Fibrous tissue overgrowth on Hancock mitral xenografts: a cause of late prosthetic stenosis.

Severe prosthetic stenosis requiring reoperation seldom occurs after mitral valve replacement with the Hancock bioprosthesis. Two cases of prosthetic stenosis caused by tissue overgrowth on the atrial aspect of the cusps are reported. The patients, who had undergone mitral replacement with a Hancock valve implanted in the subannular position, developed the signs of congestive heart failure 5 and 6 years after surgery. In both patients, the hemodynamic data supported the clinical diagnosis of mitral restenosis and both underwent successful reoperation. Histologic examination showed that the pannus was formed by dense fibrous tissue with few vessels. It is suggested that the site of the bioprosthesis insertion is likely to have favored this complication since in this condition the protection of the sewing ring from tissue overgrowth is minimal.