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An equine joint friction test model using a cartilage-on-cartilage arrangement
The Veterinary Journal 183 (2010) 148–152
Contents lists available at ScienceDirect
The Veterinary Journal journal homepage: www.elsevier.com/locate/tvjl
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 Anatomy, Liège University Faculty of Veterinary Medicine, Boulevard de Colonster, 20, B43, B-4000 Liège, Belgium Department of Aerospace Industry and Mechanics, Liège University Faculty of Applied Science, Chemin des Chevreuils, 1, B52, B-4000 Liège, Belgium c CIRALE, Centre d’Imagerie et de Recherche sur les Affections Locomotrices Équines, RD675 14430 Goustranville, France d Department of Clinical Science, Equine Clinic, Liège University Faculty of Veterinary Medicine, Boulevard de Colonster, 20, B43, B-4000 Liège, Belgium b
a r t i c l e
i n f o
Article history: Accepted 5 December 2008
Keywords: Equine articular cartilage Friction coefficient Lubrication Age Applied load
a b s t r a c t This study describes an equine joint friction test using a cartilage-on-cartilage arrangement and investigates the influence of age and load on the frictional response. Osteochondral plugs were extracted from equine shoulder joints (2–5 years, n = 12; 10–14 years, n = 15), and mounted in a pin-on-disc tribometer. The frictional response was then measured under constant conditions (2 N; 20 °C; 5 mm/s), and with increasing load (2 N, 5 N, 10 N). In all experiments, the friction coefficient of young cartilage was significantly (P < 0.001) smaller than obtained from old cartilage, while the application of a greater load resulted in a significant (P < 0.001) decrease in friction coefficient only in old cartilage. It was concluded that cartilage ageing was responsible for an increase in friction coefficient under these experimental conditions. Moreover, where young cartilage lubrication remained stable, cartilage ageing may have been responsible for lubrication regime change. The cartilage-on-cartilage model could be used to better understand lubrication regime disturbances in healthy and diseased equine joints, and to test the efficacy of various bio-lubricant treatments. Ó 2008 Elsevier Ltd. All rights reserved.
Introduction The diarthrodial joint is a self-acting and dynamic load-bearing structure consisting of a porous and elastic biomaterial (the articular cartilage) and a highly non-Newtonian lubricant (the synovial fluid). The main tribological function of articular cartilage is to provide low friction and low wear. It is well known that lubrication depends on the bearing surfaces of the material it is in contact with, the lubricant and the operating conditions (Wright and Dowson, 1976). In order to investigate the lubrication mechanism of articular cartilage, various studies (Walker et al., 1976; Murakami et al., 1998; Forster and Fisher, 1999; Krishnan et al., 2004; Ozturk, 2004; Basalo et al., 2005; Naka et al., 2005) have been carried out from animal models to measure the friction coefficient l (a dimensionless measure, which describes the ratio of frictional force and the normal force between two surfaces), using cartilage-on-glass or cartilage-on-metal setups under different operating conditions. Other more realistic, as well as challenging studies, have used cartilageon-cartilage specimens from animals (Forster and Fisher, 1996; Mabuchi et al., 1998; Caligaris and Ateshian, 2008) and from humans (Merkher, 2006). * Corresponding author. Tel.: +32 4 366 40 61. E-mail address: [email protected] (P. Noble). 1090-0233/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tvjl.2008.12.003
Due to the biphasic nature of articular cartilage (with a fluid phase representing the interstitial fluid and a porous solid phase representing collagen fibres, proteoglycans and other components), several mechanisms for joint lubrication have been proposed. Among these proposed mechanisms is, firstly, the concept of weeping lubrication (Lewis and McCutchen, 1959), in which the exudation of fluid in the loaded zone provides the necessary lubricant. A second proposed mechanism is boosted lubrication (Walker et al., 1968), which is due to the filtering action by the cartilage, where the larger sized (4000 A°) hyaluronic acid–protein complex of the synovial fluid is left behind due to the smaller pore size (20–70 A°) of the cartilage, allowing only water to enter through the surface. A third proposed mechanism for joint lubrication is biphasic self-generating lubrication (Mow et al., 1980), based on load partitioning (the fluid exuding from the leading edge was thought to provide the fluid replenishment for lubrication). Finally, there is the concept of biphasic boundary lubrication (Ateshian, 1997, 1998; Krishnan et al., 2004; Caligaris and Ateshian, 2008), variously called biphasic lubrication or self-pressurized hydrostatic lubrication. In support of this theory, cartilage lubrication has been shown to depend significantly on the pressurization of the cartilage’s interstitial water, which supports most of the joint’s contact load and helps to shift it away from the collagen–proteoglycan matrix, thereby producing a low friction coefficient (Krishnan et al., 2004). This biphasic boundary lubrication
P. Noble et al. / The Veterinary Journal 183 (2010) 148–152
mechanism has been demonstrated by direct experimental evidence of a strong negative linear correlation between interstitial pressure and friction coefficient. In addition to this current concept, a complementary approach has been proposed describing the boundary lubrication regime. In this case, the articular bearing surfaces are in partial contact through specialised molecules (i.e. lubricin, hyaluronic acid, surface-active phospholipids) found in the superficial zone of the articular cartilage (Jay, 1992; Schwarz and Hills, 1998; Elsaid et al., 2005; Bell et al., 2006; Schmidt et al., 2007; Caligaris and Ateshian, 2008). There are other lubrication mechanisms that do not depend on the biphasic nature of the cartilage such as hydrodynamic lubrication (Macconaill, 1932), in which the cartilage surfaces are kept completely apart by a fluid film. To our knowledge, no work has yet been done using equine cartilage-on-cartilage specimens. Indeed, the present study is a first attempt to measure the friction coefficient in this way, taking into account the influence of age and load, which is a prerequisite for developing an equine joint friction test to evaluate various bio-lubricant treatments. Such an experimental approach requires suitable specimens in terms of size and morphology and, taking into consideration the measurement apparatus, the shoulder joint was selected for the study.
149
It should be noted that the nominal contact pressure was obtained by dividing the normal load by the area of the smaller (upper) specimen. The friction coefficient (l) is a dimensionless measure (ffi0–1), which describes the ratio of frictional force and the normal force between two surfaces. Statistical analysis The friction coefficient in each test was calculated as an average of the first 250 data points in the first plateau region following the sliding inception (Fig. 1). The statistical significance of each of the differences in friction coefficient for experiments E1 and E2 was determined. In experiment E1, this was achieved though using a one-way analysis of variance (ANOVA) to compare values of the two groups (two levels) between the loading configurations. In experiment E2, two-way analysis of variance (ANOVA) with repeated measures was used to compare values of the two groups (two levels) between the loading configurations (three levels). In all cases, a was set to 0.001 and statistical significance was accepted for P < 0.001; post hoc testing of the means was performed with the Bonferroni correction (OriginPro 8).
Results Macroscopic examination Each joint in group A had a completely intact, smooth and stiff surface, with a shiny and bluish tint. In contrast, joints of group B had gradual, irregular and compressible surfaces, with a yellowish tint (Fig. 2).
Materials and methods
Experiments
Specimen preparation
Measurements of l in E1 (under an applied load of 2 N, at 20 °C, and with 5 mm/s sliding velocity) and E2 (under an applied load of 2 N, 5 N to 10 N, at 20 °C, and with 5 mm/s sliding velocity) are presented in Figs. 3 and 4, respectively.
Twenty-seven fresh, whole shoulder joints from 27 horses (aged 2–5 and 10–14 years) were obtained from a local abattoir, dissected and tested within 24 h of slaughter. For practical reasons, the specimens had been selected with large joints and a flat scapular glenoidal cavity. The joints were classified according to age, with group A horses being aged 2–5 years (n = 12) and group B horses aged 10–14 years (n = 15). All joints were examined macroscopically to identifying the appearance of the articular surface: smooth or rough, stiff or compressible, with a bluish or yellowish tint. A pair of cartilage specimens was extracted from each of the 27 shoulder joints, comprising a cylindrical osteochondral plug (£ 5 mm) of approximately 15 mm in length, harvested from humeral heads using a hollow drill bit of 8 mm diameter, and a flat cylindrical osteochondral plug (£ 20 mm) harvested from the scapular glenoidal cavity using a power saw. The subchondral and underlying cancellous bone was retained in order to facilitate handling and mounting of the cartilage and to ensure cartilage integrity (Forster and Fisher, 1999). Each joint was cooled and hydrated with 0.9% saline solution during drilling, and plugs were rinsed to remove debris. It has previously been shown (Hills and Monds, 1998) that the highly hydrophobic nature of surface-active phospholipids in the synovial fluid prevents their removal by rigorous rinsing with saline. The plugs were immediately moistened with 0.9% saline solution to ensure they remained hydrated prior to testing in a pin-on-disc apparatus within 1 h of harvesting. Description of the test rig A lateral-view of the apparatus is shown in Fig. 1. The pin-on-disc apparatus was used to measure the friction coefficient under unidirectional sliding at low speed. For each test, the lower flat osteochondral specimen was fixed within the rig chamber to the central shaft of a variable speed motor. The upper cylindrical osteochondral specimen was loaded into a chuck at the end of an arm and brought into contact with the flat osteochondral specimen by applying a load. The variable speed motor was used to rotate the flat osteochondral specimen in relation to the stationary cylindrical osteochondral specimen at a speed of 10 rounds per minute and with a sliding radius of 5 mm, corresponding to a speed of 5 mm/s. Tested parameters All experiments were carried out at room temperature (20 °C). Two sets of experiments were performed (Table 1). Experiment E1 compared frictional response between the A (n = 12) and B (n = 15) groups under the same conditions: under 2 N (equivalent to pressures of 0.1 MPa), at 20 °C, and 5 mm/s sliding velocity. Experiment E2 compared frictional response, within and between the A (n = 12) and B (n = 15) groups, to an increase in normal load from 2 N, 5 N to 10 N (equivalent to pressures of 0.1, 0.25 and 0.5 MPa, respectively), at 20 °C and with a 5 mm/s sliding velocity. The recovery time between successive tests without application of any load was equal to the loading time.
Effects of age and load Under the same constant operating conditions, l was found to be significantly (P < 0.001) lower in group A than in group B. Moreover, the application of a greater load resulted in a significant (P < 0.001) decrease in l. Interaction of age and load In all experiments, the effect of age within load was found to be significant (P < 0.001). Nevertheless, only in group B was there a significant (P < 0.001) effect of load within the same age group.
Discussion The main objective of the study was to measure the friction coefficient using an equine cartilage-on-cartilage setup and to investigate the influence of equine age and load on the frictional response. Results from experiment E1 clearly established that l was lower in the young cartilage group (group A) than in the old cartilage group (group B). The current theory on lubrication mechanism, called biphasic boundary lubrication, in which cartilage lubrication depends significantly on the pressurization of the cartilage’s interstitial water (Ateshian, 1997; Krishnan et al., 2004; Caligaris and Ateshian, 2008), may be connected to Basalo’s concept (Basalo et al., 2005) in which the frictional response of cartilage is not limited to a surface phenomenon and is greatly influenced by the degree of tissue degradation. Moreover, studies using experimental cartilage damage (after selective enzymatic treatment) suggest that a molecular level of matrix degradation may occur, associated with a deterioration in functional physical properties of the tissue – such as a decrease in compressive stiffness (Bonassar et al., 1995), a decrease
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P. Noble et al. / The Veterinary Journal 183 (2010) 148–152
Fig. 1. Pin on disc apparatus. (1) Functioning. Immediately after extraction, articular specimens are mounted in the pin-on-disc tribometer. The sliding frictional signal of specimens is measured by the tribometer, then transmitted to a computer and expressed as a function of time. (2) Frictional response curve. The increasing region of the curve (before the sliding inflection) represents frictional response at the point when the load is added (specimens progressively come into contact with each other). The plateau region of the curve (following the sliding inception) represents frictional response of the specimens under the load.
Table 1 List of experiments and details of testing protocols. Experiment
Lubricant
n
Test
Duration (s)
Load (N)
Velocity (mm/s)
Radius (mm)
E1
SF
27
Cartilage-on-cartilage
180
2
5
5
E2
SF SF SF
27 27 27
Cartilage-on-cartilage Cartilage-on-cartilage Cartilage-on-cartilage
180 180 180
2 5 10
5 5 5
5 5 5
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).
Fig. 2. Typical examples of the cartilage surface in the glenoidal cavity of the scapula, as encountered in young (left) and in older horses (right).
in Young’s modulus (Lyyeara et al., 1999), and alterations in the frictional response (Basalo et al., 2005). Several studies on the equine fetlock joint have shown that the total glycosaminoglycan content of articular cartilage remains relatively constant throughout life (Platt et al., 1998), as does the composition of articular cartilage collagen (Brama et al., 1999). In contrast, specific components such as hyaluronan increase in concentration with advancing age, as does the content of a structural epitope present on keratan sulfate chains (Platt et al., 1998). In addition, a moderate but very significant correlation has been demonstrated between the cartilage degeneration index (CDI) and the age of the cartilage (r = 0.41; P < 0.001) (Brommer et al., 2003). Our macroscopic examination showed differences between young and old cartilage and therefore we assumed that cartilage ageing is associated with deterioration in structural properties, tending to increase the friction coefficient, l. Experiment E2
showed that only in old cartilage, application of a greater load resulted in a significant decrease in l. Closely similar results were reported in a study using a human cartilage-on-cartilage setup (Merkher, 2006) where the friction coefficient of older patients (aged 72–86 years) was compared as a typical hydrodynamic (fluid film) lubrication regime in which the cartilage surfaces were kept apart by a fluid film made of joint fluid and interstitial fluid that weeps from the articular cartilage itself. This mechanism contrasts with the boundary lubrication regime, in which the articular bearing surfaces are in partial contact and there is only a thin film of lubricant separating them. Our macroscopic examination showed that young cartilage was less compressible than old cartilage. Under the same experimental applied loading and in comparison with old cartilage, young cartilage probably experiences less loss in thickness and less fluid escape into the intra-articular space. As a result, with increasing load, the friction coefficient of young cartilage decreased less than
P. Noble et al. / The Veterinary Journal 183 (2010) 148–152
0.07
151
disturbances in healthy and diseased joints, and to test the efficacy of various bio-lubricant treatments.
Friction coefficient
0.06 10-14 years
0.05
2-5 years
0.04 0.03
***
Conflict of interest statement None of the authors of this paper has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper. Acknowledgements
0.02 The authors would like to express their gratitude to Sylvie Salieri and the PiMW (Pôle d’engineering des Matériaux de Wallonie) for their technical assistance.
0.01 0
References
Fig. 3. Effect of age on the friction coefficient (l) of articular cartilage. Each point represents the mean ± SE; ***P < 0.001, significant difference between young and old cartilage.
Friction coefficient
0.07 0.06
°°°
°°°
0.05 0.04 0.03
***
***
***
0.02 0.01 0 2N (10-14 years)
5N (2-5 years)
2N (2-5 years)
10N (10-14 years)
5N (10-14 years)
10N (2-5 years)
Fig. 4. Effect of load on the friction coefficient (l) 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.
that of old cartilage. We assumed that because of differences in functional physical properties (and despite the same experimental increasing load) the young articular surfaces were constantly in a boundary lubrication regime with a low l, whereas the old articular surfaces changed their lubrication regime from boundary to hydrodynamic lubrication, tending to reduce l. Nevertheless, the l values of old cartilage remained higher. Obviously, the cartilage-on-cartilage setup is a simplified tribological model, which studies cartilage behaviour in a micro-system. Furthermore, the experiments were carried out at room not physiological temperatures. Because these operating conditions could have had an impact on the results (as the rheology of articular specimens was modified), the results should be interpreted with caution. Conclusions A first attempt has been made to measure cartilage friction coefficients, using an equine cartilage-on-cartilage setup. Without reference to degenerative osteoarthritis, cartilage ageing was found to cause an increase in friction coefficient under experimental operating conditions. Where young cartilage lubrication remained stable, cartilage ageing was suspected to cause a change in lubrication regime. Nevertheless, this equine cartilage-on-cartilage model could be used to better understand lubrication regime
Ateshian, G.A., 1997. A theoretical formulation for boundary friction in articular cartilage. Journal of Biomechanical Engineering 119, 81–86. Ateshian, G.A., 1998. The role of interstitial fluid pressurization and surface porosities on the boundary friction of articular cartilage. Journal of Tribology 120, 241–251. 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. Bell, C.J., Ingham, E., Fisher, J., 2006. Influence of hyaluronic acid on the timedependent friction response of articular cartilage under different conditions. Proceedings of the Institution of Mechanical Engineers [H] 220, 23–31. Bonassar, L.J., Frank, E.H., Murray, J.C., Paguio, C.G., Moore, V.L., Lark, M.W., Sandy, J.D., Wu, J.J., Eyeare, D.R., Grodzinsky, A.J., 1995. Changes in cartilage composition and physical properties due to stromelysin degradation. Arthritis and Rheumatism 38, 173–183. Brama, P.A., TeKoppele, J.M., Bank, R.A., van Weeren, P.R., Barneveld, A., 1999. Influence of site and age on biochemical characteristics of the collagen network of equine articular cartilage. American Journal of Veterinary Research 60, 341– 345. Brommer, H., van Weeren, P.R., Brama, P.A., Barneveld, A., 2003. Quantification and age-related distribution of articular cartilage degeneration in the equine fetlock joint. Equine Veterinary Journal 35, 697–701. 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. Elsaid, K.A., Jay, G.D., Warman, M.L., Rhee, D.K., Chichester, C.O., 2005. Association of articular cartilage degradation and loss of boundary-lubricating ability of synovial fluid following injury and inflammatory arthritis. Arthritis and Rheumatism 52, 1746–1755. Forster, H., Fisher, J., 1996. The influence of loading time and lubricant on the friction of articular cartilage. Proceedings of the Institution of Mechanical Engineers [H] 210, 109–119. Forster, H., Fisher, J., 1999. The influence of continuous sliding and subsequent surface wear on the friction of articular cartilage. Proceedings of the Institution of Mechanical Engineers [H] 213, 329–345. Hills, B.A., Monds, M.K., 1998. Deficiency of lubricating surfactant lining the articular surfaces of replaced hips and knees. British Journal of Rheumatology 37, 143–147. Jay, G.D., 1992. Characterization of a bovine synovial fluid lubricating factor. I. Chemical, surface activity and lubricating properties. Connective Tissue Research 28, 71–88. Krishnan, R., Kopacz, M., Ateshian, G.A., 2004. Experimental verification of the role of interstitial fluid pressurization in cartilage lubrication. Journal of Orthopaedic Research 22, 565–570. Lewis, P.R., McCutchen, C.W., 1959. Experimental evidence for weeping lubrication in mammalian joints. Nature 184, 1285. Lyyeara, T., Arokoski, J.P., Oksala, N., Vihko, A., Hyttinen, M., Jurvelin, J.S., Kiviranta, I., 1999. Experimental validation of arthroscopic cartilage stiffness measurement using enzymatically degraded cartilage samples. Physics in Medicine and Biology 44, 525–535. Mabuchi, K., Ujihira, M., Sasada, T., 1998. Influence of loading duration on the startup friction in synovial joints: measurements using a robotic system. Clinical Biomechanics (Bristol, Avon) 3, 492–494. Macconaill, M.A., 1932. The function of intra-articular fibrocartilages, with special reference to the knee and inferior radio-ulnar joints. Journal of Anatomy 66, 210–227. Merkher, Y., 2006. A rational human joint friction test using a human cartilage-oncartilage arrangement. Tribology Letters 22, 29–36. Mow, V.C., Kuei, S.C., Lai, W.M., Armstrong, C.G., 1980. Biphasic creep and stress relaxation of articular cartilage in compression? Theory and experiments. Journal of Biomechanical Engineering 102, 73–84. Murakami, T., Higaki, H., Sawae, Y., Ohtsuki, N., Moriyama, S., Nakanishi, Y., 1998. Adaptive multimode lubrication in natural synovial joints and artificial joints. Proceedings of the Institution of Mechanical Engineers [H] 212, 23–35.
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Naka, M.H., Hattori, K., Ohashi, T., Ikeuchi, K., 2005. Evaluation of the effect of collagen network degradation on the frictional characteristics of articular cartilage using a simultaneous analysis of the contact condition. Clinical Biomechanics 20, 1111–1118. Ozturk, H., 2004. The effect of surface-active phospholipids on the lubrication of osteoarthritic sheep knee joints: friction. Tribology Letters 16, 283–289. Platt, D., Bird, J.L., Bayliss, M.T., 1998. Ageing of equine articular cartilage: structure and composition of aggrecan and decorin. Equine Veterinary Journal 30, 43–52. Schmidt, T.A., Gastelum, N.S., Nguyen, Q.T., Schumacher, B.L., Sah, R.L., 2007. Boundary lubrication of articular cartilage: role of synovial fluid constituents. Arthritis and Rheumatism 56, 882–891.
Schwarz, I.M., Hills, B.A., 1998. Surface-active phospholipid as the lubricating component of lubricin. British Journal of Rheumatology 37, 21–26. Walker, P.S., Dowson, D., Longfield, M.D., Wright, V., 1968. ‘Boosted lubrication’ in synovial joints by fluid entrapment and enrichment. Annals of the Rheumatic Diseases 27, 512–520. Walker, T.W., Graham, J.D., Mills, R.H., 1976. Changes in the mechanical behaviour of the human femoral head associated with arthritic pathologies. Journal of Biomechanics 9, 615–624. Wright, V., Dowson, D., 1976. Lubrication and cartilage. Journal of Anatomy 121, 107–118.
Contents lists available at ScienceDirect
The Veterinary Journal journal homepage: www.elsevier.com/locate/tvjl
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 Anatomy, Liège University Faculty of Veterinary Medicine, Boulevard de Colonster, 20, B43, B-4000 Liège, Belgium Department of Aerospace Industry and Mechanics, Liège University Faculty of Applied Science, Chemin des Chevreuils, 1, B52, B-4000 Liège, Belgium c CIRALE, Centre d’Imagerie et de Recherche sur les Affections Locomotrices Équines, RD675 14430 Goustranville, France d Department of Clinical Science, Equine Clinic, Liège University Faculty of Veterinary Medicine, Boulevard de Colonster, 20, B43, B-4000 Liège, Belgium b
a r t i c l e
i n f o
Article history: Accepted 5 December 2008
Keywords: Equine articular cartilage Friction coefficient Lubrication Age Applied load
a b s t r a c t This study describes an equine joint friction test using a cartilage-on-cartilage arrangement and investigates the influence of age and load on the frictional response. Osteochondral plugs were extracted from equine shoulder joints (2–5 years, n = 12; 10–14 years, n = 15), and mounted in a pin-on-disc tribometer. The frictional response was then measured under constant conditions (2 N; 20 °C; 5 mm/s), and with increasing load (2 N, 5 N, 10 N). In all experiments, the friction coefficient of young cartilage was significantly (P < 0.001) smaller than obtained from old cartilage, while the application of a greater load resulted in a significant (P < 0.001) decrease in friction coefficient only in old cartilage. It was concluded that cartilage ageing was responsible for an increase in friction coefficient under these experimental conditions. Moreover, where young cartilage lubrication remained stable, cartilage ageing may have been responsible for lubrication regime change. The cartilage-on-cartilage model could be used to better understand lubrication regime disturbances in healthy and diseased equine joints, and to test the efficacy of various bio-lubricant treatments. Ó 2008 Elsevier Ltd. All rights reserved.
Introduction The diarthrodial joint is a self-acting and dynamic load-bearing structure consisting of a porous and elastic biomaterial (the articular cartilage) and a highly non-Newtonian lubricant (the synovial fluid). The main tribological function of articular cartilage is to provide low friction and low wear. It is well known that lubrication depends on the bearing surfaces of the material it is in contact with, the lubricant and the operating conditions (Wright and Dowson, 1976). In order to investigate the lubrication mechanism of articular cartilage, various studies (Walker et al., 1976; Murakami et al., 1998; Forster and Fisher, 1999; Krishnan et al., 2004; Ozturk, 2004; Basalo et al., 2005; Naka et al., 2005) have been carried out from animal models to measure the friction coefficient l (a dimensionless measure, which describes the ratio of frictional force and the normal force between two surfaces), using cartilage-on-glass or cartilage-on-metal setups under different operating conditions. Other more realistic, as well as challenging studies, have used cartilageon-cartilage specimens from animals (Forster and Fisher, 1996; Mabuchi et al., 1998; Caligaris and Ateshian, 2008) and from humans (Merkher, 2006). * Corresponding author. Tel.: +32 4 366 40 61. E-mail address: [email protected] (P. Noble). 1090-0233/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tvjl.2008.12.003
Due to the biphasic nature of articular cartilage (with a fluid phase representing the interstitial fluid and a porous solid phase representing collagen fibres, proteoglycans and other components), several mechanisms for joint lubrication have been proposed. Among these proposed mechanisms is, firstly, the concept of weeping lubrication (Lewis and McCutchen, 1959), in which the exudation of fluid in the loaded zone provides the necessary lubricant. A second proposed mechanism is boosted lubrication (Walker et al., 1968), which is due to the filtering action by the cartilage, where the larger sized (4000 A°) hyaluronic acid–protein complex of the synovial fluid is left behind due to the smaller pore size (20–70 A°) of the cartilage, allowing only water to enter through the surface. A third proposed mechanism for joint lubrication is biphasic self-generating lubrication (Mow et al., 1980), based on load partitioning (the fluid exuding from the leading edge was thought to provide the fluid replenishment for lubrication). Finally, there is the concept of biphasic boundary lubrication (Ateshian, 1997, 1998; Krishnan et al., 2004; Caligaris and Ateshian, 2008), variously called biphasic lubrication or self-pressurized hydrostatic lubrication. In support of this theory, cartilage lubrication has been shown to depend significantly on the pressurization of the cartilage’s interstitial water, which supports most of the joint’s contact load and helps to shift it away from the collagen–proteoglycan matrix, thereby producing a low friction coefficient (Krishnan et al., 2004). This biphasic boundary lubrication
P. Noble et al. / The Veterinary Journal 183 (2010) 148–152
mechanism has been demonstrated by direct experimental evidence of a strong negative linear correlation between interstitial pressure and friction coefficient. In addition to this current concept, a complementary approach has been proposed describing the boundary lubrication regime. In this case, the articular bearing surfaces are in partial contact through specialised molecules (i.e. lubricin, hyaluronic acid, surface-active phospholipids) found in the superficial zone of the articular cartilage (Jay, 1992; Schwarz and Hills, 1998; Elsaid et al., 2005; Bell et al., 2006; Schmidt et al., 2007; Caligaris and Ateshian, 2008). There are other lubrication mechanisms that do not depend on the biphasic nature of the cartilage such as hydrodynamic lubrication (Macconaill, 1932), in which the cartilage surfaces are kept completely apart by a fluid film. To our knowledge, no work has yet been done using equine cartilage-on-cartilage specimens. Indeed, the present study is a first attempt to measure the friction coefficient in this way, taking into account the influence of age and load, which is a prerequisite for developing an equine joint friction test to evaluate various bio-lubricant treatments. Such an experimental approach requires suitable specimens in terms of size and morphology and, taking into consideration the measurement apparatus, the shoulder joint was selected for the study.
149
It should be noted that the nominal contact pressure was obtained by dividing the normal load by the area of the smaller (upper) specimen. The friction coefficient (l) is a dimensionless measure (ffi0–1), which describes the ratio of frictional force and the normal force between two surfaces. Statistical analysis The friction coefficient in each test was calculated as an average of the first 250 data points in the first plateau region following the sliding inception (Fig. 1). The statistical significance of each of the differences in friction coefficient for experiments E1 and E2 was determined. In experiment E1, this was achieved though using a one-way analysis of variance (ANOVA) to compare values of the two groups (two levels) between the loading configurations. In experiment E2, two-way analysis of variance (ANOVA) with repeated measures was used to compare values of the two groups (two levels) between the loading configurations (three levels). In all cases, a was set to 0.001 and statistical significance was accepted for P < 0.001; post hoc testing of the means was performed with the Bonferroni correction (OriginPro 8).
Results Macroscopic examination Each joint in group A had a completely intact, smooth and stiff surface, with a shiny and bluish tint. In contrast, joints of group B had gradual, irregular and compressible surfaces, with a yellowish tint (Fig. 2).
Materials and methods
Experiments
Specimen preparation
Measurements of l in E1 (under an applied load of 2 N, at 20 °C, and with 5 mm/s sliding velocity) and E2 (under an applied load of 2 N, 5 N to 10 N, at 20 °C, and with 5 mm/s sliding velocity) are presented in Figs. 3 and 4, respectively.
Twenty-seven fresh, whole shoulder joints from 27 horses (aged 2–5 and 10–14 years) were obtained from a local abattoir, dissected and tested within 24 h of slaughter. For practical reasons, the specimens had been selected with large joints and a flat scapular glenoidal cavity. The joints were classified according to age, with group A horses being aged 2–5 years (n = 12) and group B horses aged 10–14 years (n = 15). All joints were examined macroscopically to identifying the appearance of the articular surface: smooth or rough, stiff or compressible, with a bluish or yellowish tint. A pair of cartilage specimens was extracted from each of the 27 shoulder joints, comprising a cylindrical osteochondral plug (£ 5 mm) of approximately 15 mm in length, harvested from humeral heads using a hollow drill bit of 8 mm diameter, and a flat cylindrical osteochondral plug (£ 20 mm) harvested from the scapular glenoidal cavity using a power saw. The subchondral and underlying cancellous bone was retained in order to facilitate handling and mounting of the cartilage and to ensure cartilage integrity (Forster and Fisher, 1999). Each joint was cooled and hydrated with 0.9% saline solution during drilling, and plugs were rinsed to remove debris. It has previously been shown (Hills and Monds, 1998) that the highly hydrophobic nature of surface-active phospholipids in the synovial fluid prevents their removal by rigorous rinsing with saline. The plugs were immediately moistened with 0.9% saline solution to ensure they remained hydrated prior to testing in a pin-on-disc apparatus within 1 h of harvesting. Description of the test rig A lateral-view of the apparatus is shown in Fig. 1. The pin-on-disc apparatus was used to measure the friction coefficient under unidirectional sliding at low speed. For each test, the lower flat osteochondral specimen was fixed within the rig chamber to the central shaft of a variable speed motor. The upper cylindrical osteochondral specimen was loaded into a chuck at the end of an arm and brought into contact with the flat osteochondral specimen by applying a load. The variable speed motor was used to rotate the flat osteochondral specimen in relation to the stationary cylindrical osteochondral specimen at a speed of 10 rounds per minute and with a sliding radius of 5 mm, corresponding to a speed of 5 mm/s. Tested parameters All experiments were carried out at room temperature (20 °C). Two sets of experiments were performed (Table 1). Experiment E1 compared frictional response between the A (n = 12) and B (n = 15) groups under the same conditions: under 2 N (equivalent to pressures of 0.1 MPa), at 20 °C, and 5 mm/s sliding velocity. Experiment E2 compared frictional response, within and between the A (n = 12) and B (n = 15) groups, to an increase in normal load from 2 N, 5 N to 10 N (equivalent to pressures of 0.1, 0.25 and 0.5 MPa, respectively), at 20 °C and with a 5 mm/s sliding velocity. The recovery time between successive tests without application of any load was equal to the loading time.
Effects of age and load Under the same constant operating conditions, l was found to be significantly (P < 0.001) lower in group A than in group B. Moreover, the application of a greater load resulted in a significant (P < 0.001) decrease in l. Interaction of age and load In all experiments, the effect of age within load was found to be significant (P < 0.001). Nevertheless, only in group B was there a significant (P < 0.001) effect of load within the same age group.
Discussion The main objective of the study was to measure the friction coefficient using an equine cartilage-on-cartilage setup and to investigate the influence of equine age and load on the frictional response. Results from experiment E1 clearly established that l was lower in the young cartilage group (group A) than in the old cartilage group (group B). The current theory on lubrication mechanism, called biphasic boundary lubrication, in which cartilage lubrication depends significantly on the pressurization of the cartilage’s interstitial water (Ateshian, 1997; Krishnan et al., 2004; Caligaris and Ateshian, 2008), may be connected to Basalo’s concept (Basalo et al., 2005) in which the frictional response of cartilage is not limited to a surface phenomenon and is greatly influenced by the degree of tissue degradation. Moreover, studies using experimental cartilage damage (after selective enzymatic treatment) suggest that a molecular level of matrix degradation may occur, associated with a deterioration in functional physical properties of the tissue – such as a decrease in compressive stiffness (Bonassar et al., 1995), a decrease
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Fig. 1. Pin on disc apparatus. (1) Functioning. Immediately after extraction, articular specimens are mounted in the pin-on-disc tribometer. The sliding frictional signal of specimens is measured by the tribometer, then transmitted to a computer and expressed as a function of time. (2) Frictional response curve. The increasing region of the curve (before the sliding inflection) represents frictional response at the point when the load is added (specimens progressively come into contact with each other). The plateau region of the curve (following the sliding inception) represents frictional response of the specimens under the load.
Table 1 List of experiments and details of testing protocols. Experiment
Lubricant
n
Test
Duration (s)
Load (N)
Velocity (mm/s)
Radius (mm)
E1
SF
27
Cartilage-on-cartilage
180
2
5
5
E2
SF SF SF
27 27 27
Cartilage-on-cartilage Cartilage-on-cartilage Cartilage-on-cartilage
180 180 180
2 5 10
5 5 5
5 5 5
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).
Fig. 2. Typical examples of the cartilage surface in the glenoidal cavity of the scapula, as encountered in young (left) and in older horses (right).
in Young’s modulus (Lyyeara et al., 1999), and alterations in the frictional response (Basalo et al., 2005). Several studies on the equine fetlock joint have shown that the total glycosaminoglycan content of articular cartilage remains relatively constant throughout life (Platt et al., 1998), as does the composition of articular cartilage collagen (Brama et al., 1999). In contrast, specific components such as hyaluronan increase in concentration with advancing age, as does the content of a structural epitope present on keratan sulfate chains (Platt et al., 1998). In addition, a moderate but very significant correlation has been demonstrated between the cartilage degeneration index (CDI) and the age of the cartilage (r = 0.41; P < 0.001) (Brommer et al., 2003). Our macroscopic examination showed differences between young and old cartilage and therefore we assumed that cartilage ageing is associated with deterioration in structural properties, tending to increase the friction coefficient, l. Experiment E2
showed that only in old cartilage, application of a greater load resulted in a significant decrease in l. Closely similar results were reported in a study using a human cartilage-on-cartilage setup (Merkher, 2006) where the friction coefficient of older patients (aged 72–86 years) was compared as a typical hydrodynamic (fluid film) lubrication regime in which the cartilage surfaces were kept apart by a fluid film made of joint fluid and interstitial fluid that weeps from the articular cartilage itself. This mechanism contrasts with the boundary lubrication regime, in which the articular bearing surfaces are in partial contact and there is only a thin film of lubricant separating them. Our macroscopic examination showed that young cartilage was less compressible than old cartilage. Under the same experimental applied loading and in comparison with old cartilage, young cartilage probably experiences less loss in thickness and less fluid escape into the intra-articular space. As a result, with increasing load, the friction coefficient of young cartilage decreased less than
P. Noble et al. / The Veterinary Journal 183 (2010) 148–152
0.07
151
disturbances in healthy and diseased joints, and to test the efficacy of various bio-lubricant treatments.
Friction coefficient
0.06 10-14 years
0.05
2-5 years
0.04 0.03
***
Conflict of interest statement None of the authors of this paper has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper. Acknowledgements
0.02 The authors would like to express their gratitude to Sylvie Salieri and the PiMW (Pôle d’engineering des Matériaux de Wallonie) for their technical assistance.
0.01 0
References
Fig. 3. Effect of age on the friction coefficient (l) of articular cartilage. Each point represents the mean ± SE; ***P < 0.001, significant difference between young and old cartilage.
Friction coefficient
0.07 0.06
°°°
°°°
0.05 0.04 0.03
***
***
***
0.02 0.01 0 2N (10-14 years)
5N (2-5 years)
2N (2-5 years)
10N (10-14 years)
5N (10-14 years)
10N (2-5 years)
Fig. 4. Effect of load on the friction coefficient (l) 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.
that of old cartilage. We assumed that because of differences in functional physical properties (and despite the same experimental increasing load) the young articular surfaces were constantly in a boundary lubrication regime with a low l, whereas the old articular surfaces changed their lubrication regime from boundary to hydrodynamic lubrication, tending to reduce l. Nevertheless, the l values of old cartilage remained higher. Obviously, the cartilage-on-cartilage setup is a simplified tribological model, which studies cartilage behaviour in a micro-system. Furthermore, the experiments were carried out at room not physiological temperatures. Because these operating conditions could have had an impact on the results (as the rheology of articular specimens was modified), the results should be interpreted with caution. Conclusions A first attempt has been made to measure cartilage friction coefficients, using an equine cartilage-on-cartilage setup. Without reference to degenerative osteoarthritis, cartilage ageing was found to cause an increase in friction coefficient under experimental operating conditions. Where young cartilage lubrication remained stable, cartilage ageing was suspected to cause a change in lubrication regime. Nevertheless, this equine cartilage-on-cartilage model could be used to better understand lubrication regime
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