**3. Biological prosthesis**

graft valves. Commonly in us are mechanical and biological prostheses.[9] When selecting between mechanical and biologic heart valves, the surgeon and patient must balance the

Charles Hufnagel in 1952. Used aortic valve ball and cage prosthesis heterotopically in the descending aorta to treat aortic insufficiency. The first aortic valve replacement with an intra cardiac mechanical prosthesis, which led to long terms survivors, was performed in 1960. Mechanical valves are classified according to their structure as caged-ball, single-tilting-disk or bileaflet-tilting-disk valves. The Starr-Edwards caged-ball valve has been available since the 1960's and comprises a silastic ball, which rests on the sewing ring when closed and moves forward into the cage when the valve opens. The single-disk valves, for example, the Bjork-Shiley prosthesis and the Medtronic-Hall prosthesis, contain a disk that tilts between two struts of the orifice housing. The most popular of the mechanical valves at present are the bileaflet valves, of which the St. Jude Medical valve and the Carbomedics valve are widely implanted. Both these devices are implanted within the aortic annulus. The two semi-circular leaflets of the bileaflet valve are connected to the housing by a butterfly hinge mechanism and swing apart during opening of the valve creating three outflow tracts, one central and two peripheral respectively. In contrast to the configuration of the latter, the Carbomedics Top Hat (Sulzer Carbomedics, Austin, TX) bileaflet aortic valve that was introduced in 1993 has a unique supra-

annular design with all its components incorporated within the aortic sinuses.[10,11]

tural valve deterioration, the major advantage of mechanical valves.3

Mechanical valves are made from carbon, Teflon, Dacron, titanium and polyester and are very durable. The current designs for the aortic and mitral positions include ball-and-cage valves, single tilting disc prostheses, and bileaflet prostheses. Bileaflet mechanical valves are the standard in current practice, with the St. Jude Medical (St. Jude Medical, Inc., St. Paul, MN) prosthesis the modern prototype, having been first implanted in 1977. Most of these valves are constructed using carbon strengthened with silicon carbide additives. Other examples of bileaflet mechanical valves include those manufactured by CarboMed‐ ics (Austin, TX); Advancing the Standard Medical (ATS, Minneapolis, MN); Medtronic, Inc. (Minneapolis, MN); and Medical Carbon Research Institute, LLC (MCRI, Austin, TX). [10,11,12] The On-XR mechanical valve (MCRI) was introduced in Europe in 1996 and dif‐ fers from other bileaflet mechanical valves in that it is made from pure pyrolytic carbon. The PROACT (Prospective Randomized On-X R Valve Anticoagulation Trial) study is an FDA-approved multicenter trial, sponsored by MCRI, currently enrolling patients to deter‐ mine whether or not defined patient groups receiving AVR (low versus high risk for TE events) with the On-X R *\_* valve may be safely maintained on lower doses of warfarin or, for patients in the lowrisk aortic valve arm, on antiplatelet drugs (aspirin plus clopidog‐ rel) alone compared with standard anticoagulation regimens. No single mechanical valve has shown superior patient outcomes, and all demonstrate extremely low rates of struc‐

risks and benefits of each choice.

366 Calcific Aortic Valve Disease

**2. Mechanical prostheses**

The biological prostheses include a wide variety of devices. Included within this broad category are the bioprostheses, a term which is used for valves with non-viable tissue of biological origin. The bioprostheses include the heterografts, composed of porcine (actual valves of a pig) or bovine tissue (pericardium of a cow) and the allografts, which are preserved human aortic valves. The initial bioprostheses were mounted on stents to which the leaflets and sewing ring were attached but subsequently stentless valves, which are sewn in free hand, have been developed.[13] Stented bioprosthetic valves, which incorporate a semi-rigid external support structure for the valve leaflets, represent the majority of tissue valves implanted in clinical practice. The external support provides accurate valve mounting, improving ease of implantation. Two types of stented bioprosthetic valves are currently available in the United States: porcine aortic valves, which incorporate chemically stabilized porcine valve leaflets mounted on a stented structure or frame, and bovine pericardial valves. The leaflets of the latter valve type are constructed from bovine pericardium and subsequently mounted on a stented frame. Available porcine valves include the Medtronic Mosaic valve (Medtronic Inc., Minneapolis, MN), the St. Jude Medical Biocor and Biocor Supra valves (St. Jude Medical, Inc., St. Paul, MN), and the Carbomedics Mitroflow valve (Carbomedics, Inc., Austin, TX). Bovine pericardial valves include the Carpentier–Edwards (C–E) Perimount (Edward Lifesciences, Irvine, CA) and the CE Perimount Magna valves as well as the Sorin Soprano (Sorin Group, Saluggia, Italy) valves. At present, based on the best available data, no one bioprosthetic valve appears superior with regard to patient outcomes and none requires systemic anticoagulation with warfarin, which is their major advantage. Their major disad‐ vantage is the incidence of structural valve deterioration and subsequent need for reoperation, although the lifespan of the latest generation of tissue valves is unknown. Recent evidence also suggests that stentless biological valves may have better coronary flow reserve compared to stented valves. Additionally, compared with stented bovine pericardial valves, stentless valves have been associated with increased transvalvular EOA and decreased pressure gradients during extended follow-up. However, as seen in other studies, LV mass regression after stentless valve implantation was not different from stented aortic bioprostheses.[3,14]

whether they are aortic or mitral valves.[16] Because of the long-term durability of me‐ chanical prostheses a valve replacement rate is less than 2% over 25 years. The most com‐ mon reasons for reimplantation are pre- and postoperative endocarditis, paravalvular leak and valve thrombosis.[17] *Bioprosthetic valves* are not as durable, have a shorter lifespan and are more susceptible to calcification than human and mechanical valves.[13] Biopros‐ theses have a significantly higher rate of reoperation due to structural valve deterioration. In large series, freedom from reoperation is > 90% at 10 years, but < 70 % at 15 years. [17,18,19] There is an important predisposition for premature bioprosthetic structural valve deterioration in younger patients, especially those under the age of 40 years.[17]

Surgical Valve Replacement (Bioprosthetic VS Mechanical)

http://dx.doi.org/10.5772/53687

369

**Nonstructural dysfunction** - Any abnormality that is not intrinsic to the valve per se, which causes stenosis or regurgitation. Examples for this include entrapment of pannus, tissue or suture; paravalvular leak; inappropriate sizing or positioning; residual leak and clinically important hemolytic anemia. This definition also excludes changes due to infection and thrombosis. Subvalvular pannus formation is rare with *mechanical bileaflet valves.*[20] Panus overgrowth and prosthetic structural degeneration interfering with normal valve opening and closure may cause hemolysis severe enough for reoperation. Paravalvular leak is an operative

**Valve thrombosis** – Any thrombus, in the absence of infection, which is attached to or near an operated valve that occludes part of the blood flow path or that interferes with function of the valve. The incidence of prosthesis thrombosis is < 0.2 % per year and it occurs more often in *mechanical prostheses.*[22] It is most commonly due to inadequate anticoagulation or noncom‐

**Embolism** – Any embolic event that occurs, in the absence of infection, after the immediate peri-operative period. This could be either a neurologic or peripheral embolic event. A neurologic event includes any new, temporary or permanent focal or global neurologic deficit. A peripheral embolic event is due to an embolus that produces symptoms from obstruction of a peripheral (non-cerebral) artery. The incidence of thromboembolic events between biopros‐ theses and mechanical prostheses are the same.[25] This is a continuous risk factor that is present through the life of patients with *mechanical valve prosthesis*, so they must maintain therapeutic anticoagulant levels. The embolic risk is highest in the first few months, before the ring and valve components have fully endothelialized.[26] Acceptable thromboembolic rates range between 0.8 and 2.3 % per patient-year.[21,22,25] 50 % of these events are neurologic, 40

**Valvular endocarditis** – Any infection involving an operated valve diagnosed by customary clinical criteria. It is rare case with prophylactic antibiotics. Around 60 % of events occur early and are associated with staphylococci. The mortality for this event is high. Freedom from endocarditis with mechanical prosthesis is 97 to 98 % at 20 to 25 years. A number of studies have reported a higher incidence of valvular endocarditis after *mechanical valve* replacement in comparison with the biologic valve replacement during the initial few months after implantation. *Bioprostheses* are less susceptible to early infection, which is often restricted to the leaflets, making cure with antibiotics more likely but increasing the chances of late failure

complication and it is related to operative technique and to endocarditis.[21]

pliance. Freedom from valve thrombosis at 20 years is > 97 %.[23,24]

% are transient and 10 % are peripheral.[21]

due to degeneration of the cusps.[27,28,29]

**Figure 3.** Biological prosthesis: a) stented porcine bioprosthesis, b) pericardial bovine bioprosthesis and c) stentless porcine bioprosthesis
