**1. Introduction**

The classic case of aortic stenosis is a healthy middle-aged patient with/without symptoms, but in practical life, patients with severe calcific aortic valve come with several and severe comorbidities such as advanced age, coronary artery disease, atherosclerotic aorta, significant left ventricular dysfunction. Aortic valve replacement (AVR) is the only options in these patients, and it requires patient-by-patient analysis of clinical, echocardiograhic, and hemo‐ dynamic data with associated pathologies. The curative treatment of calcific aortic valve stenosis is the replacement of the aortic valve with a prosthetic valve, and selection of a perfect prosthetic valve is the main goal to get a successful treatment. But, there is no any perfect heart valve prosthesis which may mimic the characteristics of the normal native aortic valve: excellent hemodynamics, life-long durability, thromboresistance, and excellent implantability. That means that native valve disease will be traded for prosthetic valve disease and the outcome of AVR is affected by the type of prosthetic valve. Mechanical valves are non-limited durable, but have a substantial risk of hematologic complications (thromboemboli, thrombotic obstruction, hemorrhage related life-long anticoagulation therapy) with/without hemolysis potential. In contract, bioprosthetic valves have a low risk of thromboembolism without anticoagulation, but their durability is limited by calcific or noncalcific tissue deterioration. Biological prostheses, especially homografts, are often believed to be the substitute of choice in AVR, but the limited availability of homografts prevents their more broadly usage. To overcome this problem and all possible complications of mechanical valves, xenogenic biological prostheses have been developed. The design of bioprosthetic valves purports to mimic the anatomy of the native aortic valve and their flow characteristics are better than mechanical valves, whereas stentless bioprostheses have hemodynamic performance similar to the healthy native aortic valve. Although stented bioprostheses can be implanted easier,

© 2013 Kirali; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

they decrease the effective orifice area due to the rigid stent and result turbulent flow through the valve. Stented valves also increase stress at the attachment of the stent which cause earlier primary tissue failure. Stentless biologic valves have been introduced into clinical practice to solve all these problems and to reproduce the anatomy and function of the native aortic valve, but their clinical use has still not exceeded the number of stented aortic bioprostheses because of more demanding technique of implantation. To gain more widespread clinical use and general recommendation of stentless bioprostheses, their advantages and simple implantation techniques must be popularized.

especially in younger patients, which are less durable than commercially available stentless bioprostheses and cannot be recommended as the ideal device [9]. The use of the patients own pericardium for constructing a heart valve prosthesis is biologically more appealing than the use of animal tissue or heterologous material. The feasibility of autologous pericardial stentless aortic valve was shown in an animal study [10]. The feasibility and durability of truly stentless autologous pericardial AVR sutured directly onto the aortic wall has been also performed in human recently [11]. Stentless porcine or pericardial xenogenic bioprostheses have been introduced to get better long-term durability and become a routine device when a stentless

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There are a lot of stentless bioprostheses with/without the aortic root in the market, but some of them are not used widespread and implantation of a few xenografts is stopped (Table 1). The first modern (first generation) stentless valves were glutaraldehyde-fixed porcine prostheses with a fully scalloped shape or a complete aortic root (Figure 1). The most preferred approach was root replacement technique because subcoronary approach needed more suture line. The second generation of stentless valves improved the technical difficulties related to free-hand implantation with two rows of sutures for subcoronary implantation of porcine bioprostheses (Figure 2). The third generation of stentless prostheses are made by xenogenic pericardium, because the pericardial valve is free from the compromises of the porcine aortic root, it is flexible, and easy to implant either with an interrupted or running suture technique (Figure 3). There are different xenogenic pericardial valves (bovine or equine), and horse pericardium is thinner, however, stronger than the bovine pericardium and also much more pliable. The fourth generation of stentless valves are produced by a proprietary process and the unique conditioning technology paves the way for autologous repopularization of the valve in patients. The durability of current bioprosthetic heart valves is diminished by glutaraldehyde-associated leaflet calcification or by the host immune reaction. As a novel tissue engineering approach to improving replacement heart valve durability, a new acellular (nonglutaraldehyde-fixed) tissue heart valve for autologous recellularization is developed to limit xenograft antigenicity. As no glutaraldehyde is used in the whole process lack of calcification and also lack of toxicity, and the method delivers a very pliable valve with very low gradients. To use of autologous pericardium fixed with glutaraldehyde avoids any immune reaction between the host and the implanted heart valve and so minimizes tissue calcification and pannus formation. The last generation of stentless valves provides avoidance of suture lines during AVR: closed [transcatheter (transfemoral or transapical)] or open

Every effort should be made to avoid moderate prosthesis-patient mismatch during AVR. Stentless valves enable to select the largest bioprosthesis to the patient's annulus and provide better aortic root and valve behavior, larger effective orifice area (EOA), reduced transpros‐

biologic valve is implanted.

(transaortic = sutureless) techniques (Figure 4).

thetic gradient and greater left ventricular mass regression.

**2. Hemodynamic recovery**

It is believed that the aortic root is probably the best stent for the native or prosthetic aortic valve. The anatomy and function of the aortic root may dampen the mechanical stress to which the leaflets are subjected during diastole. The ideal stentless prosthesis should have no synthetic materials, preserve the aortic root dynamics, restore flexibility and distensibility of the native valve annulus after decalcification, and have minimal xenograft aortic wall, short implantation time, and excellent hemodynamic performance to facilitate the recovery of left ventricular function.

#### **1.1. Historical background**

Homografts were the first biological prostheses used in clinical practice to treat aortic valve stenosis in early 1960s, and they were the first stentless valves, too [1,2]. The authors used the aortic root of the patient to secure the homograft aortic valve in the subcoronary position. The most complicated implantation technique and the restricted availability of homografts prevented their widespread usage. First stentless pig and calf xenografts were used in limited patients, but the valves were abandoned because of poor tissue fixation [3]. Stented biopros‐ theses were considered as the gold standard for several years, but abnormal stress on the leaflets was believed to decrease durability. To overcome this problem with a rigid stent on the aortic position, stentless bioprostheses were re-introduced in the middle of 80's [4], whereas new designed stentless xenografts were proposed and popularized in daily use at the begin‐ ning of 1990s [5]. The main problem (early failure of bioprostheses) was solved with new bioengineering improvement (antimineralization, zero-pressure fixation) [6]. The other problem was partial dehiscence when the heterograft contained muscular bar resulting paravalvular leakage in the area corresponding to the muscular bar, and this problem was abolished with a fine Dacron cloth covered the outside wall of the stentless porcine aortic valve along its inflow [7]. Recognizing the range of aortic root variability and disease of the root itself, the concept of stentless valve replacement was expanded to replacement of the entire aortic root. Full root replacement with a bioprosthesis brought the challenges of homeostasis and coronary reimplantation. In spite of hemodynamic advantages proven for the root replacement technique, acceptance was slowed by risk/benefit ratio concerns. The whole aortic root could be prepared and implanted with modified root inclusion or subcoronary implant techniques.

Biological stentless valve can be prepared by pulmonary autograft, homograft, xenograft, autologous or xenogenic pericardium. Pulmonary autograft has limited durability beyond the first decade [8]. The same problem has been observed with homografts in the aortic position,

especially in younger patients, which are less durable than commercially available stentless bioprostheses and cannot be recommended as the ideal device [9]. The use of the patients own pericardium for constructing a heart valve prosthesis is biologically more appealing than the use of animal tissue or heterologous material. The feasibility of autologous pericardial stentless aortic valve was shown in an animal study [10]. The feasibility and durability of truly stentless autologous pericardial AVR sutured directly onto the aortic wall has been also performed in human recently [11]. Stentless porcine or pericardial xenogenic bioprostheses have been introduced to get better long-term durability and become a routine device when a stentless biologic valve is implanted.

There are a lot of stentless bioprostheses with/without the aortic root in the market, but some of them are not used widespread and implantation of a few xenografts is stopped (Table 1). The first modern (first generation) stentless valves were glutaraldehyde-fixed porcine prostheses with a fully scalloped shape or a complete aortic root (Figure 1). The most preferred approach was root replacement technique because subcoronary approach needed more suture line. The second generation of stentless valves improved the technical difficulties related to free-hand implantation with two rows of sutures for subcoronary implantation of porcine bioprostheses (Figure 2). The third generation of stentless prostheses are made by xenogenic pericardium, because the pericardial valve is free from the compromises of the porcine aortic root, it is flexible, and easy to implant either with an interrupted or running suture technique (Figure 3). There are different xenogenic pericardial valves (bovine or equine), and horse pericardium is thinner, however, stronger than the bovine pericardium and also much more pliable. The fourth generation of stentless valves are produced by a proprietary process and the unique conditioning technology paves the way for autologous repopularization of the valve in patients. The durability of current bioprosthetic heart valves is diminished by glutaraldehyde-associated leaflet calcification or by the host immune reaction. As a novel tissue engineering approach to improving replacement heart valve durability, a new acellular (nonglutaraldehyde-fixed) tissue heart valve for autologous recellularization is developed to limit xenograft antigenicity. As no glutaraldehyde is used in the whole process lack of calcification and also lack of toxicity, and the method delivers a very pliable valve with very low gradients. To use of autologous pericardium fixed with glutaraldehyde avoids any immune reaction between the host and the implanted heart valve and so minimizes tissue calcification and pannus formation. The last generation of stentless valves provides avoidance of suture lines during AVR: closed [transcatheter (transfemoral or transapical)] or open (transaortic = sutureless) techniques (Figure 4).

#### **2. Hemodynamic recovery**

they decrease the effective orifice area due to the rigid stent and result turbulent flow through the valve. Stented valves also increase stress at the attachment of the stent which cause earlier primary tissue failure. Stentless biologic valves have been introduced into clinical practice to solve all these problems and to reproduce the anatomy and function of the native aortic valve, but their clinical use has still not exceeded the number of stented aortic bioprostheses because of more demanding technique of implantation. To gain more widespread clinical use and general recommendation of stentless bioprostheses, their advantages and simple implantation

It is believed that the aortic root is probably the best stent for the native or prosthetic aortic valve. The anatomy and function of the aortic root may dampen the mechanical stress to which the leaflets are subjected during diastole. The ideal stentless prosthesis should have no synthetic materials, preserve the aortic root dynamics, restore flexibility and distensibility of the native valve annulus after decalcification, and have minimal xenograft aortic wall, short implantation time, and excellent hemodynamic performance to facilitate the recovery of left

Homografts were the first biological prostheses used in clinical practice to treat aortic valve stenosis in early 1960s, and they were the first stentless valves, too [1,2]. The authors used the aortic root of the patient to secure the homograft aortic valve in the subcoronary position. The most complicated implantation technique and the restricted availability of homografts prevented their widespread usage. First stentless pig and calf xenografts were used in limited patients, but the valves were abandoned because of poor tissue fixation [3]. Stented biopros‐ theses were considered as the gold standard for several years, but abnormal stress on the leaflets was believed to decrease durability. To overcome this problem with a rigid stent on the aortic position, stentless bioprostheses were re-introduced in the middle of 80's [4], whereas new designed stentless xenografts were proposed and popularized in daily use at the begin‐ ning of 1990s [5]. The main problem (early failure of bioprostheses) was solved with new bioengineering improvement (antimineralization, zero-pressure fixation) [6]. The other problem was partial dehiscence when the heterograft contained muscular bar resulting paravalvular leakage in the area corresponding to the muscular bar, and this problem was abolished with a fine Dacron cloth covered the outside wall of the stentless porcine aortic valve along its inflow [7]. Recognizing the range of aortic root variability and disease of the root itself, the concept of stentless valve replacement was expanded to replacement of the entire aortic root. Full root replacement with a bioprosthesis brought the challenges of homeostasis and coronary reimplantation. In spite of hemodynamic advantages proven for the root replacement technique, acceptance was slowed by risk/benefit ratio concerns. The whole aortic root could be prepared and implanted with modified root inclusion or subcoronary implant

Biological stentless valve can be prepared by pulmonary autograft, homograft, xenograft, autologous or xenogenic pericardium. Pulmonary autograft has limited durability beyond the first decade [8]. The same problem has been observed with homografts in the aortic position,

techniques must be popularized.

ventricular function.

412 Calcific Aortic Valve Disease

techniques.

**1.1. Historical background**

Every effort should be made to avoid moderate prosthesis-patient mismatch during AVR. Stentless valves enable to select the largest bioprosthesis to the patient's annulus and provide better aortic root and valve behavior, larger effective orifice area (EOA), reduced transpros‐ thetic gradient and greater left ventricular mass regression.

**Figure 1.** First generation bioprostheses (Porcine Stentless Xenografts) A) Scalloped stentless porcine bioprostheses B) Root stentless porcine bioprostheses.

Stented valves fixe the native commissures and do not allow cyclic change of the commissural dimension as it normally occurs. This cyclic expansion of the commissural area serves reduction of stress on the leaflets, which is preserved by stentless bioprostheses. Second, the intrinsically obstructive nature of the stented bioprostheses increases pressure gradient and creates turbulent flow patterns, however, normal laminar flow patterns can be restored after AVR with stentless tissue valves. The opening and closing of the stentless biologic valve constitute a passive mechanism responding to pressure difference between the left ventricle and the aorta. Like the native aortic valve, a stress created by this difference heads toward the central coaptation area of the bioprosthesis during diastole. The negative pressure difference during diastole helps prosthetic valve to be closed. The valve opens rapidly at the beginning of ejection because of rising of pressure difference and persists to remain open as a tunnel

**Shelhigh Suprestentless**

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**Sorin Pericarbon Freedom Sorin Pericarbon Freedom SOLO 3F Therapeutics**

**3F Enable model 6000 Perceval S**

**Figure 3.** Third generation bioprostheses (Pericardial Stentless Xenografts)

**Figure 4.** Sutureless Pericardial Stentless Xenografts

**Figure 2.** Second generation bioprostheses.

To prevent early or late prosthetic failure, maintenance of the aortic root with physiological anatomy must be the primary goal during AVR with a stentless prosthesis. Any kind of bioprosthetic valve will deviate from native aortic valve in terms of leaflet dynamics. Stiffening of the aortic root either by glutaraldehyde or by stent degenerates the opening (wrinkles and blurry edges of leaflets) and closing (asynchronism) behavior of native aortic valve leaflets. Stentless Bioprostheses for Aortic Valve Replacement in Calcific Aortic Stenosis http://dx.doi.org/10.5772/55373 415

#### **Shelhigh Suprestentless**

**Figure 2.** Second generation bioprostheses.

**Figure 3.** Third generation bioprostheses (Pericardial Stentless Xenografts)

**Figure 4.** Sutureless Pericardial Stentless Xenografts

To prevent early or late prosthetic failure, maintenance of the aortic root with physiological anatomy must be the primary goal during AVR with a stentless prosthesis. Any kind of bioprosthetic valve will deviate from native aortic valve in terms of leaflet dynamics. Stiffening of the aortic root either by glutaraldehyde or by stent degenerates the opening (wrinkles and blurry edges of leaflets) and closing (asynchronism) behavior of native aortic valve leaflets.

**Figure 1.** First generation bioprostheses (Porcine Stentless Xenografts) A) Scalloped stentless porcine bioprostheses B)

(A)

414 Calcific Aortic Valve Disease

**St Jude Toronto SPV St Jude Medical-Biocor**

**St Jude SVP Root Edwards Prima Plus**

**Medtronic Freestyle Koehler Elan Root**

**Koehler Elan Labcor CryoLife-O'Brien**

(B)

Root stentless porcine bioprostheses.

Stented valves fixe the native commissures and do not allow cyclic change of the commissural dimension as it normally occurs. This cyclic expansion of the commissural area serves reduction of stress on the leaflets, which is preserved by stentless bioprostheses. Second, the intrinsically obstructive nature of the stented bioprostheses increases pressure gradient and creates turbulent flow patterns, however, normal laminar flow patterns can be restored after AVR with stentless tissue valves. The opening and closing of the stentless biologic valve constitute a passive mechanism responding to pressure difference between the left ventricle and the aorta. Like the native aortic valve, a stress created by this difference heads toward the central coaptation area of the bioprosthesis during diastole. The negative pressure difference during diastole helps prosthetic valve to be closed. The valve opens rapidly at the beginning of ejection because of rising of pressure difference and persists to remain open as a tunnel


during systole, and the aortic root may also expanse at the late diastole to help opening of the leaflets (in native aortic valve, expansion of the aortic root is about 12% and that starts opening the leaflets to about 20%). At the end of systole, the backward blood flow into the sinuses of Valsalva (behind prosthetic leaflets) and initialization of pressure difference help prosthetic leaflets to revert to their original closed position. An in-vivo-study has showed that there is no difference in opening velocities among native, stented and subcoronary stentless valves in a porcine model [12]. However, the closing velocities are significantly higher in the pericardial valves. The bending deformation increases when implanting a glutaraldehyde-treated valve subcoronary. Porcine stentless valves display a distinct folding pattern during opening resulting in an altered stress distribution and also tend to fold during opening causing

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One of the key parameters for stentless xenograft performance is the EOA. In spite of the EOA is significantly higher in stentless bioprostheses it is also dependent on the design and the implantation technique of the prostheses. The EOA will increase especially during the first year and the transvalvular gradient drops dramatically in the first 3 to 6 months after surgery, but some further drop may be seen more later [14]. The reason may be remodeling of the left ventricular outflow tract, diminished aortic root edema, and slight dilatation of the aortic root. Transvalvular gradient is closely related to the EOA: the larger orifice area the lower is the transvalvular gradient. The second reason to increase transvalvular gradient is usage of a rigid stent. Avoidance of a stent enlarges inner diameter of prosthetic valve and eliminates intralu‐ minal obstruction which increases the EOA. Several studies have shown transvalvular gradient across stentless valves is always lower than for their stented valves, especially mean and/or peek gradients [15-1617]. The third possible reason can be excessive tissue of a bio‐ prosthesis: the lesser tissue implanted within the recipient aortic root the lesser obstruction. The full root prostheses reduce the intraluminar obstruction because nothing is implanted inside, and they have larger EOA than subcoronary prostheses. The main differences of stentless biologic tissue valves are the specific gravity of the leaflets which is not equal to that of blood like native human aortic leaflets and the specific thickness of the leaflets which is thinner in pericardial tissue valves. Both parameters cause transvalvular gradient during ejection which is lesser in fully pericardial stentless valves than porcine. The other reasons may be small aortic annulus and physically active patients. The change in gradients during exercise is interesting: when cardiac output increases it also increases the transvalvular flow and raises transprosthetic gradient, but these gradients under exercise are lower with stentless valves

than stented bioprostheses, which provide better opening-closing behavior [18].

Left ventricular output is maintained by the development of the left ventricular hypertrophy which results in a large pressure gradient across the stenotic valve. The left ventricle mass increases and becomes less compliant. Left ventricular hypertrophy and increased mass can be correlated with sudden death, congestive heart failure, and other cardiovascular events. Left ventricular hypertrophy will regress after AVR regardless of the type of prostheses, and an improved hemodynamic performance of prostheses should result in a faster regression, especially in patients with severe calcific aortic stenosis and left ventricular hypertrophy, because incomplete regression after AVR is related to poor long-term outcome [19]. This

increased leaflet bending stress [13].

**Table 1.** Stentless Bioprostheses.

during systole, and the aortic root may also expanse at the late diastole to help opening of the leaflets (in native aortic valve, expansion of the aortic root is about 12% and that starts opening the leaflets to about 20%). At the end of systole, the backward blood flow into the sinuses of Valsalva (behind prosthetic leaflets) and initialization of pressure difference help prosthetic leaflets to revert to their original closed position. An in-vivo-study has showed that there is no difference in opening velocities among native, stented and subcoronary stentless valves in a porcine model [12]. However, the closing velocities are significantly higher in the pericardial valves. The bending deformation increases when implanting a glutaraldehyde-treated valve subcoronary. Porcine stentless valves display a distinct folding pattern during opening resulting in an altered stress distribution and also tend to fold during opening causing increased leaflet bending stress [13].

**A. Autograft B. Homograft C. Xenografts**

416 Calcific Aortic Valve Disease

*Dacron reinforced inflow tract*

*pericardial reinforced inflow tract*

*tri-composite design (three noncoronary leaflets)*

**II. Second generation (porcine with single suture line,**

**III. Third generation (Stentless Pericardial Bioprosthesis)**

**IV. Fourth generation (non-gluteraldayhde fixed +**

**V. Sutureless generation (Sutureless + Stentless**

Koehler Elan Root

**No-react treatment)**

*porcine pericardium*

**decellularized)**

**Pericardial Bioprosthesis)**

**D. Autologous pericardium**

**Table 1.** Stentless Bioprostheses.

Matrix A

Sorin Pericarbon Freedom SOLO *horse (equine) pericardium*

**I. First generation (Stentless Porcine Bioprosthesis)**

Toronto SPV (stentless porcine valve) St Jude Medical, Inc., St Paul, MN, USA St Jude Medical-Biocor St Jude, Belo Horizonte, MG, Brazil

Koehler Elan Koehler, Bellshill, Scotland

Labcor Labcor, Inc., Belo Horizonte, MG, Brazil

Sorin Pericarbon Freedom Sorin Biomedica Cardio SpA, Saluggia, Italy

3F Therapeutics 3F Therapeutics, Inc., Lake Forest, CA, USA

3F Enable model 6000 3F Therapeutics, Inc., Lake Forest, CA, USA Perceval S Sorin Biomedica Cardio SpA, Saluggia, Italy

Shelhigh Suprestentless Shelhigh, Inc, Millburn, NJ, USA

CryoLife-O'Brien Model 3000 CryoLife International Inc, Atlanta, GA, USA Toronto SPV Root St Jude Medical, Inc., St Paul, MN, USA Edwards Prima Plus Edwards Lifesciences, Inc., Irvine, CA,USA Medtronic Freestyle Medtronic, Inc., Minneapolis, MN, USA

One of the key parameters for stentless xenograft performance is the EOA. In spite of the EOA is significantly higher in stentless bioprostheses it is also dependent on the design and the implantation technique of the prostheses. The EOA will increase especially during the first year and the transvalvular gradient drops dramatically in the first 3 to 6 months after surgery, but some further drop may be seen more later [14]. The reason may be remodeling of the left ventricular outflow tract, diminished aortic root edema, and slight dilatation of the aortic root. Transvalvular gradient is closely related to the EOA: the larger orifice area the lower is the transvalvular gradient. The second reason to increase transvalvular gradient is usage of a rigid stent. Avoidance of a stent enlarges inner diameter of prosthetic valve and eliminates intralu‐ minal obstruction which increases the EOA. Several studies have shown transvalvular gradient across stentless valves is always lower than for their stented valves, especially mean and/or peek gradients [15-1617]. The third possible reason can be excessive tissue of a bio‐ prosthesis: the lesser tissue implanted within the recipient aortic root the lesser obstruction. The full root prostheses reduce the intraluminar obstruction because nothing is implanted inside, and they have larger EOA than subcoronary prostheses. The main differences of stentless biologic tissue valves are the specific gravity of the leaflets which is not equal to that of blood like native human aortic leaflets and the specific thickness of the leaflets which is thinner in pericardial tissue valves. Both parameters cause transvalvular gradient during ejection which is lesser in fully pericardial stentless valves than porcine. The other reasons may be small aortic annulus and physically active patients. The change in gradients during exercise is interesting: when cardiac output increases it also increases the transvalvular flow and raises transprosthetic gradient, but these gradients under exercise are lower with stentless valves than stented bioprostheses, which provide better opening-closing behavior [18].

Left ventricular output is maintained by the development of the left ventricular hypertrophy which results in a large pressure gradient across the stenotic valve. The left ventricle mass increases and becomes less compliant. Left ventricular hypertrophy and increased mass can be correlated with sudden death, congestive heart failure, and other cardiovascular events. Left ventricular hypertrophy will regress after AVR regardless of the type of prostheses, and an improved hemodynamic performance of prostheses should result in a faster regression, especially in patients with severe calcific aortic stenosis and left ventricular hypertrophy, because incomplete regression after AVR is related to poor long-term outcome [19]. This regression is related to EOA and transvalvular gradient constituted by the prosthetic valve. A significant improvement will occur in all type of valves in the first year, but this improvement is greater and faster with the stentless bioprostheses [20]. A lasting benefit beyond the first year is possible, especially in severely enlarged ventricles [21]. These improvements include mass regression, wall thickening, fractional shortening, and diastolic relaxation. Patients with small aortic annuli or with compromised left ventricular function (EF < 50%) might benefit more from stentless prostheses [22,23].
