Biomedical Engineering Reference
In-Depth Information
Perhaps most significantly, the elongation of different pieces of tissue are more
similar compared with untethered tissues. In this way, the application of a
uniaxial load during fixation can generate tissues with more consistent
mechanical properties, thereby addressing the problem of mechanical mismatch.
Further, the application of a uniaxial load during fixation generates anisotropy in
the pericardial tissue which mimics the native valve leaflet (Sacks et al., 2009a)
and porcine aortic valve leaflet (Sacks et al., 2006) tissues.
5.5.4 Non-tissue materials
The support and finishing elements of a bioprosthetic heart valve are made from
materials similar to those used in mechanical valves. All valves contain some
cloth to assist in fixation of the tissue to the stent or to the heart tissue of the
patient. The cloth is typically made of PET or PTFE and permits ingrowth of
cells into the cloth which helps hold the valve in place.
Stented heart valves can use a variety of materials for the support structure.
The Edwards PERIMOUNT
Õ
valve has a stent formed from Elgiloy
TM
wire,
while the Medtronic Hancock
Õ
valve has a stent made from Delrin
Õ
(homo-
acetal). In either case, the choice of the material must be justified and tested to
show that it has sufficient durability for the demanding job of a heart valve stent.
Early clinical explants with the Hancock
Õ
standard orifice porcine aortic valves
showed that these polymeric stents had a tendency to creep with time, ultimately
bowing down into the central orifice and reducing the area for blood flow
(Schoen and Cohn, 1986). Today, stent creep testing is a standard component of
the test regimen required of any new heart valve design.
Valves with a sewing ring may contain additional support elements to
provide annular rigidity and integrity to support the many sutures that are used
to fixate the valve in the patient's heart. The Edwards PERIMOUNT
Õ
valve
contains a silicone waffle-like ring which helps the surgeon fixate the sutures in
the sewing ring. The Medtronic Mosaic
Õ
valve contains radio-opaque markers at
the stent tips which help visualize the valve stent during imaging procedures.
All valves are held together with sutures, and the construction of a heart valve
is complex and very labor-intensive. There are so many fine stitches used to
construct the valve that the assembly workers build the valves under magnifica-
tion, and it may take a day or two for a worker to finish just one valve. The
choice of suture material(s) is an important consideration, and it may be appro-
priate to choose different types of suture materials for different applications. For
example, it may be desirable to use a stretchy suture with some compliance
when suturing the tissue to the frame. This may help prevent the type of tearing
seen with the Ionescu±Shiley valve, where the sutures tore through the tissue,
resulting in early failure. When attaching non-tissue elements together, it may be
desirable to use high tensile strength suture materials which are very non-
compliant,
to ensure a solid construction. Different
types of sutures have
