Formation of cartilage in bioprosthetic cardiac valves implanted in sheep: a morphologic study

Foci of cartilage were found in 12 of 120 bioprostheses implanted in young sheep for 13 to 24 weeks, but in none of 47 bioprostheses implanted for less than 13 weeks. Cartilage was found more frequently (p less than 0.01) in bioprotheses implanted in the tricuspid position than in those implanted in the mitral position. In porcine aortic valvular bioprostheses, the cartilage was preferentially localized in the region of the muscle shelf; in pericardial bioprostheses, it occurred in the fibrous sheaths covering the cusps. In both instances, the cartilage was found to undergo calcification and was considered to be formed by metaplasia of connective tissue cells of host origin.