WEAR ASSESSMENT IN BILEAFLET HEART-VALVES

Sorin Biomedica has accumulated 20 years of experience in designing an d manufacturing heart valves as well as in the development of turbostr atic carbons, both as bulk coating (pyrolytic carbon, PyC) and as a th in film (Carbofilm). In designing a bileaflet valve, where hinges repr esent the most critical elements since wear occurs mainly at these poi nts, we took into account: a) theoretical considerations on the geomet ry (flat-to-flat vs. curved-to-flat) of the coupling elements undergoi ng impact wear; b) kinematic coupling (sliding, rotation, rolling) bet ween the moving parts in relation to friction wear; c) experimental we ar rates of different material couples (PyC/PyC, metal/PyC) assessed b y paying attention to test artifacts due to particle contamination; an d d) wear mechanisms involving brittle fracture mechanics for bulk PyC and ductile fracture mode for metals. On the basis of the above evalu ations a Carbofilm coated titanium alloy housing and PyC leaflets were developed for the Sorin Bicarbon valve. A flat-to-flat coupling betwe en hinge stops and mating pivot surfaces for reducing the impact wear and a rolling action aimed at minimizing the friction wear were design ed for the hinge. The Bicarbon long term durability has been assessed by accelerated wear tests conducted in comparison with clinically acce pted bileaflet prostheses (CarboMedics and St. Jude Medical). The main results for the Bicarbon valve were, wear rates slightly lower or com parable to those found in the reference valves and wear morphology fre e from microfractures, while deep cracks associated with higher stress concentrations were detected on the PyC components of the reference v alves. No mechanical failure or loss of functionality occurred up to 2 ,100 million cycles (equivalent of 52.5 years).