24th International Colloquium Tribology - January 2024 261 Amorphous Carbon Coatings for Total Knee Arthroplasty - a Knee Simulator Evaluation Benedict Rothammer 1* , Kevin Neusser 1 , Marcel Bartz 1 , Sandro Wartzack 1 1 Engineering Design, Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Erlangen, Germany * Corresponding author: rothammer@mf k.fau.de 1. Introduction The use of artificial implants is substantially improving the quality of life for millions of people worldwide. However, the success of implants, such as total hip arthroplasties (THAs) or total knee arthroplasties (TKAs), depends on their ability to integrate and function in the human body without causing undesirable reactions-[1]. One way to improve the performance of artificial implants is to use amorphous carbon/ diamond-like carbon (DLC) coatings. Due to the fact that DLC is a material with excellent mechanical, chemical, and biocompatible properties, it has promising potential for various biomedical applications [2]. Some studies have investigated DLC coatings for use in joint replacements to improve wear resistance and thus extend the life of the artificial implant [3,4]. Thus, the following research gap, investigated in this contribution, arose as to whether a double-sided DLC coating of TKAs can considerably reduce wear compared to uncoated Co28Cr6Mo (CoCr)/ conventional UHM- WPE (CPE) or Ti6Al4V (Ti64)/ CPE in a knee simulator and thus substantially increase the service life of TKAs. 2. Materials and methods Basically, the unconstrained, fixed-bearing TKA BPKS Integration Knee System (Peter Brehm, Weisendorf, Germany) was used. This TKA is commercially available as a CoCr variant. Especially for this investigation, BPKS were also made of Ti64 (not commercially available) in order to characterize its tribological behavior (uncoated and coated). The spherically shaped femoral component of the left TKA(size-4) was fabricated from a cast CoCr alloy [5] and by machining a Ti64 alloy [6], respectively. The corresponding planar tibial plateau (size-4) was forged from a forged CoCr alloy [7] or manufactured additively from Ti64 by selective laser melting. The tibial inlays (size-4) were made of UHMWPE (Granular UHMWPE Ruhrchemie, GUR 1020 [8]). Subsequent γ-irradiation (BBF Sterilisation Service, Kernen im Remstal, Germany) with 31.2-±-5.5-kGy [9] to CPE was performed for the reference inlays after their production and for the coated inlays after the deposition process. The surfaces of the uncoated (Gesellschaft für Polier- und Schleiftechnologie, Bosau, Germany) and coated TKAs (Bestenlehrer, Herzogenaurach, Germany) were polished so that the surface requirements according to ISO-7207 [10] were met for metallic (R a -≤-0.1-µm) and for polymeric (R a -≤-2.0-µm) components, respectively. Articulation partners were the femur/ inlay pairs (n-=-3 plus 1 axial reference): • CoCr/ CPE as gold standard, • Ti64/ CPE the comparative pairing, • as well as Ti64/ aC: H/ CPE/ aC: H (according to [11]) as the decisive pairing to be investigated with aC: H fully coated. Experimental in-vitro testing of the implants was performed in a modified servohydraulic knee simulator with four stations (EndoLab, Thansau, Germany) [12] according to ISO 142431 [13]. In the knee simulator, walking as an everyday activity was simulated. Prior to the actual testing, the tibial inlays were preconditioned in diluted bovine calf serum (DBCS)-[14] at body temperature until the relative weight gain of the inlays was <-10-% compared to the previous week. This preconditioning prevented soaking of the inlay during the actual test, so that falsification of the wear mass of the CPE was nearly excluded. Subsequently, the implants were tested on their articulating surface areas in the simulator using DBCS tempered to 37- °C as lubricant for a total of 3.5-×-106 cycles. At intervals of 0.5-×-106 cycles, the implants were cleaned, the test fluid was retained for future particle testing, replaced by new DBCS, and the wear mass of the tibial inlay was determined gravimetrically with an analytical balance (Sartorius BP211D, Goettingen, Germany, accuracy of 0.01-mg) according to ISO-142432 [15]. In addition, the wear masses were corrected for soaking and air buoyancy using a purely axially load. In order to minimize interstation differences, the tibial inlays were rotated between stations after 0.5- ×- 106 cycles. For the characterization of the biotribological behavior, the original system (CoCr/ CPE and Ti64/ CPE) as well as the initial condition of the coated Ti64/ CPE variant before loading served as reference. 3. Results and discussion The mean values and standard deviations of the wear mass of the polymeric tibial inlays for each test interval are shown in Figure-1. With the help of the diagram, the development of the wear over the entire test can be shown for the uncoated TKAs made of CoCr/ CPE and Ti64/ CPE as well as the fully coated TKAs made of Ti64/ aC: H/ CPE/ aC: H. After the first 0.5-×-10 6 cycles, which equate to a critical runin, a comparable wear mass of approximately 2.7-mg was determined for all TKAs tested. While the uncoated references exhibited rather higher values over the test intervals with a steady, linear upward trend in wear mass, the coated tibial inlays showed a considerably milder wear with a slight upward trend. After 3.5-×-10 6 cycles, the highest wear was determined for the Ti64 (≈- 23.7- mg), followed by the CoCr (≈-20.9-mg), and the lowest wear for the coated variant (≈-9.0-mg). This clear relation was also reflected in the wear rates, which were 6.0- ±- 0.4- mg/ 10 -6 cycles for CoCr/ CPE, 7.1-±-1.0-mg/ 10 -6 cycles for Ti64/ CPE, and 2.1-±-0.7-mg/ 10 -6 cycles for Ti64/ aC: H/ CPE/ aC: H. Amorphous Carbon Coatings for Total Knee Arthroplasty -a Knee Simulator Evaluation 262 24th International Colloquium Tribology - January 2024 Figure-1: Arithmetically averaged wear mass and corresponding standard deviation for uncoated TKAs (CoCr/ CPE and Ti64/ CPE) and fully coated TKAs (Ti64/ aC: H/ CPE/ aC: H). Generally, the test showed that the CoCr/ CPE variant achieved a reduction in wear mass of about 12-% compared with the Ti64/ CPE variant. By coating both articulation partners, a significant reduction in the wear mass of roughly 57-% compared to CoCr/ CPE and roughly 62-% compared to Ti64/ CPE could be achieved, thus the service life of TKAs could be extended by DLC coatings. However, the tested components must be examined using advanced surface analysis in order to make a full statement about the exact wear behavior and particles. The obtained tibial inlay wear rates were found to be rather low for all tested TKAs compared to wear rates already published in the literature [16-18]. However, due to the revision of ISO-142431 [13], there was an adjustment of the kinematics during wear testing, allowing a higher free tibial rotation during a gait cycle. Thus, according to the current state of knowledge, a quantitative comparison of the wear rates is not expedient yet. In summary, it can be assumed that the coating can considerably increase the service life of TKAs. In this context, the DLC coatings must be specially adapted to the biotribological system TKA in order to prevent a coating-induced, adhesion-related failure. The DLC coatings on TKAs represented a compromise between the coating combinations presented in [11] and [19] - in terms of sufficiently high wear resistance and high deformation capability to prevent near-surface fatigue. The coating itself contributes to an increase in the service life by the same amount due to its thickness, provided that the coatings do not lead to undesirable (three-body) wear. Furthermore, the coating can promote a favorable running-in of the articulating partners and thus enable slow, continuous wear - avoiding the occurrence of delamination. Even though the testing of implants in the knee simulator is an important step in the process of certification of TKAs, the limitations of testing under almost ideal in-vitro conditions at the component level must be considered as well. Besides a simplified environment, ideal gait cycles were simulated. Additionally, it was ensured that clinically relevant extreme conditions were excluded. These include, for instance, the presence of ceramic particles, which can be generated during cemented implantation under non-ideal conditions and lead to three-body wear. Basically, such extreme tests could be used to assess the wear behavior of the coatings in comparison to reference pairings under more realistic conditions. In future studies, these results must be examined using precise surface analysis methods in order to be able to provide a full explanation of the wear behaviour. 4. Conclusion The current results of experimental in-vitro testing of uncoated and coated TKAs shown in this contribution demonstrated that a significant wear reduction and thus service life extension of TKAs was possible by DLC coatings on both articulating partners. The initial wear results shown were consistent with the findings in [11,19] and confirmed the biotribological effectiveness of DLC coatings. Due to the update of ISO-142431 [13], a quantitative comparison with previously published results regarding wear rates is, at present, not expedient. Also, the general limitations of a component test rig under ideal conditions must be considered in order to be able to derive well-founded, realistic wear predictions. Nevertheless, it can be stated for the investigations carried out that the service life of TKAs could be considerably increased by biotribologically effective DLC coatings. However, the current investigations from the knee simulation must be fully continued and consolidated for a holistic prognosis of the tribological performance of coated TKAs. References [1] Marian, M., et al. Adv. Colloid Interface Sci., 2022, 307. Jg., S. 102747. [2] Malisz, K., et al. Materials, 2023, 16. Jg., Nr. 9, S. 3420. [3] Birkett, M., et al. Acta Biomaterialia, 2023. [4] Shah, R., et al. Surf. Interfaces, 2021, 27. Jg., S. 101498. [5] DIN ISO 5832-4: 2015-12. [6] DIN EN ISO 5832-3: 2022-02. [7] DIN ISO 5834-2: 2020-07. [8] DIN ISO 5832-12: 2020-07. [9] DIN EN ISO 11137-1: 2015-11. [10] ISO 7207-2: 2011-07. [11] Rothammer, B., et al. Wear, 2023, 523. Jg., S. 204728. [12] Woiczinski, M., et al. Knee Surg. Sports Traumatol. Arthrosc., 2020, 28. Jg., S. 3016-3021. [13] ISO 14234-1/ Amd 1: 2009-11/ 2020-01. [14] Rothammer, B., et al. J. Mech. Behav. Biomed. Mater., 2021, 115. Jg., S. 104278. [15] ISO 14243-2: 2016-09. [16] Ezzet, K. A., et al. Clin. Orthop. Relat. 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