Abstract
In Reply.— We appreciate the opportunity to respond to the thoughtful letter by scientists with expertise in medicine and material science. They make interesting points. Our group has also experienced occasional valve strut fractures (and Pyrolite carbon-disk fractures) in spherical-disk Bjork-Shiley valves having a weld at the major strut (the valve that fractured in Dr Clark's pump was a spherical-disk valve of this type). For a number of years, Shiley Laboratories has manufactured the 60° convexoconcave valve in which the major strut is an integral part of the valve housing, ie, no major strut weld is present. Using this housing and an acetal (Delrin) disk, our group has never encountered a valve failure in extensive bench testing or in animal studies. The Food and Drug Administration had no role in our observing the early valve fractures or in our solving the valve fracture problem. The relative advantages of the Medtronic-Hall
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