Morphological variations and morphometry of the papillary muscles of the mitral valve – a cadaveric study

Recent advances and refinements in mitral valve surgery as well as interventional procedures have revived the interest in disparity in the morphology and morphometry of papillary muscles of the mitral valve. Study was taken up with an effort to extend the concepts previously presented on papillary muscles of mitral valve. Papillary muscles were studied in 320 fresh autopsied adult normal hearts. With varying number of muscle bellies. Anterolateral (ALPM) and posteromedial (PMPM) groups of papillary muscles were found. In two hearts, the ALPM and PMPM were connected by muscle tissue. According to the number of separate muscle bellies, 4 groups were categorized with increasing complexity. Single belly was the commonest form in the ALPM (73 %) and two bellies were the commonest form in the PMPM (40 %). The papillary muscles show a great variability on the nature of attachment to the ventricular wall. More than half of the hearts studied had a mixed type of papillary muscles (partly tethered and partly protruding in ALPM and PMPM groups. The mean distance separating the mitral annulus from the apex of the ALPM was 19.38 mm ± 3 SD (range 5 – 26) and from the apex of the PMPM was 20.36 mm ± 3.7 SD (range 10 – 32). The average length of ALPM was 32.24 mm ± 4.8 SD (range 20 - 45) and that of the PMPM was 29.92 mm ± 4.9 SD (range 16 - 44). The documented classical view of two papillary muscles should be adapted with 4 types of variations in morphology with increasing complexity.

This arrangement ensures a chordal attachment to both leaflets from both muscles. [5] Recent works show a spectrum of normality in the morphology of papillary muscles of mitral valve. [3 -10] Berdajs et al pointed out that mitral valve homograft is not widely used partly due to lack of information about the three-dimensional geometry of the mitral apparatus. This make it clear that thorough understanding of the structure becomes a prerequisite for a successful surgery. Therefore, a study was undertaken to establish a nominal data base of the normal variations in the papillary muscle. [6] Morphological variations and morphometry of the papillary muscles of the mitral valve -a cadaveric study

Morphological findings
We could distinguish two groups of papillary muscles: anterolateral (ALPM) and posteromedial (PMPM) in all specimens (Fig. 1). The 2 groups had varied number of muscle bellies (Fig 1 -5). The muscle bellies within each group B-Heart having equally sessile and equally protruding single bellied papillary muscles A-Heart having finger like single bellied papillary muscles were connected by fibrous or muscle tissue (Fig.   3). In two hearts, the ALPM and PMPM were connected by muscle tissue (Fig 2). Authors try to classify the papillary muscles considering the number of bellies in a group and relationship between papillary muscle-chordaeleaflet unit. Accordingly, papillary muscles were categorized into four groups. This categorization is a modified form of what described by Ramsheyi et al. [4] and Berdajs et al. [6] Group 1 : Single bellied muscle (Fig. 1 , 2) gave rice to chordae tendinae which fanned out to the corresponding hemi valve leaflet. Shape of the papillary muscles varied; conical, broad flat topped, sloping apices etc ( Fig. 1 -5). In some single bellied muscles the apex of the muscle divided into nipple like projections (Mamillated) (Fig. 3A & 4A). Commonly, the chordae to the commissural area arose from the highest point/ summit of the papillary muscle.
Group 2 : Papillary muscles with two separate bellies (Fig. 3B). The 2 bellies were either present in sagittal plane or coronal plane Group 3 : Papillary muscle with three separate muscle bellies (Fig. 4B). In this type one belly gave rise to the chordae of the posterior leaflet, another belly gave rise chordae of the anterior leaflet and the third belly to the commissural region.  Single belly is the most common form found in the ALPM group (73 %) followed by two bellies (22 %), three bellies (4 %) and complex form (1 %). Two bellies; the most common form found in the PMPM group (40 %) followed by single belly (33 %), three bellies (19 %) and complex form (8 %).
The length of attachment of papillary muscle to ventricular wall varied. In ALPM 69 % of muscles were partly attached to the ventricular wall and partly protruded into ventricular cavity (mixed A & B -Note the shape of ALPM type) and 26 % were freely protruding finger like papillary muscle and 5 % were completely tethered. In the PMPM group, 60 % of muscles had a mixed type, 32 % were finger like and 8 % were completely tethered muscles.

Measurement of papillary muscle
The mean distance separating the mitral annulus from the apex of the ALPM was 19.38 mm ± 3 SD reported. [3,5] In addition, the present study reports intergroup muscular bridge in 2 hearts. In any form of papillary muscle, the chordae distribute from a papillary muscle group to the corresponding hemi B -3 bellied PMPM A -Single bellied PMPM with 3 heads, note the basal chordae originating from separate muscle projections from ventricular wall half of both mitral leaflets. [3 -10] Therefore, rupture of a papillary muscle group, following infarction of the adjoining ventricular wall, will lead to heavy regurgitation as the half of the support of each leaflet is lost. [10] There is considerable variation in the morphology of papillary muscle. Victor and Nayak presented a classification of 100 cases of papillary muscles in self-explanatory figurative language as conical, mammillated, flat topped, grooved, wavy, stepped etc. [3] They stated that such variation in morphology can be explained by embryological Victor & Nayak pointed out; the variations in papillary muscle morphology would influence the pathophysiological effects of various cardiac disorders. [3] Shape of the muscle influence the passage of blood flow. [8,10] Conical shape of muscles facilitate smooth cardiovascular physiology. [10] Chances of left ventricular outflow tract obstruction is higher in muscles with broad apex, increase in number and size. [8,10] On the other hand, if there are only two papillary muscle in the ventricle, an ischemic event affecting one papillary muscle will lead to severe valve impairment than heart with multiple bellies. [8] Morphological variation of papillary muscles has a tremendous effect on selection of homograft on replacement of valve with homograft. [4,13] In homograft replacement, single headed muscles are easy to handle & Implanting a total homograft in multiheaded ones the respective position of different portions of the papillary muscle must be maintained in order to obtain an even distribution of traction on the leaflet tissue. [3,4,13] Therefore in tissue bank ID card, homograft can be labeled according to its morphology. The ones which are difficult to handle could be discarded.
Victor & Nayak pointed out in the mitral valve replacement with artificial valve, retention of chordopapillary support is being favoured to preserve the optimum function of the left ventricle. [3] If the native valve has too many papillary muscle bellies, these may interfere with the function of prosthetic disc or ball, especially if they are of the intraluminal type.
Muscularization of chordae were observed in 3 % of the hearts, similar to the finding reported in the literature. [2,3,8,9,11] Anomalous insertion of papillary muscle directly into anterior mitral leaflet may lead to midventricular obstruction. [8] Our findings show mixed type (partly tethered and partly protruding) is the common form of papillary muscle attachment to the ventricular wall in both ALPM and PMPM groups. Ranganathan & Burch pointed out, arterial supply to the muscle depend on the nature of muscle attachment to the ventricular wall. [12] They explained that freely protruding finger like papillary muscle have very few or no anastomotic connections with the extra papillary subendocardial plexus. On the other hand, the tethered variety of papillary muscles has a segmental distribution of the long penetrating intramyocardial vessels.These branches make connections with one another and with the extra papillary subendocardial plexus. This helps to perfuse the papillary muscles even in the presence of occlusion or narrowing of the larger epicardial feeder vessel.

Measurements of papillary muscle
In our study, we report the shortest distance between the papillary muscle and mitral valve annulus; 19.38 ± 3 mm (range 5-26) for the ALPM group and 20.36 ± 3.7 mm (range 10-32) for PMPM group. The respective values reported by Ramsheyi et al. [4] are 20 ± 3 mm (range 15 -27 mm) and 25 ± 3 mm (range 20 -31 mm) and by Acar et al. [13] are 21 ± 3 mm & 26 ± 4 mm. The results show the distance is shorter between ALPM and annulus than that between PMPM and mitral annulus. Sri Lankan values are comparatively lower than the values of Caucasians. These measurements are useful in homograft replacement. In this procedure, the placement of the donor papillary muscle in the recipient heart needs to be tailored, ensuring even distribution of traction on the leaflet tissue and optimum fanning out of chordae, to ensure its systolic and diastolic function. [3. 4, 13] The average length of ALPM in the present study is 32.24 mm ± 4.8 SD (range 20 -45) and that of the PMPM is 29.92 mm ± 4.9 SD (range 16 -44 In conclusion, the present study has found considerable variations in the number, shape, attachment to ventricular wall and dimensions and position of the papillary muscles. The findings would be of great value during endoscopic and conventional mitral valve replacement and in mitral valve homograft implantation