1. An equine joint friction test model using a cartilage-on-cartilage arrangement Prisca Noble a,*, Bernard Collin a, Jacqueline Lecomte-Beckers b, Adrien Magnée b, Jean M. Denoix c, Didier Serteyn d a Department of Veterinary Morphology and Pathology, Liège University Faculty of Veterinary Medicine, Belgium; b Department of Aerospace Industry and Mechanics, Liège University Faculty of Applied Science, Belgium; c CIRALE-ENVA, France; d Department of Clinical Science, Equine Clinic, Liège University Faculty of Veterinary Medicine, Belgium. Introduction : The main tribological function of articular cartilage is to provide low friction and low wear 1. This study describes an equine joint friction test model using a cartilage-on-cartilage arrangement and investigates the influence of age and load on the frictional response 2,  (a dimensionless measure, which describes the ratio of frictional force and the normal force between two surfaces). This is a prerequisite for developing an equine joint friction test to evaluate various bio-lubricant treatments. References : (1) Caligaris, M., Ateshian, G.A., 2008. Effects of sustained interstitial fluid pressurization under migrating contact area, and boundary lubrication by synovial fluid, on cartilage friction. Osteoarthritis Cartilage 16, 1220-1227. (2) Noble, P., Collin, B., Lecomte-Beckers, J., Magnée, A., Denoix, J.M., Serteyn, D., 2009. An equine joint friction test model using a cartilage-on-cartilage arrangement. Veterinary Journal (available on line: http://dx.doi.org/10.1016/j.tvjl.2008.12.003). (3) Basalo, I.M., Raj, D., Krishnan, R., Chen, F.H., Hung, C.T., Ateshian, G.A., 2005. Effects of enzymatic degradation on the frictional response of articular cartilage in stress relaxation. Journal of Biomechanics 38, 1343-1349. Conclusion : This equine cartilage-on-cartilage model could be used to understand lubrication regime disturbances in healthy and diseased joints, and to test the efficacy of various bio-lubricant treatments. Results and discussion : Macroscopical examination All joints of group A had a completely intact, smooth and stiff surface, with a shiny and bluish tint. In contrast, all joints of group B had a gradual, irregular and compressible surface, with a yellowish tint (Fig. 2). Materials and Methods : Osteochondral plugs were extracted from equine shoulder joints (2-5 yrs group A (n =12); 10-14 yrs B group (n =15)). For the purpose of identifying the appearance of the articular surface: smooth or rough, stiff or compressible, with a bluish or yellowish tint, a macroscopical examination was performed. Plugs were mounted in a pin-on-disc tribometer (Fig. 1). Frictional response was then measured under constant conditions (2 N; 20 °C; 5 mm/s) in experiment E1, and with increasing load (2 N, 5 N, 10 N) in experiment E2 (Table 1). Experiments In all experiments, the friction coefficient of young cartilage was significantly (P<0.001) smaller than that of old cartilage. Only in old cartilage did the application of a greater load result in a significant (P<0.001) decrease in friction coefficient. Fig. 1 Pin-on-disc: functioning Fig. 2 Typical examples of the cartilage surface in the glenoidal cavity of the scapula, as encountered in young horses (left) and in older horses (right) Fig. 3 Effect of age on friction coefficient of articular cartilage Each point represents the mean SE; ***P<0.001, significant difference between young and old cartilage. Fig. 4 Effect of load on friction coefficient of articular Cartilage. Each point represents the mean SE; °°°P<0.001, significant difference within group B; ***P< 0.001, significant differences between young and old cartilage in a given condition. According to Basalo 3, frictional response of cartilage is not limited to a surface phenomenon; it is greatly influenced by the degree of tissue degradation. Indeed, under experimental conditions, cartilage ageing was found to be responsible for an increase in friction coefficient. Moreover, in experimental conditions where young cartilage lubrication remained stable, cartilage ageing was suspected of being responsible for lubrication regime change (hydrodynamic lubrication regime versus boundary lubrication regime). ExperimentLubricantnTestDuration (s)Load (N)Velocity (mm/s)Radius (mm) E1SF27Cartilage on cartilage180255 E2 SF 27 180 2 5 10 555555 555555 Cartilage on cartilage Table 1 Each experiment was performed at room temperature (20 °C). Cartilage samples were tested immediately after harvesting with a film of synovial fluid (SF) still present. The recovery time between successive tests without application of any load was equal to the loading time (180 s). Cartilage on cartilage CIRALE_ENVA