Wear patterns from fatigue life testing of a prosthetic heart valve

Abstract

In the field of bioengineering, the design of prosthetic heart valves is optimized to avoid failure and to reduce wear as much as possible, in order to have a reliable function for a long time and to minimize the risk to activate red cell aggregation (thrombosis). Sorin Bicarbon valve was designed to improve haemodynamic performance of bileaflet valves, with regard to pressure gradient and haemolysis (turbulence) [V. Barbaro, M. Grigioni, C. Daniele, G. D'Avenio, G. Boccanera, 19 mm sized bileaflet valve prostheses' flow field investigated by bidimensional laser doppler anemometry (part I: velocity profiles), Int. J. Artif. Organs 20 (1997) 622–628; V. Barbaro, M. Grigioni, C. Daniele, G. D'Avenio, G. Boccanera, 19 mm sized bileaflet valve prostheses' flow field investigated by bidimensional laser doppler anemometry (part II: maximum turbulent shear stresses), Int. J. Artif. Organs 20 (1997) 629–636] as well as to wear, thanks to an original coupling system between housing and leaflets; the latter is the focus of this paper. The coupling system of Sorin Bicarbon allows the valve's leaflets to move, under blood action, virtually without sliding between the pivot edge on the leaflet and the corresponding point within the housing (slot). It is well known that the coupling design has a great relevance in the distribution of stresses between the two surfaces of leaflet and valve housing, determining the potential wear characteristics of the design [J.C. Bokros, A.D. Haubold, R.J. Akins, L.A. Campell, C.D. Griffin, E. Lane, in: E. Bodnar, R. Frater (Eds.), Replacement Cardiac Valves, McGraw-Hill, New York, 1992, pp. 21–48]. Thus, the Bicarbon was designed to allow the leaflet's pivot to roll without sliding within the housing slot, changing the point of contact and realizing a no-friction motion. To investigate wear patterns in the coupling zone, fatigue testing of four valves were performed at 1200 bpm. Several stages of wear were monitored, early, from the initial appearance of wear (10,000,000, 20,000,000 cycles) until the final stage (400,000,000 cycles) representing up to 11 years of fatigue life as number of beats (physiological average of 72 bpm). Wear patterns were recorded with a videomicroscope (Keyence VH-5901) and related to the motion of the valve's leaflets by means of a kinematic model of the rolling mechanism previously realized [V. Barbaro, G. Boccanera, C. Daniele, M. Grigioni, A. Palombo, Int. J. Artif. Org. 18 (8) (1995) 442; V. Barbaro, M. Grigioni, C. Daniele, G. Boccanera, in: H. Power, C.A. Brebbia, J. Kenny (Eds.), Simulation in Biomedicine IV, WESSEX Institute of Technology, 1997, pp. 349–358]. In fact, a kinematic description for the leaflets of the Bicarbon valve was obtained using the high-speed videography (HSV) technique, with a frame rate of up to 12,000 fps; the possible traces for the contact points within the coupling mechanism were determined. The kinematic model was then used to reproduce the trajectories of the contact point within the coupling and to study the motion of the valve parts producing wear: thus, the wear patterns obtained under in vitro laboratory conditions could be seen in accordance with the model. Finally, in vivo explant data were compared with in vitro fatigue wear to validate the accelerated beat fatigue testing of this valve.