'Cracking joints' - A EULAR & BMJ Rheumatology Journal · joints byFick(1911), Dittmar(1933),...

Ann. rheum. Dis. (1971), 30, 348

'Cracking joints'

A bioengineering study of cavitation in themetacarpophalangeal joint

A. UNSWORTH*, D. DOWSON**, AND V. WRIGHT***Bioengineering Group for the study ofHuman Joints, the University ofLeeds

The cracking of joints is a common phenomenonwhich interests patients and clinicians alike. Its exactnature has remained in doubt, as evidenced by arecent comment in the British Medical Journal(1969). At the knee and hip, tendons over bonyprominences can cause 'clicking' which may some-times be audible. The tensor fascia lata is particularlyapt to do this over the greater trochanter. Themajority of cracks do not appear to arise from thismechanism.The only detailed study of cracking in the meta-

carpophalangeal joint, as far as the authors areawNare, is that of Roston and Wheeler Haines (1947).Before this work bubbles had been observed injoints by Fick (1911), Dittmar (1933), and Nordheim(1938), who were interested in them as a means ofobtaining radiographs of fibro-cartilage in the kneewithout using a contrasting medium.

In the present communication it will be shown thatthis bubble is not the cause but the effect of the crack,and that fluid 'cavitation' is responsible for thecracking noise. In addition, suggestions will bemade to explain why some joints cannot be crackedand why, having been cracked, about 20 minutesmust elapse before a joint can be cracked again.

Materials and methodsA machine was designed and built to study the effects ofloading on the separation of the metacarpophalangealjoints in man (Fig. 1). The subject's arm was held in aprenyl splint attached to an adjustable table. A selectionof splints was available to ensure correct alignmentof the fingers to the loading mechanism. The fingersrested on an x-ray cassette which could slide laterallyin the table.Loads were applied by a pneumatic cylinder acting

through a transducer and twine which was connected toa ring fitted around the proximal phalanx of the middlefinger. A small adhesive dressing encircled the proximalinterphalangeal joint to prevent the ring from slipping.

The applied loads were measured using an ultra-violetrecorder whilst sequential x-ray exposures allowed thebone separation to be visualized.

X-ray exposures were taken at the beginning of thetest before loading and at increments up to about 16kg. If the joint cracked, the load trace was marked at thepoint of cracking and, after further increases in load,exposures were taken at each reduction of load to zero.Where joints did not crack the maximum load appliedto thejoint was 16 kg. During this series of tests, seventeensubjects were tested on the machine.

STUDY OF GEOMETRYTo apply the appropriate hydrodynamic equations fortheoretical analysis, the configuration of the metacarpo-phalangeal joint of the middle finger was determined innine joints. Four joints were obtained from the Depart-ment of Anatomy (having been embalmed) and five atautopsy. Silicone rubber moulds were made of themetacarpal head and the base of the proximal phalanxof each joint and acrylic plastic models were produced.These were sectioned in various planes and the radius ofcurvature measured at each plane using a projectionmicroscope.

GAS ANALYSIS OF SYNOVIAL FLUIDThis was carried out using a Van Slyke apparatus onsynovial fluid from seven patients; one had a traumaticeffusion and six had rheumatoid arthritis. The fluid wastaken in a sealed syringe and each test performed as soonas possible (usually within 1 hour). In the case of onespecimen, taken at operation, the sample had beenexposed to the air on opening the joint.

JOINT SIMULATORFrom the data obtained from the (geometrical) study, amodel was made twice this size of the metacarpo-phalangeal joint. The metacarpal head was made in nylonand the proximal phalanx in Perspex. This combinationwas used to give good photographic conditions. Synovialfluid was inserted between the surfaces and the jointplaced in compression. A sudden tension was appliedto the joint and a high speed cine camera photographedthe clearance space between the joint surfaces.

Presented at a meeting of the Heberden Society in November, 1970Accepted for publication March 25, 1971Research Fellow, Rheumatism Research Unit**Profesor of Fluid Mechanics and Tribology, Department of Mechanical Engineering***Professor of Rheumatology, Rheumatism Research Unit

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'Cracking joints' 349

FIG. 1 A machine designed to 'crack' the metacarpophalangeal joints ofhuman subjects.

THEORETICAL CONSIDERATIONSCavitation is the term used to describe the formation ofvapour and gas bubbles within fluid through localreduction in pressure. When the vapour collapses on

moving into a region of higher pressure, very high impactpressures can be generated. Consider a sample of fluidwhich suddenly has its pressure reduced. When thepressure is reduced, the vapour temperature of the fluidis also reduced; if a large pressure reduction takes placethe fluid boils at ambient temperature and reducedpressure and is converted into vapour bubbles in theselow pressure regions. In addition, the gas which waspreviously in solution forms gas cavities at reducedpressure. When these cavities or vapour-filled bubblesmove into the higher pressure areas, instant collapsetakes place with very high energy release which can giverise to extremely large stresses. It is this phenomenon thatis responsible for the erosion of ship's propellors and theblades of hydraulic turbines, and cavitation damage inmany forms of hydraulic machinery and bearings.

Results

FINGER CRACKINGOf the seventeen subjects tested, five produced cracks,seven did not, and five did not relax sufficiently toallow a test to be performed properly. This lastgroup reacted to the applied loads by tensing themuscles and so holding the joint closed.

In all the subjects who produced a crack a crescent-shaped area of high contrast was noted in theclearance space between the articular surfaces onradiography (Fig. 2, overleaf). This was absent insubjects whose joints did not crack.

Figs 3 and 4 (overleaf) show typical load-separa-tion curves for 'cracking' and non-cracking joints.Consider first of all Fig. 3. As the load increased, thejoint separation increased also. This rise was gradualat first and in three of the five cases was linear. At a



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350 Annals of the Rheumatic Diseases

FIG. 2 Radiograph of metacarpophalangealjoint after cracking. Note increased jointseparation and contrasted area within jointspace.

5.

4.EE

0

4-

0.0cU

3.~

2

0

4

0 2 4 6 8 10 12 14 16Load (kg.)

FIG. 3 Typical load-separation curve for a cracking joint.Note similarity between upper (re-load) loop and the loopofFig. 4.

load between 10 and 16 kg. a crack was heard andthe joint separation increased rapidly. The radio-graph also showed the contrasting area within thejoints. Continued loading took the separation a

little higher, and on reducing the load the uppercurve was followed (indicated by a broken line).On reloading immediately, the load-separationcharacteristics followed the middle curve and not theoriginal (lower) one. No crack was heard on thesecond and subsequent loading cycles. In the

2t

O 2 4 6 8 lb 12 14 16Load (kg.)

FIG. 4 Typical load-separation curve for a non-crackingjoint. Note absence of a sudden 'jump' in separation value(compared with Fig. 3).

non-cracking joint (Fig. 4), different load-separationcharacteristics were found. Comparing the twographs, it can be seen that the loop of Fig. 4 issimilar to the upper loop of Fig. 3, and since thearea of the loop represents the energy dissipated inthe joint, it can be concluded that the upper loopof Fig. 3 and the area enclosed by Fig 4 represent theenergy lost in extending and returning the joint. Thisenergy is due to viscous and Coulomb losses in thejoint fluid and surroulnding tissue. The lower



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Table I Observations from load-separation tests in twelve subjects

Subject Sex Age Joint cracked Dark area Resting separation (mm.)no. (yrs.) visible on

x-ray Initial Final

I M 21 Yes Yes 1*2 1*262 F 37 Yes Yes 1-17 1-443 M 22 Yes Yes4 F 19 Yes Yes 1-32 1A415 F 35 Yes Yes 1*2 1*336 F 19 No No 1*37 1*97 F 31 No No 1 608 F 34 No No 1*23 1*449 F 50 No No 1*94 2a0710 F 20 No No 1-54 1.9011 F 21 No No 0.98 1-3212 F 20 No No 1*35 1*49

approximately triangular area of Fig. 3 representsthe energy dissipated by reason of the crack itselfand is about 75 per cent. of the total energy expended.

Table I shows some of the observations from theload-separation tests.

Table II Time lapse before joint separation returnsto pre-cracking value

Time related Joint separationto crack (mm.)

Before (resting) 0-98At instant of cracking 2 505 min. after (resting) 1 4010 min. after (resting) 0.9915 min. after (resting) 0 98

Table II shows that, for the finger tested, a timelapse of about 15 minutes occurred before the finger'joint separation returned to its pre-racking value.This is probably due to the viscous effects of synovialfluid keeping the surfaces apart, together withelastic recovery within the cartilage.

STUDY OF GEOMETRYBefore the technique was used to study the geometryof joints, controlled tests were conducted onstandard specimens. It was found that the lengthof a rectangular block (1 in. steel slip gauge) wasreproduced within 0*7 per cent. A measurement ofa steel ball of 1 in. diameter was in error by 1 - 6 percent., and measurements on a glass cylinder of0 582 in. diameter gave an error of 1 2 per cent.

FIG. 5a Shadowgraph outline ofthe metacarpal head with FIG. Sb Shadow graph outline ofthe base ofthe proximala radius superimposed. phalanx with a radius superimposed.

B



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352 Annals of the Rheutmatic Diseases

FIG. 6 Convention used for defining planes of sectionl.

The radii of curvature of the model surfaces weredetermined by projecting the articulating surfaceson a projection microscope and then fitting standardradii to them (Fig. 5, previous page).The metacarpal heads were sectioned about three

axes longitudinal, transverse, and rotational definedin Fig. 6.The results of measurements on these sections are

given in Table III (opposite) and the results for themetacarpal head and proximal phalanx on thelongitudinal and transverse planes in Table IV.

It was noted that, in three of the eight fingersexamined, the metacarpal head was spherical overthe range of motion. However, it is worth notingthat two of these (F3 and F8) were spherical only byvirtue of wear taking place and causing local

adjustment of the original radius of curvature(Table IV). In all the other cases the metacarpalhead had a larger radius transversely than longitudi-nally. It therefore seems that in general the meta-carpal head is not quite spherical but has a smallerradius of curvature in the coronal plane than thetransverse plane. In this case the average differencein radius was 6 per cent., a small but definitedifference.

Table IV shows that three of the eight specimensof proximal phalanx base had equal radii in thecoronal and transverse planes (i.e. they werespherical). Two specimens had a greater radius inthe coronal plane and three in the transverse plane.The mean difference in radius was only 1 * 07 per cent.and was such that the coronal plane exhibited the

Table IV Measurements for sections or metacarpal head andproximalphalanx in eight specimens

Specimen Longitudinal radius (mm.)no.

Metacarpal Proximnalhead phalanx

Fl 8-1 8 9

F3 8.9* 9.1

6-6 7.9

6-6 7 6

7 1 7.4

7-2 7.9

.91** 9-1

7 9 7-9

Transverse radius (mm.)

Metacarpal Proximalhead phalanx

9.1 8 1

89 9.1

72 7.9

72 8 1

7 1 7.4

8 1 8-3

9.1 8-1

86 8 1

Condition ofjoint

Good, showing slight damage

Good, some wear

Good, some wear

Signs of wear

Good

Worn locally

Worn/Good

Very good

Radius of 8-9 mm. superimposed on one of 7-2 mm. *Radius of 9 1 ,-mm. superimposed on one of 7.6 mm.

F4

F5

F6

F7

F8

F9



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Table III Measurements on sectioned metacarpal heads in six specimens

Longitudinal

Angle Radius(1) (mm.)

0 8-9-10 8-9-20 8-9+l0 8-9

0-30-- 30

0+i-20+27+32

0-15-40+30

6-66-66-6

6 66-66-66-4

7-17-17-17-1

0 7-2-25 7-2-57 7-2+15 7-2+40 7-72

0

-30+26+40

7 97.97-97 9

Transverse

Angle(3)

0+20+40+80

02045

0153860

0153060

03055

0-15+30+50

Radius(mm.)

8-98 98-98-9

7-27-28-2

7-27-27-27-2

7-17-17-17-1

8-17-98-4

8-68-99.18-9

Rotational

Angle Radius(°) (mm.)+10 8-9+20 7-6+30 7-1-10 8-6-20 8-9-30 8-9

+10+20+30-10-20-30

+10+20+30-10-20-30

+10+20+30-10-20-30

+10+20+30-10-20-30

+10+20+30-10-20-30

7-27-27-27-27-67-9

7-47-47-47-17-17-1

7-17-17-16-96-97-1

7.77-98-17-97 68-1

8-68-68-98-99.49.4

larger radius. This observation is interesting becauseit appears that the base of the proximal phalanx isnearer to a sphere than the head of the metacarpal.

If the joint is now considered as a whole (i.e.fitted together), it is interesting to note the relativeradii of curvature and hence the clearance at anypoint. In the coronal plane in every case the proximalphalanx was of equal or greater radius than themetacarpal head. On average this clearance or

difference in radius was 0 5 mm. The transverseplane, however, showed much more scatteredresults. Three of the joints had larger metacarpalheads than proximal phalanx bases and five hadlarger bases than metacarpal heads. The averageclearance in the transverse plane was small andnegative. It appears that the proximal phalanx gripsthe metacarpal head very slightly by 0-025 mm. on

radius on average. Fig. 7 (overleaf) shows a joint inwhich the metacarpal head is smaller than theproximal phalanx base and Fig. 8 (overleaf) one inwhich the head is larger than the base in the trans-verse section.Although in detail thejoints are not quite spherical,

for the sake of analysis they can be considered to betrue spheres.

GAS ANALYSIS

This was carried out on samples of synovial fluidfrom seven patients (Table V, overleaf). The averagegas content was 15 per cent. by volume, and over 80per cent. of this was carbon dioxide.

In a series of experiments on four fluids, the gaswas liberated and a positive pressure of 17 cm. ofmercury was then applied to the column of gas above

Specimen

F3

F4

F5

F6

F7

F9



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longitudinallongitudinal

transverse

FIG. 7 Section showing a metacarpal head smaller than FIG. 8 Section showing a metacarpal head larger than thetheproximal base in the transverse plane. proximal base in the transverse plane.

transverse

Table V Results ofgas analysis ofsynovialfluidfromseven specimens

Specimen Type Gas content Temperatureno. (per cent. (°C.)

by volume)

Si Rheumatoid 16 23S2 Rheumatoid 15 23S3 Rheumatoid 17 23S4 Rheumatoid 16 26S5 Rheumatoid 15 26S6 Traumatic 10 26

effusionS7 Rheumatoid 16 26

the evacuated synovial fluid in a Van Slyke apparatus.A curve of the absorption of the gas with time was

drawn for each fluid (Fig. 9). Ifa rough calculation isdone for the reabsorption time, taking into accountthe difference in area of the observed bubbles injoints and the bore of the Van Slyke apparatus, thenthe gas reabsorption time would be of the order of30 min. in a metacarpophalangeal joint. This is inagreement with the clinical observation that jointscannot be re-cracked within about 20 to 30 minutes.

MATHEMATICAL CONSIDERATIONSThe geometrical study of the metacarpophalangealjoint of the middle finger showed that this could beapproximated to a sphere with little error (6 per

0-170.1640-14

t2°

234 o234C.

002

010

o--0

0

40 80 IXO) 160 20 202Time (min.)

FIG. 9 Graph of absorbtion of gas into synovial fluidagainst timeforfour samples offluld. An arbitrary pressureof17 cm. mercury was applied to the gas to help theprocess.

cent.). The hydrodynamic equations were thereforeconsidered for this configuration and it has beenshown elsewhere (Dowson, Unsworth, and Wright,1970) that the pressure at any point in a sphericaljoint under load can be written as:




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w I 127i R,2 L( -E cos 6)2 (1-e COS 61)2j

1 ln (1-E)[(1n-I-)

where p is the pressure generated within the fluid film atthe point determined by the co-ordinate 6.W is the applied load.Rs is the radius of the sphere representing the

metacarpal head.61 is half the angular extent of the bearing.

If 0 = 0 is substituted into this equation, then themaximum pressure will be obtained.

Conditions when e-*1When c approaches unity, the maximum pressuregenerated approaches minus infinity when thesurfaces are moving apart. The value of E becomesunity when the two cartilage surfaces are in contact.When a thin film of fluid is present, the pressuregenerated is very low. In general, fluid containinghigh quantities of gas cannot support low pressureswithout cavitation taking place.

JOINT SIMULATORUsing the joint simulator, an audible crack wasproduced by a sudden tension after the componentshad been compressed with synovial fluid betweenthem. Frames from the high speed cine film are shownin Fig. 10 (overleaf), in which the presence of vapourcavities can be seen.

DiscussionFrom the theoretical considerations it is clear that,subject to condition of very thin synovial fluidfilms existing between the cartilage surfaces of themetacarpophalangeal joint, it is likely that cavitationwill take place within the fluid when tension isapplied to the joint. At the instant of formationof the cavities an unstable condition is created, forthe pressure within the bubble is very low while thatof the surrounding fluid is nearer to ambientconditions. In addition, the joint separation increasesat a high rate, allowing the net flow of fluid into thelow pressure regions. This results in collapse of thevapour phase of the cavities with consequent energyrelease as noise (the 'crack' heard externally). Thegas removed during the period of low pressure,however, returns to ambient pressure but does notreabsorb for 20 to 30 minutes. This gas is readilyvisible on the radiograph, particularly on extensionof the joint but is not responsible for causing thecrack; it is there as a consequence of the crackingphenomenon.

cos 61 In(l- cos0l) sin26l 1E(1-Ef cos6l) C2 2(1-E COSI)2 J

E is the eccentricity ratio e/c.e is the displacement of the centres of the radius

of the metacarpal head and the radius of thebase of the proximal phalanx.

c is the difference between the radius of theproximal phalanx and the metacarpal head.

Roston and Wheeler Haines (1947) estimated thepressure in the joint by taking the applied load anddividing this by the area of the joint suggesting thatthe fluid was subjected to a tension of -2J atmo-spheres. The fallacy of this calculation arises fromthe fact that not all of the load is taken throughthe fluid film (some is carried by the surroundingstructures). In addition, the pressure generatedwithin the curved surface is different in all parts ofthat surface, being a minimum at the centre of thecontact: a fact which is vital to an understanding ofthe cracking phenomenon.The load-separation results from the tests described

in this communication are similar to those producedby Roston and Wheeler Haines. They differ in thatthe initial separation of the joint surfaces wasalways smaller than the final separation of the samesurfaces providing the measurement was takenimmediately after unloading. After about 15 minutesrest the two became equal. These are also importantfactors in the explanation of the phenomenon.Nordheim (1938), when explaining the shadow on

the x-ray film, used the example of a syringe with thenozzle blocked filled with water and the plungerwithdrawn. This produces a low pressure areaconsisting of water vapour and gas extracted fromthe solution in the water. On this basis Roston andWheeler Haines (1947) suggested that the suddenappearance of this low pressure bubble caused thejoint to separate at a rapid rate and that the crackwas caused by the sudden opening of the jointsurfaces producing vibrations in the joint tissue.The present work confirms that low pressures aredeveloped in the synovial fluid and these pressurescause vaporization and gas liberation from thefluid. At this point the joint space springs open,but the high speed cine camera shows that thebubble forms and collapses again in about 0 01second. It is therefore concluded that it is thecollapse of the vapour bubble which causes thecrack not its formation. This is borne out by mathe-matical considerations when the physical significanceof applying tension to the joint is considered. As thejoint is pulled apart, if the fluid film is very thin(i.e. --*I in the equation), a very low pressure is



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generated. This causes vapour cavities to be formedbut, at the same time, gas which was previously heldin solution in the fluid is released within thesecavities or bubbles. Because the pressure is muchlower in the middle of the contact (where the bubbleis formed) than towards the joint surface extremities,a pressure-induced flow will take place to fill thecavities formed. This flow causes a sudden conden-sation of the vapour previously formed because thepressure rises above the vapour pressure as thefluid flows in. This phase transformation, whichtakes place in a very short time, gives rise to freeenergy equal to the latent heat of vaporization

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ready for re-cracking. Another factor which has notreceived previous comment is that the separationbetween the joint surfaces does not return to itspre-cracking condition for a period of 15 minutes.The surfaces must be very close to give the rightconditions for cavitation at reasonable loads sothat, if a joint is separated before this space hasreduced fully, the pressure generated will be in-sufficient to cause cavitation as is apparent from thetheoretical considerations. The reasons for the jointspace taking so long to return are multiple, butprincipally the viscosity of the synovial fluid resiststhe forces trying to squeeze the fluid film to smallproportions and this resistance causes a time delay.

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©FIG. 11 Two diagrams having equalradial clearance 'c' but differenteccentricity 'e'.

(a) e-+c and therefore E-*>1. Thiscondition gives a very low pressure atthe centre ofcontact.

(b) e is small and therefore e issmall, so that the pressure generatedat the centre ofcontact is not very low.

In addition, the ligaments take some time beforethey can re-apply their initial loading because ofthe visco-elasticity of the tissue. It therefore becomesclear that joints cannot be expected to crackrepeatedly if cavitation characteristics are considered.Some joints never crack. This may be because the

joint space is too great. Examining the equation, itcan be seen that when e approaches unity, applicationof a load such that surface separation takes placeis likely to produce cavitation because of the lowpressure generation. Conversely, if 6 is very much lessthan unity, the pressure is likely to be insufficientto produce cavitation within the joint (Fig. 11).The physical explanation of a thick fluid film being

present between the articulating surfaces is that theligaments locating the joint are not strong enoughto force the two surfaces together. This point isverified by Table I which shows the resting separationof non-cracking joints to be 25 per cent. greater thanthat of cracking joints. Secondly, subjects may notbe able to relax. In this series of tests and in thoseof Roston and Wheeler Haines (1947), severalsubjects could not relax their muscles. As themachine applied the load the subject pulled backagainst this by tensing the tendons spanning thejoint. The result was that the joint did not open orat best opened erratically as the subject attemptedto relax at intervals.

SummaryA machine has been constructed to study theload-separation characteristics of the metacarpo-phalangeal joint. It was demonstrated that, in jointswhich produce a crack, an area of high contrast waspresent radiologically. This is in agreement with thefindings of other workers. The characteristics shownby a cracking joint were not the same as those of anon-cracking joint. However, the reloading curvesfor a previously cracked joint were similar to thosefor a non-cracking joint. Gas analysis, using a VanSlyke apparatus, showed that synovial fluid contains15 per cent. gas on average.

Studies of geometry demonstrated that the jointsurfaces were essentially spherical in the area ofinterest, and the hydrodynamic equations for this

configuration show that, when the joint surfacesare close together, large subatmospheric pressurescan be produced on separating the surfaces.The results support the view that 'cavitation' is

responsible for the phenomenon of cracking. Undersubatmospheric pressures, the synovial fluid vapor-izes and gas is released from solution. The collapseof the vapour cavities gives rise to the noise. Thiswas supported by high speed photography of aPerspex and nylon simulated joint.

The authors wish to record their thanks to Dr. MichaelWinn, Director of Diagnostic Radiology, for his in-valuable help in this work, and to Dr. D. I. Haslock,Department of Medicine, and Dr. W. K. J. Walls,Department of Anatomy, for the provison of specimensand helpful discussions.The group is grateful for financial support from the

Arthritis and Rheumatism Council and Messrs. Reckittand Colman.

DISCUSSIONDR. A. G. S. HILL (Stoke Mandeville) Looking at thatapparatus, there is really very little difference between itand mediaeval methods of persuasion except thatnowadays you make measurements during the process!

PROF. WRIGHT It has a safety catch the patient canoperate, and in that way it differs from the mediaevalinstrument!

DR. R. GRAHAME (London) This radiograph from ayoung man in his mid-twenties, who suffered fromPerthes' disease in his youth, supports Prof. Wright'shypothesis that the cracking of joints is due to releaseof gas within the joint. On abduction of the hip a gasbubble has appeared. I am told by my orthopaediccolleague, Mr Adrian Henry, that this induction of a gasarthrogram is known in orthopaedic circles to occurin patients with Perthes' disease. The patient in questionnoticed clicking in his hip on several occasions, and onone occasion when exercising. Orthopaedic surgeonshave always regarded this as a nitrogen arthrogram.Is there much nitrogen in synovial fluid, or is it allcarbon dioxide?

PROF. WRIGHT Our results show that it is virtuallyall carbon dioxide, though we have not estimated theexact nitrogen content.



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DR. J. A. MATHEWS (London) It is gratifying to havethe work of Roston and Wheeler Haines (1947) confirmedand extended, especially as we have been referringenquirers to it for some time. They demonstrated boneseparation and the appearance, with a 'crack', of gas at ametacarpophalangeal joint subjected to traction.

It is also possible to separate lumbar vertebrae byapplying traction. On occasions a lumbar spine can beheard to 'crack', and sometimes routine x rays show gasin the intervertebral space. Superimposed tracings ofx rays taken before and during lumbar traction can showa 2 mm. separation of the bones, but I have never observedthis separation accompanied by production of noise orgas. However, the intervertebral disc joint is not reallyanalogous to the synovial joints that Professor Wrighthas been examining. Could spinal 'cracking' arise fromapophyseal joints?

PROF. WRIGHT We are familiar with the excellentwork of Roston and Wheeler Haines, and I pointed outthat this communication was an extension of their studies.They had not appreciated that the noise was due to thecollapse of the bubbles, and had not realized that theywere observing the phenomenon of cavitation. Inter-vertebral joints do crack, but I think one should appreciatethat not all the noises that come from the body arenecessarily due to cavitation. One can, for example,have ligaments snapping over bony prominences.

DR. P. J. L. HOLT (London) These experiments were alldone by traction in extension. Under what circumstances

does this phenomenon occur in normal everyday use ofthe joints? Presumably it would have to be present duringnormal function of the joint to produce damage whichProfessor Wright put forward as a possibility. I wonderif it has any relationship to the boosted lubrication thathe has shown in the past? Is it possible?

PROF. WRIGHT You only need some tension throughgripping or lifting. This data was presented to ourMedical Sciences Club in Leeds. Many of the audiencehave said that, now they are aware that joints crack, theyhave frequently noticed its occurrence. One does nothave to perform sophisticated manoeuvres to crack one'sjoints.

DR. H. L. F. CURREY (London) Dr. Morris Ziff has casehistories of three Negro patients who had the nervoushabit of repeatedly cracking their finger joints. All hadosteoarthrosis of the finger joints and he regarded this asa cause of secondary osteoarthrosis.

[Subsequent personal communication from DR. M.ZIFF (Dallas, Texas) I have two patients with obsessiveknuckle-cracking habits, one aged 38 years and the othera teenage male. The first had undoubted osteoarthrosis,and the second had enlarged proximal interphalangealjoints but his x rays were only suggestive of sclerosis ofthe subchondral margins of the middle phalanges. Thethird patient was a deaf mute in her late 30s. She used herfingers constantly for sign language and had prominentBouchard's nodes, but did not crack her knuckles.]

ReferencesBritish Medical Journal (1969) 'Any Questions', vol. 4, p. 351.DrrTMAR, O. (1933) Verhandl. dtsch. Orthop. Ges. (27 Congr., Mannheim, 1932) (Zur Rontgenologie des

Kniegelenks).FICK, R. (1911) Anat. Hefte, Abt. 1, 43, 397 (Zum Streit um den Gelenkdruck).NoRDHmm, Y. (1938) Fortschr. Rontgenstr., 57, 479. (Eine neue Methode den Gelenkknorpel besonders dieKniegelenkmenisken rontgenologisch darzustellen)

ROSTON, J. B., AND HAINEs, R. WHEELER (1947) J. Anat., 81, 165 (Cracking in the metacarpophalangeal joint).



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