Biomedical Engineering Reference
In-Depth Information
different effects on the tissue (Paez et al., 2001), and even the use of different
sizes of needles can influence the mechanical properties of sewn tissue (Lim and
Cheong, 1994). While it may seem like a small detail, the choice of suture
material and needle used in construction can have an important effect on the
overall durability of the finished device.
5.6 Process considerations
There are a number of ways for valves to fail, and most testing plans are
designed to test a new valve design for all known failure modes. Early failure
modes include mechanical failure due to leaflet mismatch or construction issues
where tissues tear or abrade. If a valve does not fail due to early failure modes, it
can fail later due to durability issues. Late failure modes include mechanical
degradation of the tissue, with resulting tearing, which is most common for
porcine aortic valve designs (Ishihara et al., 1981; Sacks and Schoen, 2002).
Calcification is another late failure mode which is only partly due to design, and
all valves are required to be tested for calcification potential during the
development process.
5.6.1 Calcification models
Much work has been done in the early days of heart valve development on
developing various models to test for calcification. While some proposed the use
of in vitro calcification test models, these methods proved to be difficult to
execute and required constant maintenance, particularly for the super-saturated
solutions that were specified. Animal implant models proved to be more reliable,
and have been rapidly adopted for testing purposes.
Large animal models were developed first, led primarily by the pioneering
work by Drs Jones and Ferrans at the National Heart, Lung and Blood Institute
and Dr Hilbert at the Center for Devices and Radiological Health (CDRH) of the
Food and Drug Administration (FDA) (Jones et al., 1986, 1989). They utilized
juvenile sheep 4±8 months old and implanted the prostheses in the mitral
position. During the in-life portion of the study, the valves were studied for their
hemodynamic performance. After a period of 150 days, the animals were sacri-
ficed and the explanted valves were examined and analyzed for calcification.
Calcification was evaluated in a number of ways, including macroscopically,
microscopically via histopathology, and analytically using chemical methods
such as atomic absorption spectroscopy.
Since then, the model has been applied to every known valve in use today. In
particular, Professor Flameng's group in Leuven, Belgium has tested and
published results on hundreds of valve implants, looking at the effects of valve
type (stented vs. stentless), leaflet material (porcine vs. pericardial), treatment
type (anticalcification treatment vs. none), implant duration, implant location,
