Biomedical Engineering Reference
In-Depth Information
these components will be discussed in the context of the various elements of the
valve, including its design, the materials, and the processing. Ultimately, the
success of the valve is due to the combined successful interaction of all of these
components, and various valves have failed from the ultimate failure of only one
component.
5.4 Design considerations
5.4.1 Orifice area
There are a number of design considerations that must be taken into account
with a bioprosthetic heart valve as outlined in Table 5.2. Of paramount import-
ance is the orifice area of the valve, particularly in elderly women and people
with a small aortic root. A bioprosthetic valve does not have the same orifice
area as the native valve, as it requires additional support elements, such as the
stent or aortic wall tissue. Since the replacement valve is intended to improve
blood flow to the patient, every element which reduces the orifice area can be
considered to reduce the theoretical maximum benefit that a patient would
receive from the implant. The valve size should match the patient's heart
annulus size as closely as possible, to avoid prosthesis±patient mismatch.
Mismatch in the valve size implanted in the patient's annulus leads to early
structural valve degeneration (SVD) in the bioprosthesis (Flameng et al., 2010).
5.4.2
Valve design type
The tissue component is, necessarily, an essential part of a bioprosthetic valve.
In valves utilizing a synthetic material as a frame (also called the stent), the
tissue component is reserved to the leaflets, and all commercially successful
valves utilize three leaflets in their designs. Historical publications include
reports on unileaflet (Soots et al., 1986) and bileaflet (Black et al., 1986) tissue
valve designs, although durability testing showed these designs to be inferior to
trileaflet designs in long-term durability. Other types of valves utilize tissue for
the leaflet material and for the supporting structure of the leaflets, with minimal
synthetic material used at all. These valves are called stentless valves. A new
type of valve design beginning to enter the marketplace is the minimally
invasive percutaneous valve. Figure 5.3 contains photos of percutaneous valves
from Medtronic and Edwards Lifesciences that are approved for sale in Europe.
Figure 5.4 contains representative examples of tissue valves where the tissue
is supported on a synthetic frame (stented valves) or the entire porcine aortic
valve isolation is processed intact to form a stentless valve. Stented valves have
a reduced orifice area compared with stentless valves, although the surgical
implantation of a stentless valve is much more difficult than that of a stented
valve, so most surgeons will use a stented valve for its ease of use and
