CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 3, April 2012
AFRICA
127
friction-wear stress. Impact-wear stress occurs between the valve
occluder and ring, while friction-wear stress occurs between the
occluder and strut pivots. The catastrophic failure of the Bjork-
Shiley valve
6
was due to a transient occlusion impact of the disc
onto the outlet strut, causing bending stresses in excess of its
elastic material limit.
Material fatigue is another area of concern in mechanical
heart valve replacements. To this end, the majority of valve
manufacturers make use of either a silicon carbon blend, or pure
pyrolytic carbon for the formation of their leaflets. The housing
and struts are required to be manufactured from particularly
strong metals such as titanium.
Cavitation is another significant mode of failure in mechanical
heart valves. It is caused by a localised and significant drop
in pressure, equal to or below that of the blood vapour
pressure, causing the blood to vaporise and form bubbles in the
bloodstream. In mechanical heart valves, the sudden closing of
the tilting leaflet causes localised high-pressure jets of blood
to squeeze past the closing boundary and form vortices and
cavitation. Pitting and surface erosion from this cavitation were
found on failed Duromedics valves.
Tissue valves
Development in pursuit of a solution not incurring the limitations
that are placed on mechanical valves followed in the form of
tissue valve replacements. Bioprosthetic valves have the major
advantage of not being susceptible to the thrombo-embolic
effects created by the functioning of mechanical valves. Animal
tissues, most commonly bovine pericardium or porcine valves
are used for these valves. To allow for the facilitation of these
tissues to be integrated into a human heart, antigen masking
through treatment of glutaraldehyde fixation has been the status
quo since the mid 1960s.
7
Acknowledging at the outset that tissue valves require
masking of antigenicity and that the absence of living tissue
prevents any possible form of self-repair, the major research on
tissue valves has focused on the implications of this parameter.
The treatment of leaflet tissue with glutaraldehyde reduces
immunogenicity but does not totally eliminate it. This causes the
tissue to respond with cellular immune responses as the body acts
to reject the tissue. Studies indicate that the level of cross-linking
of the collagen fibres has the effect of reducing the antigenicity.
8
However, the major challenge facing tissue valve use is still the
link between immune response and calcific degeneration, and
hence to be able to trace poorly masked immunogenicity.
9
Inflammatory degradation causes explanted valves to typically
be covered in inflammatory cells, or be penetrated by giant cells
and macrophages.
10
Bovine pericardium heart valve prostheses
often have macrophages invade the prosthetic collagen, and in
porcine valves, as many as 82% of failed valves showed signs of
collagen phagocytosis.
11
Mechanical leaflet damage is a concern in tissue valves, as
the use of glutaraldehyde mitigates any option of self-repair
mechanisms. Valves in the mitral position are more stressed than
those in the aortic position. Studies by Maxwell
et al.
12
indicated
that up to 75% of failed porcine valves showed a rupture of a
free cusp edge.
In a study by Guangqiang
et al
.,
13
a comparison is drawn
between the aortic valve repair (AVR) of 518 patients using
Carpentier Edwards (CE) porcine valves between 1974 and 1996
and the AVR with CE pericardial valves from 1991 to 2002.
Ten-year survival rates for the porcine and pericardial valves
were 34
±
2 and 38
±
6%, respectively. Adverse cardiac events
of thrombo-embolism (20
±
2 and 13
±
2%, respectively) and
endocarditis (2
±
1 and 1
±
1%, respectively) over a 10-year span
were limited and similar for both. Interestingly, the 10-year follow
up requiring no re-operation was lower for porcine (90
±
2%)
than for pericardial (97
±
1%) valves. Major reasons for explant
included structural valve degeneration (SVD), endocarditis
and periprosthetic leak. The durability of the pericardial valve
offered is superior to the traditional CE porcine valve. The
pericardial valve’s freedom from SVD and re-operation makes it
a favourable bioprosthetic choice for AVRs.
Bovine pericardium
Fixation with glutaraldehyde was initially considered to greatly
reduce the immune response to tissue xenografts. It is now
evident, though, that immune responses to glutaraldehyde-
fixed pericardium do still occur,
14,15
Antibody generation and
immune rejection in bioprosthetic heart valve degeneration is
now accepted, but the actual proteins responsible for triggering
the immune response are largely unknown. Antigen masking or
antigen removal is considered and implemented to improve the
durability and lower the body’s rejection response to implanted
bioprostheses.
A study was conducted into the material properties of the two
types of unfixed bovine pericardium supplied at Glycar Pty (Ltd).
These comprised a thin and thick version of unfixed pericardium,
forming 16 randomised batches, each batch containing 10 thin
and 10 thick sheets. The pericardial sacs used were sourced in
Namibia from animals, by circumferential dissection around the
top of the heart adjacent to the major vessels. The tissue was in
the form of a cone with a spherical tip, obtained from cattle aged
between 12 and 36 months.
The mean thicknesses of the thin and thick pericardium were
0.25
±
0.03 mm and 0.34
±
0.04 mm, respectively. The tensile
strength of the samples was calculated to be 20.75
±
2.38 and
24.09
±
3.85 MPa for the thin and thick samples, respectively.
The experimental results indicate that there is an associated
increase in tensile strength with an increase in thickness.
The increase in strength that the glutaraldehyde provides is a
further benefit in support of the use of the thicker version of
pericardium, and would ultimately prolong the fatigue life of
the leaflets.
The major drawback of using glutaraldehyde-fixed porcine
or bovine tissue in prosthetic implantation is calcification.
16
Although the exact details of the methods with which such
calcification occur are not known, it is felt that the aldehyde
elements do have an impact on the rate at which the degradation
occurs.
17
In lieu of this drawback, certain pre- and post-
processing techniques are implemented to reduce the risk of
calcification. These treatments result in the modification of
the amino acids, forming a protein mass that is insoluble in
the harshest of denaturing environments.
18
Hardening of the
pericardium is prevented by immersing it in distilled water after
glutaraldehyde treatment. Any swelling that may then occur is
thus catered for once integrated into the frame geometry.
Grabenwöger
et al
.
19
conducted a study into the denaturing
of glutaraldehyde-preserved and dye-medicated photo-oxidised
