The anatomy of the mitral and its associated ... - Thorax · Thorax(1964), 19, 221 The anatomy of the mitral valve and its associated structures LOUIS A. DU PLESSIS AND PAUL MARCHAND

Thorax (1964), 19, 221

The anatomy of the mitral valve and itsassociated structures

LOUIS A. DU PLESSIS AND PAUL MARCHAND

From the Cardio-vascular Research Unit, Department of Thoracic Surgery,University of Witwatersrand, Johannesburg

Since the development of mitral valve surgery thepreviously conventional descriptions of the valvehave been reappraised, and many publicationshave appeared dealing with its detailed anatomy(Davila and Palmer, 1962; Bailey, Zimmerman,and Likoff, 1960; van der Spuy, 1958; Morris,1960; Frater and Ellis, 1961 ; Rusted, Scheifley,and Edwards, 1952; Harken, Ellis, Ware, andNorman, 1948; Brock, 1952; and Gould, 1953).Open-heart operations for correction of mitralincompetence and for treatment of the grosslydiseased stenosed valve are becoming common-place, and the results of valve replacement arenow promising. This advance is in part due tothe surgeon's improved anatomical knowledge, butsome details of the structural relations of the valveare not widely known, and there is still a need forstandardization of terminology. An effort is madein this paper to systematize the description of thiscomplex valve and to orientate it in relation to thesurrounding structures that must be avoided atoperation. We also present data of the dimensionsof the valve, which may be of interest to ana-tomists and surgeons. Familiarity with the normalmeasurements of the component parts of the valvewill, at operation, help the surgeon to assess theexact mechanical reason for valve insufficiency.

MATERIALS AND METHODS

The following descriptions are based upon ob-servations made during intra-cardiac operationsand dissections of fresh human hearts which werelater preserved in diglycerol stearate by themethod described by Kramer (1938). Thesespecimens, if correctly prepared, retain their truefeatures and relationships and can be convenientlystudied or used for teaching purposes.The dimensions of the valve were taken from

10 normal hearts. The mitral leaflets with theannulus, chordae, and papillary muscles wereremoved, and the valve was flattened out in a

single plane by dividing its ring at the lateralcommissure and by half splitting the medial papil-lary muscle mass. The opened-out valves werethen pinned to cork boards during fixation informalin and were finally waxed with diglycerolstearate.Measurements were made with point dividers

and an ordinary metric ruler. To determine thesurface areas, the leaflets were traced on graphpaper and the enclosed squares were counted.

ANATOMY

Various names have been given to the leaflet ofthe mitral valve that is related to the aorta. Ithas been called the aortic, the septal, the ventral,the antero-medial or the anterior leaflet. Multiplenames have also been given to the commissuresand papillary muscles. The terminology we sug-gest, whilst neither original nor absolutely precise,is simple and descriptive. Figure 1 shows the fourvalves orientated to the saggital plane of the body.The mitral leaflet nearest the aortic valve is theanterior leaflet, and the one opposite is theposterior leaflet. The commissure pointing towardsthe mid-line is the medial, and the one opposite isthe lateral. The papillary muscles, being relatedto the commissures, are also referred to as medialand lateral.The fibrous skeleton of the heart provides the

key to understanding the anatomical relationshipsof the mitral valve. Its keystone is the aortic root,which is the extension of the aorta below theaortic valves, and is the thickest most rigid partof the skeleton. Fibrous extensions from the aorticroot form the scaffold for the pulmonary,tricuspid, and mitral rings. Figure 2 is a diagramof the fibrous skeleton based on Fig. 3, whichshows the dissected aortic root and fibrous skele-ton. At the base of the non-coronary cusp, theaortic root is continuous with the interventricularseptum, and its downward extension forms the

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Louis A. du Plessis and Paul Marchand

Anterior the tendon of the conus. The rest of the pul-Pulmonary valve monary valve ring is supported by the base of the

Aortic valve right ventricle and the interventricular septum.The tricuspid and pulmonary valves occupy twoopenings within the right ventricle separated bya band of myocardium, the crista supraventri-cularis. On the other hand, the mitral and aorticvalves share a common opening in the base of theleft ventricle (Fig. 4). Although the aortic andmitral valves occupy the same opening in the myo-

Mitral valve Tricuspid valve cardium, they lie at an angle of 400 to each otherMitralvalve so that the anterior leaflet of the mitral valve

forms a watershed between the inflow and outflowstreams of the left ventricle (Fig. 5).

Considerable controversy centres around theexact position and even the existence of that partof the mitral annulus related to the anterior leaflet.

Pulmonary root| ~~~~~~~~~~~~~~~~~Aorticroot g

Posterior l. fibrous trigone

FIG. 1. The four valves of the heart orientated to thesaggital plane of the body. The mitral valve nearest theaortic valve is the anterior leaflet, and the one opposite is / / Undefended spacethe posterior lea-flet. The commnissure pointing towards the intervaovular spacmid-line is the medial, and the one opposite is the lateral. X- Membranousgf ~~~~~~~eptummembranous septum. Immediately above the junc-tion of the ventricular septum and the tricuspidring the aortic root is extremely thin, and this is R fibrous trigonenamed the undefended area. Occasionally this Mitral ringarea is deficient, and blood will then shunt from Tendon of To aorthe left ventricle directly into the right atrium.Posteriorly and to the left, the aortic root expands TriCUSDid rinqinto a broad membranous structure, the inter- FIG. 2. Diagram of the fibrous skeletont of the heart.valvular space, to which the anterior third of themitral annulus attaches. The intervalvular spacetherefore lies between the bases of the left andnon-coronary aortic cusps and the mitral annulus.Posteriorly and to the right, the aortic root ) *thickens to form the right fibrous trigone whichseparates the mitral and tricuspid rings and formsthe base of the inter-atrial septum. A tendinousextension from the right fibrous trigone, the ten-don of Todaro (Grant, 1958), extends between thelayers of the inter-atrial septum. On the left sidethe aortic root again thickens to form the leftfibrous trigone. This trigone is closely related tothe left coronary artery and forms part of the baseof the transverse sinus of the heart.The pulmonary valve ring is anterior and to the

left of the aortic valve ring. It is set in a planealmost at right angles to that of the aortic valve FIG. 3. Dissected aortic root andfibrous skeleton of theand is attached to the base of the aorta by a heart. The valve leaflets andpart of the left ventricle haveflimsy fibrous extension of the aortic root called been retained.

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The anatomy of the mitral valve and its associated structures

Combinedaortic and mitral

opening

Pulmonaryvalve

FIG. 4. Diagram to show that the tricuspid andpulmonatyvalves occupy separate openings within the right ventriclewhereas the mitral and aortic valves share a commonopening in the base of the left ventricle.

FIG. 5. Diagram illustrating that the mitral and aorticvalves, although occupying the same opening in the leftventricle, are set at an angle of 400 to each other. Theanterior mitral leaflet thus forms a watershed between theleft ventricular inflow and outflow streams.

Frater and Ellis (1961) state that the anteriorleaflet of the mitral valve is attached to the baseof the non-coronary and most of the base of theleft coronary cusp of the aortic valve. Van derSpuy (1958) states that the anterior mitral leaflet'is a hinged extension into the cavity of the leftventricle of the postero-lateral aortic root and

that the hinge is formed by the common fibrousorigins of the postero-lateral half of the aorticroot, of the antero-medial mitral leaflet and themajor part of two aortic cusps'. Bailey et al. (1960)maintain that this part of the mitral ring is defi-cient. They describe the annulus as a horseshoe-shaped fibrous thickening of the upper edge of theleft ventricular myocardium, which extends fromthe right fibrous trigone around to the left fibroustrigone and is deficient antero-medially. Thesethree opinions are really different ways of sayingthe same thing namely, that the anterior leafletshares a common origin with parts of two aorticcusps. We do not agree with this view.

It is convenient to describe the mitral ring bydividing it into an anterior third, to which the baseof the anterior leaflet is attached and a posteriortwo-thirds, from which the posterior leaflet arises.The posterior two-thirds of the ring is a fibrousthickening at the base of the left ventricle extend-ing from the right to the left fibrous trigone. Theanterior third of the mitral ring is closely relatedto the aortic root. It is a well-defined fibrous cordextending from the one trigone to the other andinto which the base of the left atrium also inserts(Figs. 6, 7, and 8). The part of the aortic rootenclosed by the mitral annulus and the bases ofthe left and non-coronary leaflets of the aorticvalves is the intervalvular space. We do not believethat this space can be described as part of themitral valve because it lies above the hinge ofthe anterior leaflet (point y of Fig. 8). Accordingto van der Spuy's contention, point x in Fig. 8,i.e., at the bases of the aortic leaflets, is the hingeof the anterior leaflet. However, the left atrialmuscle attaches at point y, and during atrial con-traction this point is drawn inwards whilst theleaflet flaps open, so that the intervalvular spacedoes not move with the mitral curtain. The leftatrial attachment at must anchor the anteriorleaflet, which must therefore hinge here. We agreewith Brook (1952) that this is the mitral annulus.It is a substantial structure which provides ade-quate bulk to hold the sutures of a prostheticring.The mitral valve leaflets form a continuous veil

attached as a muff to the circumference of themitral ring. The free edge of this veil hangs intothe left ventricle and is split by indentations, noneof which reaches to the mitral ring. The specimenshown in Fig. 9 is representative of the averagevalve. It shows the accentuated anterior part ofthe valve curtain which is attached to the anteriorthird of the mitral annulus. The anterior leafletis roughly triangular in shape. The base of the

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FIG. 6. Transilluminated opened-out mitral and aorticvalves seen from the ventricular aspect. Note (a) the twoaortic cusps are separatedfrom the anterior mitral leaflet(A.L.) by the intervalvular space (. V.S.); (b) the anteriorpart of the mitral annulus extends between the two fibroustrigones (L.F. T. and R.F. T.); (c) the central portion of theanterior mitral leaflet is free of chordal attachments.Chordae tendineae are attached to the whole ventricularsurface of the posterior leaflet; (d) the angleformed by theplanes of the aortic and mitral valves is approximately40°; (e) the anterior part of the mitral annulus is half thelength of its posterior portion; and (f) the depth of theanterior lea-flet is twice that of the posterior leaflet.

FIG. 8. Diagram illustrating that the point of hinging ofthe anterior mitral leaflet (y) corresponds to the attach-ment of the base of the left atrium to the posterior aorticroot. This is the anterior part of the mitral annulus.

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FIG. 7. Splayed open mitral and aortic valves seen fromthe atrial surface ofthe mitral valve. Note that the anteriorpart of the mitral ring (A.M.R.) is a definite structure towhich part of the base of the left atrium is attached.

triangle is attached to the annulus, and the apexis the free edge which is devoid of chordal attach-ments. The two sides of the triangle give attach-ment to chordae of the first order, and these areresponsible for the scalloped appearance of theleaflet edge. The atrial surface of the anterior

FIG. 9. Splayed open mitral valve. Note (a) the triangularshape ofthe anterior leaflet and the rectangular shape oftheposterior leaflet; (b) the mitral valve curtain is shortest atthe fibrous trigones.

leaflet is smooth and flat but the ventricular sur-face is ridged by the insertion of chordae of thesecond order which, however, spare the centralportion of the valve leaflet. The posterior leafletis roughly rectangular in shape, and its free edgeis usually more deeply scalloped than that of the

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anterior leaflet. At the centre of its free edgethere is also a small area devoid of chordal attach-ments. This site marks the separation between thechordae rising from the two papillary muscles.Unlike the anterior leaflet, the ventricular surfaceof the posterior leaflet is covered with chordalattachments. Apart from chordae of the first andsecond order, chordae of the third order areattached to this surface (Brock, 1952).The valve curtain is shortest near the two points

of attachment of the mitral annulus to the leftand right fibrous trigones. This is the junctionalarea between the anterior and posterior leaflets. Insome specimens (Fig. 10) the scallops in the freeedge of the ventricular curtain may be exagger-ated and may nearly reach the annulus to givethe appearance of separate leaflets, called acces-sory leaflets by some authors (Chiechi, Lees, andThompson, 1956). In rare cases the scallops inthe junctional area are so deep that they reach theannulus, and mitral regurgitation results.

; ... .'.

FIG. 10. Ventricular aspect of the mitral valve. Note thatexaggerated scalloping in the mitral valve curtain (A andB)can give the appearance of a separate commissural leaflet,sometimes called accessory leaflets.

OTHER STRUCTURES RELATED TO THE MITRAL VALVEANNULUS Figure 11 shows four important struc-tures related to the annulus which have to beavoided during replacement of the prostheticvalve. These are:

(1) The intervalvular space (Figs. 6 and 7). Thisspace affords a certain margin of safety wheninserting a prosthetic valve, but, even so, carelesslyplaced sutures may transfix the bases of the leftand non-coronary aortic cusps and cause aorticinsufficiency.

FIG. 11. Diagram illustrating the structures related to themitral valve annulus which must be avoided during insertionof the prosthetic valve.

(2) Coronary vessels. Where the lateral com-missure abuts against the left fibrous trigone, thegreat cardiac vein and the circumflex branch ofthe left coronary artery are endangered by deepsutures.

(3) Bundle of His. Where the medial commissuremeets the right fibrous trigone the atrio-ventricularbundle may be injured.

(4) The coronary sinus. The opening of thecoronary sinus into the right atrium is slightlybehind the site of the atrio-ventricular bundle.Along the posterior two-thirds of the mitralannulus the coronary sinus and left coronary veinmay be incorporated in careless suturing.The annulus provides sufficient strong fibrous

tissue in which to place sutures for anchoring aprosthetic valve ring. Good exposure and carefulsuturing will avoid damage to related structures.

DIMENSIONAL DATA

Table I lists the dimensions of the annulus andof the leaflet depth of 10 normal valves. It willbe seen that the length of the annular attachmentof the anterior leaflet is half that of the posteriorleaflet. The maximum depth of the anterior leaflet,measured from its annular attachment to its apex,is approximately twice that of the posterior leaflet.In rheumatic fever the inflammatory process andthe subsequent fibrosis is very often concentratedat the free edges of the valve leaflets, particularlyat the points of insertion of the chordae tendineae.As the depth of the posterior leaflet is half thatof the anterior leaflet, equivalent contraction of

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TABLE I

Length of Annular Maximal Length ofSpecimen Attachment ofLeaflet(cm.) Leaflet (cm.)

No.Anterior Posterior Anterior Posterior

1 3 9 7-2 2-8 1-82 30 6-4 2-2 1.03 39 6-7 2-6 1-44 3-2 6-4 3-2 1-25 2-9 6-9 2 5 1-36 3-1 5 4 2-4 1-27 3-4 5 1 2 5 1-38 4 1 8-7 2 9 1-49 4.5 7.9 30 1.510 3-2 6-0 2-6 1-3

Averages 3 5 6-7 2-7 13

the scar tissue in both leaflets will adversely affectthe function of the posterior leaflet before that ofthe anterior leaflet. This explains why shorteningof the posterior leaflet is the most common causeof rheumatic mitral regurgitation.The mitral valve ring is on average 1 1 cm.

longer than the free edge of the valve curtain(Table II). This is the reason why the mitral valveis conical in shape with a larger inlet than outlet.As the anterior and posterior leaflets are notequally deep, the plane of the outlet orifice doesnot correspond with that of the annulus. To someextent this compensates for the smaller size of theoutlet orifice (Fig. 12). Although the anterior andposterior leaflets differ in shape, annular attach-ments, and depth, their individual surface areasare approximately equal (Table II). The mitralannulus is not a perfect circle and its surface areacan only be estimated approximately. The averagelength of the mitral ring in the 10 specimensexamined was 10-2 cm., and from this the averagesurface area is calculated to be 8-1 cm.2 The aver-age surface area of the anterior leaflets was4*9 cm.2 and of the posterior leaflets 5 cm.2 (TableII). The average surface area of both leafletstherefore is 10 cm.2, which is greater than the

TABLE II

Total Length of Surface Area ofSpecimen Length of Free Edge of Leaflet (cm.)

No. Valve Ring Valve(cm.) Curtain(cm.) Anterior Posterior

111 10 5 5-2 5-32 9 4 8-5 4-0 4-53 10-6 9 5 5-5 4-84 9-6 8-5 4-5 4-85 9-8 8-4 4-3 4-66 8-5 8-0 4-4 4-57 8-5 7 9 4-5 4-28 12-8 11-7 5*9 6-59 12 4 9 3 5-6 5-810 9-2 8-3 5 0 5-0

Averages 10-2 9 1 4 9 5 0

surface area of the mitral annulus. In the cada-veric heart, the surface area of the valve orifice istherefore four-fifths that of the combined leafletarea. In addition, in life the mitral valve orificenarrows during ventricular systole so furtherreducing its surface area and leaving relativelyeven more valve leaflet surface available to closethe atrio-ventricular orifice. The normal mitralvalve thus closes not only by marginal contact butby liberal leaflet surface apposition. This is animportant concept in the recognition of themechanisms causing mitral regurgitation whereleaflet shortening, leaflet rigidity, or dilatation ofthe mitral annulus are present.

' ,1~~~~~~~~~~~'.....l

FIG. 12. Diagram showing the conical shape of the mitralvalve. Because the anterior part of the valve curtain isdeeper than the posterior, the outlet orifice lies at anoblique plane to the inlet (see text).

SUMMARY

Standardization is sought for the nomenclature ofthe mitral valve leaflets, commissures, and papil-lary muscles. We offer support for the use of theterms 'anterior' and 'posterior' for the leaflets and'lateral' and 'medial' for the commissures. As thepapillary muscles are related to the commissuresthey should also be referred to as 'lateral' andmedial'.The fibrous skeleton of the heart is described

with particular reference to its relation to theanterior part of the mitral valve annulus.The shape, chordal attachments, and inlet and

outlet orifices of the mitral valve curtain aredescribed. The mitral valve is a continuous veilof tissue, but indentations at the fibrous trigonesdivide it into a triangular anterior and rectangularposterior leaflet. The central part of the ventri-cular surface of the anterior leaflet is free fromchordal attachments, whereas the whole of the

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corresponding surface of the posterior leaflet givesattachment to chordae tendineae.The mitral valve inlet is larger than its outlet,

but an oblique setting of the plane of the outletorifice to some extent compensates for thediscrepancy in size.The proximity of the bundle of His, the

coronary sinus, the aortic cusps, and othercoronary vessels to various parts of the mitralannulus is described. These structures may bedamaged by careless suturing during total valvereplacement.

Certain dimensional data concerning the mitralvalve are presented. The annular attachment ofthe anterior leaflet is half that of the posteriorleaflet; the anterior leaflet is twice as deep asthe posterior one. Although the leaflets differ inshape, their surface areas are similar. The surfacearea of both leaflets is greater than that of themitral valve ring. This has an important bearingon the closure of the mitral valve.

The Cardio-vascular Research Unit, University ofWitwatersrand is supported by grants made to it bythe C.S.I.R. of South Africa, The Wellcome Founda-tion, and the Johannesburg City Council. We wishto thank these organizations for their support.

Some of the diagrams were drawn by the staff ofthe National Institutes of Health, Bethesda 14, Mary-land, U.S.A. We wish to thank Dr. Andrew G.Morrow for granting us these facilities.

REFERENCES

Bailey, C. P., Zimmerman, J., and Likoff, W. (1960). The completerelief of mitral stenosis: Ten years of progress toward this goal.Dis. Chest, 37, 543.

Brock, R. C. (1952). Surgical and pathological anatomy of the mitralvalve. Brit. Heart J., 14, 489.

Chiechi, M. A., Lees, W. M., and Thompson, R. (1956). Functionalanatomy of the normal mitral valve. J. thorac. Surg., 32, 378.

Davila, J. C., and Palmer, T. E. (1962). The mitral valve; anatcmyand pathology for the surgeon. Arch. Surg., 84, 174.

Frater, R. W. M., and Ellis, F. H. (1961). The anatomy of the caninemitral valve, with notes on function and comparisons with othermammalian mitral valves. J. surg. Res., 1, 171.

Gould, S. E. (1953). Pathology of the Heart, 1st ed. Thomas, Spring-field, Ill.

Grant, J. C. Boileau (1958). A Method oJ Anatomy, 6th ed., p. 565Williams and Wilkins, Baltimore.

Harken, D. E., Ellis, L. B., Ware, P. F., and Norman, L. R. (1948).The surgical treatment of mitral stenosis. 1. Valvuloplasty. NewEngl. J. Med., 239, 801.

Kramer, F. M. (1938). Dry preservation of museum specimens: areview, with introduction of simplified technique. J. techn. Meth.,18, 42.

Morris, E. W. T. (1960). Some features of the mitral valve. Thorax,15, 70.

Rusted, I. E., Scheifley, C. H., and Edwards, J. E. (1952). Studies ofthe mitral valve. I. Anatomic features of the normal mitral valveand associated structures. Circulation, 6, 825.

Van der Spuy, J. C. (1958). The functional and clinical anatomy of themitral valve. Brit. Heart J., 20, 471.

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The anatomy of the mitral and its associated ... - Thorax · Thorax(1964), 19, 221 The anatomy of the mitral valve and its associated structures LOUIS A. DU PLESSIS AND PAUL MARCHAND