Analysis of the Medtronic Intact bioprosthetic valve. Effects of "zero-pressure" fixation

The long-term performance of current-design porcine xenograft valves has not been satisfactory. These valves are generally fixed at "low pressures" of about 3 to 5 mm Hg. The Medtronic Intact (Medtronic, Inc., Minneapolis, Minn.) valve is fixed at "zero pressure" and is proposed as a better alternative to existing xenograft valves. A mechanical analysis of this valve has been carried out to determine if the Intact valve differs significantly from the low-pressure fixed xenograft. Twelve circumferential strips of tissue 5 mm wide were cut from the leaflets of four clinical-grade Intact valves. Their stress/strain, stress relaxation, and flexural behavior were examined mechanically and histologically. The Intact valve was more extensible than the low-pressure fixed xenograft (22% versus 12% strain, p less than 0.001), relaxed faster (p less than 0.001), and was more pliable than the xenograft (p less than 0.05). It did not, however, buckle less than did the low-pressure fixed xenograft during enforced bending, and it buckled significantly more than did fresh porcine aortic valve tissue (p less than 0.001). The Intact valve also relaxed significantly more slowly than did the fresh tissue (p less than 0.05). Its bending stiffness had a stronger dependence on leaflet thickness than the bending thickness of fresh tissue had (p less than 0.001) but a weaker dependence than the bending thickness of the low-pressure fixed xenograft material had (p less than 0.001). The Intact valve demonstrated a very large variability in extensibility, bending stiffness, and buckling behavior, with little correlation between these parameters. Some valves appeared to have wrinkled leaflets; others were likely fixed at different pressures. The shrinkage of the leaflet material at these low fixation pressures is likely important, since it can modify the elastic behavior of the valve cusps. Overall, the Intact valve had a more "natural" elastic behavior than had low-pressure fixed xenograft, and it should therefore experience lower stresses during normal valve function. It can be concluded that zero-pressure fixation does preserve many of the desirable stress-reducing properties of aortic valve tissue.