Polyurethane: material for the next generation of heart valve prostheses?

OBJECTIVES: The prospects for a durable, athrombogenic, synthetic, flexible leaflet heart valve are enhanced by the recent availability of novel, biostable polyurethanes. As a forerunner to evaluation of such biostable valves, a prototype trileaflet polyurethane valve (utilising conventional material of known in vitro behaviour) was compared with mechanical and bioprosthetic valves for assessment of in vivo function, durability, thromboembolic potential and calcification. METHODS: Polyurethane (PU), ATS bileaflet mechanical, and Carpentier-Edwards porcine (CE) valves were implanted in the mitral position of growing sheep. Counting of high-intensity transient signals (HITS) in the carotid arteries, echocardiographic assessment of valve function, and examination of blood smears for platelet aggregates were undertaken during the 6-month anticoagulant-free survival period. Valve structure and hydrodynamic performance were assessed following elective sacrifice. RESULTS: Twenty-eight animals survived surgery (ten ATS; ten CE; eight PU). At 6 months the mechanical valve group (n=9) showed highest numbers of HITS (mean 40/h, P=0.01 cf. porcine valves), and platelet aggregates (mean 62.22/standard field), but no thromboembolism, and no structural or functional change. The bioprosthetic group (n=6) showed low HITS (1/h) and fewer aggregates (41.67, P=1.00, not significant), calcification and severe pannus overgrowth with progressive stenosis. The PU valves (n=8) showed a small degree of fibrin attachment to leaflet surfaces, no pannus overgrowth, little change in haemodynamic performance, low levels of HITS (5/h) and platelet aggregates (17.50, P<0.01 cf. mechanical valves, P=0.23 cf. porcine valves), and no evidence of thromboembolism. CONCLUSIONS: In the absence of valve-related death and morbidity, and retention of good haemodynamic function, the PU valve was superior to the bioprosthesis; lower HITS and aggregate counts in the PU valve imply lower thrombogenicity compared with the mechanical valve. A biostable polyurethane valve could offer clinical advantage with the promise of improved durability (cf. bioprostheses) and low thrombogenicity (cf. mechanical valves).