Heart valves are dynamic structures that experience constant and cyclic mechanical stress from the hemodynamic forces intrinsic to its function. When the function of a natural heart valve declines or fails, replacement is typically required with a bioprosthetic heart valve.
One common type of bioprosthetic heart valve is a biological tissue valve, which is usually coupled to and supported by a metal frame. The metal frame can be either a wireform or a collapsible/expandable stent. Once implanted, the bioprosthetic heart valve is subjected to cyclic hemodynamic forces, causing the leaflets to open and coapt. These forces, in turn, impart mechanical stresses onto the supporting metal frame. It is therefore desirable for the metal frame to have a structural integrity that is capable of withstanding these stresses.
The surface of a metal frame, such as a wireform or stent, may often be riddled with small imperfections that can ultimately lead to reduced fatigue life and premature failure. These flaws can be in the form of inclusions (particles), draw lines, knit lines, or scratches, which are introduced during the manufacture of the wire or tube used to form the wireform or stent, respectively. It is therefore desirable to remove or ameliorate these imperfections before incorporating the metal frame into a heart valve.
One method of addressing the surface imperfections of the metal frame is to mechanically polish the surface. It is difficult, however, to mechanically polish the surface of a metal frame because the surface is not flat and typically has intricate or curved geometric configurations. It would be prohibitively difficult to uniformly polish the surface of the shaped metal frame. A similar challenge is presented with respect to electropolishing. Additionally, while mechanical polishing or electropolishing may remove certain imperfections, they may expose certain other imperfections existing below the surface of the metal frame.
What is therefore needed is a method for treating a metal frame of a bioprosthetic heart valve to improve its fatigue life, and thus, durability once implanted in a patient.