Effect of storage on tensile properties of natural heart valve tissue
Aortic homograft valves are the preferred choice of replacement valve in aortic valve
replacement procedures. The major drawback to their use is their availability. This
project addressed two of the methods by which availability can be increased by
assessing mechanical properties of the valves. Most pre-operative methods that assess
homograft valve post-operative functional ability use biochemical or histological
protocols. Rarely is their mechanical ability to sustain the pressure across them tested.
Uniaxial tensile tests were performed on radial and circumferential leaflet sections from
human and porcine aortic and pulmonary valves. The pulmonary valve has very similar
anatomy to the aortic valve, although it is thinner and there is less pressure across it in
vivo. When a patients own pulmonary valve is used to replace their aortic (their
pulmonary valve is replaced with an aortic homograft), good post-operative results are
achieved. The aim of this study was to see if pulmonary homografts would be able to
sustain aortic pressures. It is concluded that pulmonary leaflet specimens have
comparable mechanical characteristics to aortic and should therefore be suitable for
aortic valve replacement. This would double the number of valves available for
surgeons to use during valve replacements.
Porcine aortic specimens were found to be stiffer and fail at higher stresses than the
other valve types. The results from the porcine pulmonary specimen properties
indicated that bioprosthetic valve manufacturers can consider their use in bioprosthetic
valve manufacture as they are more than able to cope with human aortic valve
Some storage methods have been assigned short duration or 'sell-by' dates, with no
evidence that the mechanical integrity of the tissues has been significantly
compromised. Currently retrospective studies are used to assess whether the treatments
are detrimental to the tissues; the post-operative durability being taken as the indicator!
If the storage times can be extended then the number of valves available to surgeons
would increase. Four treatments of valves used for storage were tested over three
months and their effect on leaflet specimen tensile properties determined.
Glutaraldehyde had a significant effect on the tensile properties of the specimens and
this suggests that alternative methods should be used to treat bioprosthetic valves,
which are fixed in it. Treatment with antibiotics produced losses in stiffness of the
tissues at three months, but these were within physiological limits. Therefore valves
stored in this manner can be stored as long as three months at least with no effect on
valve function in vivo. Cryopreservation with glycerol over three months produced less
changes in specimen properties which again should not affect valve function in vivo.
Cryopreservation with dimethyl sulphoxide (DMSO) caused the least difference in
specimen properties compared to fresh tissues. It is therefore recommended as the first
choice in valve treatments for storage.
It is suggested that all new valve storage treatments be assessed for their mechanical
effects on the tissues routinely. Preliminary work on a non-destructive intact valve test
device using polarised light is introduced. This has potential for use in pre-operative
assessment of valve mechanical integrity.