Current Research

• Current research in valve replacement focuses on both mechanical and tissue valve replacements.
• One major focus of research is into reducing scar tissue which can cause future complications.
• More research is also going into finding ways to increase the durability of the implanted valves.

Figure 1: http://www.hhmi.org/biointeractive/museum/exhibit98/content/h12info.html

There are three basic types of artificial tissue valves.  The first, a homograft, is a fully functioning valve transplanted from one human to another.  Before it can be implanted it goes through an antibiotic preservation treatment (Galloway et al, par. 13).  Second, an autograft, is when the patients pulmonary valve is removed and inserted in place of the aortic valve.  Then the patient receives a homograft to replace their pulmonary valve.  This procedure is known as the Ross procedure (Galloway et al, par.14).  Third, a xenograft, is a valve transplanted from or created out of tissue from another species, porcine or bovine.  Carpentier et al. pioneered the researched they labeled Bioprosthesis, for valves of biological origin that needed preservation.  Porcine and pericardial valves are treated with glutaraldehyde to sterilize and remove the antigens from the valve tissue so they are bioacceptable (Rahimtoola et al, par 3).  The patient receiving any of these tissue valves will not have to go through immunosuppressive theory or take anticoagulant drugs. 

Figure 2: Modern tissue valves replacements; stented on the left and stentless on the right (Zilla 726)

              The homografts and autografts take very little preparation for implementation.  On the other hand, the xenografts need to be preserved and usually stented.  Porcine valves are cleaned and preserved, placed on a metal stent for support and covered with a cloth like material made from Dacron.  Dacron is a human body friendly material and easily incorporates with the cells of the heart.  Eventually, cells can actually grow through the Dacron and hold the valve in place better.  Similarly, the pericardium is cleaned and preserved.  Then the pericardium is stretched and shaped onto a stent so the tissue can function as a valve.  Additionally, a sewing ring is added so the surgeons have a way of implanting the device.  This exterior of the valve is also covered in Dacron.  Recently, engineers have been able to form these xenograft valves without stents, which are beneficial because they improve hemodynamic of the artificial valves (see Figure 3).

                Tissue valves are sometimes viewed as a better choice than mechanical valves because the chance of thrombosis is greatly reduced.  When dealing with biological tissue, blood is a lot less likely to stick to it and produce a clot (Merrick par 1).  Thus, patients do not have to take anticoagulation drugs, unlike with mechanical valves.  Mechanical valves have the advantage of lasting longer.  Since they are made from metal and other strong materials they will not erode as quickly from the force of the blood flow.  Tissue valves are vulnerable to wear from the pressure and calcification.  Tissue valves are predicted to last 15 years and will have to be replaced if the patient is young.  It has been documented that 80% of stented pericardium implants were structural failure free after 10 years (par 1).

                Calcification usually occurs on the cusps of the tissue valves.  There is a build-up of calcium that thickens the tissue and inhibits the motion of the valves.  With the invention of the stentless valves, calcification of the valve walls has also become a problem (Zilla 728).  Another problem is degradation of the valve overtime.  This degradation and tearing of the valve can be due to inflammatory cells inside the prosthetic or macrophages eating away at the valve (729).  These events usually do not occur until 10-15 years after the valve has been implanted.  Thus, tissue valve replacements are mostly recommended for older patients so the valve will not have to be replaced in their lifetime.  However, more young patients would prefer tissue valves so they will not have to be on anticoagulation therapy for the rest of their lives.