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Mixing and matching in ceramic-on-metal hip arthroplasty: An in-vitro hip simulator study
ARTICLE IN PRESS Journal of Biomechanics 42 (2009) 2439–2446
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Mixing and matching in ceramic-on-metal hip arthroplasty: An in-vitro hip simulator study Saverio Affatato a,, Michele Spinelli a, Stefano Squarzoni b, Francesco Traina a,c, Aldo Toni a,c a
Laboratorio di Tecnologia Medica, Istituto Ortopedico Rizzoli, Via di Barbiano 1/10, 40136 Bologna, Italy IGM-CNR, Unit of Bologna c/o IOR, Bologna,Italy c Dipartimento di Ortopedia e Traumatologia, Istituto Ortopedico Rizzoli, Bologna, Italy b
a r t i c l e in fo
abstract
Article history: Accepted 29 July 2009
The clinical success of second-generation metal-on-metal hip replacement and the good tribological performance of alumina ceramic revived an interest in hip articulation as a solution to reduce wear. This study was aimed at characterizing the wear behaviour of new hybrid ceramic-on-metal bearings. In particular, this study investigated the wear behaviour of ceramic-on-metal hip components (three different diameters configurations: 28, 32 and 36 mm), not specifically proposed to be coupled, in order to compare them with ceramic-on-ceramic, which is considered to be the gold standard for wear resistance. For this purpose, the weight loss over a standard wear simulation was monitored. Moreover, scanning electronic microscope observations were used to verify if any carbides removal, for the metallic components, triggered wears debris production promoting abrasive third-body wear. After five million cycles, our results showed significantly greater wear-in ceramic-on-metal compared with ceramic-on-ceramic, and significant greater wear for the 32-mm diameter compared with the 36-mm one. Our findings showed an increase in wear for the proposed hybrid specimens with respect to that of the ceramic-on-ceramic ones confirming that even in the case of ceramic-on-metal bearings, mixing and matching could not prove effective wear behaviour, not even comparable with that of the ceramic-on-ceramic gold standard. Wear patterns and roundness tolerances certainly discourage the coupling of components not specifically intended to be coupled. Unsuitable geometrical conformity could, in fact, result in a poor dynamic behaviour and lead to clinical failure. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Ceramic-on-metal Roundness measurements Hip wear rate COC COM SEM
1. Introduction Hard bearing surfaces are used for total hip replacement due to their better tribological properties. The most commonly used hard bearing surfaces are ceramic-on-ceramic (COC) and metal-onmetal (MOM); both have very good wear resistance but have some limits despite the evolution of materials. There are no doubts that ceramic implants reduce wear with respect to polyethylene and metal couplings (Affatato et al., 2001, 1999; Nizard et al., 2008; Rose et al., 1980; Willmann, 1998, 2000a, b). They have good tribological properties such as hardness, good chemical resistance, high tensile strength, and good fracture toughness (Costa et al., 1997; Kumar et al., 1991; Macchi and Willman, 2001; Willmann et al., 1996). However, the main concern remains its brittleness, which could lead to failure. Although the reported failure rate is low (0.004% head fracture (Willmann, 2000a, b) and 0.22% liner fracture (Toni et al., 2006)), it is clinically relevant (Darring, 2007; Lidgren and Robertsson,
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E-mail address: [email protected] (S. Affatato). 0021-9290/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2009.07.031
2007; Stea et al., 2009). On the other hand MOM bearings aim to ensure high wear resistance, good manufacturability and low friction torque. Despite a lower wear volume associated with such implants, very small particles are produced (Davies et al., 2004; Maloney et al., 1993; Silva et al., 2005). The amount of metal ions present in the serum and their potential toxic effects both locally and systemically (Case, 1999; Damie and Favard, 2004; Dobbs and Minski, 1980; Doorn et al., 1999; Fisher et al., 2004, 1999; Marker et al., 2008; Miki et al., 2006; Sargeant and Goswami, 2007) are a cause for concern. Moreover, polishing wear promoted by wear debris, produced by the abrasive action of carbides has been shown in retrieved Co–Cr alloy hip implants (Dahm et al., 1995). The recent introduction of an innovative hybrid hard-on-hard bearing (ceramic head and metallic insert; COM) (Barnes et al., 2008; Firkins et al., 2001a, b)) claimed to reduce ion release (Sauve et al., 2007) and wear particle production on one hand (Brockett et al., 2007), and possibly the breakage of the ceramic insert rim (Toni et al., 2006) on the other. Therefore, considering the inverse proportionality of wear rate and hardness of the bearing material (Archard, 1953; Firkins et al., 2001a, b), it might be thought that dissimilar bearing couples provide the reduced adhesive wear, typical of like-on-like commercial bearings, and
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Table 1 Geometrical details for all ceramic and metallic specimens tested in these studies. Diameter (mm)
Femoral head composition
Acetabular cup composition
Average diametral clearance (lm)
Number of componentsa
28 32 36 36
Biolox-Forte Biolox-Delta Biolox-Forte Biolox-Delta
Biolox-Forte CoCrMo Biolox-Forte CoCrMo
77 86 100 98
3+1 3+1 3+1 3+1
a
Three components for each different combination were in run onto the simulator; the last one was used as control check for ion release.
Fig. 1. Location of the roundness measurements taken on the femoral heads (Part a) and on the acetabular cups (Part b).
Fig. 2. The picture shows the weight loss for metallic and ceramic acetabular cups coupled with ceramic femoral heads. The COC combination (28 and 36 mm) wore less than the hybrid for the COC configurations tested (COC 32 ad 36 mm). For the COC configurations, it is possible observe a typical wear profile between 0 and one million cycles, containing an initial higher wear. Wear then reduced after 1–5 million cycles for the COM 36 mm while the COM 32 mm configuration shows a higher wear trend.
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Fig. 3. Incremental volumetric wear rate (7 standard deviation) for the acetabular cups of different configurations tested. The wear profile of the ceramic specimens showed an initial wear followed by a reduced wear. A different wear behaviour was observed for the two metallic size with an initial decrease in wear rate and, from two million cycles, an higher wear due probably to different clearance between heads and cups. A different wear trend was observed for the COC configuration with a continuous decrease in wear rate.
overcome polishing problems induced by carbides detachment together with a reduction in ions release and breakage of the ceramic inserts. However, short-term results of this hybrid coupling are available, with regards to the reduction of wear and ion release (Firkins et al., 2001a, b; Fisher et al., 2006; Barnes et al., 2008). The emphasis on the possible advantages of this innovative hybrid coupling might lead to a new intra-operative scenario: the use of a COM coupling with components not specifically proposed to be coupled. In the past the practice of coupling different materials led to clinical failures (Morlock et al., 2001; Valenti et al., 2007), but was poorly investigated from an experimental point of view (Barnes et al., 2008), even if it could be considered a worst-case scenario. This study investigated the wear behaviour of COM hip components, not specifically proposed to be coupled, in order to compare them with COC and MOM traditional bearings. For this purpose, the weight loss over a standard wear simulation was monitored. Moreover, scanning electronic microscope (SEM) observations were used to verify if any carbides removal, for the metallic components, triggered wears debris production promoting abrasive third-body wear. Finally, shape measurements were assessed in order to calculate diametral clearance and relevant roundness variations of the tested heads and cups.
2. Materials and methods
Fig. 4. SEM mapping of progression in ceramic surface of 28 and 36 mm COC combination (original magnification 2000; SEM variable-pressure mode; 20 KV). The picture shows typical images of the alumina matrix of heads and cups in the working region confirming that the wear test did not affect the surface texture, after five million cycles.
All ceramic and metallic specimens were commercial implants for hip surgery (Ala Ortho, Milan,Italy). Three different diameters were chosen (28, 32, and 36 mm) because they are the most commonly used by hip surgeons for conventional total hip replacement. The bearing materials tested included two types of combinations: COM and COC. In the case of COM, Biolox Delta ceramic heads were coupled with cast high-carbon (HC) CoCrMo (ASTM F-75) cups. The COC combination consisted of Biolox Forte ceramic heads and cups (Table 1). The wear test was performed using a 12-station hip joint simulator (Shore Western Mnf., USA). The kinematics and load parameters applied to our hip simulator followed a well-established internal protocol (Affatato et al., 2001, 2007, 1999, 2008). The lubricant used was 25% sterile bovine calf serum (SIGMA, St. Louis, USA) balanced with deionized water and 0.2% sodium azide (E. Merck, Darmstadt, Germany) to retard bacterial degradation. EDTA (ethylene–diaminetetraacetic acid) was also added in a concentration of 20 mMol/dm3. The specimens were removed from the simulator every 0.5 million cycles, weighed, and the test was restarted with new lubricant. The wear test lasted five million cycles.
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Fig. 5. SEM mapping of progression in ceramic surface damage of 32 mm COM combination from non-wear zone, to main wear zone and into stripe wear zone (original magnification 2000; zoomed magnification 11,000; SEM variable-pressure mode; 20 KV). On the metallic cup are emphasised mild abrasion (parallel slight scratches) and small indentations indicating a third-body abrasion mechanism. Ceramic grains were visible with SEM, and EDS analysis confirmed they were alumina particles.
We evaluated wear-in terms of weight loss measured using a microbalance ¨ (SARTORIUS AG, Gottingen, Germany) with a precision of 7 0.1 mg. Each weight measurement was repeated three times and the average weight was used for calculations. The incremental volumetric wear rate was calculated by subtracting each volumetric loss from the previous measurement and dividing it by the number of cycles corresponding to the considered step. The effects of the two bearing material combinations on wear were evaluated using a nonparametric test (Two-Sample Kolmogorov–Smirnov Test, K–S) (Armitage and Berry, 1994). Statistical significance was set at p o 0.05. After completing the test, we visually examined all specimens for scratches or damage caused by third-body wear. Eventually, we examined the most worn femoral heads and the acetabular cups under a light microscope (Nikon SMZ-2T, Tokyo, Japan) and a scanning electronic microscope (SEM) (Cambridge Stereoscan 200, UK) operating at 20 kV. In the case of ceramic specimens, a thin layer of gold coating was sputtered in order to have a better electrical conductivity and hence visibility in the SEM. Energy dispersed spectroscopy (EDS) X-ray analysis (Inca Energy-200, Oxford Instruments, UK) was used to analyse chemical composition. A state-of-the-art Coordinate Measuring Machine (CMM-ZEISS Prismo VAST 7, Germany) was used to assess the shape of the femoral heads and the acetabular cups. Calculated diametral clearance and roundness tolerance, in fact, enable the quality control of the coupling and of nominally circular products by testing the accuracy of their nominal shape. The maximum probing error of the CMM was determined to be below 1 mm. The CMM was in a temperature and humiditycontrolled room. For each specimen three profiles were taken, respectively along the transverse, the sagittal, and the frontal plane (Fig. 1), before and at the completion of the test. The number of points measured on the surfaces determines
the accuracy of the method, so the measurements were performed with a high point density distribution, which results in approximately 2000 points measured on each profile. In the pre-test measurements, all three specified tracks, for each specimen, were near the lower sensitivity threshold of the machine.
3. Results Each of the ceramic and metallic hip specimens completed 5 million cycles (Mc) of wear tests as planned. The wear rates were evaluated for the wear-in and for the wear-steady periods. In particular, the wear-in phase represents a phenomenon of accelerated wear that rigid-on-rigid bearings generally exhibit in the range between 100,000 and 1,000,000 cycles while, the wearsteady is characterized by a lowest wear than the previous phase. During the wear-in period (0–1 Mc), the wear rate was lower for the 36 mm COC articulation diameters than for the other combinations (Fig. 2). In particular, for the acetabular cups, the wear rate was lower for the 36 mm COC vs. the 32 mm COM articulations (po0.0001) and for the 36-mm COC vs. the 36-mm COM articulations (p= 0.01). The steady-state wear period showed
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Fig. 6. SEM mapping of progression in ceramic surface damage of 36 mm COM combination from non-wear zone, to main wear zone and into stripe wear zone (original magnification 2000; zoomed magnification 11,000; SEM variable-pressure mode; 20 KV). On the metallic cup a lot of tracks with precipitation inside the grains and in the grain boundaries are visible.
wear rate decreased for the 36 mm COC and 36 mm COM articular diameter while the 32-COM showed an anomalous increase in wear behaviour. The incremental volumetric wear of COM specimens showed an unsteady behaviour with respect to the COC (Fig. 3); the COM wear trend underwent great variability around two million cycles without any final plateau stabilization. SEM observations of the most worn heads revealed most scratches, holes, and micropits were situated in an annular wear zone. The micropits and holes were irregularly shaped and were surrounded by smears typical for CoCrMo alloys (Figs. 4–6). Roundness measurements prior to testing did not show any differences among the three measurement profiles; the average roundness values for all tested configurations are summarized in Table 2. The same measurements performed at the end of the test showed broad differences between the transverse plane and the other two measure profiles (Table 3).
4. Discussion The aim of this study was to look at the wear behaviour of COM hip components, not specifically proposed to be coupled, in order to compare them with COC and MOM traditional bearings. Random assembly is, sometimes, a used practice in surgical
Table 2 Roundness measurements (7 standard deviation) performed on all femoral head and acetabular cups at zero million cycles. Bearing Diameter (mm) Average roundness measurement ( 7 SD) (mm)
COC COC COM COM
28 36 32 36
Head
Cup
0.00087 0.0002 0.0006 7 0.0001 0.00057 0.0001 0.00077 0.0003
0.0006 7 0.0004 0.0006 7 0.0002 0.00087 0.0002 0.00077 0.0003
SD= Standard deviation.
theatre mainly in case of revision interventions (Morlock et al., 2001; Stiehl and Mahfouz, 2007; Valenti et al., 2007). The weight loss over a standard wear simulation was assessed. SEM observations were carried out to verify if any carbides removal, for the metallic components, triggered wears debris production promoting abrasive third-body wear. Finally, shape measurements were assessed in order to calculate diametral clearance and relevant roundness variations of the tested heads and cups. The present in-vitro study confirmed that even in the case of COM bearings, mixing and matching could not prove effective
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Table 3 Roundness measurements (7 standard deviation) performed on all femoral head and acetabular cups at five million cycles. Bearing
Diameter (mm)
Average roundness measurement ( 7 SD) (mm) Head
COC COC COM COM
28 36 32 36
Cup
Transverse plane
Sagittal plane
Frontal plane
Transverse plane
Sagittal plane
Frontal plane
0.00087 0.0002 0.00067 0.0001 0.00067 0.0001 0.00077 0.0003
0.0006 70.0001 0.0022 70.0004 0.0025 70.0001 0.0028 70.0006
0.000770.0002 0.0019 70.0002 0.0023 70.0002 0.0028 70.0001
0.00257 0.0005 0.00197 0.0002 0.00297 0.0016 0.0045 7 0.0024
0.00497 0.0014 0.00627 0.0004 0.00847 0.0014 0.00677 0.0020
0.00477 0.0005 0.00637 0.0005 0.00907 0.0014 0.00707 0.0019
SD= Standard deviation.
wear behaviour, not even comparable with that of the COC gold standard. In terms of weight loss, our findings favoured COC combinations for hip articulation, which is in disagreement with others authors (Barnes et al., 2008; Fisher et al., 2006). These authors found 100 fold wear reduction for COM pairings with respect to the MOM ones and a slight wear increase if compared to the COC configuration. SEM observations of the worn regions in the femoral heads and in the acetabular cups tested, put in evidence some relevant surface damages and alumina inclusions on the metallic cups of the hybrid pairing while no surface damages were evident on the COC specimens. Fig. 4 shows typical images of the alumina matrix of COC heads and cups (28 e 36 mm) in the working region confirming that the wear test did not affect the surface texture, after five million cycles. On the contrary, in the COM combinations it is possible to observe two different wear mechanisms. In particular, mild abrasion (parallel slight scratches) and small indentations indicating a third-body abrasion mechanism (Fig. 5), that can be due to the introduction of small hard wear debris in the contact region, either arising from fatigue wear or from detachment of the asperities present on the surface. The wear mapping of the 32 mm COM cup showed different significant features (Fig. 5); removal of original machine polishing marks, revealing grain structure and some minor pitting. A progression to disruptive type of wear was characterized by intergranular fracture and grain pullout, formation of craters and loss of surface integrity. The presence of holes and micropits from carbides removed from the bearing surfaces suggests the microstructure can also influence wear rate in COM articulation. Ceramic grains were visible with SEM, and EDS analysis confirmed they were alumina particles. The 36 mm cup had a lot of tracks with precipitation inside the grains and in the grain boundaries (Fig. 6). Roundness measurements confirmed that, for the tested specimens, a marked geometrical mismatch was evident, thus we could have expected a poor tribological behaviour. In particular, for ceramic heads, the roundness tolerance measured prior to testing (all profiles), was comparable with the machine calibration sphere ( 0.0005 mm). Roundness acquisitions at 5 million cycles showed a different behaviour between the transverse and frontal or sagittal plane (Table 3), thus confirming a different roundness tolerance on different planes. A similar tendency is evident for the cups where the roundness error greatly increased in both sagittal and frontal plane at the end of the test. The different roundness tolerances for different working planes might explain such ineffective wear behaviour. The lack of geometrical controls might have led to deviation in such parameters, so that it is impossible to distinguish in depth the contributions of bearing diameter and clearance independently. The wear rate, on the COM configurations, depended solely on mismatch between heads and cups.
Our findings showed an increase in wear for the proposed hybrid specimens with respect to that of the COC ones. Besides, the volumetric wear of the metal cups (coupled with ceramic heads—Biolox Delta) was also higher in comparison with the data reported for the same metal shells when coupled with metal heads (Affatato et al., 2007). The incremental volumetric wear rate for the tested acetabular cups showed that the respective wear profile changed between the two material configurations (Fig. 3). Whereas the ceramic specimens showed an initial wear, up to 1 million cycles followed by a reduced trend, the two metal sizes underwent an unsteady wear behaviour up to the end of the test. After an initial running in and a descending tendency of both COM diameters (up to 2 Mc), the smaller size showed a lower wear trend; then a new increase in wear was evident in the metal cup wear trend. In this second phase of the test, the bigger size showed a lower volumetric loss; this is probably because of the coupling’s geometrical head-cup mismatch at micro scale level. This clinical worst-case scenario did not produce any wear reduction for COM hips, as found by other studies (Barnes et al., 2008; Fisher et al., 2006). A comparison is not possible because of different testing conditions, different lubricants used, and differently processed materials. It has been shown that the lubricant strongly affects the friction and wear properties (Brown and Clarke, 2006; McKellop et al., 1992). A higher protein content could provide different boundary lubrication effects that unphysiologically protect against wear, therefore care should be taken with regard to protein concentration and volume of the lubricant serum. The lack of geometrical controls might have led to deviation in such parameters, such as diametral clearance between heads and cups. It is well known that the role of clearance in the wear of hip articulations is fundamental even through the selected material (Affatato et al., 2007; Brockett et al., 2008; Mabuchi et al., 2004), thus demonstrating that implant design plays a relevant role in THR (Dowson et al., 2004; Scott and Schroeder, 1997). In fact, when, after surgical procedure, a congruency mismatch is present, the clinical failure risk increases widely (Kosashvili et al., 2008; Steens et al., 2006). Moreover, if the head is too small, increased stresses over a reduced surface area ensue, leading to polar stress, erosion, prosthetic migration and pain. Even Willmann, when neither standards nor government regulations for hybrid couplings existed, proposed some guidelines to adopt for production control of THR (Willmann et al., 1996). He also suggested, approved, and prohibited couplings (based on tribological reasons) to avoid dangerous head/socket combinations (Willmann, 1998). More recently Valenti et al. (2007) observed massive metallosis in an implanted stainless steel femoral head against an alumina ceramic acetabular cup after 6 weeks. Williams et al. (2007) and Williams et al. (2008), on the other hand, observed consistent reduced ion levels in patients, who had received COM implants. In most cases, however, increased metal serum levels are associated with implant failures
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after longer clinical follow-ups (10 years) (Damie and Favard, 2004; Marker et al., 2008; Sargeant and Goswami, 2007). However, even though THR has a high success rate (Savarino et al., 2008; Willmann, 2000a, b), with a good long-term follow-up, it is possible to find several in-vivo cases of mismatching couples (Duijsens et al., 2005; Stiehl and Mahfouz, 2007; Morlock et al., 2001; Steens et al., 2006). Morlock et al. (2001) discussed the medium-term failure (42 months) of aluminium oxide/zirconium oxide pairing, where head and cup had been produced by different manufacturers; the poor fit of the two components was visible even by radiographic assessment. Stiehl and Mahfouz (2007) showed the catastrophic failure of a metal/polyethylene design mainly due to an inadequate tolerance control that caused a femoral/head mismatch despite the fact that technological evolution, both in the design phase and in the manufacturing process, should have contributed to dispelling these concerns. Finally, a case report of severe cobalt poisoning with loss of sight after COM pairing in a hip implant that, in conclusion, convinced surgeons to never mix and match (Steens et al., 2006). Firm criticism towards manufacturing techniques and tolerances was stressed also by Ito et al. (2001); their findings suggested that poor sphericity was not favourable. However, some major limitations of this study must be considered. First of all, our results, while statistically significant, were based on a small number of specimens, thus implying using caution when experimentally evaluating the wear behaviour of such occasional COM configurations. As a consequence, we cannot say whether these differences in wear would result in clinically different long-term outcomes. Moreover, no tribo-corrosion studies were carried out for the analysed worst-case scenario; in fact, it is still not clear if substituting one of the two Co–Cr bearings with a ceramic one would reduce corrosion-wear interactions. Some recent pin-on-plate wear studies indicated positive judgment in this direction (Figueiredo-Pina et al., 2008). In conclusion, on the basis of the results reported, even though hard-on-hard hybrid bearings might represent an effective solution to overcome problems related to COC and MOM couplings, self-made hybrid matching should be handled with care, unless explicitly stated by producers.
Conflict of interest All authors certify that they have not signed any agreement with a commercial interest related to this study and they have no proprietary, financial, professional or other personal interests of any nature or kind in any product, service and/or company that would in any way limit publication of any and all data generated for the study or to delay publication for any reason.
Acknowledgements We would like to thank Mara Zavalloni, Sara Cremonini, and Luigi Lena (Rizzoli Orthopaedic Institute) for their help with the set-up, the statistics, and the images, respectively. Thanks are also due to Simone Carmignato (DTG, Universita di Padova, Italy) for roundness measurements, and Ala Ortho (Milan, Italy) for the specimens supplied.
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Mixing and matching in ceramic-on-metal hip arthroplasty: An in-vitro hip simulator study Saverio Affatato a,, Michele Spinelli a, Stefano Squarzoni b, Francesco Traina a,c, Aldo Toni a,c a
Laboratorio di Tecnologia Medica, Istituto Ortopedico Rizzoli, Via di Barbiano 1/10, 40136 Bologna, Italy IGM-CNR, Unit of Bologna c/o IOR, Bologna,Italy c Dipartimento di Ortopedia e Traumatologia, Istituto Ortopedico Rizzoli, Bologna, Italy b
a r t i c l e in fo
abstract
Article history: Accepted 29 July 2009
The clinical success of second-generation metal-on-metal hip replacement and the good tribological performance of alumina ceramic revived an interest in hip articulation as a solution to reduce wear. This study was aimed at characterizing the wear behaviour of new hybrid ceramic-on-metal bearings. In particular, this study investigated the wear behaviour of ceramic-on-metal hip components (three different diameters configurations: 28, 32 and 36 mm), not specifically proposed to be coupled, in order to compare them with ceramic-on-ceramic, which is considered to be the gold standard for wear resistance. For this purpose, the weight loss over a standard wear simulation was monitored. Moreover, scanning electronic microscope observations were used to verify if any carbides removal, for the metallic components, triggered wears debris production promoting abrasive third-body wear. After five million cycles, our results showed significantly greater wear-in ceramic-on-metal compared with ceramic-on-ceramic, and significant greater wear for the 32-mm diameter compared with the 36-mm one. Our findings showed an increase in wear for the proposed hybrid specimens with respect to that of the ceramic-on-ceramic ones confirming that even in the case of ceramic-on-metal bearings, mixing and matching could not prove effective wear behaviour, not even comparable with that of the ceramic-on-ceramic gold standard. Wear patterns and roundness tolerances certainly discourage the coupling of components not specifically intended to be coupled. Unsuitable geometrical conformity could, in fact, result in a poor dynamic behaviour and lead to clinical failure. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Ceramic-on-metal Roundness measurements Hip wear rate COC COM SEM
1. Introduction Hard bearing surfaces are used for total hip replacement due to their better tribological properties. The most commonly used hard bearing surfaces are ceramic-on-ceramic (COC) and metal-onmetal (MOM); both have very good wear resistance but have some limits despite the evolution of materials. There are no doubts that ceramic implants reduce wear with respect to polyethylene and metal couplings (Affatato et al., 2001, 1999; Nizard et al., 2008; Rose et al., 1980; Willmann, 1998, 2000a, b). They have good tribological properties such as hardness, good chemical resistance, high tensile strength, and good fracture toughness (Costa et al., 1997; Kumar et al., 1991; Macchi and Willman, 2001; Willmann et al., 1996). However, the main concern remains its brittleness, which could lead to failure. Although the reported failure rate is low (0.004% head fracture (Willmann, 2000a, b) and 0.22% liner fracture (Toni et al., 2006)), it is clinically relevant (Darring, 2007; Lidgren and Robertsson,
Corresponding author. Tel.: + 39 51 6366864; fax: + 39 51 6366863.
E-mail address: [email protected] (S. Affatato). 0021-9290/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2009.07.031
2007; Stea et al., 2009). On the other hand MOM bearings aim to ensure high wear resistance, good manufacturability and low friction torque. Despite a lower wear volume associated with such implants, very small particles are produced (Davies et al., 2004; Maloney et al., 1993; Silva et al., 2005). The amount of metal ions present in the serum and their potential toxic effects both locally and systemically (Case, 1999; Damie and Favard, 2004; Dobbs and Minski, 1980; Doorn et al., 1999; Fisher et al., 2004, 1999; Marker et al., 2008; Miki et al., 2006; Sargeant and Goswami, 2007) are a cause for concern. Moreover, polishing wear promoted by wear debris, produced by the abrasive action of carbides has been shown in retrieved Co–Cr alloy hip implants (Dahm et al., 1995). The recent introduction of an innovative hybrid hard-on-hard bearing (ceramic head and metallic insert; COM) (Barnes et al., 2008; Firkins et al., 2001a, b)) claimed to reduce ion release (Sauve et al., 2007) and wear particle production on one hand (Brockett et al., 2007), and possibly the breakage of the ceramic insert rim (Toni et al., 2006) on the other. Therefore, considering the inverse proportionality of wear rate and hardness of the bearing material (Archard, 1953; Firkins et al., 2001a, b), it might be thought that dissimilar bearing couples provide the reduced adhesive wear, typical of like-on-like commercial bearings, and
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Table 1 Geometrical details for all ceramic and metallic specimens tested in these studies. Diameter (mm)
Femoral head composition
Acetabular cup composition
Average diametral clearance (lm)
Number of componentsa
28 32 36 36
Biolox-Forte Biolox-Delta Biolox-Forte Biolox-Delta
Biolox-Forte CoCrMo Biolox-Forte CoCrMo
77 86 100 98
3+1 3+1 3+1 3+1
a
Three components for each different combination were in run onto the simulator; the last one was used as control check for ion release.
Fig. 1. Location of the roundness measurements taken on the femoral heads (Part a) and on the acetabular cups (Part b).
Fig. 2. The picture shows the weight loss for metallic and ceramic acetabular cups coupled with ceramic femoral heads. The COC combination (28 and 36 mm) wore less than the hybrid for the COC configurations tested (COC 32 ad 36 mm). For the COC configurations, it is possible observe a typical wear profile between 0 and one million cycles, containing an initial higher wear. Wear then reduced after 1–5 million cycles for the COM 36 mm while the COM 32 mm configuration shows a higher wear trend.
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Fig. 3. Incremental volumetric wear rate (7 standard deviation) for the acetabular cups of different configurations tested. The wear profile of the ceramic specimens showed an initial wear followed by a reduced wear. A different wear behaviour was observed for the two metallic size with an initial decrease in wear rate and, from two million cycles, an higher wear due probably to different clearance between heads and cups. A different wear trend was observed for the COC configuration with a continuous decrease in wear rate.
overcome polishing problems induced by carbides detachment together with a reduction in ions release and breakage of the ceramic inserts. However, short-term results of this hybrid coupling are available, with regards to the reduction of wear and ion release (Firkins et al., 2001a, b; Fisher et al., 2006; Barnes et al., 2008). The emphasis on the possible advantages of this innovative hybrid coupling might lead to a new intra-operative scenario: the use of a COM coupling with components not specifically proposed to be coupled. In the past the practice of coupling different materials led to clinical failures (Morlock et al., 2001; Valenti et al., 2007), but was poorly investigated from an experimental point of view (Barnes et al., 2008), even if it could be considered a worst-case scenario. This study investigated the wear behaviour of COM hip components, not specifically proposed to be coupled, in order to compare them with COC and MOM traditional bearings. For this purpose, the weight loss over a standard wear simulation was monitored. Moreover, scanning electronic microscope (SEM) observations were used to verify if any carbides removal, for the metallic components, triggered wears debris production promoting abrasive third-body wear. Finally, shape measurements were assessed in order to calculate diametral clearance and relevant roundness variations of the tested heads and cups.
2. Materials and methods
Fig. 4. SEM mapping of progression in ceramic surface of 28 and 36 mm COC combination (original magnification 2000; SEM variable-pressure mode; 20 KV). The picture shows typical images of the alumina matrix of heads and cups in the working region confirming that the wear test did not affect the surface texture, after five million cycles.
All ceramic and metallic specimens were commercial implants for hip surgery (Ala Ortho, Milan,Italy). Three different diameters were chosen (28, 32, and 36 mm) because they are the most commonly used by hip surgeons for conventional total hip replacement. The bearing materials tested included two types of combinations: COM and COC. In the case of COM, Biolox Delta ceramic heads were coupled with cast high-carbon (HC) CoCrMo (ASTM F-75) cups. The COC combination consisted of Biolox Forte ceramic heads and cups (Table 1). The wear test was performed using a 12-station hip joint simulator (Shore Western Mnf., USA). The kinematics and load parameters applied to our hip simulator followed a well-established internal protocol (Affatato et al., 2001, 2007, 1999, 2008). The lubricant used was 25% sterile bovine calf serum (SIGMA, St. Louis, USA) balanced with deionized water and 0.2% sodium azide (E. Merck, Darmstadt, Germany) to retard bacterial degradation. EDTA (ethylene–diaminetetraacetic acid) was also added in a concentration of 20 mMol/dm3. The specimens were removed from the simulator every 0.5 million cycles, weighed, and the test was restarted with new lubricant. The wear test lasted five million cycles.
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Fig. 5. SEM mapping of progression in ceramic surface damage of 32 mm COM combination from non-wear zone, to main wear zone and into stripe wear zone (original magnification 2000; zoomed magnification 11,000; SEM variable-pressure mode; 20 KV). On the metallic cup are emphasised mild abrasion (parallel slight scratches) and small indentations indicating a third-body abrasion mechanism. Ceramic grains were visible with SEM, and EDS analysis confirmed they were alumina particles.
We evaluated wear-in terms of weight loss measured using a microbalance ¨ (SARTORIUS AG, Gottingen, Germany) with a precision of 7 0.1 mg. Each weight measurement was repeated three times and the average weight was used for calculations. The incremental volumetric wear rate was calculated by subtracting each volumetric loss from the previous measurement and dividing it by the number of cycles corresponding to the considered step. The effects of the two bearing material combinations on wear were evaluated using a nonparametric test (Two-Sample Kolmogorov–Smirnov Test, K–S) (Armitage and Berry, 1994). Statistical significance was set at p o 0.05. After completing the test, we visually examined all specimens for scratches or damage caused by third-body wear. Eventually, we examined the most worn femoral heads and the acetabular cups under a light microscope (Nikon SMZ-2T, Tokyo, Japan) and a scanning electronic microscope (SEM) (Cambridge Stereoscan 200, UK) operating at 20 kV. In the case of ceramic specimens, a thin layer of gold coating was sputtered in order to have a better electrical conductivity and hence visibility in the SEM. Energy dispersed spectroscopy (EDS) X-ray analysis (Inca Energy-200, Oxford Instruments, UK) was used to analyse chemical composition. A state-of-the-art Coordinate Measuring Machine (CMM-ZEISS Prismo VAST 7, Germany) was used to assess the shape of the femoral heads and the acetabular cups. Calculated diametral clearance and roundness tolerance, in fact, enable the quality control of the coupling and of nominally circular products by testing the accuracy of their nominal shape. The maximum probing error of the CMM was determined to be below 1 mm. The CMM was in a temperature and humiditycontrolled room. For each specimen three profiles were taken, respectively along the transverse, the sagittal, and the frontal plane (Fig. 1), before and at the completion of the test. The number of points measured on the surfaces determines
the accuracy of the method, so the measurements were performed with a high point density distribution, which results in approximately 2000 points measured on each profile. In the pre-test measurements, all three specified tracks, for each specimen, were near the lower sensitivity threshold of the machine.
3. Results Each of the ceramic and metallic hip specimens completed 5 million cycles (Mc) of wear tests as planned. The wear rates were evaluated for the wear-in and for the wear-steady periods. In particular, the wear-in phase represents a phenomenon of accelerated wear that rigid-on-rigid bearings generally exhibit in the range between 100,000 and 1,000,000 cycles while, the wearsteady is characterized by a lowest wear than the previous phase. During the wear-in period (0–1 Mc), the wear rate was lower for the 36 mm COC articulation diameters than for the other combinations (Fig. 2). In particular, for the acetabular cups, the wear rate was lower for the 36 mm COC vs. the 32 mm COM articulations (po0.0001) and for the 36-mm COC vs. the 36-mm COM articulations (p= 0.01). The steady-state wear period showed
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Fig. 6. SEM mapping of progression in ceramic surface damage of 36 mm COM combination from non-wear zone, to main wear zone and into stripe wear zone (original magnification 2000; zoomed magnification 11,000; SEM variable-pressure mode; 20 KV). On the metallic cup a lot of tracks with precipitation inside the grains and in the grain boundaries are visible.
wear rate decreased for the 36 mm COC and 36 mm COM articular diameter while the 32-COM showed an anomalous increase in wear behaviour. The incremental volumetric wear of COM specimens showed an unsteady behaviour with respect to the COC (Fig. 3); the COM wear trend underwent great variability around two million cycles without any final plateau stabilization. SEM observations of the most worn heads revealed most scratches, holes, and micropits were situated in an annular wear zone. The micropits and holes were irregularly shaped and were surrounded by smears typical for CoCrMo alloys (Figs. 4–6). Roundness measurements prior to testing did not show any differences among the three measurement profiles; the average roundness values for all tested configurations are summarized in Table 2. The same measurements performed at the end of the test showed broad differences between the transverse plane and the other two measure profiles (Table 3).
4. Discussion The aim of this study was to look at the wear behaviour of COM hip components, not specifically proposed to be coupled, in order to compare them with COC and MOM traditional bearings. Random assembly is, sometimes, a used practice in surgical
Table 2 Roundness measurements (7 standard deviation) performed on all femoral head and acetabular cups at zero million cycles. Bearing Diameter (mm) Average roundness measurement ( 7 SD) (mm)
COC COC COM COM
28 36 32 36
Head
Cup
0.00087 0.0002 0.0006 7 0.0001 0.00057 0.0001 0.00077 0.0003
0.0006 7 0.0004 0.0006 7 0.0002 0.00087 0.0002 0.00077 0.0003
SD= Standard deviation.
theatre mainly in case of revision interventions (Morlock et al., 2001; Stiehl and Mahfouz, 2007; Valenti et al., 2007). The weight loss over a standard wear simulation was assessed. SEM observations were carried out to verify if any carbides removal, for the metallic components, triggered wears debris production promoting abrasive third-body wear. Finally, shape measurements were assessed in order to calculate diametral clearance and relevant roundness variations of the tested heads and cups. The present in-vitro study confirmed that even in the case of COM bearings, mixing and matching could not prove effective
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Table 3 Roundness measurements (7 standard deviation) performed on all femoral head and acetabular cups at five million cycles. Bearing
Diameter (mm)
Average roundness measurement ( 7 SD) (mm) Head
COC COC COM COM
28 36 32 36
Cup
Transverse plane
Sagittal plane
Frontal plane
Transverse plane
Sagittal plane
Frontal plane
0.00087 0.0002 0.00067 0.0001 0.00067 0.0001 0.00077 0.0003
0.0006 70.0001 0.0022 70.0004 0.0025 70.0001 0.0028 70.0006
0.000770.0002 0.0019 70.0002 0.0023 70.0002 0.0028 70.0001
0.00257 0.0005 0.00197 0.0002 0.00297 0.0016 0.0045 7 0.0024
0.00497 0.0014 0.00627 0.0004 0.00847 0.0014 0.00677 0.0020
0.00477 0.0005 0.00637 0.0005 0.00907 0.0014 0.00707 0.0019
SD= Standard deviation.
wear behaviour, not even comparable with that of the COC gold standard. In terms of weight loss, our findings favoured COC combinations for hip articulation, which is in disagreement with others authors (Barnes et al., 2008; Fisher et al., 2006). These authors found 100 fold wear reduction for COM pairings with respect to the MOM ones and a slight wear increase if compared to the COC configuration. SEM observations of the worn regions in the femoral heads and in the acetabular cups tested, put in evidence some relevant surface damages and alumina inclusions on the metallic cups of the hybrid pairing while no surface damages were evident on the COC specimens. Fig. 4 shows typical images of the alumina matrix of COC heads and cups (28 e 36 mm) in the working region confirming that the wear test did not affect the surface texture, after five million cycles. On the contrary, in the COM combinations it is possible to observe two different wear mechanisms. In particular, mild abrasion (parallel slight scratches) and small indentations indicating a third-body abrasion mechanism (Fig. 5), that can be due to the introduction of small hard wear debris in the contact region, either arising from fatigue wear or from detachment of the asperities present on the surface. The wear mapping of the 32 mm COM cup showed different significant features (Fig. 5); removal of original machine polishing marks, revealing grain structure and some minor pitting. A progression to disruptive type of wear was characterized by intergranular fracture and grain pullout, formation of craters and loss of surface integrity. The presence of holes and micropits from carbides removed from the bearing surfaces suggests the microstructure can also influence wear rate in COM articulation. Ceramic grains were visible with SEM, and EDS analysis confirmed they were alumina particles. The 36 mm cup had a lot of tracks with precipitation inside the grains and in the grain boundaries (Fig. 6). Roundness measurements confirmed that, for the tested specimens, a marked geometrical mismatch was evident, thus we could have expected a poor tribological behaviour. In particular, for ceramic heads, the roundness tolerance measured prior to testing (all profiles), was comparable with the machine calibration sphere ( 0.0005 mm). Roundness acquisitions at 5 million cycles showed a different behaviour between the transverse and frontal or sagittal plane (Table 3), thus confirming a different roundness tolerance on different planes. A similar tendency is evident for the cups where the roundness error greatly increased in both sagittal and frontal plane at the end of the test. The different roundness tolerances for different working planes might explain such ineffective wear behaviour. The lack of geometrical controls might have led to deviation in such parameters, so that it is impossible to distinguish in depth the contributions of bearing diameter and clearance independently. The wear rate, on the COM configurations, depended solely on mismatch between heads and cups.
Our findings showed an increase in wear for the proposed hybrid specimens with respect to that of the COC ones. Besides, the volumetric wear of the metal cups (coupled with ceramic heads—Biolox Delta) was also higher in comparison with the data reported for the same metal shells when coupled with metal heads (Affatato et al., 2007). The incremental volumetric wear rate for the tested acetabular cups showed that the respective wear profile changed between the two material configurations (Fig. 3). Whereas the ceramic specimens showed an initial wear, up to 1 million cycles followed by a reduced trend, the two metal sizes underwent an unsteady wear behaviour up to the end of the test. After an initial running in and a descending tendency of both COM diameters (up to 2 Mc), the smaller size showed a lower wear trend; then a new increase in wear was evident in the metal cup wear trend. In this second phase of the test, the bigger size showed a lower volumetric loss; this is probably because of the coupling’s geometrical head-cup mismatch at micro scale level. This clinical worst-case scenario did not produce any wear reduction for COM hips, as found by other studies (Barnes et al., 2008; Fisher et al., 2006). A comparison is not possible because of different testing conditions, different lubricants used, and differently processed materials. It has been shown that the lubricant strongly affects the friction and wear properties (Brown and Clarke, 2006; McKellop et al., 1992). A higher protein content could provide different boundary lubrication effects that unphysiologically protect against wear, therefore care should be taken with regard to protein concentration and volume of the lubricant serum. The lack of geometrical controls might have led to deviation in such parameters, such as diametral clearance between heads and cups. It is well known that the role of clearance in the wear of hip articulations is fundamental even through the selected material (Affatato et al., 2007; Brockett et al., 2008; Mabuchi et al., 2004), thus demonstrating that implant design plays a relevant role in THR (Dowson et al., 2004; Scott and Schroeder, 1997). In fact, when, after surgical procedure, a congruency mismatch is present, the clinical failure risk increases widely (Kosashvili et al., 2008; Steens et al., 2006). Moreover, if the head is too small, increased stresses over a reduced surface area ensue, leading to polar stress, erosion, prosthetic migration and pain. Even Willmann, when neither standards nor government regulations for hybrid couplings existed, proposed some guidelines to adopt for production control of THR (Willmann et al., 1996). He also suggested, approved, and prohibited couplings (based on tribological reasons) to avoid dangerous head/socket combinations (Willmann, 1998). More recently Valenti et al. (2007) observed massive metallosis in an implanted stainless steel femoral head against an alumina ceramic acetabular cup after 6 weeks. Williams et al. (2007) and Williams et al. (2008), on the other hand, observed consistent reduced ion levels in patients, who had received COM implants. In most cases, however, increased metal serum levels are associated with implant failures
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after longer clinical follow-ups (10 years) (Damie and Favard, 2004; Marker et al., 2008; Sargeant and Goswami, 2007). However, even though THR has a high success rate (Savarino et al., 2008; Willmann, 2000a, b), with a good long-term follow-up, it is possible to find several in-vivo cases of mismatching couples (Duijsens et al., 2005; Stiehl and Mahfouz, 2007; Morlock et al., 2001; Steens et al., 2006). Morlock et al. (2001) discussed the medium-term failure (42 months) of aluminium oxide/zirconium oxide pairing, where head and cup had been produced by different manufacturers; the poor fit of the two components was visible even by radiographic assessment. Stiehl and Mahfouz (2007) showed the catastrophic failure of a metal/polyethylene design mainly due to an inadequate tolerance control that caused a femoral/head mismatch despite the fact that technological evolution, both in the design phase and in the manufacturing process, should have contributed to dispelling these concerns. Finally, a case report of severe cobalt poisoning with loss of sight after COM pairing in a hip implant that, in conclusion, convinced surgeons to never mix and match (Steens et al., 2006). Firm criticism towards manufacturing techniques and tolerances was stressed also by Ito et al. (2001); their findings suggested that poor sphericity was not favourable. However, some major limitations of this study must be considered. First of all, our results, while statistically significant, were based on a small number of specimens, thus implying using caution when experimentally evaluating the wear behaviour of such occasional COM configurations. As a consequence, we cannot say whether these differences in wear would result in clinically different long-term outcomes. Moreover, no tribo-corrosion studies were carried out for the analysed worst-case scenario; in fact, it is still not clear if substituting one of the two Co–Cr bearings with a ceramic one would reduce corrosion-wear interactions. Some recent pin-on-plate wear studies indicated positive judgment in this direction (Figueiredo-Pina et al., 2008). In conclusion, on the basis of the results reported, even though hard-on-hard hybrid bearings might represent an effective solution to overcome problems related to COC and MOM couplings, self-made hybrid matching should be handled with care, unless explicitly stated by producers.
Conflict of interest All authors certify that they have not signed any agreement with a commercial interest related to this study and they have no proprietary, financial, professional or other personal interests of any nature or kind in any product, service and/or company that would in any way limit publication of any and all data generated for the study or to delay publication for any reason.
Acknowledgements We would like to thank Mara Zavalloni, Sara Cremonini, and Luigi Lena (Rizzoli Orthopaedic Institute) for their help with the set-up, the statistics, and the images, respectively. Thanks are also due to Simone Carmignato (DTG, Universita di Padova, Italy) for roundness measurements, and Ala Ortho (Milan, Italy) for the specimens supplied.
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