Biomedical Engineering Reference
In-Depth Information
fabrication. The tissues were also matched for thickness, as one would do in
manufacturing. As is immediately evident from the data, not all tissues with
uniform thickness and acceptable cosmetics behave similarly. The consequences
of this have been long appreciated since pericardial valves were first used
clinically; that is, that this mismatch in mechanical properties between leaflets
can cause asynchronous closing of the leaflets and result in poor valve func-
tioning. As early as 1986, manufacturers implemented selection criteria `accord-
ing to standardized techniques and exacting methods for tissue selection and
correct matching of leaflets for uniformity of function' (Ionescu et al., 1986).
Every manufacturer which produces a pericardial valve must address the
issue of mechanical property mismatch between leaflets and each have deve-
loped different approaches to this problem. One particularly effective method
that has been implemented by Edwards Lifesciences involves measuring the
displacement of an isolated leaflet when subjected to a standardized dynamic
load (Nguyen et al., 2002). An example of a leaflet displacement tester is shown
in Fig. 5.17. Use of such a system enables close matching of leaflet mechanical
properties and can be utilized to match tissue properties across the range of in
vivo forces to which the valve is exposed.
Another approach to the problem of mechanical mismatch is to subject the
tissue to an applied force during fixation, thereby removing some of the intrinsic
variability in elongation of the tissue prior to valve fabrication. Numerous
studies have been published which document the effects of the exposure of
applied forces to pericardial tissue, in either one or two directions, and compared
with untethered values (Lee et al., 1989a,b). Figure 5.18 demonstrates the
effects of a load when applied in one direction; the extensibility of the tissue
along the axis of the applied load is reduced compared with untethered tissues.
5.18 Stress±strain curves for nine random samples of bovine pericardial tissue
crosslinked in glutaraldehyde with an applied load for 30 minutes. Note the
significant reduction in variability in elasticity between tissues, compared with
Fig. 5.16. (Figure reprinted with permission from Arbor Surgical Technologies,
Inc. (now owned by Medtronic.)
