Three-dimensional imaging of fractures in outlet struts of Björk-Shiley convexo-concave heart valves by micro-computed tomography in vitro

Background and aims of the study: For implanted Björk-Shiley convexo-concave (BSCC) heart valves, structural failure of the valve's U-shaped outlet strut results in embolization of its blood flow-regulating disc (occluder), with consequent patient morbidity and mortality. After a variable and unpredictable number of cardiac cycles, one strut leg may fatigue ('single-leg separation'; SLS); subsequently the other strut leg may also fatigue, resulting in full structural failure ('outlet strut failure'; OSF). Some BSCC valves are believed to be at more risk of SLS and OSF than others. As valves may function in the SLS condition for some time before OSF occurs, several investigators have sought non-invasive methods to differentiate valves with SLS struts from valves with intact struts in order to provide a rationale for prophylaxis. Herein, we report the use of X-ray microcomputed tomography (micro-CT) to image and characterize SLS strut fractures, including fracture faces otherwise visible only by means of physical sectioning. Methods: An X-ray micro-CT system was adapted to provide high-resolution, three-dimensional (3D) images of intact and fractured BSCC valve outlet struts in vitro. System modifications included use of a tungsten anode X-ray source to achieve sufficiently high X-ray energies to overcome attenuation within the metal structures, and a hafnium filter to minimize the imaging artifact caused by X-ray beam hardening. For rotating the valve for tomographic scanning, special alignment procedures were developed to maintain the region of interest within the field of view. Typical 3D images of the outlet struts were composed of cubic voxels, 10 μm on a side. Image analysis and display software was used to view the outlet struts and the fractures from several perspectives, including en-face images of fracture surfaces. Results: 3D volume data representations of the SLS and intact outlet struts were obtained, facilitating identification of fracture location and geometry. En-face images of the fracture surfaces were also generated. Several different fracture geometries were observed, such as fractures with and without longitudinal gaps between the fracture faces, and fractures with and without lateral displacement between the faces. En-face views showed varying degrees of roughness on fracture faces. Conclusion: This application of micro-CT to image outlet strut fractures in BSCC valve explants demonstrates the value of this method for fracture characterization in vitro, including visualization of fracture faces of SLS struts without physical sectioning. Although the method is not suitable for clinical use because it requires high-intensity X-rays, micro-CT can serve as a tool to understand further any failure mechanisms, and to aid the development of clinical differentiation methods.