Annals of the Rheumatic Diseases, 1980, 39, 354-358

Anatomy and function of the communicationbetween knee joint and popliteal bursaeWOLFGANG RAUSCHNING

From the Department ofOrthopaedic Surgery, University Hospital, S-75014 Uppsala, Sweden

SUMMARY The anatomy and function of the opening between the knee joint cavity and the gastro-cnemio-semimembranosus bursa was studied in 120 necropsy specimens of knee joints both byconventional knife dissection and by a newly modified technique of serial cryosectioning of unde-calcified joints frozen at various angles of flexion. The communication invariably took the shape of atransverse slit separating the capsule from the undersurface of the gastrocnemius tendon. Onflexion it opened as the pull from the semimembranosus tendon and the medial meniscus widenedthe gap. On extension the communication was closed by compression by the overlying tendons. Afunctional closing action was invariably demonstrated, whereas no unidirectional valve mechanismwas found.

In the 1840s Adams observed cystic swelling in thepopliteal region of knees affected by 'chronicrheumatoid arthritis' so frequently that he consideredit to be a reliable symptom of rheumatoid affectionof the joint. He observed that the swelling was due toan enlargement of the bursa beneath the medial headof the gastrocnemius and that it communicated withthe joint cavity by 'a species of valvular opening'." 2Poirier3 also ascribed the popliteal cysts to fluiddistension of the compound gastrocnemio-semimembranosus bursa, which he consequentlycalled 'la bourse des kystes poplites'.A different mode of popliteal cyst formation was

proposed by Gruber,4 who considered that theenlarged bursae as well as the popliteal cysts resultedfrom protrusion of the synovial membrane throughthe fibrous capsule. His concept was adopted inBillroth's textbook,5 which was quoted by Baker.6Although Baker was uncertain about the origin ofthe cysts whose clinical course he described in 2classical papers,6'7 his name has since been epony-mously used for popliteal cysts, especially thoseoccurring in rheumatoid arthritis, commonlyimplying a hernial origin.

In 1856 Foucher8 observed that by manualcompression the fluid from the cyst could be forcedinto the joint when the knee was flexed, whereas thiswas never possible when the knee was extended.Although a true unidirectional valve mechanism isAccepted for publication 22 August 1979Correspondence to Dr Rauschning.

operative in some patients, with constantly distendedpopliteal cysts impeding the flow of fluid, even whenthe knee joint is in flexion (negative sign of Foucher),communicating bursae can usually be emptied intothe joint. This has frequently been demonstrated atarthrography,9 with simultaneous recording of fluidpressures developing in the joint cavity and thebursa in various knee positions and during the use ofthe knee joint.'0"11

Considering the high frequency of the communica-tion and its importance in the formation of poplitealcysts, very little has been reported on its morphologyand function in the main textbooks of anatomy.

Materials and methods

At necropsy 120 fresh knee joint specimens wereselected from subjects for whom a study of themedical records and inspection revealed no historyof joint disease or knee surgery. Sixteen knees werefrom subjects younger than 50 years of age and theremainder from older subjects.

KNIFE DISSECTIONConventional knife dissection was carried out on108 knees. With the body lying prone the postero-medial aspect of the knee capsule was exposed byresection of the gastrocnemius, semimembranosus,and semitendinosus tendons. The frequency of acommunicating opening, its shape and size, and itstopographic location were studied, as well as the

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Anatomy andfunction of the communication between knee joint and popliteal bursae 355

deformations of the orifice occurring on simulatedpull of the tendons and on extension and rotation ofthe joint. The whole capsule overlying the medialfemoral condyle was cut out, inspected from within,and searched for weak portions by fibre-lighttransillumination.

SERIAL CRYOSECTIONING OFUNDECALCIFIED SPECIMENSTwelve knees with communicating popliteal bursaewere filled with 60-80 ml of water stained withmethylene blue and frozen in situ at various angles offlexion or in extension by repeated application ofcarbon dioxide snow. The specimen was then cut outand embedded and frozen on a large microtome stagecontaining carboxymethyl-cellulose. Specimens werecut in the horizontal or sagittal planes on a heavy-duty cryomicrotome (LKB 2250 PMW, Bromma,Sweden). Sections 20-50 Vm thick were cut from theblock, and at intervals of 1 mm the surface of thespecimen contained in the block was thawed and

Fig. 1 Intra-articular view of the posteromedialjoint capsule. Femoral condyle (fc) above. A probe isinserted into the broad capsular gap.

photographed. This modification of the technique oflarge-specimen cryosectioning12 prevented deforma-tions which might have resulted from conventionalcollection of the sections on tape or glass plates andthe subsequent histological procedures of fixationand staining.

Results

KNIFE DISSECTIONSIn 58 of the 108 knees the gastrocnemio-semimem-branosus bursa communicated with the knee jointcavity. The capsular opening invariably had theshape of a transverse slit opposite the upper lateralcircumference of the medial femoral condyle and2 0-5 * 5 cm distal to the origin of the gastrocnemiustendon (Fig. 1). The lower rim of the slit was sharp,whereas no definite upper rim was found on thebroad arch of the gastrocnemius tendon lyingbehind and above the slit, which in the mediolateraldirection measured 4-24mm (mean 18 mm).

In 50 specimens no communication was found. Attransillumination a constant semicircular area ofcapsular thinning was seen distal to the straight lineat which the gastrocnemius tendon merged with thecapsule. This weak portion measured 1-2 cm2. Insome cases the junction between the capsule and thegastrocnemius tendon was extremely thin and

Fig. 2 Extra-articular view of the communication site.The tendons of the gastrocnemius (g) andsemimembranosus (sm) muscles are divided andretracted. Note the thin upper rim of the capsule andthe underlying femoral condyle (fc).

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356 s Wolfgang Rauschning

ruptured during the dissection. No other weak areaswere observed in the capsule. When a slit-shapedcommunication was found, the capsule was alwaysvery thin and transparent adjacent to its upperrim (Fig. 2), which had the appearance of a heartvalve.

Rotation of the tibia caused diagonal deformationsof the slit, especially on slight flexion of the knee, andpulling on the semimembranosus and gastrocnemius

tendons markedly widened the gap in the cranio-caudal direction, especially with the greater degreesof knee flexion. The semimembranosus, by theaction of its capsular arm (i.e., the oblique poplitealligament), pulled the capsule downwards.

CRYOSECTIONSWith increasing degrees of knee flexion thecommunication became progressively wider as the

Fig. 3 Two sagittalcryosections. A: through thelateral portion of the capsularopening; B: at its medialedge. The large cyst displacesthe tendons. The asteriskindicates the canal into thejoint cavity.

Fig. 4 Horizontal cryosectionat the level of the thin uppercapsular rim (small arrows).Note the thickness of thegastrocnemuis tendon (g).The large arrow indicates whyon lateral arthrograms thecanal may be misinterpretedas a narrow neck.

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Anatomy andfunction of the communication between knee joint and popliteal bursae 357

Fig. 5 Detail from a sagittal plane cryosection of aknee joint specimen which was frozen at extension.The thin upper rim of the capsule (arrow) is firmlycompressed between the femoral condyle and theoverlying tendons of the gastrocnemius (g),semimembranosui. (sm), and the semitendinosus (st)muscles.

gastrocnemius tendon retracted dorsally away fromthe subjacent femoral condyle (Fig. 3). In horizontalsections the portion of the bursa situated betweenthe gastrocnemius tendon and the capsule was

filled with fluid (Fig. 4).On extension of the knee the broad tendinous arch

of the gastrocnemius muscle was first firmly stretchedover the upper convexity of the femoral condyle, andon further extension the upper rim of the capsule wascompressed between the tendon and the condyle.When the knee was extended yet further, the fluidfrom the posterior recess of the knee as well as fromthe subgastrocnemial portion of the bursa was

progressively emptied, and on final extension theheavy tendons of the semimembranosus and semi-tendinosus muscles, owing to their divergentdirections of pull, were firmly twisted around themedial and posterior aspects of the gastrocnemiustendon. This compression between the tendonsappeared to afford a further degree of closure andobviously increased the compression of the capsularrim (Fig. 5). In horizonal sections the posterior

portion of the bursa bulged maximally beneath thepopliteal fascia on full extension.

Discussion

Communication between the joint cavity and thegastrocnemio-semimembranosus bursa was foundin over 50% of cases, a figure consistent with thefindings of previous investigators.13 None of theother posteromedial bursae communicated with thejoint. The opening displayed constant anatomicalfeatures with respect to location, shape, and width.Lindgren and Willen14 found that the communica-tion was rare in younger subjects owing to theinterposition of a broad layer of elastic connectivetissue between the joint and the bursa. During agingprogressive rarefaction and thinning of this barrieroccurred. The results of this study favour the viewthat the degenerative changes at the junctionbetween the capsule and the gastrocnemius tendonare due to the capsular tearing forces emanating fromthe pull of the meniscus and the semimembranosustendon. The progressive thinning and the finalcommunication would consequently represent aphysiological, acquired condition.The alternating opening and closing of the

communication provides a morphological explana-tion for Foucher's sign and also explains why thevast majority of communicating popliteal bursaewhich are accidentally detected at arthrography9do not develop into clinically significant andpalpable popliteal cysts.By means of simultaneous recording of the intra-

articular and intrabursal fluid pressures occurring inknees with chronic or simulated effusions duringjoint use, Jayson and Dixon'0 convincingly demon-strated that a considerable pressure rise occurs in thejoint as well as in the bursa during knee bending andsquatting and that no pressure rise is transmittedbetween these cavities when the knee is extended.The present study provides a morphological explana-tion for these pressure changes. During kneebending with weight bearing, for example during thestance phase of gait, the fluid pressures are extremelyhigh in the joint as well as in the communicatingbursa'5 During extension the communication issuddenly blocked, entrapping the fluid at this highpressure. Further extension of the knee appears tocause even higher intrabursal pressures as the deepportions of the bursa are firmly compressed by thecross-action of the overlying tendons (Fig. 5).

This mechanism would make it understandablewhy patients have most discomfort from theirpopliteal cysts during extension of the knee and whythey usually experience relief when the knee isflexed, as, for example during the swing phase of gait.

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358 Wolfgang Rauschning

In this phase the opening of the communicationallows the fluid pressures to equilibrate by reflux offluid to thejoint. This closing mechanism is consistentwith the significantly higher fluid pressures in thecyst and the fact that the volume of joint effusion issignificantly smaller in knee joints with a communi-cating cyst.10 As the entrapment of fluid in the cyst isnot due to an anatomical structure causing a true,unidirectional obstruction of flow in either directioneven during knee flexion, it should be referred to as a'functional valve mechanism'.The valve-like aspect of the communication

(Figs. 1 and 2) has caused some authors to assumethat a true valvular mechanism is constantlyoperative at this location.2"6 But if the free rim of thecapsule was forced against the gastrocnemius itwould rather impede flow from the joint to thebursa.Anothercommon morphological misinterpretation

is the existence of a curved narrow neck, pedicle, orstalk between the joint cavity and the bursa. Thisidea has arisen from the shape of the communicationon lateral arthrograms (it is not visible in the frontalview, as it is concealed by the femoral condyle andthe contrast medium in the joint and the bursa). Asseen in Figs. 3A and 4 the canal (i.e., the sub-gastrocnemial portion of the bursa) is a cavity,2-3 cm broad the walls of which are fairly rigid andstraight and cannot be expected to collapse to form avalve of the Bunsen type. The alternating closing andopening of the communication would explain why,during pressure measurements, Lindgren" did notfind a progressive increase in pressure in the bursaeas would be expected if a true valve mechanism waspresent. The observation of Pinder17 in the course of14 anterior synovectomies that all popliteal cystscould be emptied into the joint cavity even thoughthey contained fibrin masses is in accordance withown observations, and it indicates a fairly widecommunication. Nor does it suggest a true valvularmechanism of the ball type in all rheumatoid cases.

References

Adams R. Abnormal conditions of the knee joint.Anat Physiol (Lond) 1839, 1847; 3: 48-78.

2 Adams R. Chronic rheumatic arthritis of the knee joint.DublinJ Medi Sci 1840; 17: 520-522.

3 Poirier P. Bourses sereuses du genou (r6gion post6rieure).Bourses sereuses de la r6gion poplitee. Arch Gen Med1886; 38: 539-575.

4 Gruber W. Ueber die Ausstuelpungen der Synovialkapseldes Kniegelenkes und ueber die chirurgische Wichtigkeitder Communication derselben mit einigen benachbartenSchleimbeuteln. Vierteljahres prakt Heilk 1845; 5:95-105.Billroth T. General Surgical Pathology and Therapeutics,trans. C E Hackley. London: Lewis, 1875.

6 BakerW M. On the formation of synovial cysts in the legin connection with disease of the knee joint. St Bart HospRep 1877; 13: 245-261.

7 BakerW M. The formation ofabnormal synovial cysts inconnection with the joints (second communication). StBart Hosp Rep 1885; 21: 177-190.

8 Foucher E. M6moire sur les kystes de la region poplit6e.Arch Gen Mid 1856; 8: 313-335.

' Doppman J L. Baker's cyst and the normal gastrocnemio-semimembranosus bursa. AJR 1965; 94: 646-652.

10 Jayson M I V, Dixon A ST J. Valvular mechanisms injuxta-articular cysts. Ann Rheum Dis 1970; 29: 415-420.

"I Lindgren P -G. Gastrocnemio-semimembranosus bursaand its relation to the knee joint. III Pressure measure-ments in joint and bursa. Acta Radiol (Diagn) (Stockh)1978; 19: 377-388.

12 Ullberg S. The technique ofwhole body autoradiography.Cryosectioning of large specimens. Sci Tools (specialissue) 1977; 2: 2-29.

13 Wilson P D, Eyre-Brook A L, Francis J D. A clinical andanatomical study of the semimembranosus bursa inrelation to popliteal cyst. J Bone Joint Surg 1938; 20:963-984.

14 Lindgren P -G, Will6n R. Gastrocnemio-semimem-branosus bursa and its relation to the knee joint. I.Anatomy and histology. Acta Radiol (Diagn) (Stockh)1977; 18: 497-512.

15 Dixon, A St J, Grant C. Acute synovial rupture inrheumatoid arthritis. Clinical and experimental observa-tions. Lancet 1964; 1: 742-745.

16 Taylor A R, Rana N A. A valve. An explanation of theformation of popliteal cysts. Ann Rheum Dis 1973; 32:419-421.

17 Pinder I M. Treatment of the popliteal cyst in therheumatoid knee. J Bone Joint Surg 1973; 55B: 119-125.

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