**6. Stress analysis**

Current state of biomechanical and micro structural characterizations of the chordae tendineae, and shear stress areas are discussed in this with a new Indian

**303**

accompaniment.

*Nonresectional-Graded Neo Chordal Dynamic Repair of Mitral Valve: Stress Analysis Induced…*

Chordae can be classified as true or false or as basal, marginal or strut chordae [8–10]. The commissural chordae also is a name that we have added to this as it is slightly thicker and helps to modify the surgical repair technique. Interestingly mitral valve (MV) anterior leaflet chordae are thicker than the MV posterior leaflet chordae [11]. Load or stress bearing is the strut chordae in the normal valve and it shifts to marginal chordae in leaftlet prolapse and makes this susceptible to rupture. On an average 25 chordae attached to an atrioventricular valve dissipates off the shear stress of systolic closure. The fact that stress and chordal thickness are linked can be understood by proper analysis of fetal hearts which shows thinner chordae. Mitral chordae microstructure shows forms of collagen: (i) a mostly straight, dense, collagen fiber core (ii) widely spaced collagen fibers that wrap around the straight collagen fiber core with some angle of alignment on the primary axis. Fiber size and stress are again related. Tricuspid chordae have a greater collagen fiber density and a smaller fiber diameter, as these are subjected only to right heart pressures significantly lower than the right. From out in the arrangement is elastin sheath with fiber orientation at angles to longitudinal axis, straight fibers, undulated collagen fibers with circumferential orientation and a longitudinal structure in the centre. Diseases like myxomatous degeneration affect chordae more than the leaflets increasing its water and glycosaminoglycan contents. Proteoglycans contribute to calcification also. This promotes defective formation of elastin and collagen. In rheumatic involvement the collagen core is affected and fibrosis is an

Chordae even have blood supply and could be vehicles through which nutrient supply also reaches the leaflets. Distinct elastin layer and planar undulated collagen fibers are unique to human species showing some evolutionary advantage even in the micro structural design. Structure and mechanics decide on material properties in nature and emulating them could be the reason for success in surgical approaches to the valve too. Even while studying strut chordae the radial pattern of collagen

surgical repair technique that takes advantage of this knowledge to repair the valve with better long term outcomes. This almost eliminates chordal failure in repairs making it durable. Customarily, finite element analysis (FEA) is used to predict material stress and strain fields rendered by applying a load on an initially unloaded model. MV leaflets are relatively non deformed during systolic loading. Leaflet strain in vivo is measured using sono micrometry in an ovine model, hybrid models of normal human MVs as constructed using transesophageal real-time 3-D echocardiography (rt-3DE) loaded repeatedly using FEA, and serial rt-3DE images of normal human MVs used to construct models at end diastole and end isovolumic contraction to detect any deformation during isovolumic contraction. Human MV deformed minimally during isovolumic contraction, as measured by the mean absolute difference calculated over the surfaces of both leaflets between serial MV models: 0.53 ± 0.19 mm. FEA modeling of MV models derived from in vivo highresolution truly 3-D imaging is reasonable and useful for stress prediction in MV pathologies and repairs. Customarily, FEA is used to predict material stress and strain fields rendered by applying a load on an initially unloaded model. In general, the approach has been to use highly idealized and simplistic geometric models to analyze ex vivo or anatomically idealized MVs to assess physiology; to analyze standardized pathological valves to predict stress distribution and, potentially, the integrity and failure behavior of repair techniques; to "evaluate proposed surgical repairs" using idealized computational models, including models that attempt integration of fluid-structure interactions; or to evaluate pathological alterations on the function of idealized computational models. Superimposed tissue formation which is a main determinant of leaflet thickening in MVP, is related to increased

*DOI: http://dx.doi.org/10.5772/intechopen.94433*

stresses over the leaflets.

#### *Nonresectional-Graded Neo Chordal Dynamic Repair of Mitral Valve: Stress Analysis Induced… DOI: http://dx.doi.org/10.5772/intechopen.94433*

surgical repair technique that takes advantage of this knowledge to repair the valve with better long term outcomes. This almost eliminates chordal failure in repairs making it durable. Customarily, finite element analysis (FEA) is used to predict material stress and strain fields rendered by applying a load on an initially unloaded model. MV leaflets are relatively non deformed during systolic loading. Leaflet strain in vivo is measured using sono micrometry in an ovine model, hybrid models of normal human MVs as constructed using transesophageal real-time 3-D echocardiography (rt-3DE) loaded repeatedly using FEA, and serial rt-3DE images of normal human MVs used to construct models at end diastole and end isovolumic contraction to detect any deformation during isovolumic contraction. Human MV deformed minimally during isovolumic contraction, as measured by the mean absolute difference calculated over the surfaces of both leaflets between serial MV models: 0.53 ± 0.19 mm. FEA modeling of MV models derived from in vivo highresolution truly 3-D imaging is reasonable and useful for stress prediction in MV pathologies and repairs. Customarily, FEA is used to predict material stress and strain fields rendered by applying a load on an initially unloaded model. In general, the approach has been to use highly idealized and simplistic geometric models to analyze ex vivo or anatomically idealized MVs to assess physiology; to analyze standardized pathological valves to predict stress distribution and, potentially, the integrity and failure behavior of repair techniques; to "evaluate proposed surgical repairs" using idealized computational models, including models that attempt integration of fluid-structure interactions; or to evaluate pathological alterations on the function of idealized computational models. Superimposed tissue formation which is a main determinant of leaflet thickening in MVP, is related to increased stresses over the leaflets.

Chordae can be classified as true or false or as basal, marginal or strut chordae [8–10]. The commissural chordae also is a name that we have added to this as it is slightly thicker and helps to modify the surgical repair technique. Interestingly mitral valve (MV) anterior leaflet chordae are thicker than the MV posterior leaflet chordae [11]. Load or stress bearing is the strut chordae in the normal valve and it shifts to marginal chordae in leaftlet prolapse and makes this susceptible to rupture. On an average 25 chordae attached to an atrioventricular valve dissipates off the shear stress of systolic closure. The fact that stress and chordal thickness are linked can be understood by proper analysis of fetal hearts which shows thinner chordae.

Mitral chordae microstructure shows forms of collagen: (i) a mostly straight, dense, collagen fiber core (ii) widely spaced collagen fibers that wrap around the straight collagen fiber core with some angle of alignment on the primary axis. Fiber size and stress are again related. Tricuspid chordae have a greater collagen fiber density and a smaller fiber diameter, as these are subjected only to right heart pressures significantly lower than the right. From out in the arrangement is elastin sheath with fiber orientation at angles to longitudinal axis, straight fibers, undulated collagen fibers with circumferential orientation and a longitudinal structure in the centre. Diseases like myxomatous degeneration affect chordae more than the leaflets increasing its water and glycosaminoglycan contents. Proteoglycans contribute to calcification also. This promotes defective formation of elastin and collagen. In rheumatic involvement the collagen core is affected and fibrosis is an accompaniment.

Chordae even have blood supply and could be vehicles through which nutrient supply also reaches the leaflets. Distinct elastin layer and planar undulated collagen fibers are unique to human species showing some evolutionary advantage even in the micro structural design. Structure and mechanics decide on material properties in nature and emulating them could be the reason for success in surgical approaches to the valve too. Even while studying strut chordae the radial pattern of collagen

*Cardiac Diseases - Novel Aspects of Cardiac Risk, Cardiorenal Pathology and Cardiac Interventions*

Mitral valve has stood the test for evolution and the extreme dynamic nature brings forth a great concept of engineering skills to repair and hold on to this precious tissue. Resection creates extreme stress and should be avoided at all costs. To replace when repair is feasible is a sin with our current understanding and technological evolution. Long term durability and SAM were intriguing concepts which made surgeons adopt technological modifications, but reparability rates remained constant in the last decade in most advanced cardiac centers around the world. The understanding of the intervalvular triangle as an important part of anterior leaflet and the concept of avoiding placing a horizontal stiff ring across it was emphasized by the American correction version of mitral repairs. Mitral valve stress analysis shows at the beginning of systole the marginal chordae carries the maximum stress. Stress increases now on the strut chordae in mid systole with more of leaflet coaptation with entire stress transfer to annulus during late systole with good leaflet coaptation [2, 3]. With annular dilatation stress is evenly distributed to all valvular structures and that is the reason why mitral regurgitation tends to be a progressive disease. Normal valve dynamics ensure optimal diastolic locking, proper zone of coaptation with excellent left ventricular outflow dynamics and smooth leaflet and chordal stress distribution. Of the various geometric, kinetic and structural factors that can lead to SAM, impaired aorto mitral coupling dynamics are most significant. It is important to avoid rigid and undersized rings which not only alter coupling dynamics but reduce the aorto mitral angle [4–7] also that lead to both LV inflow and outflow obstructions. Failures to recognize the interventricular component of anterior leaflet and aortomitral coupling dynamics are important reasons for failure of repair of this segment. Avoid resection and true sized annuloplasty rings that take the interventricular triangle are keys to success. Ischemic mitral regurgitation often with sagging P2 P3 areas require annuloplasty to correct this portion and then bringing the papillary muscles to within 2cm of each other before placing the ring – for which a true sized ring would be most effective. The

goals of the Indian method of correction would be explained as follows

5.Restore stress ratios to normal thereby enhancing durability of repair.

6.Graded Neochordal reconstruction of the valve chordae for natural stress

It is an excellent reproducible and safe procedure with 0.2% mortality [8]. Failure with repair techniques to due to leaving behind areas of stress which has to

Current state of biomechanical and micro structural characterizations of the chordae tendineae, and shear stress areas are discussed in this with a new Indian

be meticulously avoided by proper assessment and optimal repair [9].

4.Maintain normal left ventricular outflow dynamics

1.Eliminate mitral regurgitation

2.Ensure normal leaflet coaptation

3.Restore normal annular dynamics

redistribution

**6. Stress analysis**

**5. Discussion**

**302**

fibers is seen for those inserted more anteriorly while circumferential pattern is noted in those inserting closer to annulus. Organized cross networking of collagen is noted at papillary muscle chordal junction also.

The microstructure of artificial chordae should also be looked into. The smooth micro porous structure which reduces thrombogenicity is the cause of calcification and rupture later.

Chordal biomechanics can be evaluated and demonstrated in labs using (i) uniaxial tensile testing; (ii) stress-relaxation testing; (iii) chordae-leaflet insertion region testing; and (iv) in vitro flow loop testing. Chordal ruptured at a strain of 21.4% and a stress of 3.1 × 108 dyne/cm<sup>2</sup> . Tricuspid chordae are less extensible when compared with mitral chordae in uniaxial tensile testing. Aging causes the chordae to become stiffer. Interestingly rupture strains of myxomatous and normal chordae are similar. Here shifting of strain from basal to marginal chordae may be the precipitating factor for chordal rupture.

strut chordae to have the fastest and greatest relaxation behavior (49.1 ± 5.4%), followed by the basal chordae (42.4 ± 8.3%), and then the marginal chordae (33.2 ± 4.7%). Strut chordae are stiffer than the marginal and basal chordae; (ii) the basal chordae has greater extensibility than the marginal chordae; (iii) the mitral valve chordae were stiffer than their tricuspid valve counterparts; and (iv) the chordae attaching to the tricuspid valve septal leaflet are more extensible than the chordae attaching to the other two tricuspid valve leaflets. Studies using tensile testing devices have shown that marginal chordae ruptures at 68% less load and 28% less strain than the basal chordae. Posterior leaftlet chordae of mitral valve ruptures at 43% less load and 22% less strain compared with anterior leaftlet chordae. These factors clearly support the graded neochordal reconstruction of the mitral valve retaining the full advantages of the non resectional approach. Marginal chordae have the largest glycosaminoglycan concentrations and the smallest relaxation pattern while the strut chordae had the greatest relaxation pattern, but the lowest glycosaminoglycan content. Biaxial testing shows that the leaflet and papillary muscle insertions have a higher molecular strain than the rest of the chordae. Chordae experienced a strain rate of 75.3 ± 3.43% during systolic closure and a strain rate of −54.8 ± −56.6% during diastolic opening. Constant plateau of the chordal strain between 3.75% and 4.29% during valve closure is also noted in various studies. In this regard the greater compliance of e PTFE chordae could in fact be a surgical advantage [12, 13].

Knowledge is evolving about better understanding and importance of the chordae tendineae of the atrioventricular valves. Morphology and microstructure of the chordae and chordal subsets have been well-defined. There are no standard protocols for investigating chordae mechanics or microstructure as of now, though some investigators like us have ventured out into this area which will help future researchers as time progresses and better evaluation methods are available. Tissue mechanics of MV strut chordae have been well characterized, but future studies are warranted regarding the mechanics of TV chordae. These should be linked to stress analysis and microstructure changes for human chordae. Improved therapy and treatment outcomes with long term effects would result from such studies.

Chordae tendineae are critical to distributing shear stress during systolic movements of leaflets to the papillary muscles, preventing leaflet prolapse and regurgitation. Suboptimal outcomes with repair techniques are due to inability to understand [14–16]. Mechanics and microstructure of the chordae tendineae of these atrioventricular heart valves should be kept in mind and linked to the shear stress analysis on an individual patient basis and surgical repair techniques tailored according to it.

**305**

**7. Conclusion**

*Nonresectional-Graded Neo Chordal Dynamic Repair of Mitral Valve: Stress Analysis Induced…*

The chordae structures redistribute strain to the papillary muscles during systolic closure and prevent leaflet prolapse. The major stress occurs in the coapting leaftlet belly. In the case of chordae failure, such as elongation or chordal rupture with prolapse the shear stress points align along the tips of the leaflets. Uncontrolled regurgitation ends in cardiac failure. Leaftlet prolapse secondary to chordal rupture can be triggered by lower ventricular pressures. Major surgical approaches to counter chordal failure include shortening, transposition, and replacement. Shortening has been traditionally described to the superior to transposition in terms of freedom from late significant regurgitation [17]. Vulnerability to rupture is a concern which is overcome with the construction of neochordae. There is still a 13% incidence of recurrent regurgitation with such techniques [18]. Concerns include elongation of the synthetic chordae, rupture of the native chordae, calcification, or recurrent prolapse potentially caused by an elastic modulus higher than that of the native chordae still weigh in the mind of the surgeon [19–22]. Refinement of computational modelling methods and simulation tools may bring forth greater points that could be useful in the modification of surgical techniques and make treatment more individualized [23, 24]. Amalgamation of knowledge of morphology, microstructure and mechanics with material property knowledge of replacement materials will give surgical techniques the much needed

support of durability in mitral valve repairs in the years to come [25–27].

long run and surgery would remain the gold standard in future.

The Indian Dynamic Correction of mitral valve differs from the French correction that there is no resection of the valve and from American correction in that a complete physio ring is used preserving the aorto mitral dynamics with graded Neochordal reconstruction which would simulate the natural stress redistribution dynamics. This would in future ensure 100% reparability and would increase the percentage of valve repairs in all centres. Stress dynamics enforce the need for proper surgical correction and the fallibility of the developing percutaneous concept in ignoring the aorto mitral dynamics..There is growing evidence showing that the "non-resection" technique has some potential advantages including: (I) preserved leaflet mobility; (II) larger surface of coaptation; (III) no changes in annular geometry; and (IV) implantation of larger prosthetic annuloplasty ring. The leaflet is the most precious part of the valve so preserve it. Mitraclips placed severely damage the valve and placing it has only options of replacing the valve if it fails. Current percutaneous methods fail to relieve the stress ratios and would certainly fail in the

This paper was presented at the video Micro-symposium of Indian Association

of Cardiothoracic Surgeons Held at Visakhapatanam India from Feb 1-5 -2018.

*DOI: http://dx.doi.org/10.5772/intechopen.94433*

*Nonresectional-Graded Neo Chordal Dynamic Repair of Mitral Valve: Stress Analysis Induced… DOI: http://dx.doi.org/10.5772/intechopen.94433*

The chordae structures redistribute strain to the papillary muscles during systolic closure and prevent leaflet prolapse. The major stress occurs in the coapting leaftlet belly. In the case of chordae failure, such as elongation or chordal rupture with prolapse the shear stress points align along the tips of the leaflets. Uncontrolled regurgitation ends in cardiac failure. Leaftlet prolapse secondary to chordal rupture can be triggered by lower ventricular pressures. Major surgical approaches to counter chordal failure include shortening, transposition, and replacement. Shortening has been traditionally described to the superior to transposition in terms of freedom from late significant regurgitation [17]. Vulnerability to rupture is a concern which is overcome with the construction of neochordae. There is still a 13% incidence of recurrent regurgitation with such techniques [18]. Concerns include elongation of the synthetic chordae, rupture of the native chordae, calcification, or recurrent prolapse potentially caused by an elastic modulus higher than that of the native chordae still weigh in the mind of the surgeon [19–22]. Refinement of computational modelling methods and simulation tools may bring forth greater points that could be useful in the modification of surgical techniques and make treatment more individualized [23, 24]. Amalgamation of knowledge of morphology, microstructure and mechanics with material property knowledge of replacement materials will give surgical techniques the much needed support of durability in mitral valve repairs in the years to come [25–27].

## **7. Conclusion**

*Cardiac Diseases - Novel Aspects of Cardiac Risk, Cardiorenal Pathology and Cardiac Interventions*

The microstructure of artificial chordae should also be looked into. The smooth micro porous structure which reduces thrombogenicity is the cause of calcification

. Tricuspid chordae are less extensible when

Chordal biomechanics can be evaluated and demonstrated in labs using (i) uniaxial tensile testing; (ii) stress-relaxation testing; (iii) chordae-leaflet insertion region testing; and (iv) in vitro flow loop testing. Chordal ruptured at a strain of

compared with mitral chordae in uniaxial tensile testing. Aging causes the chordae to become stiffer. Interestingly rupture strains of myxomatous and normal chordae are similar. Here shifting of strain from basal to marginal chordae may be the

strut chordae to have the fastest and greatest relaxation behavior (49.1 ± 5.4%), followed by the basal chordae (42.4 ± 8.3%), and then the marginal chordae (33.2 ± 4.7%). Strut chordae are stiffer than the marginal and basal chordae; (ii) the basal chordae has greater extensibility than the marginal chordae; (iii) the mitral valve chordae were stiffer than their tricuspid valve counterparts; and (iv) the chordae attaching to the tricuspid valve septal leaflet are more extensible than the chordae attaching to the other two tricuspid valve leaflets. Studies using tensile testing devices have shown that marginal chordae ruptures at 68% less load and 28% less strain than the basal chordae. Posterior leaftlet chordae of mitral valve ruptures at 43% less load and 22% less strain compared with anterior leaftlet chordae. These factors clearly support the graded neochordal reconstruction of the mitral valve retaining the full advantages of the non resectional approach. Marginal chordae have the largest glycosaminoglycan concentrations and the smallest relaxation pattern while the strut chordae had the greatest relaxation pattern, but the lowest glycosaminoglycan content. Biaxial testing shows that the leaflet and papillary muscle insertions have a higher molecular strain than the rest of the chordae. Chordae experienced a strain rate of 75.3 ± 3.43% during systolic closure and a strain rate of −54.8 ± −56.6% during diastolic opening. Constant plateau of the chordal strain between 3.75% and 4.29% during valve closure is also noted in various studies. In this regard the greater compliance of e PTFE chordae could in fact be a surgical

Knowledge is evolving about better understanding and importance of the chordae tendineae of the atrioventricular valves. Morphology and microstructure of the chordae and chordal subsets have been well-defined. There are no standard protocols for investigating chordae mechanics or microstructure as of now, though some investigators like us have ventured out into this area which will help future researchers as time progresses and better evaluation methods are available. Tissue mechanics of MV strut chordae have been well characterized, but future studies are warranted regarding the mechanics of TV chordae. These should be linked to stress analysis and microstructure changes for human chordae. Improved therapy and treatment outcomes with long term effects would result

Chordae tendineae are critical to distributing shear stress during systolic movements of leaflets to the papillary muscles, preventing leaflet prolapse and regurgitation. Suboptimal outcomes with repair techniques are due to inability to understand [14–16]. Mechanics and microstructure of the chordae tendineae of these atrioventricular heart valves should be kept in mind and linked to the shear stress analysis on an individual patient basis and surgical repair techniques tailored

fibers is seen for those inserted more anteriorly while circumferential pattern is noted in those inserting closer to annulus. Organized cross networking of collagen

is noted at papillary muscle chordal junction also.

21.4% and a stress of 3.1 × 108 dyne/cm<sup>2</sup>

precipitating factor for chordal rupture.

and rupture later.

advantage [12, 13].

from such studies.

according to it.

**304**

The Indian Dynamic Correction of mitral valve differs from the French correction that there is no resection of the valve and from American correction in that a complete physio ring is used preserving the aorto mitral dynamics with graded Neochordal reconstruction which would simulate the natural stress redistribution dynamics. This would in future ensure 100% reparability and would increase the percentage of valve repairs in all centres. Stress dynamics enforce the need for proper surgical correction and the fallibility of the developing percutaneous concept in ignoring the aorto mitral dynamics..There is growing evidence showing that the "non-resection" technique has some potential advantages including: (I) preserved leaflet mobility; (II) larger surface of coaptation; (III) no changes in annular geometry; and (IV) implantation of larger prosthetic annuloplasty ring. The leaflet is the most precious part of the valve so preserve it. Mitraclips placed severely damage the valve and placing it has only options of replacing the valve if it fails. Current percutaneous methods fail to relieve the stress ratios and would certainly fail in the long run and surgery would remain the gold standard in future.

This paper was presented at the video Micro-symposium of Indian Association of Cardiothoracic Surgeons Held at Visakhapatanam India from Feb 1-5 -2018.
