THEORY OF WEAR AS RELATED TO THE BJORK-SHILEY DELRIN(R) DISC HEART-VALVE

Background and aims of the study: Mechanical wear is an important cons ideration for the Bjork-Shiley Delrin(R) (BSD) heart valve, the disc o f which is periodically impacted against the inlet strut by the moment um of the blood flow during closure. Impact wear theory was used in de signing experiments as well as establishing theoretical evaluation and projections of wear life. Materials and methods: The experimental app aratus involved a pivotal hammer device where the striking face could be varied by the inclusion of distinct spherical shapes; the sharper t he radius, the higher the contact stress induced. The striking speed c ould be varied between one and three m/s, and the repetition rate betw een 2-50 Hz. blood was simulated with an aqueous 42% glycerol solution . The same wear mechanism was induced on Delrin occluder disc specimen s as the one governing the wear observed in Delrin occluder discs of s ome of the explanted BSD heart valves; thus two wear parameters, c and g, were established for the prediction of wear in BSD implants. The e xperimental runs were repeated on as many as four simultaneous specime ns, for up to 7.5 x 10(7) cycles. Results: The wear history was found to comprise three distinct regions, corresponding to (a) initial plast ic deformations; (b) zero wear; and (c) measurable wear. The zero wear region produced very little change in the contact dimensions, and rep resented a fatigue threshold to the progress of a wear scar. Procedure s for analytical establishment of the zero wear limit are outlined in the paper. Measurable wear was governed by the process of increasing c onformance between the repetitively contacting surfaces. The different ial equation of wear was then used with the physical parameters c and g previously determined experimentally. Conclusion: The maximum depth of the wear scar after cycling the equivalent of 20 years was predicte d to be 278 mu m. This value appears to represent a safe dimension for BSD heart valves.