The dominance of water on lubrication properties of articular joints Hakan Göçerler, Bernhard Pfeil and Friedrich Franek AC2T research GmbH, Wiener Neustadt, Austria, and Christoph Bauer, Eugenia Niculescu-Morzsa and Stefan Nehrer Danube University Krems, Krems, Austria Abstract Purpose – The paper aims to focus on the individual contribution of water to the lubrication regimes in articular joints because understanding of these regimes is crucial not only for the treatment of diseases like osteoarthritis but also for the development of new implants to have a longer service cycle. Design/methodology/approach – Cartilage specimen and the synovial fluid were both isolated from bovine knee joints that are enclosed by a synovial membrane under sterile conditions of a biosafety Level 2 (BSL2) cabinet. Subchondral bone was removed from the cartilage because it not only creates a stable base to place the specimen on the holder, but it also acts as a primary shock absorber protecting the overlaying cartilage under high-impact loads. A specimen holder specially designed for tests and was attached to the linear oscillation (SRV) test machine. The SRV test machine provides a reciprocating sliding motion between the cartilage samples that are submerged into the selected biological media. The entire system can be mounted on the BSL2 cabinet, sealed with the convoluted gaiter and transported to the SRV machine with a specifically designed handle for the entire system. The process ensures sterile conditions for tests on biological samples that are highly sensitive to the environmental conditions. Findings – A remarkably low coefficient of friction value for distilled water constitutes more evidence to support the assumption of the impact of water in the friction behaviour of the cartilage-against-cartilage contact. As the fluid in articular cartilage (AC) effectively serves as a synovial fluid reserve and 70-80 per cent of its composition is distilled water, it can be stated that the tribotest system mimics the natural working conditions of an actual knee joint adequately. Research limitations/implications – Time and limited availability of the animal-driven samples led to a focus on certain parameters mentioned in the approach. A planned scan of parameter matrix, such as variation of load and speed, would allow deeper knowledge on the lubrication regimes. Practical implications – Study of relevant tribological contact in human joints might give ideas on new designs for artificial joints. Social implications – Understanding of lubrication regimes is crucial not only for the treatment of diseases like osteoarthritis but also for the development of new implants to adapt motion of related joint. Originality/value – Exclusion of water and application as the primary lubricant in the test system brings a new perspective to joint lubrication. Keywords Biotribology, Articular cartilage, Lubrication regimes, Ex vivo tissue, Synovial fluid, Tribometry Paper type Research paper 1. Introduction Enhancements in material and surface finishing technologies ensured arthroplasty applications to be an alternative solution for people who have various complications related to articular joints. However, recent reports reveal complications long before the expected lifetime. Major issues are material wear, aseptic loosening, various infections, arthrofibrosis, malalignment or malposition (Lum et al., 2018). One of the main reasons for these issues is the mismatch between the mechanical properties of artificial materials and the host tissue under in situ tribological conditions. Even though a wide portfolio of biomaterials incorporated with cell-based technologies was introduced in clinical applications, inconsistency in success rates is apparent (Escobar Ivirico, 2017). To increase success rates, a deeper knowledge about highly loaded biomechanical systems in the human body, natural functions of the tissues and advanced biomedical investigations in the field of tribology are required. Tribological analysis in natural joints focuses on the lubricating mechanisms and the actual working conditions where a failure of cartilage is generally observed. Researchers have proposed various tribological theories such as boundary lubrication, hydrodynamic lubrication, weeping, squeeze film lubrication, boosted lubrication, biphasic lubrication, elastohydrodynamic lubrication and complex-mode The current issue and full text archive of this journal is available on Emerald Insight at: https://www.emerald.com/insight/0036-8792.htm Industrial Lubrication and Tribology 72/1 (2020) 31–37 © Emerald Publishing Limited [ISSN 0036-8792] [DOI 10.1108/ILT-02-2019-0064] This work was funded by the Austrian COMET Program (Project K2, XTribology, no. 824187/849109) and carried out at the “Excellence Centre of Tribology” (AC2T research GmbH). Conflicts of interest: The authors declare no conflict of interest. Received 25 February 2019 Revised 4 July 2019 Accepted 4 July 2019 31