Calcification of tissue heart valve substitutes: progress toward understanding and prevention

Calcification plays a major role in the failure of bioprosthetic and other tissue heart valve substitutes. Tissue valve calcification is initiated primarily within residual cells that have been devitalized, usually by glutaraldehyde pretreatment. The mechanism involves reaction of calcium-containing extracellular fluid with membrane-associated phosphorus to yield calcium phosphate mineral deposits. Calcification is accelerated by young recipient age, valve factors such as glutaraldehyde fixation, and increased mechanical stress. Recent studies have suggested that pathologic calcification is regulated by inductive and inhibitory factors, similar to the physiologic mineralization of bone. The most promising preventive strategies have included binding of calcification inhibitors to glutaraldehyde fixed tissue, removal or modification of calcifiable components, modification of glutaraldehyde fixation, and use of tissue cross linking agents other than glutaraldehyde. This review summarizes current concepts in the pathophysiology of tissue valve calcification, including emerging concepts of endogenous regulation, progress toward prevention of calcification, and issues related to calcification of the aortic wall of stentless bioprosthetic valves.