Ann. rheum. Dis. (1968), 27, 512

"BOOSTED LUBRICATION" IN SYNOVIAL JOINTSBY FLUID ENTRAPMENT AND ENRICHMENT

BY

P. S. WALKER,1 D. DOWSON,2 M. D. LONGFIELD,3 AND V. WRIGHT4From the Bioengineering Group for the Study ofHuman Joints, Rheumatism Research Unit, Department of Medicine,General Infirmary, Leeds; The Institute ofTribology, Department ofMechanical Engineering, University ofLeeds; and the

Royal Bath Hospital, Harrogate

The manner in which a human load-bearingsynovial joint is able to carry the applied loads hasbeen the subject of speculation for many years. Inthe past, a number of theories have been advancedeach using as a basis one of several modes of lubri-cation well known to mechanical engineets. Morerecently some of the more unique characteristics ofhuman cartilage have been recognized, and theorieshave been modified accordingly.Thus MacConaill (1932) proposed an explanation

based upon a type of lubrication known to engineersas "hydrodynamic", Charnley (1960) proposed anexplanation based upon "boundary lubrication",and McCutchen (1962, 1966) explored the possi-bilities of "weeping lubrication". In the light offurther study an explanation has been proposed byLinn (1967). Davies, Barnett, CocIrane, andPalfrey (1962) and Edwards (1967) have investigatedthe properties of articular surfaces. Negami (1964)along with Davies (1966) has studied the rheologicalproperties of synovial fluid. Rydell (1965) andPaul (1967) have investigated the forces which actthrough joints in normal circumstances.

Certainly, if an understanding is to be reachedconcerning the role of synovial fluid in a humanload-carrying joint, the physical characteristics ofthe articular surfaces, in addition to those of thesynovial fluid, must be taken into account.

In the present study, a number of friction experi-ments were designed to investigate the extent towhich the various modes of lubrication were presentin physiological conditions. The overall geometryof the fluid-filled gap between the mating articularsurfaces has been investigated, and the microscopicsurface quality of cartilage has been recorded.On the basis of these observations, the features of

normal joint lubrication are described and thefactors which encourage full fluid film lubricationare listed. "Boosted lubrication", the name givento a mode of lubrication not previously described,

Research Fellow.2 Professor of Engineering, Fluid Mechanics and Tribology.Manager, Industrial Unit of Tribology.Senior Lecturer in Medicine, Consultant Physician in Rheuma-tology.

is suggested as an explanation for the load-carryingcapability of weight-bearing synovial joints.

Theoretical ConsiderationsSome of the various types of lubrication which are

known to mechanical engineers are shown in Fig. 1.

(1) Fluid Film LubricationThe description implies that the two solid surfaces

are separated by the lubricant at all times. Thefilm thickness is sufficiently great to prevent anyisolated asperity on one surface from touching asimilar peak on the other surface. In "hydrodyna-mic lubrication" (Fig. li) the classical representationis that of two rigid surfaces set at an angle to eachother producing a wedge-shaped gap. When onesurface is caused to move tangentially relative tothe other in the direction shown, the fluid which ispresent is drawn in to the diminishing gap byviscous forces. A pressure is generated within thefluid of a magnitude sufficient to support a trans-verse load.

If the working conditions become more arduous,the pressure within the fluid may rise to such anextent that elastic deformation may take place inthe bounding solids. If the deformation is sufficientto alter the overall geometry to a significant degree,the description "elastohydrodynamic lubrication"is used. Softer, or more elastic, boundaries havethe effect of allowing a greater volume of fluid to bedrawn into the converging gap, thus resulting in anenhanced lubricant film thickness. It can be shownthat the film thickness is given by an equation ofthe form:

h= C1 ... Equation 1

where h is the film thicknessR is the effective radius of curvature between the

two surfaces- is the viscosity of the lubricantu is the mean surface sliding speedE is a measure of the elastic properties of the

surfacesC1 is a constant

512

copyright. on F

ebruary 7, 2020 by guest. Protected by

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.27.6.512 on 1 N

ovember 1968. D

ownloaded from

"BOOSTED LUBRICATION" IN SYNOVIAL JOINTS

FLUID FILMSurfaces

completely separatedby a fluid film

i) Hydrodynamic_ ,rgdmaterial _

- *>motion

ii) Elastohydrodynamic

soft materialdeformed

-* motion

iii) Squeeze filmnormal

-a pproach- -

-__ fluid, -o

BOUNDARY

Load supported bysurface-to-surface contact

v) Molecular protection

vi) Dry contactpoints of contact

MIXEDLoad support shared byfluid film pressure andby boundary contacts

ivi HydrostaticFig. 1.-Types of lubrication between two surfaces, loaded together

It follows, therefore, that the film thicknessincreases with increases in the relative radius ofcurvature, the viscosity, the sliding speed, and thesoftness of the materials. Elastohydrodynamiclubrication is represented in Fig. 1 ii.A further type of fluid film lubrication is covered

by the term "squeeze film lubrication", representeddiagrammatically in Fig. liii. In this case, theupper solid is caused to approach the lower in a

normal direction without transverse movementbeing necessarily present. The approaching solidstend to squeeze out the intervening fluid, but thisaction is strongly resisted by viscous forces. Fluidpressures are generated and in normal circumstancessolid-to-solid contact does not occur until a longtime has elapsed. Moore (1965) gives the equationwhich defines the time required for the fluid filmthickness to reduce from ho to h. Where the contactis of a circular shape,

t C2 p Lh2-h02j * * Equation 2

where t is the timeD is the contact diameterP is the load-q is the viscosity

It will be seen that the time taken is very sensitiveto the diameter of the contact D and that, theoreti-cally, the time required is infinite if the final thick-ness, h, is set to zero.

"Hydrostatic lubrication" occurs when a supply

of fluid is injected between a pair of opposingsurfaces. An external pump is used to build up afluid pressure sufficient to prevent contact betweenthe surfaces. In the case of a porous material, suchas articular cartilage, fluid may seep away throughthe matrix, thus reducing the load-carrying capacityfor a given supply of lubricant.

(2) Boundary LubricationIt has been found that certain substances, when

added in small proportions to the lubricating fluid,migrate to the solid boundaries and form protectivelayers on the surfaces. In many cases, an additiveconcentration of only 1 per cent. is sufficient toprovide a tenacious coating. It is believed that thelong chain molecules of the additive become attachedat one end to the surfaces in the manner shown inFig. 1v. In this way, intimate contact between therubbing surfaces may often be prevented, thusminimising the possibility of damage. It is worthyof note that the proportion of hyaluronic acid-protein complex in synovial fluid is much greaterthan the proportion of additive normally used inengineering applications.

(3) WearThree types of wear have been identified in

situations where components are rubbed togetherwhile under load:

"Adhesive wear" takes place when direct contactoccurs between asperities. If the strength of the

513

copyright. on F

ebruary 7, 2020 by guest. Protected by

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.27.6.512 on 1 N

ovember 1968. D

ownloaded from

ANNALS OF THE RHEUMATIC DISEASES

junction is greater than that of the underlying mat-erial, a fragment will be removed from one surfaceand adhere to the other. Such a fragment may wellbecome detached at a later stage to become a freeparticle of wear debris. This type of wear isparticularly likely to take place when the rubbingcomponents are metallic.

"Abrasive wear" may occur in two different ways.If one of the surfaces is significantly harder thanthe other, particles may be scraped or ploughedoff the softer surfaces by the harder surface. Alter-natively, the presence of loose hard particles in thespace between the surfaces may result in a scouringaction.

"Fatigue wear" occurs when a material is repeated-ly loaded and unloaded. Repetitions of a stresscycle may result in material failure at stress levelswell below those required under static loading.Certainly, this type of loading is present in humanhip and knee joints.

Material and MethodsThe characteristics of surfaces used in engineering

applications are commonly assessed with the aid of astylus instrument. A very lightly loaded diamond havinga tip radius of 0 0001 in. is drawn across the surfaceunder inspection. An electrical signal representing theundulations of the surface is amplified and may bedisplayed on graph paper using appropriate scales.While in many engineering cases it is possible to workdirectly with the surface under scrutiny, in the presentinvestigation castings of the articular surfaces were madewith an acrylic material. Subsidiary tests were madeto confirm that the acrylic casting reproduced the true

profile of the articular cartilage to a sufficient accuracy.Castings were taken of cartilage surfaces of hips andknees immediately after opening the joints at autopsy.Before pouring the casting material on to the cartilage,the surface was cleaned with tissue moistened withsaline. Typical results are shown in Fig. 2.

In order to determine the geometry of the cavitybetween the acetabulum and the femoral head, thejoint was exposed. The casting material was thenpoured in and the joint was returned to a natural position.The hardened cast was removed after about 30 minutes.

Friction experiments were carried out using a modifiedsledge microtome (Fig. 3, opposite). Essentially, thespecimen was held at the end of a pivoted arm andloaded against a reciprocating flat glass plate. Themaximum speed of the reciprocating glass plate could bevaried between 0 3 and 4 in. per sec., and the load fromzero to 8 lb. The pivoted arm was supported in a"frictionless" air bearing, the extreme end being re-strained by a load transducer which recorded the force offriction acting on the specimen.

GeometryA. Surface Quality

Fig. 2 shows stylus tracings taken from acryliccastings of subjects ranging from an 8-month foetusto a 57-year-old man. On each trace a straightcentre line has been drawn. The graphs have beendrawn to scales of x 1,000 and x 100 in the verticaland horizontal directions respectively for con-venience in reproduction. A frequently-used meas-ure of the magnitude of the undulations on surfacesis the "centre line average" (c.l.a.). The c.l.a. valuefor a surface is an indication of the average height

Femoral condyle, 8-month foetus

Femoral condyle. 2byears

Avi

ScalesVertical x 1001Horizontal x 11

Femoral head, b7 years

D00

V VYIOsteo-arthrosic femoral condyle, 63years

Fig. 2.-Typical Talysurf traces taken from acrylic castings of cartilage.

"A M2 ..

514

A-C

copyright. on F

ebruary 7, 2020 by guest. Protected by

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.27.6.512 on 1 N

ovember 1968. D

ownloaded from

"BOOSTED LUBRICATION" IN SYNOVIAL JOINTS

Fig. 3.-Reciprocating friction machine.

of the peaks or depths of the valleys from thegeometrical centre line of the profile. The c.l.a.value is almost always found electrically from thestylus signal. It is to be expected that an averagepeak-to-valley height will be two or three timesgreater than the c.l.a. value for the surface. Thec.l.a. value for engineering bearing surfaces liesnormally in the range 5 to 15 pin. The c.l.a. valueof the articular surfaces ranged from a minimum ofabout 30 pin. to more than 200 pin. for cases showingevidence of chondromalacia or osteoarthrosis.Specimens from younger subjects displayed anundulating surface of wavelength about 0-001 in.with a c.l.a. of about 30 ztin. With advancing age,the wavelength of the undulations increased up toabout 0*01 in., superimposed upon which weresmaller scale roughnesses.

B. Overall GeometrySo far as lubrication is concerned, it is not realistic

to consider the curvatures of the acetabulum or thefemoral head separately. It is rather the geometryof the gap between the two opposing surfaces whichinfluences the action of the joint. The determina-tion of the geometry of the gap rather than that ofthe solids led to a measure of the nominal contactarea. The acrylic castings taken from the cavitiesbetween articular surfaces in the hip and knee weremeasured at a number of points to determine thelocal thickness. Contour maps were then drawnto indicate the variation in surface-to-surface dis-tance.

The load-bearing area of normal hip joints wasobserved to be of a horse-shoe shape disposed nearthe periphery of the acetabulum. In the centre ofthe joint, between the top of the femoral head andthe acetabulum, there was a cavity of average width0o02 in. under conditions of no load. Assumingthat no gross distortion takes place when bodyweight is applied to the joint, it was concludedthat when standing or walking the area of closecontact will be about 0 75 sq. in. Consequentlythe nominal contact pressure may be expected torise to about 450 lb./sq. in.The corresponding figures for the knee joint were

harder to assess because of the double contact atthe two condyles. A further difficulty arose becausethe opposing surfaces conform less closely than thoseof the hip. Consequently the synovial fluid willbe squeezed out more easily and the area of contactmay be more dependant on the duration of loading.With these reservations, a typical total contact areawas observed to be about 0 - 3 sq. in. with a maximumpressure of about 900 lb./sq. in.

These estimations of the contact pressures wereused to determine the test conditions in the frictionexperiments.

Frictional CharacteristicsA. Boundary LubricationThe first experiments were carried out to determine

the coefficient of friction between cartilage and glassunder conditions which could reasonably be des-cribed as boundary lubrication. The test specimenswere discs of cartilage cut from human femoralcondyles. The discs were thick enough to ncludea substantial amount of the bony structure below thesurface. The nominal surface area at the er';A of thecylindrical specimen was 0 * 02 sq. in. Synovialfluid, usually taken from the same joint as thecartilage, was used as the lubricant and was appliedliberally to the glass plate at the start of a test. Thereciprocating glass plate was set in motion with amaximum cycle speed of 1 in./sec. and a load of8 lb. was applied. Under this compressive load,the fluid within the pores of the cartilage was grad-ually squeezed out over a period of about 30 min.Values of the friction force were then measured fora range of loads, allowing 10 min. at each stage forconditions to stabilize. The falling curve shown inFig. 4 is plotted through a number of points corres-ponding to the different loads. It will be observedthat the coefficient of friction diminished as the loadwas increased. Further tests up to a pressure of1,000 lb./sq. in. yielded coefficients of friction in theregion of 0-1. When the test was repeated for aspecimen saturated with synovial fluid, the coeffi-

515

copyright. on F

ebruary 7, 2020 by guest. Protected by

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.27.6.512 on 1 N

ovember 1968. D

ownloaded from

ANNALS OF THE RHEUMATIC DISEASES

cient of friction was observed to have a value ofabout 0 01.The rising curves of Fig. 4 show the variation in

the frictional shear stress with the nominal contactpressure for four specimens of cartilage taken fromdifferent subjects. In all cases, the frictional shearstress rose fairly steeply to a point where the nominalcontact pressure reached about 150 lb./sq. in.Thereafter, the frictional shear stresses tendedtowards a constant value of about 60 lb./sq. in.

These results may be used to draw conclusionsconcerning the real area of contact as opposed tothe nominal area. The theory of elasticity predictsthat if an elastic body possessing a number ofspherical asperities on the surface is pressed on to aflat rigid plate, then the real area of contact isproportional to (load)n, where n is a number lyingbetween 2/3 and 1 (Archard, 1957). It will nowbe assumed that the ultimate shear strength of thesurface film in boundary lubrication conditionsremains constant. In this event, the friction forceis also proportional to (load)n while the asperitiesare being flattened under increasing load. Afterthe point when the asperities have been compressedout of existence, intimate contact between the elasticsolid and the rigid plate takes place over the wholearea of nominal contact. Thereafter, the frictionforce is insensitive to further load increases.Examination of the rising curves shown in Fig. 4

suggests that the cartilage behaved elastically up toa nominal contact pressure of about 150 lb./sq. in.Beyond that pressure, the area of real contactapproximated to that of nominal contact. A valuein the region of 60 lb./sq. in. is seen to be the ultimatefrictional shear stress when boundary lubricationoccurred between cartilage and glass.

Tests conducted with diseased cartilage yieldedresults which did not differ significantly from thoseof healthy cartilage. It is presumed that thesynovial fluid provided similar boundary lubrication.

Similar tests carried out with rubber specimensof the same geometry gave values for the ultimateshear stress greater than those for cartilage by about30 per cent. It was concluded that synovial fluidwas acting as a boundary lubricant for rubber onglass.

(B) Fluid Film LubricationFurther experiments were carried out in order to

detect whether or not a fluid film could be generatedbetween cartilage and glass. A "converging wedgecontact" was made from a portion of femoral con-dyle of one inch radius of curvature. The specimenwas loaded and held against the reciprocating glassplate until the friction force had reached a constant

80 -

.42cr

,, 20-

b0

Nominal contact pressure300 400

Pav (LBF/in2)Fig. 4.-Boundary lubrication. The specimens of cartilage came fromhuman femoral condyles, lubricated with synovial fluid, sliding on

glass, reciprocating motion with maximum speed 1 in./sec.

value. At this stage, as in previous tests, it was

assumed that most of the fluid contained in thepores of the cartilage had been squeezed out. Theforce of friction was then observed as the speed wasincreased for various loads. In all cases the frictionforce diminished as the speed was increased, indi-cating the presence of at least some hydrodynamicload carrying capability.

Subsequently, the specimen was trimmed with ascalpel to form a flat-ended contact face whichwas essentially parallel to the glass plate. Themeasured friction forces were found to be three orfour times greater than those occurring when a

wedge profile was present. Under these circum-stances it was concluded that parallel contactsurfaces could not generate a full fluid film.

(C) Squeeze FilmsSpecimens of cartilage affixed to bone were cut

from femoral condyles and the cartilage cut awayleaving an area of 0-02 sq. in. The specimen waspositioned in the reciprocating friction machine andthe cartilage lowered on to a pool of synovial fluidso that complete soaking and adsorption occurred.When the specimen was now loaded against the

reciprocating plate and friction plotted againsttime, the frictional force rose steadily until a limitwas reached when the maximum amount of fluidhad been wrung out from the cartilage at that load.In these experiments, the value of friction at anystage during the wringing out process was calledthe "boundary value" of friction at that stage ofwring-out.To obtain squeeze film results at any stage, the

cartilage specimen was lifted clear of the glass plate

0-8

C-)_060

A

*04 °._.n

020-2 '

O

516

copyright. on F

ebruary 7, 2020 by guest. Protected by

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.27.6.512 on 1 N

ovember 1968. D

ownloaded from

"BOOSTED LUBRICATION" IN SYNOVIAL JOINTS

to allow synovial fluid to enter the space betweenthe surfaces, friction readings being recorded.

After a short run with the cartilage fully saturated,the friction reading was taken to be the first "boun-dary value". The cartilage was momentarily liftedfrom the plate and the motion of the plate continueduntil the friction values became greater than thefirst "boundary value". The friction force reachedwas called the second "boundary value".Another squeeze film test was done, reaching and

exceeding the second boundary value, as before.The experiment proceeded in this way until thelimiting friction force at full wring-out was reached.Squeeze film curves for healthy cartilage from a26 year old subject are shown in Fig. 5. The moststriking feature of the results is the large squeezefilm times.

It is of great interest to estimate the averageoperating viscosity of the synovial fluid, using thestandard Reynolds equation for parallel sinkage, aniso-viscous incompressible fluid and a circularcontact (Equation 2 in Theoretical Considerations).A typical time of 40 sec. is substituted. Theminimum film thickness can be estimated by con-sidering that the surfaces are separated only byhyaluronic acid-protein complexes, which underload can be expected to give a separation of about10 pain. The average viscosity works out to beabout 20 poises, which is very large indeed, andmany times greater than any value which has beenmeasured for synovial fluid rheologically. Thisresult suggests that a very viscous substance hasformed on the cartilage surface during the squeezingprocess. For comparison, experiments were carriedout with various rubbers and it was found that thesqueeze film times were consistently about 0 - 5 sec.,compared with the values of up to 100 sec. forcartilage. The same result was obtained when arubber replica was made of cartilage surface. Theaverage viscosity of the synovial fluid works out tobe about 0 2 poise, which is an average figure fromrheological measurements.

It is concluded that the surface of the cartilageplays an essential part in prolonging the squeezingout of fluid from between the surfaces.The magnitude of the squeeze film times for actual

joints are considered in relation to the results of theexperiments. In the hip joint, if the contact area is1 in 2 and the load 200 lb., the squeeze film timeshould be at least twenty times the experimentalones. It is hard to make a similar estimate for theknee because of the uncertainty of the area of con-tact, but it seems that squeeze film times will be atleast as great as the experimental ones so thatboundary lubrication will be prevented duringwalking.

"~~~~~~~~~~~~~~~~~-4 m= X 4 inl.

U,

2-4

L_ 3

2 t.

5sec. lOsec. 3Osec. Imin. 2min.Squeeze film time

Fig. 5.-Squeeze film times to reach boundary lubrication conditionsat stages during "wring-out", for cartilage with synovial fluid onglass. The dark spots are at the values of boundary frictional shearstress. The times marked on the curves are those at the full load of

400 lbf/in .

For specimens of diseased cartilage, the squeezefilm times at full wring-out were similar to thosefor healthy cartilage, but at low wring-out theywere much less. The reason for this is probablybecause the fluid was able to escape easily throughthe large surface roughnesses during the squeezefilm process, so that boundary lubrication couldmuch more easily occur.

DiscussionLubrication Mechanism

Little attention has been paid in the literature tothe fundamental geometry ofjoint surfaces. Never-theless the effective radius of curvature is well knownto be an important feature in the operation of mecha-nical bearings. Profile recordings taken fromacrylic castings of joints have shown that articularsurfaces are extremely rough compared withbearing surfaces used in normal engineering practice.In general terms, the smoother articular surfaceswere found to be from 3 to 15 times more roughthan an engineering bearing surface. Osteo-arthrosic cartilage was observed to be between 20and 200 times as rough.

Synovial fluid has been described as a dialysateof blood plasma, with the addition of a mucopoly-sacharide, hyaluronic acid, most of which is boundto protein. Ogston and Stanier (1951) have sugges-ted that the resulting molecules, when suspended ina liquid, are spherical in shape and have a diameter

517

copyright. on F

ebruary 7, 2020 by guest. Protected by

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.27.6.512 on 1 N

ovember 1968. D

ownloaded from

ANNALS OF THE RHEUMATIC DISEASES

of about 20 pin. or 0 5 p. Consequently, if thefluid film thickness between cartilage surfaces fallsbelow about 40 ,lin., these large molecules can beexpected to influence the sliding behaviour and,perhaps, to act as a boundary lubricant. Thepresent experiments suggest that a substantialmeasure of boundary lubrication action will onlyoccur if the opposing cartilage surfaces are loadedtogether sufficiently long for the fluid to be squeezedout of the pores. It has been observed that thissituation will seldom develop in healthy joints.Nevertheless it appears to be probable that boundarylubrication will always be present to protect tosome extent the highest peaks of the cartilagesurface.The experimental results support the deductions

made by Dowson (1967), that elastohydrodynamicaction is unlikely to occur in the knee or hip jointsduring the weight-bearing phase in walking. Onthe other hand, during periods of sliding underlow load, as in an unloaded swing of the leg, asubstantial fluid film is likely to be generatedbetween the surfaces. The fluid present betweenthe cartilages would then act as a squeeze film duringsubsequent application of load. Indeed the squeezefilm experiments provide very strong evidence thatthis type of fluid film lubrication is the predominantmeans by which joint surfaces are lubricated.The possibility that, under pressure, a gel formed

of concentrated synovial fluid collects on thecartilage surface has been suggested by Maroudas(1967). The present experiments provide data tosubstantiate this interpretation and it is furthersupported by studies of the cartilage surface andsynovial fluid with a scanning electron microscope(Walker, Sikorski, Dowson, Longfield, and Wright,1968). When two cartilage surfaces (Fig. 2) areplaced in close proximity, contact will take placebetween the peaks and the opposite surface. As thesurfaces are pressed together under load, the area ofcontact will increase, and further peaks of lesserheight will touch the opposite surface. Over thewhole area of nominal contact, many such peakswill make contact as the load is increased. Alterna-tively, it can be said that the nominal contact areais sufficiently large to contain a large number ofsubstantial depressions in the cartilage surface.This implies that there will be trapped pools ofsynovial fluid as illustrated in Fig. 6 (opposite). Itis suggested that these pools will develop a highconcentration of hyaluronic acid-protein complexbecause of the diffusion of water and low molecularweight substances through the cartilage pores andfrom the edges of the contact area.

It will be seen that the cartilage surfaces will bemore effectively preserved when there is a film of

thick or viscous fluid between them. The opposingjoint surfaces, particularly in the hip, are sufficientlycongruent to form large areas of nominal contact.This geometry results in very long squeeze filmtimes. Since the cartilage has pronounced un-dulations on the surface, trapped pools of fluid areformed which take a significant time to leak away.This explanation of the action of a weight-bearingjoint, depending on the formation of trapped poolsof synovial fluid, has been termed "boosted lubri-cation".

Wearing of CartilageIt has been concluded that the cartilage surfaces

of normally functioning joints do not suffer damagebecause they are separated by a fluid film. It issuggested that the occurrence of boundary lubrica-tion is an important factor in producing surfacedamage because it imposes relatively high shearforces along the surfaces. Meyer (1931) hasobserved that small flaps or strips were lifted fromthe surface of the cartilage. Zarek and Edwards(1963) showed that, even in normal use, tensilestresses are set up along the surface because ofcartilage deformation. The addition of a shearstress of magnitude approximately 60 lb./sq. in. maywell be sufficient to rupture the surface.Boundary lubrication of the whole or part of the

nominal contact area might occur in several differentways. If a joint remains statically loaded for aconsiderable time and then is caused to movesuddenly, boundary lubrication may occur. Good-fellow and Bullough (1967) have shown that thoseareas of cartilage which are not normally load-bearing become gradually softer and the structurebecomes open or "feathery" as in chondromalacia.If such a surface is used for load-bearing purposes,boundary lubrication is likely to occur with subse-quent wear.Boundary lubrication may also occur in the hip

when the muscular forces which push the femoralhead into the acetabulum are weakened for anyreason. There will then be a tendency for thefemoral head to "ride on the rim", imposing greatercontact pressures. If this is accompanied by smalleroverall contact areas, then the squeeze film timeswill be reduced significantly.The success of osteotomies may be due, at least

in part, to the restoration of larger contact areasby reseating the femoral head in the acetabulumwith a return once more to a squeeze film type oflubrication rather than boundary lubrication.

SummaryThe overall geometry of the articular surfaces of

the hip and knee joints has been determined by

518

copyright. on F

ebruary 7, 2020 by guest. Protected by

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.27.6.512 on 1 N

ovember 1968. D

ownloaded from

"BOOSTED LUBRICATION" IN SYNOVIAL JOINTS

load

fluid escapinq i lar e moleculesthrouqh pores una%Ie to escape

trapped poolsof concentrated boundarysynovial fluid molecules

_= ~~~~AUFig. 6.-Pictorial representation of the lubrication of cartilage with

synovial fluid.

acrylic castings. A stylus instrument tracing hasdetermined the surface quality of the cartilage andshown that normal joint surfaces are three to fifteentimes as rough as normal engineering bearings,while osteo-arthrosic joints are twenty to twohundred times as rough.Contour maps of the load-bearing area show that

in the hip this is horseshoe-shaped, disposed near the

periphery of the acetabulum. It has been deducedthat the contact pressure will rise to maximumvalues of 450 lb./sq. in. in the hip and 900 lb./sq.in. in the knee.A reciprocating friction machine has been de-

signed and used to study different modes of lubri-cation of cartilage on glass. It has been shown thatunder conditions of boundary lubrication thecoefficient of friction falls as load increases and thatat 1,000 lb./sq. in. load it is in the region of 0 1.Under boundary conditions, a value of about 60lb./sq. in. is the ultimate frictional shear stress.Synovial fluid also acted as a boundary lubricantfor rubber on glass.A converging wedge contact produced fluid film

lubrication between cartilage and glass. Duringa period of sliding under low load, such as the swingphase of a leg, a substantial fluid film is likely to begenerated in the joint.

Large squeeze film times were obtained experi-mentally. By substituting these values in the stan-dard Reynold's equation, very high values for theviscosity of synovial fluid were obtained. Itis suggested that this occurs by fluid entrapmentbetween irregularities of the cartilage surface, thesqueezing of water through the small pores of thecartilage, and the production of concentrated syno-vial fluid on the cartilage surface. It is proposedthat this action is important in the load-bearingphase of joint action, and the mechanism of fluidentrapment and enrichment has been termed"boosted lubrication".

The authors wish to record their thanks to Dr. W.Goldie and Dr. H. G. Kohler for supplying specimens andMr. H. P. Jones for assistance with problems of measure-ment.

REFERENCESArchard, J. F. (1957). Proc. roy. Soc. A., 243, 190 (Elastic deformation and the laws of friction).Chamley, J. (1960). Ann rheum. Dis., 19, 10 (The lubrication of animal joints in relation to surgical

reconstruction by arthroplasty).Davies, D. V. (1966). Fed. Proc., 25, 1069 (Synovial fluid as a lubricant).

,Bamett, C. H., Cochrane, W., and Palfrey, A. J. (1962). Ann. rheum. Dis., 21, 11 (Electronmicroscopy of articular cartilage in the young adult rabbit).

Dowson, D. (1967). In "Lubrication and Wear in Living and Artificial Joints: A Symposium",Institution of Mechanical Engineers Proceedings, vol. 181, Part 3J, pp. 45-54. London(Modes of lubrication in human joints).

Edwards, J. (1967). Ibid., pp. 16-24 (Physical characteristics of articular cartilage).Goodfellow, J. W., and Bullough, P. G. (1967). J. Bone Jt Surg., 49-B, 175 (The pattern of ageing

of the articular cartilage of the elbow joint).Linn, F. C. (1967). Ibid., 49-A, 1079 (Lubrication of animal joints. I. The arthrotripsometer).MacConaill, M. A. (1932). J. Anat. (Lond.), 66, 210 (The function of intra-articular fibrocartilages,

with special reference to the knee and inferior radio-ulnar joints).McCutchen, C. W. (1962). Wear, 5, 1 (The frictional properties of animal joints).

(1966). Fed. Proc., 25, 1061 (Boundary lubrication by synovial fluid: demonstration andpossible osmotic explanation).

519

copyright. on F

ebruary 7, 2020 by guest. Protected by

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.27.6.512 on 1 N

ovember 1968. D

ownloaded from

ANNALS OF THE RHEUMATIC DISEASES

Maroudas, A. (1967). In "Lubrication and Wear in Living and Artificial Joints: A Symposium",Institution of Mechanical Engineers Proceedings, vol. 181, Part 3J, pp. 122-124. London(Hyaluronic acid films).

Meyer, A. W. (1931). J. Bone Jt Surg., 13, 341 (The minuter anatomy of attrition lesions).Moore, D. F. (1965). Wear, 8, 245 (A review of squeeze films).Negami, S. (1964). M. Sc. Thesis, Lehigh Univ., Bethlehem, Penn. (Dynamic Mechanical Properties

of Synovial Fluid).Ogston, A. G., and Stanier, J. E. (1951). Biochem. J., 49, 585 (The dimensions of the particle of

hyaluronic acid complex in synovial fluid).Paul, J. P. (1967). In "Lubrication and Wear in Living and Artificial Joints: A Symposium",

Institution of Mechanical Engineers Proceedings, vol. 181, Part 3J, pp. 8-15. London (Forcestransmitted by joints in the human body).

Rydell, N. (1965). In "Biomechanics and Related Bio-engineering Topics: Proceedings of a Sym-posium held at Glasgow, 1964", ed. R. M. Kenedi, pp. 351-358. Pergamon Press, Oxford(Forces in the Hip-Joint. Part II. Intravital Measurements).

Walker, P. S., Sikorski, J., Dowson, D., Longfield, M. D., and Wright, V. (1968). Unpublished data.Zarek, J. M., and Edwards, J. (1963). Med. Electron. biol. Engng, 1, 497 (The stress-structure

relationship in articular cartilage).

"Lubrication de renfort" des articulations synoviales paremprisonnement et enrichissement du liquide

REsuMEOn a determine la geometrie complete des surfaces

articulares de la hanche et du genou a l'aide des moulagesacryliques. Un trace obtenu par un instument pourvud'une pointe fine de diamant a permis l'evaluation de lasurface du cartilage. On a'observe que la surface ducartilage normal est trois a quinze fois plus rugueuseque celle des billes pour roulement et dans l'osteoarthrosecette rugosite est 20 a 200 fois plus accentu6e.Un trace du contour de la surface d'appui montre

que dans la hanche elle est en fer de cheval et qu'elleest situee vers la peripherie de l'acetabulum. On acalcule que la pression de contact peut atteindre unmaximum de 31,5 kg. par cm2 dans la hanche et 63 kg.par cm2 dans le genou.On a construit une machine a mouvement alternatif

pour etudier la friction du cartilage contre le verre sousdifferents modes de lubrication. Dans les conditions delubrication limitrophe le coefficient de friction tombe amesure que la charge augmente et se trouve dans laregion de 0,1 avec une charge de 70 kg./cm2. Dans lesm8mes conditions, 4,2 kg./cm2 constituent la dernierelimite de cisaillement frictionnel. Le liquide synovialfonctionnait aussi comme lubrifiant pour la friction ducaoutchouc contre le verre.Un contact par coin convergent produisait une couche

lubrifiante liquide entre le cartilage et le verre. Pendantle glissement sous un poids bas, par exemple lors del'oscillation de la jambe, une couche appreciable peutetre engendree dans l'articulation.On a etudie experimentalement le temps necessaire

pour exprimer du cartilage le liquide synovial par frictionet on a trouve ce temps tres long. En introduisant lesvaleurs ainsi obtenues dans l'equation de Reynold on aobserve que les chiffres pour la viscosite du liquidesynovial etaient tres eleves. On suggere que cela seproduit parce que le liquide se touve emprisonne entreles irregularites de la surface cartilagineuse, l'eau estexprimee par les petits pores du cartilage laissant a lasurface un liquide synovial concentre. On enonce laproposition que cette action est importante lorsquel'articulation en mouvement supporte une charge et ace m6canisme d'emprisonnement et d'enrichcissementdu liquide on donne le nom de "boosted lubrication"("lubrication de renfort").

"Lubricaci6n de refuerzo" en las articulaciones sinovialespor encarcelaci6n y enriquecimiento del liquido

SUMARIOSe estableci6 la geometria completa de las superficies

articulares de la cadera y de la rodilla con la ayuda demoldes acrilicos. Un trazado obtenido con un instru-mento provisto de una puntilla de diamante permiti6 lavaloraci6n de la calidad de la superficie del cartilago.Se observ6 que la superficie del cartilago normal es tresa quince veces mas aspera que la de una bola metalicaindustrial; en la osteoartrosis esta asperidad es 20 a 200veces mayor.Un trazado del contorno de la superficie de apoyo en

la cadera muestra que este contorno reviste la forma deherradura y que esta situado en la periferia del acetabulo.Fue estimado que la presi6n de contacto puede alcanzarun maximo de 31,5 kg. por cm2 en la cadera y 63 kg.por cm2 en la rodilla.

Se ha construido una maquina de movimiento alter-nativo para estudiar la fricci6n del cartilago contra elvidrio bajo diferentes condiciones de lubricaci6n. Enlas condiciones de lubricaci6n limitrofe el coeficiente defriccion baja cuando la carga aumenta. Con una cargade 70 kg./cm2 este coeficiente se halla en la region de 0,1.En las mismas condiciones, una carga de 4,2 kg./cm2constituia el ultimo limite del esfuerzo cortante friccional.El liquido sinovial actuo tambien como lubricante en lafriccion del caucho contra el vidrio.Un contacto en forma de cuna convergente producia

una tela lubricante liquida entre el cartilago y el vidrio.Durante una accion de deslizamiento, con una cargabaja, por ejemplo cuando la pierna oscila, una telaapreciable puede formarse en la articulaci6n.

Se estudi6 experimentalmente el tiempo necesario paraexprimir del cartilago, por movimiento de fricci6n, elliquido sinovial y se hallo este tiempo muy largo. Sesustituyeron los datos asi obtenidos en la ecuaci6n deReynold y se hallaron muy altas cifras para la viscosidaddel liquido sinovial. Se sugiere que estos fen6menosocurren porque el liquido se ve encerrado entre lasirregularidades de la superficie cartilaginosa, el aguaexprimida por los pequefios poros del cartilago, dejandoen la superficie un liquido sinovial muy concentrado.Se enuncia la proposici6n de que esta acci6n es importantecuando una articulaci6n movil Ileva una carga; a estemecanismo de encarcelacion y de enriquecimiento delliquido se da el nombre de "boosted lubrication" ("lubri-caci5n de refuerzo").

520

copyright. on F

ebruary 7, 2020 by guest. Protected by

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.27.6.512 on 1 N

ovember 1968. D

ownloaded from