An investigation of the aseptic loosening of an AISI 316L stainless steel hip prosthesis

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Biomed. Mater. 5 (2010) 045012 (8pp) doi:10.1088/1748-6041/5/4/045012

An investigation of the aseptic loosening ofan AISI 316L stainless steel hip prosthesisMatjaz Godec1, Aleksandra Kocijan1, Drago Dolinar2,Djordje Mandrino1, Monika Jenko1 and Vane Antolic2

1 Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia2 Orthopaedic Clinic Ljubljana, Zaloska 9, 1000 Ljubljana, Slovenia

E-mail: matjaz.godec@imt.si

Received 11 May 2010Accepted for publication 12 July 2010Published 3 August 2010Online at stacks.iop.org/BMM/5/045012

AbstractThe total replacement of joints by the implantation of permanently indwelling prostheticcomponents has been one of the major successes of modern surgery in terms of relieving painand correcting deformity. However, the aseptic loosening of a prosthetic-joint component is themost common reason for joint-revision surgery. Furthermore, it is thought that wear particlesare one of the major contributors to the development and perpetuation of aseptic loosening.The aim of the present study was to identify the factors related to the aseptic loosening of anAISI 316L stainless steel total hip prosthesis. The stem was evaluated by x-ray photoelectronspectroscopy, with polished and rough regions being analyzed in order to establish thedifferences in the chemical compositions of both regions. Specific areas were examined usingscanning electron microscopy with energy dispersive x-ray spectroscopy and light microscopy.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The total replacement of joints by the implantation ofpermanently indwelling prosthetic components has been oneof the major successes of modern surgery in terms of relievingpain and correcting deformity [1]. The aseptic looseningof a prosthetic-joint component is the most common reasonfor joint-revision surgery. However, there have been a fewreported cases where a fracture of the stem neck has occurred,most likely due to the development of an inappropriatemicrostructure [2, 3]. The lifetime of cemented implantscan be improved by enhancing the bond between the implantand the cement [4]. Nevertheless, it remains an openquestion as to whether the cellular reactions that occur nearthe implant are solely responsible for this form of jointfailure. While abnormal wear at the bearing surface mightgive rise to excessive particle generation, the bone loss inrelation to inflammation caused by these particles might inturn result in the loosening of the device and subsequentabnormal mechanical loading. In any case, wear particlesare now considered to be one of the major contributors to thedevelopment and perpetuation of aseptic loosening [5, 6].

Particles of all the biomaterials used so far can provoke anadverse biological reaction in periprosthetic tissues involvingthe formation of osteolytic foreign-body granulomas, theinhibition of bone formation and joint-fluid production [7].Bone resorption leads ultimately to implant loosening, andthere is a clinically evident association between the wearrate and the incidence of periprosthetic osteolysis [8]. Thewear between the primary bearing surfaces is considered themost important source of prosthetic particles due to micro-abrasions and micro-adhesions, regardless of the prostheticdesign or the material characteristics [9]. The wear ratesamong the individual bearers are different, depending onthe implant types, the surgical techniques and patient-relatedfactors [9]. The wear debris is formed at the prosthetic-jointarticulations, the modular interfaces and the non-articulatinginterfaces [10, 11]. Although a wide range of particles hasbeen found, the majority of particles formed are less than5 μm in diameter and randomly shaped. Studies havesuggested that the cellular response to particles may varywith the size, shape, composition, charge and the numberof particles [12, 13]. Furthermore, it was proposed thatparticle phagocytosis represents an important component of

1748-6041/10/045012+08$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK

Biomed. Mater. 5 (2010) 045012 M Godec et al

the cellular response to implants. The particles generatedare shed directly into the synovial fluid, from where they areremoved by macrophages in the fluid and the synovial liningcell layer.

Biological and mechanical factors have been implicatedin the early and late stages of the development of osteolysisfollowing joint replacement. Inflammatory reactions developat early stages as part of the resolution process. These reactionsprimarily include an increased circulation and elevated fluidlevels in the vicinity of the affected tissue [14, 15]. Inaddition, apart from a massive recruitment of macrophagesto the site of the injury, some studies identified the recruitmentof lymphocytes, indicating that the immune system may beinvolved in this inflammatory response [14, 16].

Metal implants usually exhibit a low level of wear (40–100times lower than ultra-high-molecular-weight-polyethyleneUHMWPE), a good surface finish and excellent mechanicalproperties. The drawbacks of metals are the metallic,electrochemical corrosion risks (low bio-compatibility)accelerated by the ions present in the body fluids and thepresence of oxygen [17–19]. Metal implants are also subjectedto the processes of adhesive and abrasive wear. Thisadhesive wear occurs at the interface of two materials withsufficient ductility, where small particles are extracted fromboth materials as a result of the adhesive forces. In abrasivewear, hard particles originating from the materials or from anexternal source cause abrasive action, which is demonstratedby a scratchy surface. Wear polishing is an additional type ofabrasive wear, which is caused by the movements of micron-to submicron-sized abrasive particles. The wear regions givea very polished appearance, characteristic for small loadingsof the joints. These conditions generate chemically active andtoxic degradation products and produce wear particles. Thewear particles are very small (mostly 10–25 nm in diameter),but the number of particles exceeds those of UHMWPE (10 to500 times more). The small size and the large number of theparticles raise a new issue of remote distribution in the humanbody and the biological effects on various cells and tissues.Some particles may even attach or dissolve in the lymphaticvessels. The hematological spread of the metal particles mayaccess any tissue in the body, such as the liver, kidneys andeven brain. Metal debris in the lymph nodes causes structuralchanges, including necrosis and fibrosis. There is also anincreased risk of the development of tumours of the lymphaticsystem, hypersensitivity and toxicity reactions. [16]

In the present study an AISI 316L stainless steeltotal hip prosthesis with a larger content of non-metallicinclusions than usually used in an orthopaedic applicationwas evaluated by x-ray photoelectron spectroscopy (XPS),with polished and rough regions being analyzed in orderto establish the differences in the chemical compositions ofboth regions. Specific areas were examined using scanningelectron microscopy with energy dispersive x-ray spectroscopy(SEM/EDX) and light microscopy.

2. Experimental details

A total hip endoprosthesis was analyzed after a revisionoperation on a male patient, 63 years aged, who had undergone

total hip arthroplasty for osteoarthritis of the left hip some106 months earlier, because of severe pain due to asepticloosening of both the femoral and acetabular components.The femoral component was AISI 316L stainless steelstem (�0.03 wt% C, 16–18 wt% Cr, 10–14 wt% Ni, 2–3 wt% Mo) and UHMWPE acetabular cup cemented withpolymethylmethacrylate (PMMA). The hip prosthesis wasimmediately cleaned, using only distilled water, after surgicalremoval. The specimens for the surface examination were cutfrom the hip stem and the polyethylene cup and cleaned inethanol in an ultrasonic bath.

The metal specimen for the microstructural examinationwas metallographically prepared by a standard procedure using3 and 1 μm diamond polishing paste, and then etched byglyceregia to reveal the grain boundaries. The specimens forelectron backscatter diffraction (EBSD) analyses were, afterthe polishing, given an additional polish with colloidal silicaoxide for 5 min and cleaned in an ultrasonic bath. EBSDanalysis is a relatively new technique which, in combinationwith scanning electron microscopy, enables texture analysisand most recently also phase analysis on the basis of electrondiffraction from the crystal structure of the analyzed surface.The specimens were analyzed using a light microscopeMicrophot FXA (Nikon) and with a FE-SEM JEOL JSM6500F field-emission scanning electron microscope withattached EDX (an INCA X-SIGHT LN2 type detector, INCAENERGY 450 software and an HKL Nordlys II EBSD camerausing Channel5 software). For the EDX analysis, a 15 kVaccelerating voltage and a probe current of 0.8 nA were used.The parameters were chosen because they represent a goodcompromise between the size of the analyzing volume and theovervoltage needed to detect the chemical elements present.The EBSD was performed at a 20 kV accelerating voltageand 2.7 nA probe current. The polyethylene specimen wascovered with a 4 nm AuPd conductive layer for a subsequentSEM examination. The surface morphology of the damagedacetabulum cup was investigated using a Carl Zeiss LSM 780confocal laser microscope.

The XPS analysis was performed in a VG Microlab 310FSEM/AES/XPS. For the XPS measurements, Mg Kα radiationat 1253.6 eV with an anode voltage × emission current =12.5 kV × 16 mA = 200 W power were used. For the ion-beam sputtering, Ar+ with a 3 keV energy and a 1 μA (order ofmagnitude) ion current over an area of 4 × 4 mm2 were used.A rough estimate of the sputtering rate for these parametersis approximately 1 nm min−1, which is not inconsistent withsome calibration measurements performed on metallic andoxide-type samples, as well as with some reference datafor the sputtering rates of Fe and Cr and their oxides [20].The acquired VG Microlab 310F data were processed usingthe Avantage v3.41 software, produced and supplied by themanufacturer of the instrument, and using the commerciallyavailable CasaXPS software [21] for the XPS and AES spectra.

3. Results and discussion

An AISI 316L stainless steel total hip prosthesis wasinvestigated microscopically and spectroscopically following

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(a) (b)

(c) (d )

Figure 1. (a) Polyethylene acetabular cup after being explanted from the patient. (b) Surface modification of the polyethylene acetabularcup’s inner wall, SE image. (c) Polished surface of the inner wall of the polyethylene acetabular cup, SE image. (d) Cavities of the innerwall of the polyethylene acetabular cup, SE image.

Figure 2. Cross-section of the acetabular cup over a large cavityoriginated by adhesive wear, SE image.

a surgical replacement in a patient after 106 months. Thesurface wear of the polyethylene acetabular cup (figure 1(a))and the stainless-steel femoral stem (figure 4(a)) wasstudied.

During the functional life of the hip prosthesis the wearbetween the polyethylene acetabular cup and the metallic headcaused the formation of polyethylene particles. Figure 1(b)shows the surface topography of the acetabular inner wall.Two phenomena occurred during the tribological activity ofthe acetabular cup and the metallic head. The first is thewear polishing of the polyethylene acetabular cup and theformation of very fine, irregularly shaped, elongated particles,

Figure 3. Surface topography of the acetabular cup over a largecavity obtained with the confocal microscope with a line profileshowing the topography along the marked line.

only a few microns in length (figure 1(c)). The naked-eyeexamination of the acetabular cup indicated a very smooth,glossy-like appearance of the surface (figure 1(a)). In contrast,in overloaded situations during the hip prosthesis’ lifetime,the adhesive wear plays a significant role and larger pieces

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Biomed. Mater. 5 (2010) 045012 M Godec et al

(a) (b)

(c) (d )

Figure 4. (a) Hip prosthesis with marked area of SEM/EDX surface examination where the upper part of the specimen is rough and thebottom part is polished; (b) SE image of the rough surface showing the pits at high magnification; (c) SE image of the polished surface,(d) EDX spectra performed over a larger area of the rough and polished surfaces.

of the inner wall of the polyethylene acetabular cup surfacecan be pulled out (figures 1(b), (d)). The cross-section oversuch a large cavity exposes its depth as being up to 300 μm(figure 2). A similar result was obtained when usingthe confocal microscope, with which the complete surfacetopography was revealed (figure 3). Approximately 30% ofthe entire surface is damaged by the adhesive wear, and if itis supposed that the average depth of the cavities is 200 μm,the volume loss caused just by the adhesive wear is estimatedaround 120 μm3.

The areas of the analyses of the metallic femoral stemwere selected according to the visually detectable rougherand polished spots marked in figure 4(a). The rougherareas are related to the manufacturing process of the stem.Figure 4(b) shows a rough surface area with plenty of smallmechanically produced pits visible at a higher magnification,while a polished surface is shown in figure 4(c). The startingrough surface of the metallic hip-prosthesis stem becomessmoother with friction at the cement/metal interface, whilethe metallic particles and ions accumulate in the body tissuesand fluids. PMMA or bone cement as a fixation medium forthe hip prostheses is an excellent type of fixation, especiallyfor elderly, relatively inactive patients who are usually alsosuffering from osteoporosis. Small cement fractures arethought to play a significant role in the initiation of the cementfailure [22]. Debonding between the cement and the implantis also thought to reduce the longevity of the implant fixation[22]. In hips, the stresses between the stem implant and thecement are mostly shear, whereas compressive-stress forces

prevail in the acetabulum. In polished stems, where debondingoccurs more easily, the forces transmitted to the cement aredifferent from those in pre-coated or blasted stems designedto avoid debonding. The wear particles from the cement,the metal and the polyethylene (PE) are claimed to play amajor role in the aseptic loosening. Extensive osteolysis inthe cemented hip artroplasties without any signs of infectionis related to the release of PMMA debris in the tissues bymicromotion, which initiates the cement failure. This will inturn cause a foreign-body reaction, initiate osteolysis and leadto loosening. The metal particles also cause third-body wearby themselves, resulting in more wear particles. In summary,one can assume that despite the mild foreign-body reactioncaused by the metal particles, these particles even induce acytokine release from the macrophages, which means thatmetal particles may also be involved in the process of asepticloosening [22].

The EDX microchemical analyses were performed in therough and smooth regions of the hip prosthesis (figure 4(d)).Both the EDX spectra are very similar; however, the roughsurface has a larger amount of carbon and oxygen on thesurface. Due to the larger surface area of the rough region,it is expected that the carbon contamination and the surfaceoxidation are more pronounced. The EDX analysis of largerareas on both surfaces showed increased Al-content comparedto the concentration in cross-section. The increased Alconcentration in the order of 1 wt% on the surface is related tothe surface treatment of the implant during the manufacturingprocess. Figure 5 presents two different types of alumina

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Biomed. Mater. 5 (2010) 045012 M Godec et al

(a)

(c)

(b)

(d )

Figure 5. (a) SE image of alumina particles in surface irregularities with marked spot of the EDX analysis; (b) EDX spot analysis of aparticle in (a); (c) SE image of a non-metallic inclusion with marked spot of the EDX analysis; (d) EDX spot analysis of a particle in (c).

particles: Al2O3 particles originating from the contaminationin the manufacturing process (figures 5(a), (b)) and Al2O3 non-metallic inclusions (figures 5(c), (d)). The former particlesare captured in larger surface irregularities, while the latterones are distributed evenly in the bulk and are smallercompared to the surface contamination. Light microscopy,as well as scanning electron microscopy images obtained inbackscattered mode, clearly confirms the aluminum oxideinclusions (figure 6). However, the number of these inclusionsis too large for the AISI 316L steel grade which is unusualin an orthopaedic application. The presence of both typesof alumina particles greatly accelerates the abrasive wear andcorrosion during the patient’s use of the prosthesis due to theirhardness and brittleness during micromotions at the interfacebetween the implant and the cement.

The microstructure of the examined hip-prosthesis stemconsists of austenite grains, with the grain boundaries beingfree of any carbides. Due to the low carbon content thepossibilities of intercrystalline corrosion are negligible incontrast to the stainless steels with a higher carbon content,where Cr23C6 carbides along the grain boundaries are formed[23]. Figure 7 shows the microstructure close to the surface,where one of the pits is shown in cross-section. Some sliplines can be seen on the very surface due to the higher surfacedeformation, most likely caused by sand-blasting. In somecases the formation of stress-induced martensite might lead tomicro-crack formation, which, especially in combination withintercrystalline corrosion, leads to material fracture in overloadsituations [24]. The EBSD technique was used to establish thepossible material texture and presence of unwanted phases

Figure 6. BE image of the polished specimen surface, where theAl2O3 non-metallic inclusions are darker due to thelower-atomic-number elements constituting the inclusions.

(e.g. delta ferrite, brittle sigma and chi phases). The EBSDanalyses performed for the larger areas showed equiaxedaustenite crystal grains with twin grain boundaries that haveno texture. The EBSD mapping also revealed no other phases(figure 8).

Both types of surfacees (rough and polished) were alsoinvestigated by XPS, which is a surface-sensitive technique.The XPS spectra from the rough as well as the polished parts ofthe surface were first measured on the ‘as-received’ samples.Then after 900 s of Ar+ sputtering a new set of XPS spectra

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Biomed. Mater. 5 (2010) 045012 M Godec et al

Table 1. Atomic concentrations of rough and polished surfaces obtained by XPS from the as-received and the sputter-cleaned rough andpolished surfaces.

Al C Ca Cr Fe Mo Ni O P Si

Rough surface0 s 1.6% 63.0% 1.2% 1.0% 1.3% 0.1% 0.6% 27.4% 1.3% 2.5%

900 s 9.1% 31.6% 1.8% 8.7% 18.8% 1.2% 9.4% 17.5% 0.7% 1.2%1800 s 6.2% 30.8% 1.8% 7.8% 23.9% 1.7% 11.4% 14.7% 0.5% 1.5%

Smooth surface0 s 7.6% 44.9% 1.6% 2.6% 2.2% 0.2% 0.9% 33.3% 2.2% 4.4%

900 s 7.1% 15.7% 2.1% 11.2% 26.3% 1.8% 10.9% 22.9% 1.1% 0.9%

Figure 7. SE image of hip microstructure close to the surface,where one of the pit cross-sections is shown, and the slip lineshaving different orientations depending on the grain orientation.

Figure 8. EBSD mapping shows the equiaxed austenite grains withtwin grain boundaries.

was measured. On the rough surface an additional 900 s ofsputtering and the spectra acquisition cycle were performed.The compositions obtained from the XPS survey spectra areshown in table 1. An additional sputtering cycle was applied tothe rough surface because of its morphology, however, without

much success, as can be seen from table 1 (by comparing thecarbon concentrations after 900 and 1800 s of sputtering).

Figures 9 and 10 show detailed XPS scans over selectedbinding-energy ranges for rough and polished surfaces ofthe as-received sample, respectively. Figure 9(a) shows thatthe surface chromium is nearly entirely bound as Cr2O3 (Cr2p3/2 and Cr 2p1/2 binding energies of 577.2 ± 0.2 eV and587.2 ± 0.2 eV) [26, 27], which is typical of chromiumstainless steel with an oxide layer [25–27]. Figure 9(b) showsthe complex nature of the surface-layer molybdenum thatappears in metallic form as well as in the form of two oxides,close to MoO2 and MoO3; the corresponding Mo 3d5/2 andMo 3d3/2 binding energies are 228.2 ± 0.3 eV and 231.3 ±0.3 eV, 230.8 ± 0.3 eV and 233.9 ± 0.3 eV, 232.6 ±0.3 eV and 235.7 ± 0.3 eV [26, 27]. In figure 9(c)Ni 2p3/2 at 853.0 ± 0.2 eV corresponds to metallic Ni[27, 28], while there are no oxide components. Infigure 9(d) there are O 1s components at 530.3 ± 0.2 eV,corresponding to the metallic oxide [26, 27], presumablyCr oxide, and another component at 532.2 ± 0.4 eV. Thecomponent at 533.5 ± 0.2 eV, observed only on non-sputtered surfaces, corresponds to loosely adsorbed surfacecontamination as these binding energies are characteristic ofthe oxygen in water and some complex hydrocarbons [28, 29],which form the majority of the surface contaminants.Figure 9(e) shows Ca 2p with Ca 2p3/2 at 348.1 ± 0.2 eVand Ca 2p1/2 at 351.7 ± 0.3 eV. Figure 9(f ) shows Si 2p at102.8 ± 0.3 eV corresponding either to SiO2 or some ternaryAl-Si-O compound. Figure 9(g) shows Al 2p at 75.1 ± 0.3 eVcorresponding to Al2O3. Figure 9(h) shows P 2p at 133.8 ±0.3 eV.

Qualitatively identical conclusions can be made fromfigures 10(a)–(h) about the polished surface of the as-receivedsample. The binding-energy values for the Ca 2p and P 2p areindependent of the sputtering time or the surface type. Theycorrespond reasonably well with calcium phosphate, as doesthe O 1s component at 532.2 ± 0.4 eV from figures 9(d), 10(d)[28, 29].

From the sputtering times, the sputtering-rate estimatesand the metallic-versus-oxide component ratios, the effectivethickness of the Cr oxide layer can be roughly estimated to bein the 5–10 nm range.

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(a) (b) (c) (d)

(e) (f ) (g) (h)

Figure 9. High-resolution XPS spectra from the rough-type surface; Cr 2p (a), Mo 3d (b), Ni 2p (c), O 1s (d), Ca 2p (e), Si 2p (f ), Al 2p(g), P 2p (h).

(a) (b) (c) (d)

(e) (f ) (g) (h)

Figure 10. High-resolution XPS spectra from the polished-type surface; Cr 2p (a), Mo 3d (b), Ni 2p (c), O 1s (d), Ca 2p (e), Si 2p (f ), Al2p (g), P 2p (h).

4. Conclusions

Metal implants can be subjected to the processes of adhesiveand abrasive wear as well as to wear polishing. Based on theinvestigations performed on an AISI 316L stainless steel hipprosthesis it was found that in the present case wear polishingand adhesive wear played an important role. Wear polishing ofthe polyethylene acetabular cup caused a very smooth, glossy-like appearance of the surface, in contrast to areas exposed

to adhesive wear, with rougher regions where larger pieceswere pulled out. Wear polishing of the femoral stem wasobserved to occur unevenly over the stem surface. This wasincreased at the locations with lower radii of curvature. Thewear polishing of the surface caused a loss of material from thesurface of the implant, the formation of small polyethylene andmetallic particles and the possibility of metal-ion formation.

The microstructure of the AISI 316L stainless steelhip prosthesis consisted of equiaxed austenite grains with

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twin grain boundaries, but with no carbides along the grainboundaries. An unusually large number of aluminum-basednon-metallic inclusions were found, corresponding to the pitsobserved in the SEM. The long-term wear polishing ‘digsout’ these hard, non-metallic inclusions, thus complementingthe wear-polishing mechanism with abrasive wear. Thewear process was additionally accelerated by the presence ofalumina particles on the surface of the implant originated fromthe manufacturing process.

The XPS results show scant differences between the roughand polished surfaces. The passive layers established on thesurface of both regions contained the oxides of two mainelements, i.e. Fe and Cr. The alloying elements Ni and Moalso contribute to the passive layer. Calcium phosphate wasalso detected at both surfaces.

The loosening of the components in such cases is triggeredby the micromotion of the cemented components which causesthe abrasion of the implant’s surface, producing metal-weardebris and a proliferative soft-tissue reaction. Based on theresults the aseptic loosening of the investigated hip-prosthesisstem was induced by excessive content of Al2O3 particlesoriginating from surface treatment during manufacturing andfrom non-metallic inclusions accompanied by long-term wearpolishing. The calcium phosphate found on the prosthesismay have been deposited there after the loosening started.It is difficult to separate the reaction of the tissues fromthe individual components: metal, polyethylene and cement.However, in the manufacture of total hip-endoprostheses,intended to improve the implant fixation, attention must bepaid to avoiding an excessive number of aluminum-basednon-metallic inclusions and the ability of these surfaces towithstand abrasive wear and their potential for producingirritating and potentially toxic metal-wear debris.

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