CURRENT COMMENT Synovial Fluid as a Lubricant By ERICL. &DIN S John Chamley,’ a decade ago, shattered the view that synovial fluid acts hydrodynamically, like the oil in most rotating bearings, it has been clear that synovia must have lubrication properties of a different kind. But the attention of most investigators in this area has been turned to the articular cartilage, which seems to hold one of the keys to the extremely low friction of animal joints. The experimental evidence clearly supports the concept, initially put forth by McCutchen,2 that joints are basically lubricated hydrostatically. Articular cartilage, when pressure is applied to it, “weeps” interstitial fluid. The greater the load on the cartilage, the greater the fluid pressure. Although the opposing surfaces rub, they rub only lightly, the bulk of the load being borne by the interstitial fluid. McCutchen2Jr4 has gone on to show that intra-cartilaginous liquid can be expressed, that cartilage is inherently elastic and if unloaded will recover its fluid, that “weeping” elastic surfaces, under load, caii achieve coefficients of friction of the same order of magnitude ( as those of animal joints, and that the coefficient of friction decreases as the load increases. It has also been shown that the elastic properties of cartilage are excellent,“ that fluid is held in part “osmotically” in the cartilage,’J; that joint motion does compress the cartilage, and that articular cartilage, wiped free 01 synovial fluid, retains much of its low fricAs to whether there is tional propertie~.~*T an elasto-hydrodynamic component to the lubrication mechanism, as proposed by Dintinfass,8 Tanner: and Dowson,lo is as yet unclear. Then why bother with synovial fluid? Although synovia on metallic surfaces is a worthless lubricant,ll the fact remains that cartilage is better lubricated by synovial fluid than by physiological ~ a l i n e . ~ This ,~ lubricating advantage can be wiped or This washed off the cartilaginous evidence supports E. S. Jones’12 original hypothesis, popularized by Chamley1 that the synovial mucin actually binds to thc cartilage surface, interposing a thin layer of molecules between the joint surfaces, somewhat as teflon bonded to metal imparts slipperiness to the surface of a frying pan. Balazsl3 has even seen this layer electronmicroscopically. Using a mechanical set-up which measures instantaneous coefficients of friction of animal joints in uitro,” investigators at The National Institutes of Health, in a paper in this issue15 report on experiments studying the binding mechanism of synovial mucin. From their results it appears as if the protein portion of the hyaluronate-protein complex of synovial mucin is absolutely essential to the binding phenomena. The hyaluronate portion can be depolymerized without loss of lubricating advantage. Severe hyaluronidase digestion does diminish the effectiveness of the lubrication, but there is some question as to the integrity INCE ~~~ ~ ~~~ ERICL. -IN, M.D.: Instructor in Orthopedic Surgery, Haruard Medical School. Current address: Orthopedic Research Laboratories, Massa- ARTHRITISAND &iEUhfATIShI, VOL. chusetts General Hospital, Boston, Massachusetts 02114. 11, NO. 5 (OCTOBER 1968) 693 694 ERIC L. W I N of the protein moiety of the complex under those conditions. This work confirms earlier reports that synovia’s lubricating action i s not viscosity-dependent.uvlG It was also possible to reduce mucin’s lubricating advantage with heparin. This suggests that either the heparin deactivated the mucin, or that the cartilaginous binding sites were sensitive to a mucopolysaccharide configuration and were blocked by heparin. It was also shown that formalin fixation, which markedly stiffened the cartilage and greatly increased its coefficient of friction, did not significantly affect the binding of the mucin. These investigators went on to carry out experiments designed to elucidate cartilage’s contribution to joint lubrication. Their findings that hydrogen ion concentration and molarity affect cartilage compressibility and coefficient of friction are not surprising, in view of earlier What is surprising is that the coefficient of friction is not always related to cartilage compressibility, suggesting that mutliple factors operate to curtail the flow of interstitial fluid, or that other mechanisms are involved in the lubricating phenomenon. The major unanswered question raised by these recent experiments is why synovial mucin occurs as such a large molecule if fragments of it alone suffice? Is its thixotropica nature merely an accident, one which has led investigators down the wrong path for many years? This seems unlikely. The large molecule may provide a safety valve, in times of severe, unremitting exercise when the molecules are subject to severe shear and breakdown, in excess of the synovial cells’ ability to synthesize them. The large molecules may act as shock absorbersl8 to dampen the effect of sudden high load. Finally, Barnett and Cobboldlo have presented evidence that the peri-articular structures create high resistance to joint activity, especially in wide arcs of motion. It has been suggested that perhaps the thixotropic nature of the joint fluid mucin, which is directly related to its large molecular size and configuration, is useful in acting as a lubricant for the overlying soft tissues.2’J Clearly much more work needs to be done. ‘Thixotropy-A property, exhibited by certain fluids, of having their viscosity diminish as their flow rate is increased. Ketchup is an example of such fluid. REFERENCES 1. Charnley, J.: The lubrication of animal joints. In: Symposium on Biomechanics. London, Institute of Mechanical Engineers, 1959, p. 12. 2. McCutchen, C. W.: Sponge-hydrostatic and weeping bearings. Nature 184: 1284,1959. 3. Lewis, P. R. and McCutchen, C. W.: Experimental evidence for weeping lubrication in animal joints. Nature 184:1285, 1959. 4. McCutchen, C. W.: The frictional properties of animal joints. Wear 5: 1, 1962. 5. Elmore, S. M., Sokoloff, L., Norris, G . and Carmeci, P.: The nature of “imperfect” elasticity of articular cartilage. J. Appl. Physiol. 18:393, 1963. 6. Linn, F. C. and Sokoloff, L.: Movement and composition of interstitial fluid of cartilage. Arthritis Rheum. 8:481, 1965. 7. Linn, F. C.: Lubrication of animal joints. 2. The mechanism. J. Bioinech. (in press), 1968. 8. Dintinfass, L.: Lubrication in synovial joints: a theoretical analysis. J. Bone Joint Surg. 45A: 1241, 1963. 9. Tanner, R. I.: An alternative mechanism for the lubrication of synovial joints. Phys. Med. Biol. 11: 119, 1966 10. Dowson, D.: Modes of lubrication in human joints. Symposium on Lubrication and Wear in Living and Artificial Human Joints. Paper No. 12. London, Institute of Mechanical Engineers, 1967. 11. Tanner, R. I. and Edwards, F. J.: The lubricating properties of synovial fluid. Appendix to the Paper by J. Charnley, 1959 (ref. 1). 12. Jones, E. S.: Joint lubrication. Lancet 1: 3426, 1934. 13. Balazs, E. A.: Unpublished data. 14. Linn, F. C.: Lubrication of animal joints. SYNOVIAL FLUID AS A LUBRICANT I. The arthrotripsonieter. J. Bone Joint Surg. 49A: 1079, 1!367. 15. Linn, F. C. and Radin, E. Id.: Lubrication of animal joints. 111. The effect of certain chemical alterations of the cartilage and lubricant. Arthritis Rheum. 11:674, 1968 (this issue). 16. McCutchen, C. W.: Boundary lubrication by synovial fluid: Demonstration and possible osmotic explanation. Fed. Proc. 25: 1061, 1966. 17. Sokololf, L.: Elasticity of articular cartilage, effect of ions and visions solutions. Science, 141: 695 1055,1963. 18. White, H. K.: The rheology of synovial fluid. J. Bone Joint Surg. 45A:1084, 1963. 19. Barnett, C. 13. and Cobbold, A. F.: Lubrication within living joints. J. Bone Joint Surg. 44B:662,1962. 20. McCutchen, C. W.: Why did nature make synovial fluid slimy? P. 117. In: Proceedings of the Workshop on Cartilage Degradation and Repair. Washington, D. C., National Research Council, 19G7.