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Synovial Fluid as a Lubricant.

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Synovial Fluid as a Lubricant
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 . ~
lubricating advantage can be wiped or
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
M.D.: Instructor in Orthopedic
Surgery, Haruard Medical School. Current address: Orthopedic Research Laboratories, Massa-
chusetts General Hospital, Boston, Massachusetts
11, NO. 5 (OCTOBER 1968)
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.
property, exhibited by certain
fluids, of having their viscosity diminish as their
flow rate is increased. Ketchup is an example of
such fluid.
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,
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.
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:
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.
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.
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synovial, lubricants, fluid
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