Wear patterns in the Sorin Bicarbon mechanical heart valve: a clinical explant study.

BACKGROUND AND AIMS OF THE STUDY: Mechanical heart valve prostheses are subject to wear due to impact and friction between the occluder(s) and valve housing. Reference data on the extent of wear in vivo is lacking. Therefore, the aim of this study was to describe the wear pattern in Sorin Bicarbon valves explanted from humans for comparison with the findings established in an earlier in vitro study. METHODS: We investigated eight valves explanted from seven patients, which had been functioning for more than six months and were returned to the Sorin Company for investigation. The protocol comprised: (i) initial visual inspection on receipt; (ii) stereomicroscopy after cleaning and disassembly; (iii) computed planimetry of the worn areas in the housing; (iv) pivot surface profilometry; and (v) scanning electron microscopy. RESULTS: All wear depths and exposed areas of titanium were less than that predicted from in vitro figures. No valves were explanted due to mechanical failure and none of the patients had reported peripheral embolic events which could be attributed to valve wear. The extent of wear was comparable with that of other valves. CONCLUSIONS: This descriptive study did not indicate any association between the Bicarbon valve wear characteristics and clinical complications. Clinical studies on valve performance are of paramount importance for elucidating this issue. The establishment of an independent body for studying explanted valves in collaboration with the manufacturers may promote the publication of findings which are of interest for the entire medical community.

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).

Relationship between some design characteristics and wear in the Bicarbon heart valve prosthesis.

The hinge design of a new bileaflet valve (Bicarbon) is presented in relation to the long-term durability characteristics. Theoretical considerations supported by experimental findings, lead to the identification of two different wear mechanisms acting in bileaflet valves, i.e. impact and friction wear. Impact wear, caused by the collision between the hinge stops and inflow surface of the leaflet pivots, is the predominant phenomenon; it is mostly influenced by the design of the coupling elements. The wear due friction is significantly less important, given a proper kinematic coupling and a thorough washout. These different mechanisms can significantly affect the wear resistance and long-term reliability of bileaflet valves; therefore, they have been properly taken into account in designing the new valve model. The safety of the overall material and design solutions adopted in the Bicarbon valve has been confirmed by extensive accelerated durability tests resulting in no mechanical failure or loss of functionality.

Properties of a new carbon film for biomedical applications.

The physical and chemical properties of carbon films, about 0.5 mu thick, deposited at low temperature (40-200 degrees C) by a new physical vapour deposition technique are presented, in comparison with those of pyrolytic carbon normally used in biomedical devices. Several analytical techniques have revealed that the thin carbon films present structural properties similar to those of pyrolytic carbon. For example, similar chemical bondings of C atoms are observed in both samples. Moreover the data suggest the presence in thin C films of both tetrahedral and graphitic bondings, as in the turbostratic structure typical of pyrolytic carbon. These results confirm the similar biocompatibility properties of both types of materials already established by in vivo experiments.