New method for in-vitro wear assessment of materials used in mechanical heart valves

Abstract

Accelerated life testing has been in use for a number of years to study the long-term in-vitro durability of heart valves. These tests were designed to test concurrently the durability of the materials and design of the heart valves. However, problems interpreting the results and relating them to actual clinical applications were encountered. Current view is that the life testing of mechanical heart valves can best be performed by partitioning the in-vitro wear assessment of the material separate from that due to the overall design of the heart valve. A new method, simulating the impact-cum-sliding action in mechanical heart valves, at a low frequency, was developed to test a variety of candidate materials: polyoxymethylene (Delrin), polyetheretherketone (PEEK), polyethersulphone (PES), and ultrahigh molecular weight polyethylene (UHMWPE). The results were compared with those reported earlier in published literature. The damage features seen on the different test specimens and the wear debris produced were similar to those found on an explanted heart valve of more than 17 years old, supporting that the present technique could simulate the wear conditions in mechanical heart valves. The technique saved time and was amenable to wear mechanisms study using the scanning electron microscope (SEM). The wear results showed that, in terms of maximum wear depth, UHMWPE wore the least followed by Delrin, PEEK, and PES. There does not appear to be any correlation between wear and the molecular weight of Delrin. The incorporation of 0.3% carbon did not improve the wear performance of Delrin. Both UHMWPE and PES produced debris that were significantly larger than the rest. Because of the possibility of dislodgement of large debris in the range of 100 μm or more, UHMWPE was not considered to be a good material for use as occluder in mechanical heart valves even though it wore the least. At present, Delrin is still considered to be the best occluder material examined. PEEK showed good potential but longer experiments need to be carried out in order to show its full potentiality. PES showed the worst wear characteristics.