Attempts to replace diseased human valves with
prostheses began more than 30 yrs ago.
Heart valve prostheses can be broadly classified into mechanical
prostheses (made out of non-
biological materials) and
bioprostheses made out of
biological tissue.
Biological valves are made from animal tissue bovine pericardium and porcine valves. The use of these tissues became commercially available after the introduction of the
glutaraldehyde (GA) fixation technique. GA reacts with tissue
proteins to form inter- and intramolecular crosslinks, resulting in improved durability. The advantage of
bioprostheses compared with mechanical valves is the freedom from
thromboembolism; and therefore, the avoidance of long-term anticoagulation
therapy. These
prostheses are preferable in elderly people and in patients who do not tolerate
anticoagulants. However, tissular calcification and primary tissue failure (caused by the mechanical stress) are the main unresolved problems. The causes of calcification are numerous and, to date, a satisfactory
solution to this question has not been found, although chemical treatments with
metal cations,
diphosphonates and treatments eliminating
phospholipids have proved to mitigate calcification. In addition, alterna-tive approaches to GA chemical treatment fixation are being proposed to provide the tissue with greater resistance to this process. Studies are under way using polyepoxy compounds, derivates of amino
oleic acid (AOA), agents such as diphenylphosphorylazide,
carbodiimide,
amino acids etc. Further improvements in fixation techniques, as well as in
bioprosthesis design (stentless valves) are being made to improve the durability and functional characteristics of bioprosthetic heart valves. The development of a
biomaterial capable of withstanding calcification and mechanical stress, while being as durable as mechanical
prostheses, would convert the
bioprostheses into the replacement of choice by eliminating the need for anticoagulation
therapy.