literature [15] and mesh characteristics are summarized in Table 1. After visual inspection, the external side of the acetabulum appears constrained. An external load of 2300 N was applied from the head to the liner [16] and it was applied in steps of 15°, beginning in the axis of symmetry (0°) and following up to 75°. Two interfacial conditions between the metalbacked and the bone tissue were simulated: (i) bonded and (ii) with friction coefficient of 0.15. These simulations were prepared to verify the effect of the stable fixation in short term and the interfacial loosening manifestation on the screw stress level, respectively. All the other interfaces were considered as bonded. The maximum Von Mises stress experienced by the screw thread was correlated to the load angle, material (Ti6Al4V or Cr–Co) and the interface condition between the metalbacked and the bone tissue (bonded or 0.15 friction coefficient).
3. Results 3.1. Preliminary analysis The radiographic analysis showed similar features for both cases. The metal-backed loosening and screw fractures were observed. The radiography achieved prior to revision of the case 1 shows wear of the liner, since the stem head was displaced upward in the proximal direction, as showed in Fig. 2. Rotation of the metal-backed was encouraged by a distal screw rupture. The image also shows the screw tip lodged into the trabecular bone tissue. Visual analysis indicated both metal-backed are porous coated. Fig. 3a shows the metal-backed of case 1. Despite some wear degree of the liners, they had not reached the metal/metal contact yet, as showed in Fig. 3b. The rupture occurred in the cross section corresponding to the first thread from the screw hub in one screw for case 1 (Fig. 4a) and in two screws for case 2 (Fig. 4b). Thread crest fractures were also observed in a third screw for case 2, as illustrate in Fig. 4c. Another evident aspect in these screws of case 2 is scoring marks at the longitudinal axis and secondary crack at the threads. The heads and femoral stems showed no signs of significant damage related to the revision.
3.2. Fracture and EDS analyses The screw in case 1 presented a unidirectional bending fracture which started in a specific region at the thread root and propagated by a large amount until the final rupture was reached. It was related to shear of the thread section (Fig. 5a). Fig. 5b shows an SEM image of the nucleation site. Fatigue features (striations) were identified by high magnification analysis (SEM) in the fracture surface near the nucleation site (Fig. 5c), whereas dimples formation was observed next to the
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Fig. 2. Radiography obtained just before the revision of the case 1. It is possible to observe the metal-backed rotation. The tip of the fractured distal screw is lodged in the acetabulum (arrow).
Fig. 3. Images showing (a) case 2 porous coated metal-backed, and (b) the case 1 liner concavity. Both metal-backed have 50 mm outside diameter.
propagation end (Fig. 5d). Dimples are often seen at the end of the fatigue propagation and it may occur due to increased stress intensity factor at the crack tip. Both screws fractured in the cross section of the case 2 showed a rough and brittle fracture. This feature is commonly seen in coarse grain material fractures. Linear steps also were seen on the fracture surface in both screws of case 2. These steps have similarity with beach marks; however they are straight and not semi-elliptical as expected. Fig. 6 shows a SEM analysis
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Fig. 4. Image (a) shows the fractured screw of the case 1. Fracture takes place in the region of the first thread. Image (b) shows the screws of the case 2: the cross section of two fractured screws and the remaining 24 mm length screw. Image (c) shows fractures in the thread crests of the remaining screw. Longitudinal scoring marks also can be seen.
Fig. 5. Images of case 1 screw: (a) the smoothness aspect of the fracture surface. The arrow points to the fracture initiation site. (b) Radial lines emanating from the fracture initiation. (c) The appearance of fatigue propagation on the surface near the fracture initiation, with striations (white arrows) and some shear between the deformed grains (black arrows). (d) Formation of dimples on the fracture surface near the end of the fatigue propagation.
and can be observed the separation without plastic deformation, the fatigue appearance and the dimples formation. The remaining screw of the case 2 had fractures in the thread crests. Fig. 7 shows the formation of interdendritic fracture (striations which indicate fatigue propagation), the scoring marks and longitudinal secondary cracks on the thread crests. Fig. 8 shows the EDS analyses for the screw in case 1, indicated titanium, aluminum and vanadium and for case 2 screws, the analysis shows peaks in chromium, cobalt, molybdenum and silicon. 3.3. Microstructural analysis The case 1 screw presented a typical Ti6Al4V alloy microstructure transformed at temperatures above the beta transition, with the formation of platelike alpha phase and beta intergranular. It was observed the formation of a lighter, thin surface
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Fig. 6. Images at left showing brittle aspect of cross section fracture of both case 2 screws. The roughness appearance and flat, parallel steps over the surfaces can be seen. The images at right show the respective fracture micromechanisms, with particle separation, fatigue appearance and dimples outlining carbide particles (white arrows).
Fig. 7. Thread crest fractures, secondary cracks and the transversal scoring marks of the remained case 2 screw (a). High magnification of interdendritic separation and fatigue striations of the fractured crests (b).
Fig. 8. Microprobe EDS spectrum of the case 1 screw (left) and the case 2 screw (right).
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Fig. 9. Fracture initiation of case 1 screw associated to a lighter surface layer (a). The microstructure consists of alpha phase plates and intergranular beta phase (b). Etchant: 0.2%, 0.4% HF and HNO3 diluted in water.
Fig. 10. As polished metallographic transversal section of the case 2 screw head showing shrinkages alignment.
Fig. 11. Representative microstructures of the case 2 screws. Austenite dendritic matrix contains eutectic carbide chains and scattered carbide particles. The right image shows in detail the patch of lamellar constituent at a grain boundary. The globular inclusions are chiefly sulfide and silicate. 10 mL HNO3, 20 mL HCl, 30 mL glycerol.
layer, indicating a greater amount of alpha phase. This feature is common for oxygen-rich atmosphere heat treatment in titanium alloys. The fracture nucleation is related to this layer, as could be observed in Fig. 9. The analysis of the screws of the case 2 indicated the formation of shrinkages oriented across the section, as showed in Fig. 10. The microstructure is typical of the as cast, Co–Cr high temperature resistant alloys, consisting in cobalt-rich solidsolution matrix containing carbide within grains and at grain boundaries (Fig. 11).
3.4. FE analysis The results of the FE simulation are shown in Fig. 12. The friction produces increased stress to values around 150 MPa and 200 MPa, respectively, for the Ti6Al4V and Cr–Co alloy. Von Mises stress remains almost the same with angle up to
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Fig. 12. The graphic shows the Von Mises stress experienced by de screw thread regarding to the load angle, metallic material tested (Ti6Al4V or Cr–Co alloy) and interface condition between the metal-backed and bone tissue (bonded or with friction coefficient l = 0.15).
30°. Moreover, the stress dramatically increases with increasing the angle above 45°, especially for the case of the Ti6Al4V alloy in friction.
4. Discussion Total hip arthroplasty is considered one of the safest and most successful orthopedic procedures. Polished and tapered prosthetic stem models such as original Charnley and Exeter have been showing excellent results in hip function restore and patient satisfaction, with survivor rate higher than 90% for more than 20 years [17–25]. However, the greatest current challenge in total hip arthroplasty is to increase the survival rate for younger and more active people. Innovative projects of cementeless components have been tried in the last years, as the acetabular component using metal-backed. As was exposed before, previous studies confirm a useful life of only 5.1 years in this case. This rate still not satisfactory and point to the necessity of improve the knowledge in prosthesis project. This paper analyzed two cases of failed metal-backed screws with five patients in less than 5 years after the surgery. The radiographic analysis showed metal-backed displacement after screw fractures. The removal of the screw tip inside the bone may lead to further bone loss and the necessity for bone grafting, impairing the interface quality of a new revision. The biological fixation using metal-backed is very dependent on short-term interaction between bone and metal. Screws are used to increase the stability and provide the initial short-term fixation. In cases where biological fixation deficiency manifests, screws function is maintain the implant stability. Therefore, the materials and processes used to manufacturing the screws should be chosen from those that produce the higher fatigue performance. There are some contributions in the literature dealing with the study of the fatigue performance of biomaterials. The Cr–Co alloys may have increased twice their fatigue performance if manufactured from forging or Hot Isostatic Pressure (HIP) instead of conventional casting [26–28]. The Ti6Al4V alloy shows something as 40% fatigue strength reduction of the lamellar or platelike microstructure when compared to the finely dispersed. In all cases studied here the fracture occurred near the first thread from the screw hub. In this specific region is expected to have higher stress concentration due to tightening and to loads applied when the metal tries to sliding against to the bone [29]. In case 1, the screw had aspect of bending stress fatigue, with striations and some shear between the deformed platelike grains. The failure was encouraged by the oxygen-rich Ti6Al4V alloy high temperature transformation microstructure. According to the literature, it has a low performance on fatigue nucleation [30]. Furthermore, transformed lamellar Ti6Al4V alloy presents corrosion fatigue strength reduction in tests simulating body environment [31], denoting a tendency to high sensitivity to environmentally assisted cracks [32,33]. Perhaps due to these features, the ASTM F136 standard determines the microstructure of the Ti6Al4V alloy to use in surgical implants should be a fine dispersion of alpha and beta phase, resulting from processing in the alfa + beta field. The microstructure should not contain neither continuous alpha network in beta primary grain boundary nor coarse elongated alpha plates. Indeed, the material with a fine alpha + beta dispersion has better fatigue properties. The cross section rupture of two screws was seen in case 2. The fractures had rough aspect with straight and parallel steps along the surface. The fracture surface morphology as well as the micromechanics indicates brittle fracture. It can be seen smooth regions of fracture propagation, with striations in the matrix phase and some dimples outlining carbides. The metallographic showed typical as cast cobalt–chromium alloys gross dendritic structure and the formation of shrinkages outlining the dendrites in some regions. Shrinkages are typical casting defects and substantially reduce the fatigue performance of the material by acting as stress risers. This crosswise aligned dendritic structure may be related to the steps formation on the fracture surface and was also responsible for the scoring marks and the brittle fractures. Fatigue propagation mechanism
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was also found between the dendrites of some of the thread crests of the third screw, indicating it was supporting a high load and was also bound to break or to be pulled out of the bone. The manufacture using this alloy was for compatibility with the acetabular metal-backed which is also chrome-cobalt. Materials in load bearing contact must be compatible (preferably be the same material) to prevent the formation of galvanic cell corrosive processes. However, the screws would have resisted more if they had been made from a finer cobalt–chromium alloy structure as those obtained by forging or Hot Isostatic Pressure (HIP) to prevent fractures and scoring marks in machine process. The 2D model used in the simulation was sufficient to evaluate the stresses in the screw thread. We have noted that the effect of the load applied in one side of the hemisphere do not affect significantly the stresses of the screw in the other side. The stress experienced by the screws is lower when the interface between the metal-backed and the bone is bonded and in the angle range between 0° and 30°. The friction coefficient produces increased stress to values around 150 MPa and 200 MPa, respectively, for the Ti6Al4V and Cr–Co alloy. The fatigue strength of the as cast Cr–Co alloy is in the range of 250–300 MPa [26]. If one considers the fatigue strength reduction due to the shrinkages, the stress of 200 MPa could be enough to provide the fatigue failure. The same analysis can be evaluated for the case of the Ti6Al4V alloy if it is considered the published results of the fatigue strength reduction due to microstructural transformation and body environment effect [26– 28]. The simulation showed also that the Von Mises stress dramatically increases with increasing the angle above 45°, especially for the case of Ti6Al4V alloy in friction. The condition when the load is imposed by the head in the periphery of the liner simulates the prelude for dislodgement of the head and it is considered a critical situation in the clinical practice. The results of the present study indicate that this situation enhances the stress and can promote the screws fracture in a very short term. 5. Conclusions Screws are often used in cementless acetabular metal-backed to increase the fixation stability. Although they are sometimes seen as secondary elements, the rupture of the screws can provide a premature failure. Therefore, the most resistant materials and manufacturing process should be selected in the design of these elements. In the two cases it could be observed microstructural features that lead to poor performance in fatigue and corrosion fatigue: (a) titanium alloy microstructure consisting of gross plates of alpha phase with oxygen-rich alpha case, (b) cobalt chrome as cast alloy with dendritic microstructure and formation of unfavorably aligned microvoids.
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