Pure pyrolytic carbon: preparation and properties of a new material, On-X carbon for mechanical heart valve prostheses.

BACKGROUND AND AIM OF THE STUDY: Historically, the pyrolytic carbon used in mechanical prosthetic heart valves contained small amounts of silicon, this being a necessary additive to achieve consistently the hardness required for wear resistance. New processing technology has allowed the deposition of pyrolytic carbon without silicon, while maintaining adequate hardness to ensure wear resistance. METHODS: A parametric study of coating parameters identified the conditions necessary to produce the optimal pure carbon material. RESULTS: In comparison with silicon-alloyed carbon, the pure carbon was found to be about 20% stronger, have a strain-to-failure about 25% higher and have a greater toughness. CONCLUSIONS: The enhanced strength, deformability and toughness of the new carbon permits designers to utilize component shapes and dimensions that could not be manufactured using the silicon-alloyed carbons. Such design features have hemodynamic benefits resulting in valve performance improvements.

Early postoperative echocardiographic hemodynamic performance of the On-X prosthetic heart valve: a multicenter study.

BACKGROUND AND AIMS OF THE STUDY: This study aimed to investigate the early postoperative Doppler-derived hemodynamic results from the first patients receiving the On-XR prosthetic heart valve, a new bileaflet, pyrolytic carbon valve. METHODS: Data were derived from 111 patients included in a 10-center international trial between September 1996 and December 1997. RESULTS: The effective orifice area (EOA) for the valve, when implanted in the aortic position, ranged from 1.5 to 2.7 cm2 in 19 mm to 25 mm valves. The corresponding mean pressure gradients ranged from 11.8 to 7.6 mmHg. Mitral EOA was 2.3 cm2 for all sizes, these values being combined because the housing was identical for all mitral valves used in this study. The mitral mean gradient was 4.7 mmHg. CONCLUSIONS: Early results of the study show the good hemodynamic performance of the On-X valve when implanted in the aortic or mitral positions.

Pyrolytic carbon indentation crack morphology.

In studying fatigue and fracture behavior of brittle materials, Vickers diamond indentation cracks are often used. Many of the studies of indentation cracks use crack system models such as the radial-median crack or Palmqvist crack. These systems are also used to study small crack growth in brittle materials, and have been studied for pyrolytic carbon. However, the true morphology of these cracks in pyrolytic carbon coatings on graphite substrates have not been described. This study examined Vickers diamond and spherical ball indentation cracks in pyrolytic carbon coatings using several techniques, including serial metallographic cross sections, indentation fracture in bending, acoustic emission, and residual surface indentation scanning. The crack systems developed using these techniques were not typical of either radial median or Palmqvist systems. The morphology is unique to this material, possibly because of the coating thickness limitations. Given the difference in crack system, the application of standard indentation crack equations in studying fracture mechanics, especially for small cracks, must be questioned.

Finite element analysis of indentation tests on pyrolytic carbon.

The stresses which cause failure at contact areas between leaflets and orifices in pyrolytic carbon heart valves are evaluated. These contact stresses have previously been studied using Hertzian crack models that apply to monolithic material. Many heart valves are not monolithic pyrolytic carbon but a pyrolytic carbon deposited on graphite. Contact loads on these layered structures cause initial cracking in the pyrolytic carbon at the interface between pyrolytic carbon and graphite rather than Hertzian surface cracks. Increasing the load on layered structures will cause a secondary cracking (of Hertzian cracks) on the surface. The contact loading was simulated with a 5.1 mm diameter ball pressing against a flat sample of graphite coated with 0.26 mm of pyrolytic carbon on each surface. Finite element analysis of this model calculated the stresses associated with a range of loads causing no cracks through initial interface cracks and secondary surface cracks to complete failure. The calculated stresses are correlated with parallel laboratory experiments. A failure criterion for contact stresses is developed. The initial cracks at the graphite/pyrolytic carbon interface occur when the tensile stress in the pyrolytic carbon reaches 207 to 276 MPa and the compression stress in the graphite reaches 414 to 483 MPa. These initial cracks do not propagate immediately to the surface since they run into a high triaxial compression stress field. Circular surface cracks occur at the edge of the ball/pyrolytic carbon contact area at higher loads. These cracks require a shear stress of about 241 MPa and also require a tensile stress component. The results provide a criterion for designing contact regions in pyrolytic heart valves.

Effect of cavitation on pyrolytic carbon in vitro.

It has long been known that mechanical heart valves, when tested for durability using non-physiologic conditions common in accelerated testers, would cavitate. Until recently, cavitation was never observed in vivo. The discovery that a small number of Edwards-Duromedics heart valve explants indicated signs of cavitation erosion prompted a reassessment of the cavitation erosion potential of pyrolytic carbon (PyC). Analyses of the explanted valves indicated that cavitation may be accentuated by porous regions in the pyrolytic carbon coating from which most mechanical heart valves are constructed. Early studies have shown that for PyC (a) the resistance to cavitation erosion is comparable to that of aluminum, (b) the resistance to cavitation erosion is high initially, but with time the erosion rate accelerates, and (c) the cavitation erosion resistance is somewhat variable. In this study, similar experiments were performed utilizing polished pyrolytic carbon as well as microporous surfaces since microporous surfaces have been implicated as accelerating erosion. Within the accuracy of the measurement, we found no contributing acceleration due to the microporous nature of the pyrolytic carbon surfaces tested when compared to the polished surfaces. Examination of cross sections of samples exposed to cavitation conditions revealed the presence of extensive microcracking even without the presence of substantial surface erosion.