Biomedical Engineering Reference
In-Depth Information
this valve may require a permanent pacemaker in order to maintain normal sinus
rhythm postoperatively (Baan et al., 2010).
In each of these cases, the unique characteristics of the elderly sick patient
were not duplicated in young healthy animals during the preclinical studies, and
thus these problems could not be detected until clinical use. Another aspect of
the service environment that has to be considered is the hospital or surgical
environment. How are the sizers going to be resterilized between cases? What
particular challenges does your new product pose to the institution? Will you
require refrigeration of the product or the use of new equipment during rinsing
or implantation? Changes like these can lead to unforeseen problems which can
be disastrous and result in early clinical failures. Because of the importance of
the service environment, sponsoring companies are required to conduct exten-
sive risk analysis and risk management processes as a part of the product
development process.
5.5 Material considerations
5.5.1
Leaflet material
The choice of leaflet material is one that has been hotly debated throughout the
history of bioprosthetic valves. In the early period of valve development, valves
were fabricated from various autologous tissues including the diaphragm, veins,
and aortic wall. Pericardium was most often utilized because of its availability
and its properties of pliability, resistance, and thinness, which was very similar
to the aortic valve tissue (Duran, 1986). Despite the lack of immunogenicity
issues with an autologous tissue, and the apparent suitability of autologous
pericardium, these tissues ultimately failed as valves, as they were rapidly
absorbed by the body. Thus, chemical stabilization of the tissue is an essential
component to its use as a valve, regardless of the source. Because of the imprac-
ticality of using glutaraldehyde in the operating room (the need for freshly made
dilute solutions, the time required for crosslinking, and the chemical residuals in
the tissue), homologous tissues were quickly abandoned in favor of xenogeneic
tissues, which could be prepared by commercial laboratories under standardized
conditions. Allograft valves and valved conduits are still in use today, however
due to the limited supply of cadaver tissue these do not represent a practical
option for most patients requiring treatment.
5.5.2
Tissue type
Since 1976, with the commercialization of the Ionescu±Shiley bovine peri-
cardial valve, there has been a proliferation of tissue valves of various designs
made with a variety of animal tissues. Practical considerations such as supply,
size, and other factors generally limited the choices to porcine aortic valve
