Multi-Scale Evaluation of Wear in UHMWPE-Metal Hip Implants Tested in a hip Joint Simulator

Biotribology 4 (2015) 1–11

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Multi-Scale Evaluation of Wear in UHMWPE-Metal Hip Implants Tested in a hip Joint Simulator R.M. Trommer a,⁎, M.M. Maru a, W.L. Oliveira Filho c, V.P.S. Nykanen a, C.P. Gouvea a, B.S. Archanjo a, E.H. Martins Ferreira a, Rui F. Silva b, C.A. Achete a a b c

Materials Metrology Division, INMETRO, Duque de Caxias, Brazil CICECO, Department of Materials and Ceramic Engineering, University of Aveiro, Aveiro, Portugal Mechanical Metrology Division, INMETRO, Duque de Caxias, Brazil

a r t i c l e

i n f o

Article history: Received 19 June 2015 Received in revised form 18 August 2015 Accepted 29 August 2015 Available online 3 September 2015 Keywords: Wear characterization Hip joint wear simulator UHMWPE Total hip implant

a b s t r a c t Wear is a critical issue related to the performance of hip joint implants, namely for ultra-high molecular weight (UHMWPE) fabricated components. A greater knowledge and understanding of the attributes and capabilities of UHMWPE related to wear, at macro to nano scale levels, is crucial in the context of engineering design aiming the improvement of the implants’ behaviour. Various multi-scale characterization techniques (gravimetry, geometrical analysis using coordinate measuring machine, profilometry, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, atomic force microscopy and Raman spectroscopy) were combined for the wear assessment of UHMWPE/metal (stainless steel and cobalt–chromium) implants tested in a hip joint simulator. The wear rate of the UHMWPE was about 48 mg/106 cycles, equivalent to a linear wear rate of 0.16 mm/year, independently of the femoral head material. Two main mechanisms determined polymer wear: a) abrasion, by second-body action of counterface metal asperities and by third-body debris; b) adhesion/fatigue, disclosed by micro-scale ripples, resulting from cyclic plastic strain accumulation. Going deeply into the analysis by AFM and Raman spectroscopy it was also observed that the structure of the material changes after wear but in distinct modes: the scratched areas became more crystalline while the smooth areas remained without structural modifications. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Total Hip Replacement (THR) has been a widely used surgical procedure in Orthopedics with a remarkable success [1–3]. Basically, a THR involves the use of devices in which an adequate choice of materials plays an important role in its performance [4]. Ultra-high molecular weight polyethylene (UHMWPE) has been recognized as a suitable bearing material for artificial joints since the 1960s [5–7]. The system consisted by an UHMWPE cup paired with a metal head is the most widely used prosthesis in implant design [8]. However, wear of UHMWPE remains as the primary cause of failure in such soft-on-hard hip implant forcing a revision surgery [9,10]. The problem related to wear is the production of debris due to the progressive loss of material. Such debris causes tissue inflammation, joint pain and implant loosening. Aiming to minimize the problems related to wear, the improvement of hip implants performance involves new ideas and designs from the durability point of view [11]. Concerning the materials, the use of new ceramics (e.g., Zirconia-Toughened

⁎ Corresponding author. E-mail address: [email protected] (R.M. Trommer).

http://dx.doi.org/10.1016/j.biotri.2015.08.001 2352-5738/© 2015 Elsevier Ltd. All rights reserved.

Alumina (ZTA) or Si3N4) and coatings (e.g., TiN, “diamond-like” carbon) have been considered for the improvement of hip implants [12]. A sophisticated engineered material such as nanocrystalline diamond (NCD) coated on Si 3 N 4ceramic has also been investigated [13]. It should be pointed out that every material combination intended for implant application requires specific analyses prior to clinical use. The most common preclinical test performed in both research laboratories and industry is the wear simulation, which evaluates new bearing designs and materials to be used in hip implants [9,14,15]. To simulate wear, simulators are employed to mimic the biomechanics of hip human joints (loading and angular displacement cycles) observed in vivo, considering the gait of a typical patient [16]. In addition, several factors that affect the wear behavior of UHMWPE acetabular cups, including design, material, amount and type of protein lubricant, direction of motion and applied force, and appropriate implant positioning, can be assessed using simulators [5,17]. The methodology of the simulation wear test has been refined with its accuracy been improved over time, even though it still lacks a strong explanation for the difference in the wear rate values found in tests conducted in simulators around the world [14,18]. Since there is no agreement within the orthopedic community upon which parameters are fundamental in the analysis of

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UHMWPE wear, the discrepancy in the results is probably related to the methodological approach, together with the characteristics of the specific simulation apparatus [16]. This scenario reinforces is the importance to obtain a particular database for each simulator placed at a specific laboratory, allowing further comparisons of results when different designs of hip prosthesis are tested, or under different test parameters. Long-term follow-up studies through regional or national registries are of great value in ascertaining the wear performance of implant systems [16]. This is particularly important to provide data on the wear behavior of THR systems to local regulatory bodies and to the health care systems, especially in the case of implants manufactured and commercialized in the local market. In the field of orthopedics, the UHMWPE material is still considered as the gold standard for articulating surface for total joints [19]. An important feature concerning the wear performance of material combinations that use UHMWPE is the damage mechanism that causes the wear of the polymeric component, due to its direct effect on the durability and on the generation of debris that causes osteolysis [20]. By understanding the underlying mechanisms, and also the biomechanics of the joint, engineers, scientists and researchers are able to improve the design of the material and so develop more durable prosthetic components [4,8]. In this work, the wear behavior of non-crosslinked UHMWPE articulating against two metallic counterfaces, namely cobalt-chromium alloy (CoCr) and stainless steel (SS), was assessed by performing tests in a hip joint simulator. This work combines data resulting from a full set of multi-scale techniques, aiming at a better understanding of the wear phenomena. Classical macro-analyses, such as gravimetric and dimensional measurements, micro-scale techniques of profilometry and optical/electron microscopy, were combined with advanced analyses based on atomic force microscopy and Raman spectroscopy for morphological and structural characterization at a micro/nano level.

2.2. Hip joint wear simulator The equipment used in the wear tests is a hydraulic simulator (AMTI H52-6-1000 model) with six working stations, operating at 1 Hz of angular displacement frequency, and two active control stations. All the sets of acetabular cup and femoral head specimens were mounted in the test equipment in the so-called anatomical position. To evaluate the fluid absorption by the polymeric acetabular cup component, active soak controls for each material combination were also tested, submitted to the same compressive loading but without angular displacement. The cleaning procedure of the specimens and the subsequent gravimetric measurements were performed following the recommendations of the ISO 14242–2 [24] standard, at a maximum interval of 0.5 million cycles, up to 5 million cycles of testing. The compressive loading (maximum load of 3000 N and minimum load of 300 N), the angular displacements (flexion/extension (F/E), abduction/adduction (AB/AD) and internal/external rotation (IR/OR)) and the lubricant conditions of the wear tests were in accordance to the recommendations of ISO 14242–1:2002 [25] standard. The angular values were 2 ° internal rotation, 10 ° external rotation, 25 ° flexion,18 ° extension, 4 ° abduction and 7 ° adduction. The lubricant was bovine calf serum (LGC Biotecnologia, batch n° 02141-VT), diluted with deionized water to a protein concentration of 26.6 g∕L and filtered through a 0.1 μm filter to remove contaminants. For the minimization of bacterial and fungal degradation, a mass fraction of 0.2 % of sodium azide was added to the lubricant. Ethylenediaminetetraacetic acid (EDTA) was also added to the lubricant in a concentration of 20 mM in order to bind calcium in solution and minimize precipitation of calcium phosphate on the bearing surfaces. To prevent fluid evaporation and contamination, the wear tests were performed in a controlled environment by using individual plastic bag and container for each pair of specimens. The lubricant was maintained in closedloop circulation at (37 ± 2) °C, being replaced at every 0.5 million cycles of test.

2. Materials and methods

2.3. Mass loss and geometrical analysis

2.1. Hip joint implants

After the tests, the macro-analyses of wear comprised the evaluation of the wear rate and the wear depth. For the bulk mass loss data, gravimetric measurements were performed in a high precision analytical balance (Sartorius MSA225S-0CE-DI model), with a resolution of 0.01 mg. The net mass loss at every n million cycles (Wn) was calculated according to Eq.1.

Two commercial soft-on-hard material combinations were tested in this work, one of non-crosslinked UHMWPE acetabular cup and metallic head of stainless steel (SS, ASTM F138 standard [21]), and the other one of non-crosslinked UHMWPE paired with CoCr alloy (ASTM F75 [22]). For each combination of material, three pairs were used, i.e. a total number of six specimens were simultaneously tested in the simulator. The femoral heads had 28 mm in diameter and a maximum initial Ra surface roughness of approximately 0.005 μm. The UHMWPE acetabular cup material was in accordance to ISO 5834–2 [23]. All specimens were machined from GUR 1020 extruded bar stock, with 28 mm inner diameter and 50 mm outer diameter, a maximum initial Ra surface roughness of 2 μm and thickness of 9 mm. All cups were sterilized by ethylene oxide (EtO). All head-cup articulating components were supplied by Ortosíntese Indústria e Comércio Ltda- Brazil. For the tests, the acetabular cups were attached to the inner surface of a metallic machined replica of a real metal back. To replicate the clinical implant positioning recommended by the hip implant supplier, and assuming that the angles used in this wear test represents the configuration mostly advised by the company to the surgeons, the acetabular set (cup plus metal back) was placed at 45° inclination in the sagittal plane. The cups also had an elevated rim of 10 degrees, to extend the femoral head coverage and to prevent the dislocation of the hip implant in vivo. The inclination of the femoral heads was set to 40°, in the sagittal plane, relatively to the vertical loading axis, with the aid of a stainless steel device, especially machined for the test. This angle was chosen to replicate the anatomical positioning of the implant, following the recommendation of the supplier.

Wn ¼ Wan þ Sn

ð1Þ

Where n is the number of cycles into consideration, Wan is the mass loss in the wear test and Sn is the mass increase due to fluid absorption. The wear rate for each specimen is calculated by linear regression using the least square method of the net mass loss data obtained at every 0.5 million cycles, up to 5 million cycles [24]. The average wear rate of each set of materials is calculated and the Student´s t-test is used to evaluate the statistical difference between the means. 2.4. CMM analysis Geometrical approaches using a three-dimensional CoordinateMeasuring Machine (CMM), coupled to a suitable software, have been reported in the literature for the assessment of wear in UHMWPE orthopedic components after in vitro tests. This method was used in this work to estimate the wear depth and the linear wear rate of the polymeric acetabular cups [15,26–29]. The 3D surfaces of two representative acetabular cups (one tested against a SS head and the other one tested against a CoCr alloy head) were obtained using a CMM (Mytutoyo Legex 9106 model), with a scanning ball probe of 4 mm diameter. From sixteen scanning

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measurements crossing the pole of the sample, sets of data arranged in 3D Cartesian coordinates were produced. By the aid of the Geomagic Studio® software, the measured points were used to construct a 3D surface of the acetabular cup after the test. In this 3D surface, it was assumed that wear was localized in a specific region in the cup. According to the literature, an area without wear can be used as a suitable reference of the pristine cup surface [15,27,28]. In the present work, the data obtained from the unworn area identified in the initial 3D surface were used to fit a 3D reference (pristine) spherical surface. After this, the deviation maps and the visual observation of the location of the wear damage on the acetabular cup can be done by the direct comparison between the 3D surface constructed from all the measured points and the 3D reference spherical surface, using the Geomagic Studio® software. With the aid of these deviation maps, the maximum wear depth in the acetabular cup was estimated, assuming that the femoral head penetrated the cup following a linear trajectory [19]. The linear wear rate of the acetabular cup was estimated by dividing the maximum wear depth by the total number of cycles of the wear test (i.e. 5 millions). 2.5. Roughness measurements Following the multi-scale analysis proposed in the present work, the surface roughness (Ra) of the tested femoral heads and acetabular cups was evaluated with the aid of a contact profilometer (Taylor Hobson PGI 830 model), using a 2 μm radius diamond tip stylus. The advantage of the profilometer used in this investigation is the large length of measurement on curved surfaces, which allows assessing long and continuous regions of convex/concave components. The measurements were performed in the flexion∕extension direction as well as in the abduction∕adduction direction passing through the pole region, with the maximum length of 10 mm and 15 mm for the acetabular cups and the femoral heads, respectively. The acquired data was analyzed using a 0.08 mm cut-off, following the ISO 7206–2 standard, [30] and Gaussian filtering. Regarding microscopic analyses, the surface damage mechanisms resulting on the femoral heads (CoCr alloy and SS) and the UHMWPE acetabular cups were firstly assessed by optical microscopy (Olympus model BX51M), and further in more detail by scanning electronic microscopy (SEM) technique coupled with energy dispersive X-rays spectroscopy (EDS) analysis (FEI Helios Nanolab 650 model) operating at 10 keV .

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Fig. 1a and b, respectively. The wear increased noticeably with increasing number of wear cycles, independently of the mating surfaces. At a first glance, this independence was unexpected because the difference in hardness [31] of the head materials would confer different susceptibility to scratching of the metal surface and consequently a difference in the wear rate of the polymer [32]. This will be further discussed in Section 3.3. Furthermore, the results show a relative linearity or a steady-state wear rate after the first evaluation at 0.5 million cycles. The linearity and increased wear during the test cycling is the typical behavior of non-crosslinked UHMWPE acetabular cups [33,34], meaning that the wear takes place under a steady mechanism along the test cycles. The wear rate is an important parameter determined from the simulated wear test, since the failure of artificial joints by loosening has been shown to occur when the total volume of wear debris reaches about 600 mm3 [35]. Considering a density of approximately 0.93 g.cm− 3 for the UHMWPE material [34,36–38], this gives a total mass loss of around 560 mg. The average wear rate of the UHMWPE cup specimens of the present study, determined from the slope of the linear regression fitting to the mass loss data in Fig. 1, was (48.45 ± 9.30) mg/106 cycles and (48.16 ± 4.07) mg/106 cycles when paired with CoCr and SS, respectively. A Student´s t-test is used to evaluate the statistical difference between the two averaged wear rates. The tvalue calculated is 0.045 b 2.776 (t-tabulated value for a 5% level of confidence and 4 degrees of freedom), implying that there is no significant statistical difference between the two average wear rates. Considering that a million of simulated cycles represents one year of clinical use, it is expected that both tested implant designs last at least 10 years. In

2.6. Atomic force microscopy (AFM) and Raman spectroscopy The wear in micro/nano-scale level was assessed by atomic force microscopy (AFM) in a WITec (Alpha300 system) equipment, operating in alternated contact mode (tapping mode), using a rectangular silicon cantilever with nominal spring constant of 42 N.m−1. One representative tested cup sample was roughly cut into square shaped specimens to achieve flat enough surfaces suitable for the analysis of worn regions of approximately 30 x 30 μm. Modifications in the molecular structure of the polymer (UHMWPE) due to wearing process were evaluated by Raman spectroscopy using the same WITec (Alpha300 system) equipment. Spectra were recorded using backscattering configuration and 1800 lines/mm grating, excitation source of 2.33 eV diode laser, wavelength of 532 nm. The microscope objective used has a numerical aperture of 0.95 (100x magnification), and the laser spot at the sample has a diameter of approximately 1 micrometer. 3. Results and discussion 3.1. Gravimetric data The cumulative wear denoted by mass loss of the tested acetabular cups after 5 million cycles grouped in SS and CoCr, are shown in

Fig. 1. Cumulative wear of non-crosslinked UHMWPE acetabular cups paired against SS (a) and CoCr alloy (b) femoral heads, as a function of the test cycles.

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terms of the wear rates resulting from simulated wear of metal-polymer hip implants, the literature reports values around 50 mg/106 cycles, close to those found in the present work for both material combinations. For instance, using an orbital bearing motion simulator, the wear rate reported for non-crosslinked UHMWPE paired against a 28 mm CoCr femoral head was (57.8 ± 0.6) mg/106 cycles [39]. With a biaxial rocking motion simulator, the wear rate of the same material combination and diameter was (56.4 ± 13.3) mg/106 [37], very close to the previous result. However, in this last test the UHMWPE was gammairradiated, which would confer more wear resistance to the polymer material. Here, the type of motion in the wear simulation test is crucial when comparing the results from different simulators. Another work reported that non-crosslinked UHMWPE acetabular cups in conjunction with 28 mm CoCr femoral heads tested for 2.5 million cycles presented a wear rate of 52.74 mg/106 cycles [40]. In this particular test, abrasive particle of radiopaque PMMA were intentionally added to the serum to simulate third-body abrasive wear. This should have caused more wear to the non-crosslinked UHMWPE by abrasion mechanism when compared to a test condition without such particles. Other reasons that may explain the variation seen in the wear rate values is the inclination angle of the acetabular cup (45° in the present study), once the angle plays an important role in the hip implantation surgery by affecting the contact mechanics and potentially increasing the polyethylene wear [2,8]. Nevertheless, considering the design of new material combinations intended to be used in hip implants, lowering the wear rate plays a key role for improving of the performance of the artificial joint, but care should be taken to evaluate the wear under the same wear testing conditions. 3.2. Geometric analysis One of the problems related to the use of purely gravimetric method in the analysis of wear is the potential error arising from the material transfer from the metallic femoral head to the polymeric acetabular cup, or the effects of fluid sorption [28,41]. Thus, in comparison to the gravimetric wear measurements, geometric analysis is a versatile tool having the advantage of precisely identifying the wear location and the corresponding depth in the acetabular cup throughout a color coded image [1,29]. However, during CMM and 3D modeling analyses of acetabular cups, it is necessary to consider that the dimensional wear of the polymer component is a result from the combination of creep, cold-flow and mass loss [41,42]. Using the 3D models, geometric analyses were performed by ascertaining the difference in the vertical position (z axis error or z-shift) between the worn surface and the reference surface [43]. This can be done by the use of a simple fitting operation in the software employing the reference surface as its datum. Consequently, the z-shift represents the penetration of the metallic femoral component into the UHMWPE acetabular cup [41]. Fig. 2a shows the representative macroscopic image of the bearing surface of an acetabular cup after the wear test with two distinct zones: the unworn (left) and the worn one (bright region, in the right). The results of the geometric analyses (z-shift) considering representative acetabular cups after the wear test against SS (Fig. 2b) and CoCr (Fig. 2c) corroborates with the macroscopic observation of Fig. 2a, where the bearing surfaces have a particular area in which the wear damage was located (in light blue color), in comparison to the remaining surface of the UHMWPE cup (in light green color). The existence of such particular worn area is the result of the inclination of the acetabular cup (45°) that intensifies an asymmetric mechanical contact with the femoral head [28]. Observing the color coded images, the maximum penetration depth is roughly estimated in 0.8 mm for both acetabular cups. The relevance of wear depth evaluation is related to the fact that, after the surgery, clinical researchers and surgeons are interested in the in vivo assessment of UHMWPE wear, which is achieved by means of radiographs [19]. Surgeons usually quantify polyethylene wear

Fig. 2. Macroscopic top view of a representative cup after the test with the worn area highlighted (a), and three-dimensional model of the spherical deviation caused by wear, with the worn area delineated in light blue color, in the cups paired with SS (b) and CoCr (c).

in vivo by the direct measurement of two plain radiographs obtained at different follow-up times (paired analysis), where the displacement of the femoral head with reference to the acetabular component surface (also named penetration depth) is measured [44]. A common practice for clinical wear evaluation is the analysis of wear depth per year, i.e., the linear wear rate [45]. Thus, after in vitro tests, it is interesting to estimate the acetabular cup depth for further comparisons with the radiographic method used in vivo.

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Based on the geometrical measurements, a linear wear rate (mm/year) can be calculated dividing the estimated penetration depth by 5 million cycles (test interval), also assuming that one million cycles in a simulator approximates the average number of steps taken by a patient in one year in vivo [14,36]. The penetration depth was previously estimated through the observation of the color coded images in Fig. 2a and b, giving a value of approximately 0.8 mm for both situations. Thus, the calculated maximum linear wear rates were about 0.16 mm/year for both UHMWPE bearings paired with SS and CoCr. The linear wear rates determined in this work are in accordance with those reported in literature, in both simulated and in vivo studies. In a wear simulated test after 2 million cycles, the penetration of the femoral head into the acetabular cups was approximately 0.30 mm, considering a 26 mm-diameter SS femoral head articulating against UHMWPE, giving a linear rate of 0.15 mm/year [46]. Further, with gamma-irradiated UHMWPE cups paired against 28 mm CoCr heads an average linear wear rate of 0.14 mm per million cycles was reported [36]. In a different work with wear simulated tests, the linear rate of non-crosslinked UHMWPE was approximately 0.1 – 0.15 mm/year [17]. From in vivo studies, a mean linear wear rate of 0.15 mm/year was calculated for the metal-on-UHMWPE bearing [16]. Most of the hip implants that failed due to osteolysis presented a linear wear of 0.1 mm/year [47]. Consequently, this value is usually known as osteolysis threshold [34,48], and one might assume that the value of the linear wear rate estimated in the present work for the UHMWPE paired with either metallic material could cause osteolysis, even though the gravimetric analyses indicated that these material combinations have success for clinical use at least for 10 years without loosening. In the linear wear rate analysis we must consider that wear is localized in a specific region in the polymeric surface, without uniform penetration. 3.3. Profilometric evaluation Fig. 3 shows the topographic profiles of a representative polymer cup (Fig. 3a-c) and a metal head (Fig. 3d-f) after the wear test. Regarding the tested UHMWPE cup, the raw profile, i.e., the profile measured from the surface without any filtering is shown in Fig. 3a, which depicts the wide length measured on the concave surface of the cup and allowed assessing different aspects, from the pristine to the worn surface. The specific characteristics of each zone on the surface are better identified after applying a filter to remove the circular form, resulting

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in the primary profile shown in Fig. 3b. It is clear that the measured surface has three curved-like profiles, revealing deviations from the circular form, which are attributed to deformation suffered by the polymer surface. Besides that, each one of the three profiles has a different level of roughness. This is better identified in the roughness profile obtained by applying a Gaussian filter and using a 0.08 mm cut-off, as shown in Fig. 3c. In the roughness profile, the region at the right-hand side was identified as the pristine one. The Ra value of this region was around 0.65 μm, a little lower than the original, unworn surface, in which the Ra value was around 0.80 μm. This small difference in Ra is due to the occurrence of small deformations since this is an area close to the loaded zone. This is later discussed with the aid of the SEM micrographs. The two last regions refer to the worn zone, comprising a very smooth surface at the center, and a region at the left-hand side corresponding to a severely roughened area. With this, a simple profile measurement clearly revealed that the soft surface of the UHMWPE experienced different levels of damage after the wear test. Using the multi-scale evaluation proposed in this work, the three different regions are further discussed, aiming at a better understanding of the morphology and structure of the polymer worn surface. Concerning the femoral head, the raw profile also shows a wide length of measurement on the convex surface (Fig. 3d), but in this case the primary profile revealed that the form errors (in relation to the circle) were limited in an amplitude range of less than 4 μm (Fig. 3e). Also, the surface of head resulted much smoother and homogeneous (Fig. 3f) in comparison to the cup surface (Fig. 3c), which is evidently a consequence of the difference in hardness among the tested materials. The average Ra surface roughness of the SS and CoCr alloy femoral heads after 5 million of cycles was (0.012 ± 0.005) μm and (0.008 ± 0.002) μm, respectively. Compared to the pristine head surface condition (0.005 μm Ra), an increase in Ra was observed, meaning that the heads were scratched during the wear cycles. If one considers that the scratches in the heads can affect UHMWPE wear [16,49,50], the rougher SS head would lead to more cup wear. Conversely, it is clear in the present work that the resulting difference in scratching in both of the head materials, seen by their Ra values, does not account for a difference in the wear of the UHMWPE cups [35]. However, it is possible that, with the extension of the test until a larger number of cycles, a difference in the wear rate of UHMWPE when paired with SS or CoCr alloy could be observed as a result of the different roughening of the metal component. This may influence the wear of UHMWPE since much larger debris

Fig. 3. Topographic profiles of a representative polymer cup (a-c) and a metal head (d-f) after the wear test. Profiles measured from the surfaces without any filtering (a, d), primary profiles after circular form removal (b, e) and roughness profiles after applying Gaussian filter and 0.08 mm cut-off (c, f). Schematics in (a) and (d) show the measurement position in the cup (with the worn zone at right) and in the head. Red color ellipses in (b) highlight the three distinct topographic zones of the cup profile.

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particles might be expected [35,40]. Therefore, further studies are necessary to investigate if the roughness of the femoral head has influence on the particle size of the debris produced in the wear test. 3.4. Morphological analyses: optical microscopy, SEM and AFM The top view optical microscopic observation of the CoCr (Fig. 4a) and SS (Fig. 4b) femoral heads after the wear test indicates that the increase of the surface roughness occurred mostly due to the presence of scratches in different directions (Fig. 4a-d). The scratching can be

attributed to the action of third body particles entrapped in the contact between the metallic surface of the femoral head and the polymeric surface of the acetabular cup. Such third body particles can be, for example, debris originated from the bone cement used to fix the metal back of the cup, or worn material that is expelled from the contact between the two surfaces in the form of debris [37,38,40,51]. In the literature it is reported that the presence of scratches in the surface of the metallic head influences the wear of UHMWPE [16,50,51]. Another important observation is the existence of some areas on the femoral heads of both materials with apparent material deposition, showing some cracks

Fig. 4. Images from optical microscope of the top view of the CoCr alloy (a, b) and SS (c, d) femoral heads after the test, revealing some scratches (a-d) and deposited material (b, d). Chemical analysis by EDS of the material deposited on the surface of the CoCr alloy (e) and SS (f).

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(Fig. 4b and d), denoting that the deposited material has a brittle behavior. Analysis using EDS reveals the presence of phosphorous and calcium, besides the typical elements of CoCr alloy (Fig. 4e) and SS (Fig. 4f), suggesting that calcium phosphate precipitated on the femoral heads surface even though EDTA was added to the bovine serum to minimize this precipitation. In the case of the tested components it is possible that, like the bone cement, the third body can be also produced from the calcium phosphate deposit and its detachment during the wear test. Similar behavior can be also seen in porous hydroxyapatite coatings, where hydrolysis, debonding, and delamination of the coating may release third body particles [52]. Concerning the cups, the surface observation using optical microscope revealed that UHMWPE cups, paired with SS and CoCr, presented the same morphology, suggesting that the wear mechanism is identical, no matter the type of metal contacting the polymeric cup. The macroscopic view of a wear tested cup is shown in Fig. 5a, denoting a shiny polished worn region at the right hand side of the image. Higher magnification of this region by optical microscope is presented in Fig. 5b, where a morphology with dominant ripple-like structure is identified. This figure also shows few scratches along random directions suggesting the occurrence of third body abrasion. This can occur in vivo, resulting in an increased surface roughness of the metal head, and consequently in a more pronounced wear rate of polyethylene [35], namely in the absence of crosslinking [5]. Fig. 5c corresponds to a transition region from the pristine to the worn affected area. Three distinct morphologies are seen: a region with the machining marks of the pristine surface (region I), another one with smoothed feature (region II) and a last one with innumerous parallel scratches (region III). Regions II and III are magnified in Fig. 5d and e, respectively, being later discussed with the aid of AFM and Raman techniques.

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Following the multi-scale evaluation, scanning electron microscopy (SEM) is a powerful technique for a better understanding of the morphologies above mentioned. Fig. 6a presents details of a scratched area. The scratches were probably created due to the existence of protuberances from the metallic counterface that were forced to move against and along the bearing surface of the cup. Such protuberances can be identified as the form error in Fig. 3e. In the ploughing mechanism that proceeds during the test, the cup surface experienced several cycles of deformation by the passing protuberances and asperities of the femoral head. Also in Fig. 6a, transverse cracks to the ploughing direction are apparent. This may be a result of exceeding the UHMWPE ductility limit [53]. Close observations of the region presented in Fig. 5b using SEM (Fig. 6b) confirm the presence of the ripples on the acetabular cup. In the literature, this surface morphology is addressed to the UHMWPE material tested in a hip wear simulator or in a wear screening machine [53–57]. In addition, ripples are an important feature observed in worn acetabular cups because debris is produced from the fibrils created from the rippled protrusions [52]. Furthermore, the wavelength between the ripples was close to the values reported in literature [54,57]. The origin of ripples can be justified by a set of sequential factors, as follows. Firstly, the existence of ripples in the worn area of the acetabular cup can be attributed to the nature of the contact among the asperities, where every asperity of the UHMWPE surface was cyclically traversed by passing asperities of the metallic counterface, representing an adhesive/fatigue wear mechanism [53]. The multi-directional motion caused by the angular displacements (flexion/extension, internal/external rotation and abduction/adduction) and the compressive load in the wear test result in a repeated loading cycle in each point among the contacting surfaces. In this region, the contact areas of the asperities of the UHMWPE

Fig. 5. Representative images of a UHMWPE cup after the wear test. a) Macroscopic view; (b-d) optical micrographs of selected regions: b) inside the worn area with ripple-like morphology; c) transition between pristine and worn areas with machining marks from the pristine surface (region I), with smoothed characteristic area (region II)-, and a parallel scratched area (region III); d) inset of region II; e) inset of region III. The squared marks in (d) and (e) indicate regions where a deeper analysis by AFM and Raman techniques was performed.

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Fig. 6. SEM images of a representative cup showing innumerous parallel scratches with transversal cracks (a); ripples in the polished zones (b); transition between the very rough surface (left), with remaining machining marks, and the smooth surface (right) (c); pristine surface (d).

bearing surface would be plastically strained probably via plastic deformation in the amorphous regions and crystallographic slip in the lamellae [54,57]. Consequently, the ripple-like structure can be the result of the plastic strain accumulation process, which is caused by the continuous sliding of the hard counterface (metallic femoral head) during the wear test [56]. Ripples lead to layer peeling off from the UHMWPE surface [55]. The accumulation of plastic strain promotes the development of parallel arrays of tongue-like features. As the test proceeds, the gradual accumulation of strain causes the lateral growth of these tongs that became thick and form the ripples morphology [54]. Considering the acetabular cup, the magnitude of plastic deformation is limited to the sites of intimate asperity contacts being that the material loss is defined by a critical strain criterion [53]. The left side area in Fig. 5c (region I) was also analyzed by SEM (Fig. 6c) in a higher resolution. From this image, it must be pointed out that this region has actually been modified with respect to the pristine morphology (Fig. 6d). Interestingly, the detailed observation of Fig. 6c reveals the existence of cracks in the remaining machining marks and some blunting of their crests, contrarily to the pristine surface in Fig. 6d where these features are not observed. This morphological change can be the result of material flow (i.e. creep) taking place under relatively lower loading with respect to the severely worn area [58]. Surface analysis of extreme wear level areas, corresponding to the squared marks in Fig. 5d and e, was carried out by AFM and presented in Fig. 7. The first information extracted from the topographic images concerns the nanometric roughness of the surface. Both areas, smooth and scratched, presented morphological features similar to those

observed by SEM and optical microscopy. Fig. 7a corresponds to the scratched region of Fig. 5e, with peak-to-valley heights as high as 200 nm. In this area, even at the nanometric scale, it is possible to observe transverse cracks to the ploughing direction, denoting that this wear mechanism exists in particular areas, which cannot be identified by using microscopy techniques. Fig. 7c presents the topographic image of Fig. 5d. This region is characterized by nanometric roughness in a homogeneous surface, with peaks not higher than 50 nm. Phase contrast images, also obtained by AFM in intermittent contact mode, allow the identification of phases having different structural nature within the surface of a material, and suggest differences in the mechanical properties of distinct areas [59,60]. For instance, crystalline and amorphous phases in semi-crystalline polymers, like UHMWPE, can be identified in the phase images with nanometric resolution. It is known that the polymer chains of polyethylene and other semi-crystalline polymers can undergo an induced orientation due to mechanical action, which affects brittleness and the fracture toughness properties [6]. The phase image of the smoothed worn area of Fig. 5d is presented in Fig. 7d, revealing homogeneous surface with isotropic features. Contrary, the phase image in Fig. 7b of the scratched area of Fig. 5e presents alternated dark and light regions forming an anisotropic structure, suggesting that UHMWPE chains were oriented due to the stress experienced during the wear test. Moreover, there is an angle of 45° between alignment and scratching directions. This can be explained by the friction the soft polymer surface underwent in contact with the metal head, causing shear stress between the crystalline planes of the polymer in its surface and, consequently, a slip direction shifted at 45° to the scratching direction.

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Fig. 7. AFM images acquired from de marked square zones (Fig. 5) in the insets of region II (7c and 7d) and III (7a and 7b). Images at left side correspond to the topography and at right side to the phase contrast. The inset curves show the roughness of the surfaces obtained from topographic profile. Note the angle of 45° between the alignment and scratching directions in topographic image and the alignment in phase image, at region III or rough region.

Fig. 8. Mean Raman spectra acquired from the surface to 19 μm deep into the bulk of PE (a), and variation of crystalline contents with the depth (b) of regions I, II and III (see Fig. 5).

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3.5. Structural Analysis: Raman spectroscopy Vibrational spectra of semi-crystalline polymers have a high degree of definition and changes in its crystallinity imply spectral changes such as broadening or even appearance (or disappearance) of bands [61,62]. As only the polymer chains on the surface of the sample are exposed to the stress imputed by the wear test, it becomes clear that the structure of the polymer in the surface will differs from the bulk after the test. Confocal Raman Depth Scan was performed to identify the structural modifications from surface into the bulk material. The Raman spectra were acquired in the same regions analyzed by AFM starting at the surface and going down to 19 μm into the bulk. Fig. 8a shows the typical Raman spectra of UHMWPE acquired from the 3 regions (I, II and III). Although very similar, there are some differences in the spectra that can be observed in the intensity and shape of the bands. For instance, the band at 1130 cm−1 attributed to skeletal vibrations has its intensity increased in the worn regions compared to the non-worn region. More specifically, differences in the intensity of the band at 1414 cm− 1 (− CH2-, bending), commonly attributed to the orthorhombic phase, can be noted, which is more intense in the rough region than other regions analyzed (see inset of Fig. 8). Slight changes can also be noticed in the band related to the amorphous PE at 1080 cm−1. The spectra are normalized by the intensity of the band at 1300 cm− 1 (− CH2-, twisting) which is phase independent and can be used as an internal standard. Therefore, one may assess the degree of crystallinity of PE by the relative integrated intensity of the band at 1414 cm−1 with respect to the integrated intensity of the band at 1300 cm− 1[63–65]. Fig. 8b shows the results of such analyses confirming that the degree of crystallinity of UHMWPE is higher in worn regions than in non-worn in all scanning area extent. The increase in the degree of crystallinity at the worn regions is larger at the surface than in the bulk material. This confirms that the wear process provokes a reorientation of the polymeric chains mainly at the surface. Those results are in good agreement with the AFM results and also the finding from other authors [6,65,66] as they reported that the degree of crystallinity at the surface is higher than in deeper regions of similar UHMWPE cups. 4. Conclusions • The wear rates determined for the UHMWPE paired with CoCr and SS were (48.45 ± 9.30) mg/106 cycles and (48.16 ± 4.07) mg/106 cycles, respectively. The material of the femoral head (SS or CoCr alloy) had no influence on the wear rate of the acetabular cups, and no significant statistical difference between the two average wear rates was found. The surface of SS and CoCr femoral heads presented several scratches probably resulting from third body particles, but it also did not influence the polymer wear. • The wear of the UHMWPE cup was located in a specific region inside the contact area and the calculated linear wear rates were 0.16 mm/year for the UHMWPE bearings paired with both the SS and CoCr counterfaces. • In the region associated with wear, three different topographies and morphologies were observed. The first one was the pristine like, which exhibited cracks in the remaining machining marks and some blunting of their crests. The second one was smooth with ripple-like morphology resulting from adhesion/fatigue mechanism and cyclic plastic strain accumulation. Finally, the last one was the scratched region and resulting from ploughing mechanism by protuberances of the metallic counterface, with transverse cracks to the ploughing direction. • An important feature when dealing with UHMWPE wear resistance, related to the molecular chains orientation phenomena, could be clearly identified by AFM and Raman spectroscopy. • The combination of a set of different techniques is a powerful tool for the assessment of wear in UHMWPE acetabular cups tested in hip joint simulators, allowing a deep understanding of the whole wear,

from macro to micro/nano scale phenomena. This is a key issue in improving the quality of the prostheses, thus improving the life quality of the patients.

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