Bioprosthesis Dysfunction
Biological heart valves consist of fixed or decellularized human or
animal (usually porcine or bovine) tissue, attached or not to a stent
(without a stent in the stentless prosthesis) and a sewing ring (without
sewing ring in the sutureless prosthesis). Structural dysfunction is a
logical and expected consequence of the chemical, mechanical and
immunological processes that occur during fixation, fabrication and
implantation.
Beside all the technological development in the past decades,
bioprosthesis only replicate native heart valve structure. In
bioprosthesis, 1) the cusps are constituted by subendothelial connective
tissue; 2) the cusps are locked in a static geometry and 3) have
nonviable cells due to the fixation and crosslinking processes (21, 22).
They lack the functional capacities of heart valves and the preservation
techniques severely decrease functional activity of the natural matrix
(17). Thus, they are not able to maintain the normal remodeling
processes.
Over the past 50 years, glutaraldehyde (Glut) has been the most used
chemical and is nowadays widely used to preserve and stabilize
biological prosthetic tissues. Glut is an aldehyde with fixative and
preservative functions (43), firstly introduced in 1963 by Sabatini et
al as a fixative for electronic microscopy. Glut is responsible for the
chemical crosslinking (creation of covalent chemical bonds to stabilize
tissues and terminate any ongoing biochemical reactions), enhancing
material stability and reducing antigenicity (29, 30). However, Glut is
also partly responsible for prosthesis dysfunction, directly because of
its toxicity and indirectly through the processes described ahead.
Valve degeneration is a multifactorial process, including chemical,
mechanical and immunological factors (Figure 2). The contribution of
each mechanism remains poorly understood, but represents an active and
attractive field of investigation. The common endpoint of all these
processes is calcification and degradation, culminating in the failure
of the structure with bioprosthetic dysfunction.