Wear assessment in bileaflet heart valves

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