A preliminary experimental investigation on the biotribocorrosion of a metal-on-polyethylene hip prosthesis in a hip simulator

Corrosion at the taper/trunnion interface of total hip replacement (THR) often results in severe complications. However, the underlying mechanisms of biotribocorrosion at the taper/trunnion interface during the long-term walking gait cycles remain to be fully understood. In this study, a hip joint simulator was therefore instrumented with an electrochemical cell for in-situ monitoring of the tribocorrosion evolution in a metal-on-polyethylene (MoP) THR during a typical long-term walking gait. In addition, the biotribocorrosion mechanism was investigated via surface and chemical characterizations. The experimental results confirmed that the taper/trunnion interface dominated the contemporary MoP hip joint corrosion. Three cyclic variations in the open circuit potential (OCP) were observed throughout the long-term electrochemical measurements, attributed to the formation and disruption of the adsorbed protein layer. The corrosion exhibited an initial increase at each period, peaking at approximately 0.125 million cycles, followed by a subsequent gradual reduction. Surface and chemical analyses revealed the formation of a tribochemical reaction layer (tribolayer) on the worn surface of the taper/trunnion interface. The surface/chemical characterizations and the electrochemical measurements indicated that the adhesion force of the adsorbed protein layer was weaker than that of the tribolayer. In contrast, the opposite was true for the corrosion resistance. Based on the observations from this study, the tribocorrosion mechanism of the taper/trunnion interface under the long-term walking gait cycles is deduced.


Introduction
Charnley's metal-on-polyethylene (MoP) total hip replacement (THR) is widely regarded as the gold standard for end-stage arthritis treatment. Further improvements include modular designs, consisting of (taper/trunnion) interface, which is convenient for surgeons when treating a wide range of patients [1]. However, due to the cyclic loading, fretting-corrosion at the taper/trunnion may result in adverse local tissue reactions (ALTRs) [2], leading to pseudotumours and THR failure. An increasing number of ALTRs in MoP THRs has recently been reported. For example, the prevalence of revision surgery caused by ALTRs in MoP THRs is 0.5% and 3.2% at a mean follow-up of 7 years [3] and over 10 years [4], respectively. Due to the excellent clinical results of contemporary hip implants using highly cross-linked polymer (PE) or vitamin E diffused cross-linked PE, it seems that the wear problem of THR has been overcome. However, the fretting-corrosion at the taper/trunnion interface is still an engineering issue that has not been properly solved. As a result, much research focuses on understanding the fretting-corrosion at the taper/trunnion interface, including the visual inspection of fretting-corrosion damage from retrieved implants [5,6], electrochemical assessment of material combination corrosion behavior in pin/ball-on-disc tests [7][8][9], and volume loss measurements of the hip joint taper/trunnion interface [10].
Retrieved implant analysis provides helpful information on the corrosion behavior under real circumstances. However, the limited number of samples and difficulty in changing the implant parameters result in challenges in identifying the evolution and mechanism of corrosion by this method. Combining a pin/ball-on-disc tribometer with an electrochemical cell is widely employed in research on the corrosion mechanisms of material combinations. However, the contact geometry, motion, and loading conditions at the modular junctions of THRs are distinct from those of simple tribometers. Thus, this method is unable to reflect the in-vivo environment. Simulator testing is the closest approach to clinical practice compared with other in-vitro test methods. Implementing a coordinate measuring machine allows directly determining the volume loss at the hip joint taper/trunnion interface from the hip joint simulator. Thus, volume loss measurements can evaluate the fretting-corrosion damage at the taper/trunnion junction more quantitatively than other assessments.
Owing to the protection of the taper/trunnion interface from assembly and disassembly, the volumetric wear was measured prior to and after testing. Consequently, the volume loss measurements cannot trace the tribocorrosion evolution at the taper/trunnion junction during the test.
The tribocorrosion of metal-on-metal (MoM) THR has been well studied in an instrumented hip simulator [11][12][13][14]. However, to the best of the authors' knowledge, there is a lack of research that employs hip simulator tests to investigate the long-term biotribocorrosion at the MoP THR modular junctions. Therefore, this paper aims to investigate the long-term biotribocorrosion behavior of the MoP hip implant via an instrumented hip joint simulator and to attempt to explain the biotribocorrosion mechanism from the experimental observations.

Test materials and specimens
Four sets of 28-mm diameter MoP hip prostheses were tested in this study. The taper/trunnion interface comprised a CoCrMo head (12/14 taper; ISO 5832-12) and a Ti-6Al-4V (TC4) stem (12/14 taper; ISO 5832-3). The stem and spigot ( Fig. 1) were employed to represent the clinical femoral stem as they use the same morse taper design. The femoral heads and stems were assembled in a torsional testing machine (E10000, Instron, USA) with a 2-kN axial force at a stroke rate of 0.04 mm/s (ISO 7206-10). The ultra-high molecular weight polyethylene (UHMWPE) liner was integrated with the femoral head to form the MoP articulations. All the test samples were provided by Beijing Chunlizhengda Medical Instruments Co., Ltd., China. The lubricant calf serum solution was prepared following the recommendation of ISO 14242-1 and Ref. [15]. Briefly, it was diluted to a total protein content of 30 g/L with deionized water and supplemented with ethylene diamine tetraacetic acid (EDTA), gentamicin, and amphotericin B to retard the precipitation of calcium phosphate and bacterial growth.

Experimental setup
The test samples were mounted on a 6-station hip joint simulator (Prosim, Simulation Solutions, UK) under a cup inclination angle of 60°. All the test specimens were subject to time-varying twin-peak load and relative articulating motion per ISO 14242-1 standard. The in-situ measurements of the MoP hip joint electrochemical reactions adopted a hip simulator with three-electrode electrochemical cells [11]. Figure 1 presents the schematic diagram of the experimental setup. The working electrode (WE) connection was placed at the stem bottom, and Ag/AgCl reference and platinum counter-electrodes were inserted into the container connecting the test cell. All possible sources of metallic tribocorrosion were sealed with silicone sealant and urethane coating. At the same time, sample mountings of polyether ether ketone (PEEK) were used to prevent any contributions to the electrochemical measurements. Sometimes, the machine stops due to the breakage of the silicon rubber case or the overvoltage of the power supply. The test samples were not removed from the simulator when dealing with the stop of the machine.

Biotribocorrosion of MoP THR during long-term walking gait cycles
The first test consisted of two sets of MoP THRs, which was conducted with the same test configuration to improve the reliability of the tests. These tests were referred to as Samples 1 and 2, and they were run for 1.5 million cycles at 1 Hz under the typical walking gait cycle. The in-situ corrosive degradation was monitored throughout the tests. Following ISO 14242-1, the fluid test medium was replaced every 0.5 million cycles. At each serum changing point, the serum was drained from the test cell and flushed three times with deionized water before refilling with fresh serum [16]. To minimize disturbances to the tribochemical reaction and protein adsorption layers at the taper/trunnion interface, the test samples were in place in the simulator when replacing the calf serum.

Biotribocorrosion of the MoP THR bearing surface
In the experimental setup ( Fig. 1), the WE represented the entire exposed surfaces of the taper/trunnion interface and the head/liner articulation. Therefore, an additional series of tests was introduced to examine the articulation effects on the electrochemical measurements. The tests were based on several considerations. Firstly, to the best of the authors' knowledge, there are no reports of MoP THR clinical failure caused by corrosion from MoP bearing surfaces. Furthermore, Hesketh et al. [8] determined a 30-fold reduction in the MoP corrosion current corr ( ) I compared to MoM material combinations. Hence, this study assumed that the taper/trunnion interface www.Springer.com/journal/40544 | Friction (metal-metal contact) was the major source of metal ion release and MoP THR tribocorrosion. To further confirm this, the surface characterization of the femoral head bearing surface was examined before and after testing. Moreover, the clear cyclic variation of the open circuit potential (OCP) trends was observed at each serum changing point of the long-term tests. Thus, a test that ran for 0.5 million cycles (a typical serum changing point) was considered long enough to demonstrate the entire corrosion cyclic variation and distinguish the articulation effect on corrosion for the entire system. Last, since the movement during testing prevented the full sealing of the MoP articulation, the taper/trunnion interface was sealed with silicone sealant instead. This excluded any electrochemical measurements of the taper/trunnion interface, ensuring that only signals from MoP bearing surfaces were measured. In summary, this test scenario employed two new sets of samples with a sealed taper/trunnion interface tested for 0.5 million cycles. The two repetitions of the same test configuration in this test scenario were Samples 3 and 4. The electrochemical measurements of bearing surfaces (Samples 3 and 4) were compared to those of the long-term tests (Samples 1 and 2). The data comparison and surface analysis can potentially determine whether the bearing surface or taper/trunnion interface is the dominant potential source of the metallic tribocorrosion.

Effect of changing the serum solution on biotribocorrosion
Beadling et al. [12] combined an electrochemical cell with a hip simulator to investigate the corrosion rate of MoM bearings and determined that changing the serum could repeat OCP patterns. We also observed this phenomenon in the current study. The third series of tests were performed following the long-term test to improve our understanding of the relationship between the serum state and corrosion. More specifically, the test cell was rinsed three times with deionized water and continuously ran with the original serum solution. The fresh and used serum solutions were visually compared to investigate variations in their protein structure. Following this, the protein concentrations of the used lubricant from the soak and test stations were measured using a Pierce bicinchoninic acid assay kit [17] to trace the status of serum. Note that at each serum changing point, the soak and test stations were refreshed with the same fresh serum solution (i.e., the same batch).
In summary, Samples 1 and 2 were run for 1.5 million cycles to investigate the long-term biotribocorrosion of MoP THR. Sample 2 was tested for additional 0.5 million cycles with the used serum solution to determine the effect of the serum state on biotribocorrosion. Samples 3 and 4 with a sealed taper/trunnion interface were tested for 0.5 million cycles to examine the bearing surfaces' effects on biotribocorrosion.

Electrochemical measurements
All electrochemical measurements were conducted using a Princeton Applied Research P4000A electrochemical workstation (AMETEK, USA) at an acquisition rate of 1 Hz. The OCP was monitored during the entire test process. The OCP monitoring did not require the polarization of the test samples. Thus, the surfaces exhibited free-corrosion conditions, and a net current between the working and counter electrodes was absent. An OCP change represented a change in anodic and cathodic half-reactions rates, providing a semi-quantitative assessment of the WE passivation level.
Linear polarization resistance (LPR) was employed to determine the polarization resistance P ( ) R . The P R and Stern and Geary equation [11] can be adopted to assess the corr I of the system as Eq. (1): where a  and c  are Tafel constants. The LPR was measured every 10,000 s, with a measurement duration of 200 s, a scan range of ±20 mV from OCP with the scan rate of 0.33 mV/s, and a  and c  equal to 120 mV [11]. Since the polarization from OCP was of a low range, linear polarization was considered as a non-destructive technique.

Surface/chemical characterizations
The surface topography and appearance of each stem trunnion were examined using the laser scanning and scanning electron microscope (SEM; JSM-IT500, JEOL, Japan), respectively. The stem trunnion's threedimensional surface roughness Z ( ) R was measured using the LSCM (MFT-500B, Rtec, USA). Chemical analysis of the worn and unworn surfaces was performed using the energy-dispersive X-ray spectroscope (EDS; Ultim Extreme, UK). The chemical composition of the carbonaceous tribolayer was observed via the SEM and EDS, with more in-depth measurements performed using the confocal Raman spectroscope (LabRAM HR, HORIBA, Japan) with a record range of 1,000-2,000 cm -1 . The Raman spectra were fitted using the D and G lines of graphitic carbon [18] to distinguish the adhering proteins from carbonaceous tribofilms. All the surface/chemical measurements of the test specimens were performed before and after testing.

Wear measurements
The gravimetric mass changes for the UHMWPE liner were recorded. The mean wear rate for 1.5 million cycles was 42.824.65 mg/10 6 cycles. The recorded wear rate was similar to a 28-mm head diameter UHMWPE liner [19,20]. The details of the wear measurements of the articulating surfaces will be published in future work.  corrosion state. The OCP variation trend repeated twice over the next two serum changes. Figure 3 depicts the corr I for Samples 1 and 2, determined via the P R and Stern Geary equation [11].

Biotribocorrosion of MoP THR during long-term walking gait cycles
corr I values exhibited an upward trend for both tests during the static to dynamic phase, and then kept relatively stable during the movement phase. The reduction in the amplitude of the first two OCP cycles in Sample 1 exceeded that of the last cycle, and corr I variations exhibited a similar trend. For Sample 2, the third OCP cycle presented the largest decline, and corr I also indicated the corrosion to be the strongest in the third cycle. Thus, corr I variations agree with the OCP trends. www.Springer.com/journal/40544 | Friction Figure 4 demonstrates that Samples 3 and 4 presented a stable OCP trend. This indicates the limited bearing surface corrosion rate variation over the test period. Thus, the tribocorrosion from the taper/trunnion interface primarily affected the MoP THR corrosion rate variation. Moreover, the OCP values were greater, and the corr I corrosion currents values were much smaller for Samples 3 and 4 than those for Samples 1 and 2 (Figs. 4 and 5). This implies the dominance of the metallic corrosion from the metal-metal contact (taper/trunnion interface) during  long-term testing, which is in agreement with the clinical reports [3,4] and previously published tests [8]. The electrochemical results of Samples 3 and 4 imply the taper/trunnion as the key active interface to be monitored in the electrochemical cell.

Effect of changing the serum solution on biotribocorrosion
As detailed in Section 2.2, the third series of tests investigated the key influencing factor of the repeated OCP patterns. We measured the OCP variations in the test station filled with the used serum solution and kept the sample unchanged for 0.5 million cycles. The electrochemical measurements were compared with the previous OCP trends using the same serum solution. The light-yellow area (Stage I, Sample 2) in Fig. 6 was the last cyclic variation of Sample 2 in the long-term test, representing that the serum solution was changed with fresh serum, while the light-green area (Stage II, Sample 2) in Fig. 6 represents that the serum solution was unchanged. The OCP of Stage II displayed an initial reduction to −270 mV and quickly recovered to the final state of the previous OCP repetition (around −185 mV, Stage I). This state was maintained until the test terminated. Visual comparisons reveal a difference in color between the fresh and used serum solutions of canary yellow and yellowish-white, respectively. Furthermore, according to the requirement of solution preparation from ISO 14242-1, the fresh serum passed the 2-μm filter while the used one did not. The previous research  reported that the proteins within the serum solution greatly impact corrosion [21]. The current study results revealed that the serum change induces the repetition of OCP trends in the MoP hip prosthesis. Thus, the protein variations within the serum solution altered the corrosion rate. Table 1 reports the protein concentrations in the used serum solution of the soak and test stations throughout the long-term test to investigate the relationship between the protein concentration and wear-corrosion. The protein concentrations of the soak station exceeded those of the test stations. The soak station was only subjected to the axial force without the applied motion compared with the test station. Thus, the protein concentration declined due to wearing. Figure 7 presents the surface topographies of the femoral head bearing surface (CoCrMo) obtained using the SEM. No significant corrosion signs were observed at the bearing surface of the post-test femoral head compared with the pre-test case. However, scratches appeared at the post-test bearing surface, mainly attributed to mechanical sliding and wear. Following the test, the stem trunnion was disassembled from the femoral head based on ISO 7206-10. The pre-and post-test taper trunnions were examined by the LSCM and SEM. The pre-test trunnion surface exhibited clear machining marks. The wear debris and the corrosive particles were mounded beside the worn area of the post-test trunnion, which increased Z R from 12.14±0.29 to 14.96±1.48 μm (Fig. 8). This agrees with the retrieved implant results of Cadel et al. [5]. The deposited layer was also observed in the SEM images (Fig. 9). The EDS observations further revealed that the layers on the worn surface were  www.Springer.com/journal/40544 | Friction carbon-rich and contained oxygen and femoral head constituents (e.g., cobalt, chromium, and molybdenum) (Fig. 10). The EDS analysis also indicates that the appearance of the carbon-rich deposited layer was attributed to the synergies between the fretting wear and corrosion. The Raman spectroscopy confirmed that the deposited layer was the nanocrystalline graphite and amorphous sp 2 carbon tribological layer [18]. The D-and G-peaks were present at around 1,404.0 and 1,575.2 cm -1 , respectively (Fig. 11).

Evolution of the corrosion behavior in long-term walking gait
For Samples 3 and 4, the OCP values were larger, and the corrosion currents were much smaller than those of Samples 1 and 2, while the OCP trend of the former was more stable. This indicates that the tribocorrosion from the taper/trunnion interface was the principal factor influencing the corrosion rate variations of MoP THR. In addition, the surface characterizations identified only mechanical damage on the post-test bearing surface, with no significant signs of corrosion. Consequently, the dominance of biotribocorrosion at the taper/trunnion interface of MoP THR was confirmed. Such findings were consistent with the retrieved clinical studies [3,4,22]. Under the long-term walking gait cycles, three clear periodicities in the OCP trends were observed for Samples 1 and 2. This observation may be attributed to three factors: 1) the termination of the | https://mc03.manuscriptcentral.com/friction machine, 2) the flushing of the test cells by deionized water, and 3) the change of serum solution, all of which could damage the passive film or break the corrosion status balance. The OCP repetition caused by the "machine stop" was firstly excluded, because its effect on the OCP was insignificant, as shown in Fig. 2. Therefore, following the long-term test, Sample 2 was continuously tested with the same serum solution to determine which subsequent factors were the underlying cause. Then the electrochemical measurements of Sample 2 with fresh (Stage I) and used (Stage II) serum solutions were compared. The corrosion evolution trend from Fig. 6 indicates that although flushing affected the corrosion variations, the serum change influence was dominant. The used serum was able to protect the test samples from corrosion.
Talha et al. [21] determined that proteins significantly influenced corrosion as they could be adsorbed on the metal surface, creating a barrier between the lubricant and metal surface. Wang et al. [23] reported that proteins that were less stable and close to their isoelectric point, or those with a larger size and unfolding rate, could be readily adsorbed on the metal surface. The visual observations in this study demonstrated a color change in the used serum solution. Furthermore, unlike the fresh serum solution, the used serum could not pass the 2-μm filter, indicating protein precipitation and aggregation. By integrating our results with those of the literature, it was assumed that the precipitated and aggregated proteins with a larger molecule size were more likely to be adsorbed on the metal surface, resulting in the formation of the adsorbed protein layer. This adsorbed protein layer inhibited corrosion by isolating the metal surface from the serum solution, while the fresh serum did not. Thus, the adsorbed protein layer could not be regenerated with the application of the fresh serum. Furthermore, the adhesion force between the adsorbed protein layer and the metal surface was weak. Hence the adsorbed protein layer could be easily rinsed out from the surface via deionized water. This assumption could explain the corrosion evolution trend between the fresh and used serum solution in Fig. 6. The adsorbed protein layer was damaged after flushing, consequently increasing the corrosion corresponding to Stage II of Sample 2. The precipitated and aggregated proteins within the used serum solution rebuilt the adsorbed layer, implying that the corrosion quickly recovered to the final state of the last OCP repetition (Stage I, Sample 2) and subsequently maintained stable at this state. The adsorbed protein layer of the fresh serum solution was permanently damaged, preventing its rapid regeneration. This was the primary reason for the repeated corrosion trends.
At each cyclic variation of the corrosion, the initial corrosion increase indicated the mechanical damage of the material oxide layer after applying the force and motion. This is a common phenomenon of MoM or MoP bearings for both hip simulators and pin/ball-on-disc tribometers [8,11,16,24]. The depassivation of the taper/trunnion interface was dominant until the turning point (at approximately 0.125 million cycles). This was linked to the fretting wear and corrosion synergism, leading to the passive film breakdown and the material degradation. Repassivation subsequently became the dominant factor. This may be attributed to the faster recovery rate of the passive film than the rates of alloy dissolution and mechanical damage. Following the gentle decline in corrosion, a relatively steady-state stage was observed. This stage, which indicates a balance between the depassivation and repassivation, was not observed in the second and third cycles of the OCP patterns in Sample 2. Such a phenomenon reveals a change in the serum or the termination of the test before the degradation and passivation rates could achieve equilibrium.

Formation and function of the tribolayer on the taper/trunnion interface
The protein concentration of the soak and test stations indicates that the proteins within the serum solution reacted with the test samples under the wear condition. The tribochemical reaction layer (termed tribolayer) observed on the taper/trunnion interface's worn surface may have been one of the reaction products. The Raman spectral signals were similar to tribolayer signals with hard-on-hard bearing surfaces (carbonaceous layer) [18], confirming the appearance of carbonaceous tribolayers at the taper/trunnion interfaces. The tribolayer is commonly assumed to be made of denatured proteins [25][26][27], with graphitic www.Springer.com/journal/40544 | Friction carbon as the principal chemical component [18]. The tribolayer may be formed via a two-step process under continuous sliding: 1) The proteins are adsorbed to the metal surfaces serving as solid lubricants [14] and 2) this adsorption layer is converted to the tribological film (tribolayer) under the wear condition [28,29]. The carbon-rich tribolayer firmly adheres to MoM bearing surfaces [27,30] with lubricating and isolating effects that can lower the prostheses' wear rate and corrosion rate [8,[30][31][32][33]. Similar tribolayers were identified on the taper/trunnion interface compared with MoM bearing surfaces [18]; thus, it was assumed that the tribolayer formation and function on the taper/trunnion interface were equal to those on MoM bearing surfaces. As a result, the tribolayer formed on the worn surface could contribute to the noble shifts in potential and consequently inhibit corrosion.

Hypothesis of the tribocorrosion mechanism
Based on the current study and previous literature, it is possible to infer the tribocorrosion mechanism at the taper/trunnion interface. After the test started, the bulk material oxidation layer was damaged by either mechanical wear or chemical dissolution. Chemical and mechanical reactions subsequently occurred between the exposed bare metallic surface and the corrosive medium, resulting in a sudden increase in corrosion. During the tests, two passivation films formed on the surface of the taper/trunnion interface, namely, an adsorbed protein layer from the precipitated and aggregated proteins and the tribolayer from the denatured proteins and wear debris. The reduction in corrosion was likely the result of the formation of the two types of layers, indicating the corrosion inhibitory ability of both layers. The adhesion force of the adsorbed protein layer was weaker than that of the tribolayer, while the opposite was true for the corrosion resistance. This explains why deionized water rinsed the adsorbed protein layer at the serum changing point, while the tribolayer was preserved on the worn surface after the test. Moreover, the adsorbed layer damage immensely influenced the system's corrosion.
The precipitated and aggregated proteins that made the adsorbed protein layer possibly started forming at approximately 0.125 million cycles, marking the turning point of corrosion.

Limitations
This study did not measure the femoral head and stem trunnion volume losses. Furthermore, according to ISO 14242-1 requirement, the full hip wear test should complete 5 million cycles. Since this study was a preliminary test, it ran for just 1.5 million cycles. However, the test duration used here was still the longest compared with the published literature. Future work will focus on the complete 5-million-cycle test and the effect of the test duration on the tribocorrosion behavior. An additional limitation is the lack of an in-depth investigation of the tribolayer evolution mechanism. The transmission electron microscopy (TEM) can be employed to comprehensively evaluate the tribolayer structure and composition by analyzing its underlying surfaces. Nano test techniques, such as the atomic force microscopy (AFM) and nanoindentation, can also be introduced to assess the mechanical properties of the tribolayer to better understand its role in the tribocorrosion process.

Summary
The current study reports the first MoP THR in-situ electrochemical measurements under the long-term walking gait. These live in-situ measurements provide valuable insight into the biotribocorrosion mechanism. Key observations of this work are summarized as follows: 1) The modular taper/trunnion junction was the main cause of MoP THR biotribocorrosion.
2) Changing the serum solution was the main factor for the repetition of the corrosion variation at the taper/trunnion junction in the long-term walking gait cycles. The typical serum changing point 0.5 million cycles) was considered long enough for the degradation and passivation rates to achieve the dynamic equilibrium.
3) Each OCP repetition exhibited a similar corrosion rate trend. The corrosion was immediately enhanced after applying motion and force, reaching a maximum at approximately 0.125 million cycles, followed by a gradual reduction.
4) The adsorbed protein layer formation and disruption markedly affected the corrosion evolution in MoP THR. This protein adsorption layer principally caused the long-term passivation. 5) A carbon-rich proteinaceous layer was observed on the worn surface of the stem trunnion. This layer was similar to the tribolayer observed on the MoM bearing surfaces.