Prosthetic valve degeneration and stent creep: “Stent Creep” seen in some types of bioprosthesis is due to inward bending of the stent struts . It may not be obvious on 2-D echocardiographic imaging, but increases the transvalvular gradient as measured by Doppler echocardiography. Stent creep tends to increase with time after implantation . It was due to an ultrastructural deformity of polypropylene used in the valve stent . This was noted in explanted Hancock bioprosthesis which led to the development of newer stent materials resistant to such deformity.
Valve-in-valve transcatheter aortic valve implantation has been used to treat bioprosthetic failure due to stent creep . They reported three cases, of which two had Carpentier Edwards Perimount prosthesis while one had Mitroflow. Authors suggested that valve oversizing could be the reason for the permanent inward deflection of the stent posts.
Three dimensional echocardiography geometric analysis has been used for analysis of failed bioprosthesis to determine stent creep . Three stent posts are used in bioprosthetic valves to provide support to the leaflets. These stent posts are perpendicular to the annular plane and situated at the corners of a bottom triangle. The tips of the posts also are at the corners of a tip triangle. Normally the areas of both these triangles are equal. When stent creep develops, top triangle shrinks due to inward bending of the stent posts. When stent creep develops, the ratio between the top and bottom triangle areas (stent creep ratio) becomes less than one. This can be measured by 3-D echocardiography and gives an assessment of stent creep non-invasively .
Mechanical valve malfunction
Mechanical valve malfunction could be due to ball degeneration, ball variance, strut fracture, thrombus or pannus. Pannus is the growth of fibrous tissue into the valve structure resulting in increase of transvalvular gradient.
Akiyama K, Sawatani O, Imamura E, Endo M, Hashimoto A, Koyanagi H. Clinical study of “stent creep” in porcine bioprosthesis. Nihon Geka Gakkai Zasshi. 1988 May;89(5):747-51.
Valente M, Bortolotti U, Thiene G, Arbustini E, Milano A, Mazzucco A, Gallucci V. Post bending of the polypropylene flexible stent in mitral Hancock bioprostheses. Eur J Cardiothorac Surg. 1987;1(3):134-8.
Attia R, Papalexopoulou N, Hancock J, Young C, Thomas M, Bapat V. Successful treatment of failing biological prosthesis because of “Stent creep” with valve-in-valve transcatheter aortic valve implantation. Catheter Cardiovasc Interv. 2015 Aug;86(2):E119-25.